I  TP761
    .C65A53
 -  v.z                              OOOR90002
 I
 I
 I
 I    ENVIRONMENTAL  PROTECTION
 •    AGENCY

                            SPECIAL REPORT
 |                    OFFICE OF MOBILE SOURCES
 I
 I
                Analysis of the Economic and
 •          Environmental Effects of Compressed
 •              Natural Gas as a Vehicle Fuel
 •                      Volume li
 I                 Heavy-Duty  Vehicles
                        April 1990
I
I
I

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     This  report  addresses  the  economic   and   environmental
issues associated with the  use  of compressed  natural gas  as  a
motor vehicle  fuel.   Volume  I  analyzes  the  use  of  compressed
natural  gas  as  a  fuel   for  passenger  cars   and  light  trucks.
Volume  II  considers  the use  of  compressed  natural  gas  as  a
heavy-duty vehicle fuel.

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Analysis of the Economic and Environmental Effects
    of Compressed Natural Gas as a Vehicle Fuel
                       Volume  II
                  Heavy-Duty Vehicles
                       April  1990
                            U.S. Environmental Protection Agency
                            Por^-iu 5, L'bv.u-y (iPL-16;
                            R">\: •).  i/o.-x. ter.~  Lh--eet, Loom 1670
                            Kdoago,  IL   60504

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                             Table of Contents


 Chapter                                                              Page

I  1        Introduction                                               1-1

   2        Uses of CNG in Heavy-Duty  Vehicles
           I.    Historical Use of CNG  in  Heavy-Duty
                  Vehicles                                           2-1
           II.   Recent  Progress  in CNG Heavy-Duty
                  Vehicle Applications                               2-3
           III.  Potential Future Markets  for  CNG
                  Heavy-Duty Vehicles                                2-11

   3        CNG Heavy-Duty Vehicle Technology
           I.    Introduction                                         3-1
           II.   Fuel Properties  of Natural Gas                       3-1
           III.  Current Heavy-Duty CNG Technology                    3-3
                A.    Engines                                        3-3
                B.    Fuel Storage                                   3-7
                C.    Emissions  from Current  Technology
                        CNG vehicles                                  3-9
                      1.    Dual Fuel  Vehicles                       3-9
                      2.    Dedicated  CNG Vehicles                   3-14
                D.    Engine Efficiency                              3-21
                E.    In-use Performance                             3-25
           IV.   Optimized Vehicle Projections       '                3-26
                A.    Emissions                                      3-26
                B.    Future Optimized Engine Efficiency            3-30
           V.    Safety  Issues  for CNG  uses in Heavy-Duty
                   Application                                        3-31
                A.    Introduction                                  3-31
                B.    Fuel  Properties  and General
                        Considerations                              3-32
                       1.    Toxicity                                3-32
                       2.    Flammability                             3-32
                       3.    Hazards  Associated With a  Fire          3-35
                      4.     Issues  of  Special Concern  in
                               The Use  of  OIG                        3-36
                C.     Implications  for Vehicle Safety                3-37
                       1.     Refueling                                3-37
                       2.    Vehicle  Operation and  Crashes           3-38
                       3.    Maintenance                             3-40
                 D.     Summary                                        3-41

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                       Table o£ Contents  (cont'd)_

lhapter                                                              Page

  14        Economics of Using CNG in Heavy-Duty Application
          I.     Introduction                                         4-1
          II.    Domestic Natural Gas Supply and Price                4-1
          III.   CNG Refueling Station Cost                           4-2
                A.    CNG Refueling Station Hardware                 4-2
                B.    CNG Refueling Station Hardware Cost            4-5
                C.    Total CNG Refueling  Station Cost               4-8
          IV.    Heavy-Duty CNG, Gasoline and Diesel
                 Vehicle Fuel Cost                                  4-12
                A.    Basis of Comparison                            4-13
                B.    Compression  and Station Maintenance Costs      4-13
                C.    Capitalized  Service  Station Cost               4-14
                D.    Relative Fuel Prices                           4-15
                E.    Relative Vehicle Fuel Costs                    4-15
          V.     Heavy-Duty CNG Engine and  Vehicle Costs              4-20
                A.    Engine Costs                                   4-20
                B.    Vehicle Costs                                  4-20

  5        Air  Quality Benefits
          I.     Introduction                                         5-1
          II.    Urban Ozone Level                                    5-1
          III.   Air Toxics                                           5-9
          IV.    Global Warming                                       5-15
          V.     Other Air Quality  Impacts                            5-20

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                           CHAPTER 1

                          INTRODUCTION
     This report  is one  in a  series of  reports which  EPA is
preparing in  order  to  describe  the  environmental and  economic
impacts of various  clean alternative fuels.   It  deals  with -he
use of  compressed  natural gas  (CNG)  as  a  fuel  for  heavy-duty
vehicles.    CNG,   of  course,   also   is  used   in   light-duty
vehicles.   However,  the nature and issues  associated  with these
two  areas  are  sufficiently different  that  EPA  has chosen to
issue  separate  volumes  on  light-duty  and  heavy-duty  uses  of
CNG.   Other   reports  in  this  series  already  issued  or  being
prepared deal with  methanol,  ethanol, liquified  petroleum gas,
electricity  and  reformulated  gasoline.   In  sum,   they  will
provide  a  comprehensive   review  of  the   choices  of   clean
alternative fuels  which might  be used  to move  America  toward
cleaner cars and trucks and reduced dependence upon petroleum.

     This  report  was  provided  in draft  form  for   review  and
comment to  other  EPA offices  as well as  a  variety  of  external
organizations.  A number  of  comments  were received  in  response
to  this  request  for  review.   To  the  extent  possible  these
comments have been  incorporated  into  this  final  version  of the
report

     When  considered  as  a  vehicle   fuel,   CNG  is  distinctly
different from conventional gasoline  or  diesel  fuels  in that it
is  a  gas  at  all normal  temperatures and pressures.   Thus, it
requires different  approaches  to vehicle refueling and to  fuel
storage on the  vehicle.   On the other hand, being gaseous means
that  the  entire  CNG  fuel  system  must   be  a  closed  one,
eliminating evaporative  emissions entirely.   Because  of  this,
and other  clean burning  characteristics of  CNG, it  offers the
potential for significant emission reductions in vehicle uses.

     Considerable  experience  with  CNG  use  in  a   variety  of
heavy-duty   applications   already   exists.    CNG   has   been
successfully  used to power  vehicles  ranging  from light delivery
trucks  to  full-sized  urban buses.   These  applications  have
generally been  based upon conversions of existing truck engines
to  run  on  both  gasoline  and CNG;  which  has  allowed  the  use of
CNG  fuel  even  though  it  is  not widely  available  at fueling
stations.  However, when considered from the perspective of use
as  a  clean   alternative  fuel,   the use  of  dedicated  vehicles,
which  can  run  only  on  CNG,  assumes  a  dominant  role.   Such
vehicles,  because  they  can  be  optimized to  make   use  of the
specific  combustion properties  of CNG,  hold  promise  of  much
greater  emission  reductions and fuel consumption gains than do
dual-fueled vehicles.

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e
                              1-2

     The remainder  of this  report examines  in greater  detail
the  use  of  CNG  in  heavy-duty  vehicles,   both  current  and
future.   It begins with  an  introductory overview of current and
historical     applications    for   CNG    vehicles,     and    the
identification of  types  of  uses where  the most  ready  growth of
CNG  would  be  possible.    Chapter   3  describes   CNG   engine
technology   and   presents   emissions  data   for   CNG   fueled
heavy-duty  engines.   Based  upon  projections  of  potential
progress in  emissions control  and fuel  consumption,  emissions
estimates  for  more  advanced   future  CNG   engines  are  also
developed.    Chapter   4  reviews  the  costs  associated with CNG
use.  These  include  vehicle  costs  for  engine  and  fuel  tank
hardware,  fueling  station  costs and  fuel  costs.   Since  heavy
duty applications  for CNG  represent  a relatively  small  impact
on  overall  natural gas use, fuel  cost  estimates  are based upon
the  assumption  that  current   natural  gas   supplies  to  the
domestic market  can  be  used,   with  negligible  impact  on gas
prices.   Finally,  Chapter 5 provides  a comparative  analysis of
the   environmental   benefits   of   CNG   compared   to   both
gasoline-fueled  and  diesel  engines.   This  analysis  focuses
principally  on  the  areas  of  ozone,   air   toxics  and  global
warming  and  shows  that CNG  indeed has  the potential to  provide
significant emissions benefits.

     As  in the other reports in this  series, this   report  deals
with  the  topics  of  CNG   technology   and   the  economic  and
environmental  impacts of heavy-duty  CNG use.   There  are  other
matters  related  to the introduction of  alternative fuels  which
are  not specifically  addressed.   These include  such  things as
establishing   emissions    standards    and   test   procedures,
overcoming  institutional barriers,  etc.   While  such questions
are  important, they are outside  the scope of  these  reports.

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                           CHAPTER  2

               USES OF CNG IN HEAVY-DUTY VEHICLES
     The  purpose  of  this  chapter   is  to  provide  background
information  on  the  use  of  compressed  natural  gas  (CNG)
heavy-duty  vehicles  in  the  United  States,   and  to  discuss
generally the potential for  future  growth  in  this  area.   The
Chapter  will address  economic,  technological,   and  regulatory
factors  only   peripherally  as   needed   to  explain   certain
projections  and conclusions;  later chapters  in this report will
detail findings  in these areas.

     This chapter is  organized into  three sections.   Section I
addresses  the  historical  use  of CNG  in heavy-duty vehicles,
Section  II  describes  recent  activities  in  the conversion  of
heavy-duty  vehicles  to  CNG,  and  Section  III  describes  the
potential   future   market   for  CNG   in    heavy-duty   vehicle
applications.

I.   Historical  Use of CNG in Heavy-Duty Vehicles

     Experience  with  and  interest   in  CNG  as   an  alternative
motor-vehicle fuel differs  significantly around  the globe.   Use
of CNG  in heavy-duty vehicles throughout  the mid-1970s  in the
United States was  sporadic and  isolated  at  best, while  use of
CNG  in  parts of Europe,  South America, and Asia has been more
enthusiastic.   While   economics   was  the  primary  motivating.
factor  for  CNG -use both  abroad and  in  the   United  States,
environmental  and   political   concerns  have   also  played  an
important role in the United States.

     CNG as  a vehicular fuel has been popular in other parts of
the world  since  the mid-1930s.   In  Italy,   for  example,  use of
CNG  as  an  alternative  fuel   began  in  1935  as  part   of  the
National Economic Policy,  which  called for  self-sufficiency in
all   materials.    Gasoline   shortages   during   World   War  II
increased  the demand for  CNG,  which was  used for  both public
transportation  and  private vehicles.  At  its  peak  from 1945 to
1960,  CNG captured  about ten  percent  of  the  Italian  vehicle
fuel  market.[1]   When  gasoline became less expensive  and more
readily  available  in  the 1960s,  however,   CNG's  share  of  the
fuel market  fell.   The international appeal of CNG continues to
be strong: New Zealand has  125,000 cars  and  trucks  converted to
natural  gas; Canada has  20,000 natural  gas  vehicles; Argentina
has  15,000  natural  gas  vehicles;  and  the  Soviet  Union  has
200,000  vehicles,  with plans  to  convert  another 300,000 by the
end of 1990.[2]

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                              2-2

     Use of CNG  in  heavy-duty  vehicles in the United States was
given a boost in the 1970s with the general  increase in concern
for the environment.  Auto  emissions  were identified as a rr.ajor
source of air pollution,  spurring  some interest:  in development
of  alternative   fuels.   During this  period,  several  companies
offered systems  to  convert vehicles  from traditional  fuel  use
to natural  gas.   The market was  not  well  defined or financed,
however,  and the  demand  for  vehicles  fueled  by  natural  gas
remained  low.    An  estimated  15,000  CNG-fueled  vehicles  were
operating  at this  point   in  time  in  the  United  States,  a
fraction of which were heavy-duty vehicles.

     The  "energy crises"  of the  1970s generated  a concern for
energy  and  oil   security  in the  United States,  and temporarily
increased;  interest  in  alternative  fuels.   The  United  States
made a  concerted effort  to find substitutes  for  foreign  oil in
order  to  weaken  the hold of  the OPEC  cartel  on world  oil
prices.   The  transportation sector  appeared to  be the  key to
meaningful   reductions    in  oil   consumption.     Interest   in
alternative fuels waned,  however,  as  significant  investments in
energy   efficient  technologies   between   the   mid-1970s   and
mid-1980s, combined with an  increase  in  the world's oil supply,
eventually forced lower oil prices.

     Motivated  by  the  potential  for substitution of  CNG for
gasoline  and   diesel  fuel  to   reduce   fuel   costs,   improve
declining  urban  air quality,   and create  energy  security,  a
variety  of  interrelated  market • and  regulatory  forces  caused
renewed  interest in  CNG  use in heavy-duty  vehicles during the
1980s.     This    renewed    interest    stimulated   CNG-related
development,  resulting   in a  viable CNG   heavy-duty  vehicle
market.   This market  currently includes  an  estimated 1,500 CNG
heavy-duty  vehicles*  served (along with light-duty vehicles and
trucks)  by up  to  300 refueling stations  (mostly  owned  by gas
utilities),  and  supported  by several utilities,  approximately
80 CNG  vendors,  and related gas interest  associations.


*Forpurposes  of  this report,   "heavy-duty"  vehicles are
     defined  to have a  gross vehicle  weight (GVW) of more  than
     8,500  pounds.   There is,  however, a  disparity between  this
     definition and  the  definition used  by  the Department of
     Transportation  (DOT).   Dot defines  a  "heavy"  truck as: 1)
     a  single-unit truck  with  GVW greater  than  26,000 pounds,
     2)  a  tractor-trailer  combination,   3)  a truck with  cargo
     trailers,  and 4) truck-tractor  pulling  no  trailer.    Some
     trucking  associations use alternative  definitions, such as
      a   GVW  in  excess  of   20,000   pounds.    Differences  in
     definitions may make  comparisons  of  vehicle statistics
     difficult.

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                               2-3

II.   Recent Progress in CNG Heavy-Duty Vehicle Applications

     As shown in Figure  2-1,  the number of  heavy-duty vehicles
in  the  United States,  which  represents  the maximum  potential
market  for  CNG-fueled  heavy-duty  vehicles,  is  quite  large.
Efforts  by  the  CNG  industry  to  penetrate  this  heavy-duty
vehicle market have met with  mixed success.  Figure  2-2 shows
the  CNG heavy-duty  vehicles  that  exist  today  in the  United
States  by  application,  including  dump trucks  and  heavy-duty
pickup  trucks,   school  buses,  transit  buses,   United  Parcel
Service (UPS) delivery trucks,  and  trash  collection trucks.   Of
the   estimated   1,500   heavy-duty   vehicles  that   have  been
converted  to  CNG,  about 43  percent are heavy-duty  dump  trucks
and   pickup   trucks,   and   18  percent   are   school   buses.
Unidentified vehicles  in the Figure represent that  fraction of
the  total  number  of CNG heavy-duty vehicles that  the American
Gas  Association  estimates  exists  but  which  have  not  been
identified to date  in  their  surveys.   That number  should be
considered only an approximate value.

     As  detailed   in  Table   2-1,   the   conversion   programs
completed  to date  were  primarily  sponsored (and predominantly
funded)  by  gas  utilities,  frequently  in  conjunction   with  a
transit authority or  school  district,  and have been designed to
test  evolving  diesel   conversion  technology.    Most  of  the
conversion projects have  involved  only  one or  two  vehicles.
There  are  a few   notable  exceptions  to  these  generalities,
including -a  cooperative effort between two  Ohio  gas companies
and  Flxible  Corporation  to  develop  prototypes  for   use  in
transit  districts,  and  a  Garland, Texas  school  district that
decided on its own to dedicate  its fleet of buses to CNG.

     Texas itself  is  worthy of  further  note.   This state is
attempting to  bring its four  non-attainment areas (Dallas-Fort
Worth,  Houston-Galveston-Brazoria,  Beaumont-Port   Arthur,   El
Paso)  into  compliance  through  proactive  legislative   action.
Texas  Senate  Bill  769   requires  the  use  of  CNG  or  other
alternative  fuels  that  reduce  emissions  in rapid transit buses
and,  if  necessary,  certain  local  government and  private fleet
vehicles   in  non-attainment  areas  for  either  ozone,  carbon
monoxide  (CO),  oxides of  nitrogen,  or particulates.   By 1994,
940   transit  buses   are  to  be  converted  to  CNG  with  the
possibility  of  4,038  school  buses  to be  added  to  that  figure
(122  of which  are  presently CNG  fueled).   Details  on  Texas'
legislative  implementation schedule are given in Figure 2-3.[4]

      The  use of  both prototypes  and entire  fleet conversions
over  the  last  decade  has  had the  effect  of  chronicling  the
evolving  technology of  CNG-conversion  vehicles,  beginning with

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                                FIGURE 2-1


                      U.S.  Heavy-Duty Vehicle Market
         Number of  Vehicles (thousands)
           Intercity
             But«*
School
 Bus«t
Transit
 Butts
Trucks
          1977 & 1987 Heavy-Duty Vehicles by  Application
*      Defined to include Class I, II, III interstate earners, all of which report to the Interstate
      Commerce Commission.


"*     Figures for trucks are estimated.  DOT designation of single unit trucks is based on the
      carrying load, not the gross vehicle weight.


Source: U.S. DOT/RSPA, NATIONAL TRANSPORTATION STATISTICS. Annual Renort. 1989.
      Cambridge, MA.

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                                     2-5


                                FIGURE 2-2

             Distribution of Existing CNG  Heavy-Duty Vehicles


         Number of  Vehicles
          Trucks
School
 Buses
Transit
 Buses
UPS
Trash    Unidentified
Trucks     Vehicles
                CNG Heavy-Duty Vehicle Applications
Sources:      American Gas Association, the Natural Gas Vehicle Coalition, Representatives of
            Gas Utilities and Representatives of Vehicle Conversion Suppliers.

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                               2-6

                           TABLE 2-1

          Existina CNG Heavv-Dutv Vehicle .-.cclicazior.s
diesei/20% CNG)
NumiDer of vehicles:   1
Location:  Phoenix,  A2
Goals   of   application:   Emissions   reductions,   technology
demonstration
Participant:   City of Phoenix
Sponsor:  Southwest Gas Corporation
Source:  Southwest Gas Corporation
Type of vehicle:  CNG-dedicated two-stroke diesel transit bus
Number of vehicles:  1
Location:  Tucson, AZ
Goal of application:  Technology demonstration
Participant:  City of Tucson
Sponsor:  Southwest Gas Corporation
Source:  Southwest Gas Corporation
Type of vehicle:  Heavy-duty truck
Number of vehicles:  630
Location:  Arizona
Goal of application: Economics
Participant:  Southwest Gas Corporation
Sponsor:  Not Applicable
Source:  Southwest Gas Corporation
Type of vehicle:  Two-stroke  diesel  transit  bus
Number of vehicles:  2
Location:  Tacoma, WA
Goal of application:  Demonstration  project
Participant:  Pierce Transit
Sponsors:  Washington Natural Gas  Company, Pierce Transit
Source:  American Gas Association
 Type of  vehicle:   Transit  bus
 Number  of  vehicles:   2
 Location:   Los  Angeles,  California
 Goal of  Application:  UMTA demonstration project
 Participant:  Southern  California Regional  Transit District
 Source:   American Gas Association

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                              2-7

                           TA3LE 2-1

      Zxistinc CNG Heaw-Dutv Vehicle Asolicaticr.s (ccr. t;
Type of vehicle:   Transit bus
Number of vehicles:   2
Location:  Mew York City
Goal  of  application:    CNG  bus  demonstration  program  begun by
New   York   City's   Department   of    Transportation   and  the
Department of Environment Protection
Participant:   Command Bus Company
Sponsors:  Brooklyn Union Gas  Company  provided engines; partial
funding from   UMTA
Source:  American Gas Association, Natural Gas Vehicle Coalition
Type of vehicle:  Trash collection truck
Number of vehicles:  1
Location:  New York City
Goal of application:  Demonstration program
Participant:  Department of Sanitation
Sponsor:  Brooklyn Union Gas Company
Source: Natural Gas Vehicle Coalition
Type of vehicle:  Delivery trucks (UPS)
Number of vehicles:  10
Location:  New York City
Goals of application:  Demonstration program
Participant:  United Parcel Service
Sponsor:  Brooklyn Union Gas Company
Source:  Natural Gas Vehicle Coalition
Type of vehicle:  Dual-fueled trucks
Number of vehicles:  2
Location:  Minneapolis-St. Paul, MN
Goal of application:  Demonstration program
Participant:  Twin Cities Metropolitan Transit Authority
Sponsor:  Minnegasco
Source:  American Gas Association

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                              2-8

                       Table 2-1 (cor.t'a)

           xisting CNG Heavy-Puny Vehicle Aggl icar icr.s
Type of vehicle:  Gasoline-fueled school bus
Number of vehicles:   122
Location:  Garland,  TX
Goal of application:  Economics
Participant:  Garland, TX School District
Sponsor:  Unknown
Source:   Transportation   Department   and  Garland  Independent
School  District,    "Compressed  Natural   Gas  System,  Two-Year
Summary," June  1985.
Type of vehicle:  School bus
Number of vehicles:  24
Location:  Tulsa, OK
Goal of application:  Economics
Participant:  Tulsa, Oklahoma School District
Sponsor:  Oklahoma Natural Gas Company
Source:  American Gas Association
Type of vehicle:  School bus
Number of vehicles:  90
Location:   Indiana
Goal of application:  Economics
Participant:  Evansville & Vanderburg  School  Corporation
Sponsor:  Not available
 Type  of  vehicle:   School  bus
 Number of  vehicles:   40
 Location:   Erie,  Pennsylvania
 Goal  of  application:   Economics
 Participant:   Harbor  Creek  School  District
 Sponsor:   Not available

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                               2-9

                           TABLE 2-1

      Exist ing CNG Heavy-Duty Vehicle Applica t: c r. s
Type of vehicle:   Dual-fuel CNG/diesel dump-trucks
Number of vehicles:   18
Location:  Columbus,  OH
Goals of application:  Economics
Participant:   Columbia Gas Corporation
Sponsors:  Not applicable
Source:  American Gas Association
Type of vehicle:   Garbage truck
Number of vehicles:  2
Location:  North Miami, FL
Goals of application:  Testing program
Participant:  City of North Miami
Source:  Natural Gas Vehicle Coalition
Type of vehicle:  Transit bus
Number of vehicles:  1
Location:  Cleveland, OH
Goals of application:  Environmental, Economic
Participant:  Consolidated Gas Corporation
Sponsors:  Flxible Corporation, Consolidated Gas Corporation
Source:  Flxible Corporation
Type of vehicle:   Transit bus
Number of vehicles:  1
Location:  Columbus, Ohio
Goals of application:  Environmental, Economic
Participant:  Central Ohio Transit Authority
Sponsors:  Columbia Gas Company; Flxible Corporation
Source:  Flxible Corporation

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                                     2-LO
                                 FIGURE 2-3

                   Projected Use of CNG Buses In Texas

      Number of Vehicles  (thousands)
      Transit  School  Transit  School  Transit  School  Transit  School
       Bu«««     But€t  Bua««    Bu«««   Bu«««     Bu«««   Buset     Bus«9
           1989           1994           1996           1998

                  Legislative Implementation Schedule
source:
Texas Senate Bill 763 requires the use of CNG, or other alternative fuels r.hat reduce
emissions, to be used in rapid transit buses and, if necessary,  certain local
government and private fleet vehicles in non-attainment areas. Conversions of school
buses as well as other fleets may not be necessary if Texas' 4 non-attainment areas
(Dallas-Fort Worth, Houston-Galveston-Brazona, Beaumont-Port Arthur, and El Paso;
reach and maintain required attainment levels.

The Economic Costs and Benefits of Proposed Amendments  to the Texax  Clean  Air
Act, the State Purchasing and General Services Act, che Metropolitan Rapid Transit
Authority  Act,  the  Regional Transportation  Authority  Act  and   the City
Transportation Department Act, prepared for Texas General Land Office. March
1989.

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                              2-11

CNG conversions  of  gasoline or  diesel  fuel  systems  (dual-fuel
vehicles*)  and  moving   into   CNG-specific  fuel  systems  and
engines  (i.e.,  dedicated  vehicles).    The  initial  conversions
were  from gasoline  vehicles  to  dual-fuel vehicles.   Gasoline
conversions to  dual-fueled  vehicles  are no  longer under  such
active  consideration,   however,   because  such  vehicles  do  not
offer the  cleanest or  most fuel-efficient  alternative,  as  will
be discussed in Chapter  3.   Most gasoline conversions today are
to dedicated CNG vehicles.

     Complications  with  the  diesel  engine  have  historically
limited  diesel  conversions  to primarily bi-fuel applications.
Recent  efforts  by various  gas  utilities  (e.g., Columbia  Gas
Co., Brooklyn Union  Gas  Co., Southwest  Gas Corp.) have resulted
in  the  successful   conversions   of  2-stroke  diesel  buses  to
bi-fueled  vehicles,   and  a  4-stroke   diesel  bus  to  bi-fuel.
These  prototypes  have  proven  that  the  conversions  could  be
accomplished and that  the characteristic  black smoke from older
diesel-fueled   vehicles    could   be    eliminated.**     Diesel
conversions to dedicated  vehicles, however, look more promising
in the long run, with the advent of the Cummins  NG  L 10  engine,
which  is  derived  from  an  existing  Cummins  diesel  engine.
Already  being  tested in  buses,  the  Cummins  design  is  able  to
meet the 1991 bus emissions  standards.[3]

III. Potential Future Market for CNG Heavy-Dutv Vehicles

     The   heavy-duty  CNG   market   currently   appears   to  be
undergoing  a  resurgence  of  interest.   There   are  about  170
confirmed  CNG vehicles  that have been contracted or planned for
the near future.   Many  of  these vehicles  involve  diesel  buses
*    The  term  "dual-fuel"  is  used  to  refer  to  both  a  fuel
     system  that  enables  the  user  to switch  back  and forth
     between  CNG  and gasoline  use using  a manual  switch,  as
     well as  to  CNG/diesel engines  (sometimes  called bi-fuel),
     which  idle  on  100  percent  diesel and  run on  80  percent
     CNG/20 percent  diesel.   This  term should not  be confused
     with  what  are   commonly  called "flexible  fuel" vehicles.
     These latter  vehicles,  which operate on liquid  alternative
     fuels,  have  the ability  to  inter-mix  fuels   in  the  same
     fuel  tank.    In  contrast  to   this,  CNG,  being  gaseous,
     requires  its  own distinct fuel  storage and delivery system.
**   Given  both  the  difficulty  of  2-stroke  engine conversion
     and  the  presence  in  the   exhaust  of  increased unburned
     fuel,  the technological focus  currently is on converting
     4-stroke  diesel  engines.

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                              2-12

that will be  converted  to dedicated  CNG  use using  the  Cummins
NG  L  10  engine.    Table  2-2  briefly  describes  the  future
projects which  have been  identified.  Some  of these  projects
are  partially  funded  by   UWTA.    UMTA   is  providing  grants
totaling  535.1   million  to   purchase  buses   that   run   on
alternative  fuels;  approximately  20 percent  is  earmarked  for
CNG-fueled  buses  (although  80  percent   of  the  applications
received have been for CNG).

     Besides  basic  economic  factors, the future  market  for  CNG
use in heavy-duty vehicles  is  also dependent on  the parameters
of  the   regulatory  environment  surrounding  use of  alternative
fuels.    Depending on  the particular  requirements  in the  final
version  of  the  Clean Air Act  amendments, and implementation of
the 1991 and  1994 emissions standards for  buses and trucks, the
potential size of the CNG market could grow significantly.

     The President's proposed  amendments  also include a plan to
require  the  use  of  clean alternative fuels  in new  urban  buses
in  all  cities with a  1980  population  of over  1  million.   This
proposal would affect  over  80  percent of all new  bus purchases
by "the  mid-1990s.   Given the  current high  level  of  interest in
CNG applications for buses, this would  appear  to  be  a promising
market for CNG.

     More broadly,  CNG appears  to be a  readily  adaptable fuel
for  a   variety  of   heavy-duty  uses.    Heavy-duty  vehicles
generally have sufficient  space available for CNG fuel cylinder
placement and, as  will be  seen in  subsequent  portions  of this
report,  are  not  greatly affected by the  added weight  of  the
fuel  cylinders.    CNG   appears   especially   suited  to  those
heavy-duty  applications  which  have  access  to  central  fueling
stations  and would  not  require  development  of  a   significant
fuel   delivery   infrastructure.   Thus,  promising   areas  for
potential  increased  future  use  of dedicated  CNG  heavy-duty
vehicles  include  many municipal applications such as school and
transit  buses,  delivery  trucks,  and trash  collection  trucks.
Other  centrally  fueled  applications also exist  in  the private
sector   for   such   areas  as   utility  service  vehicles,  local
delivery trucks,   etc.    Dual-fueled  vehicles  may  see  even
broader  usage than  this because of  their ability  to operate on
both  CNG  and conventional  fuel.   However,  as  noted  earlier,
most current interest seems  to be directed  at dedicated vehicle
development.

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                              2-13

                           TABLE  2-2

              New CNG Heavv-Dutv  Vehicle  Prefects
Type of vehicle:   CNG-dedicated transit bus
Number of vehicles:   1
Location:  Phoenix,  AZ
Participant:   Phoenix Transit
Sponsor:   Southwest Gas Corporation
Comments:  Southwest  Gas  has  given  the  "okay"  to  order  the
Cummins  engine for  the  bus;  Southwest Gas will  lease bus at no
cost to Phoenix Transit for one or two years.
Source:  Southwest Gas Corporation
Type of vehicle:  Trash collection trucks
Number of vehicles:  8
Location:  New York City/Staten Island
Participant:  Snug Harbor Cultural Center
Sponsor:  Brooklyn Union Gas Company
Comments:  None
Source:  Natural Gas Vehicle Coalition
Type of vehicle:  Heavy-duty trucks
Number of vehicles:  50
Location:  New York City
Participant:  NYC Department of Parks
Sponsor:  Brooklyn Union Gas Company
Comments:  None
Source:  Brooklyn Union Gas Company
Type of vehicle:  Postal vehicles
Number of vehicles:  50
Location:  New York City/Staten  Island
Participant:  Post Office
Sponsor:  Brooklyn Union Gas Company
Comments:  None
Source:  Brooklyn Union Gas Company

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                              2-14

                           TABLE  2-2

          New CNG Heavy-Duty Vehicle Projects  (
Type of vehicle:   Transit bus
Number of vehicles:   10
Location:  Southern California
Participant:   Southern California Transit Districts
Sponsor:  Flxible Corporation
Comments:   Contract  is  on  back-order  because  of  performance
testing  to  be conducted on  existing  converted  buses  and  the
volume of conversion contracts.
Source:  Flxible Corporation
Type of vehicle:  Transit bus
Number of vehicles:  3
Location:  Fort Worth, IX
Sponsor:  Flxible Corporation
Comments:   Contract  is  on  back-order  because   of  performance
testing  to  be  conducted  on existing  converted buses  and the
volume of conversion contracts
Source:  Flxible Corporation
Type of vehicle:  Transit bus
Number of vehicles:  2
Location:  Dallas, TX
Sponsor:  Not available
Comments:   Contract  is  on back-order  because  of performance
testing  to be  conducted on  existing converted  buses  and  the
volume of conversion contracts.
Source:  Flxible Corporation
 Type  of  vehicle:  Transit bus
 Number of  vehicles:   5
 Location:   New  Jersey
 Sponsor:   Not available
 Comments:   Contract  is  on  back-order  because  of  performance
 testing  to  be   conducted  on  existing  converted  buses  and  the
 volume of  conversion  contracts
 Source:   Flxible Corporation

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                               2-15

                            TA3LZ  2-2

           New CNG Heavy~Puty Vehicle  Projects  (cor.  ~ )
Type of vehicle:   Transit bus
Number of vehicles:   2
Location:  Los Angeles,  CA
Participant:   L.A. County Transportation Commission
Sponsor:  Southern California Gas Company
Comments:  None available
Source:  American Gas Association
Type of vehicle:  Transit bus
Number of vehicles:  5
Location:  Rochester, NY
Participant:  Rochester-Genesee Regional Transportation Authority
Sponsor:  UMTA (75% of cost)
Comments:  None available
Source:  Rochester Gas & Electric, Rochester, New York
Type of vehicle:  Transit bus
Number of vehicles:  23
Location:  Buffalo, Syracuse, and Long Island, NY
Participants:   Transit  authorities  in   Buffalo,   Syracuse,   and
Long Island, NY
Sponsor:  UMTA  (75% of cost)
Comments:   Buffalo,  Syracuse,  Long  Island,  and  Rochester  (see
previous  entry)  transit  authorities are  involved  in  a group
purchase   of   28   natural  gas-fueled   transit   buses.     UMTA
assistance  totaling  $6.3 million will  pay  for  75% of the cost,
with state and  local funds providing  a 25%  share.
Source:  Department of Transportation
Type of vehicle:  Transit bus
Number of Vehicles:
Location:  Tacoma, Washington
Participant:  Pierce Transit
Sponsor:  UMTA  ($8.3 million)
Source:  New York Times, March  1989

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                              2-16

                      References Chanter 2
     1.    Pietro    Magistris,     "Compressed    Natural    Gas
Distribution   System   in    Italy,"    in    Symposium   Papers:
Nonpetroleum   Vehicular   Fuels   II,    presented   in   Detroit,
Michigan, June 15-17, 1981.

     2.    American  Gas  Association,   "Natural   Gas  Vehicles
Bulletin," Cat. No. G92238,  1989.

     3.    Conversation  between   ICF   Incorporated   and   Jeff
Seisler, Natural  Gas Vehicle Coalition, October  1989.   Phase I
engine  tested at  Southwest  Research  Institute  with  oxidation
catalyst.

     4.    Gross   and   Weinstein,  "The   Economic   Costs   and
Benefits of Proposed Amendments to the  Texas  Clean Air Act,  the
State  Purchasing  and  General  Services Act,  the  Metropolitan
Rapid   Transit  Authority  Act,   the   Regional   Transportation
Authority  Act,  and the City  Transportation  Department Act"  for
the Texas General Land Office, March 1989.

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                           CHAPTER 3

               CNG HEAVY-DUTY VEHICLE TECHNOLOGY
I.    Introduction

     In this chapter the  hardware,  emissions and performance of
heavy-duty  CNG vehicles  will  be  discussed.   First,  a  brief
discussion  of  the  properties of  natural  gas as  a vehicle fuel
will be presented,  followed by a  description  of  the hardware on
current  dual-fuel   and  dedicated  CNG  vehicles.   Next,   the
emissions   and  performance   of  current  technology   will  be
presented.    Finally,   projections   of   the    emissions   and
performance of  future  optimized  dedicated  CNG  engines will  be
derived.  The  emissions information presented here will be used
in Chapter  5  for  a comparison  of the environmental  impacts of
heavy-duty   CNG  vehicles   and   their   gasoline   and  diesel
counterparts.

II.  Fuel, Properties of Natural Gas

     The  most  fundamental  difference between  natural gas and
conventional motor  fuels  (i.e.,  gasoline,  diesel fuel) is that
natural  gas,  unless cryogenically stored  (i.e.,  at  extremely
low  temperatures)   under  pressure,  is  in  gaseous  form  rather
than  a liquid.  As can  be seen  from the  fuel  properties  in
Table  3-1,  the energy  density (Btu/gal)  of natural gas as  it is
stored on a vehicle is very  low  compared  to liquid fuels.  This
has  significant  impacts in  the  area  of  onboard fuel storage as
will be discussed s'hortly.

     The  composition of natural gas varies,  as  can be seen from
Table  3-1.   The  methane  content  of  natural gas  is typically
over  90  percent  although  it  can  be lower.   This  results  in
unburned  hydrocarbon   (HC)  emissions  that  are  largely methane,
as  will be seen later.   In terms  of ozone  forming potential,
methane  is  essentially unreactive.   Thus,  on a mass basis, CNG
HC  emissions  are much  less  ozone  forming that  the HC emissions
from  gasoline  and  diesel  engines.  This is discussed further in
Chapter  5.

      Two  of the more  important properties of  fuels in relation
to  engine design are  the octane  and  cetane ratings.  The octane
rating of  a  fuel  is  a  measure of  its  resistance  to   knock
 (spontaneous   combustion  away  from  the spark-initiated   flame
front).   Good antiknock   properties  are   important   in   spark
 ignited  engines because  they  allow  for  increased compression
ratios and a  resultant  increase  in  engine  efficiency.   As is
shown  in  Table  3-1  the   octane  rating  of  natural  gas  is
 significantly  higher than for gasoline.

-------
                                            3-2

                                         Table  3-1

                Properties o£ Natural Gas and Conventional ?eiro 1 eu.^ F ^e 1 s
Properties

Chemical
Constituents
Boiling Range
 (3? 3 1 atm)

Specific Gravity

Btu/ft3 of Mixture
  (LHV)

Btu/gal (LHV)
 Btu/lb   (LHV)
                     Commercial
                     Unleaded
                     Gasoline

                     Mixture of
                      Hydrocarbons
                      (chiefly
                     80 to 420
No. 2 Diesel
    Fuel

Mixture of
 Hydrocarbons
 (chiefly
320 to 720
                     0.71 to 0.73[2]   0.79 to 0.38**

                     95.5****          96.9****
                     114,132
                     13,900
 129,400
 13,310
     CN
90-98"-= Methane
 Remainder,
 Ethane and
 Other
 Paraffins,
 C02,  H2,
 He,  N2

-259*
                            0.13***

                            37 .
19,750 8  2400 psi
70°F

21,300
 Octane  Number
  (R*M)
    2

 Cetane  Number
  Range

 Stoichiometric
  Air/Fuel  Ratio
  (weight)

 Peak Flame Temp
                     37-93
                      5-20
                      14.5  to  15.5
                      3900-4100
 N/A
 38-51
 14.5  to 15.1
                                                                    120*****
H/A
 17.2
                                                                   3410
**
***
****
Source:
        Pure Methane.   Other minor  constituents  (ethane,  propane,  sec.)  boil ac
        higher temperatures.
        At 60^ with respect to water at 60°F.
        At 80"F with respect to water at 6o"F.
        At  Stoichiometric  gaseous  air/fuel ratio,  14.7  psia,  SO^F,  lower (net)
        heating value.
        Octane  number  ratings above  100 are  correlated  with given concentration
        of tatraethyl lead  in iso-octane.

          EA-Mueller,  Inc.  and  "Gaseous  Fuel  Safety Assessment
          for  Light-Duty  Automotive Vehicles," M.C.  Krupka,  A.T.
          Peaslee,   and  H.L.   Laquer,   Las  Alamos    National
          f alirtT-a^nr-rf . N/ivamh«r 1983.

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                              3-3

     The cetane rating of a fuel is a measure of  its  ability to
autoignite when  compressed and heated.   A  high cetane  fuel is
essential  for  the proper  operation of  a  compression  ignition
(i.e.,   diesel)   engine   where  no  external   ignition  source  is
used.    Although  no   actual  cetane  number  is  available  for
natural gas,  its cetane rating  is very  low  compared to diesel
fuel.    This  is  reasonable given  that  in  a  sense  octane  and
cetane  are opposites and  natural  gas  has   a  very high octane
rating.  For  this  reason  natural  gas  generally cannot  be used
in  an  internal  combustion engine without  an  external  ignition
source  (which  could  be  a  spark  plug  or a pilot  injection of
diesel fuel).

Ill. Current Heavy-Duty CNG Technology

     A.    Engines

     As  noted  in  Chapter  2,  CNG  has  been  used  on  a  limited
scale as a heavy-duty vehicle fuel  for many years  in  the United
States.  Generally,  a conversion  kit consisting of fuel storage
cylinders,  high pressure  fuel  lines,  fuel   pressure  regulation
equipment  and  some type of  fuel-air mixer  has  been  retrofitted
onto  a vehicle  originally designed to  operate  on  some  other
fuel,   usually gasoline.   Most  often the capability  to  operate
on  the  original fuel was  retained,  giving  rise to  a dual-fuel
vehicle which was clearly  not optimized for CNG use.

     The  operation  of  gasoline-derived  dual-fuel vehicles is
somewhat  different  than  for   diesel-derived  versions.   Since
gasoline  engines  already  have  a spark  ignition system,  the
dual-fuel gasoline/CNG engines are  designed for  the use  of only
one fuel  at a  time  and usually  have  a  switch  on the dashboard
to  allow  the  driver   to  choose  which  fuel  to   run   on.   In
contrast   to   gasoline   engines,   diesel   engines   rely   on
compression  of  the  charge  to   autoignite  the  fuel.    Since
natural  gas  has low  cetane  ratings,  compression  ignition is
difficult  to  initiate   and  control  and an external   ignition
source  is  required.   Generally,   in  conversion  situations,   a
pilot  injection of diesel fuel is  used  to  initiate  combustion.
With  this  type of conversion the engine  retains the ability to
run  on  pure  diesel  fuel  or  varying  amounts  of natural  gas
replacing  diesel fuel (up to 90  percent  at high loads)  but not
on  pure natural  gas.[l]   In  any  case,  as  will  ba described
further  below,  to  date  very   little   effort  has  gone   into
accurately characterizing  the  emissions,  performance,  or  the
long   term  durability   of  converted  heavy-duty  CNG  powered
vehicles.

     Only   recently   has   serious   effort   been   directed   at
developing heavy-duty vehicle  engines dedicated  and optimized

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                              3-4

for CNG,  taking  advantage of  the unique  qualities  of  natural
gas as  a  vehicle  fuel.   In this regard, there are two  distinct
approaches being  used,  which this report  will  characterize  as
"lean-burn   combustion"   and   "stoichiometric    combustion."
Lean-burn  combustion  engines  have been  derived  from  current
diesel  engines,   which  operate  with  a substantial  amount  of
excess  air  in  the  combustion  chamber,   giving  rise  to  the
lean-burn  designation.    The   stoichiometric  approach,   which
utilizes  a chemically  correct fuel-air mixture  with no  excess
air, has  been derived  from gasoline-fueled engine designs using
a three-way catalyst control system.   Such  engines must operate
very close to  ideal,  stoichiometric, air-fuel mixtures for the
three-way catalyst to  perform properly.

     Each  of  these combustion  systems has  its   advantages and
disadvantages,  and both are  considered  good candidates for more
widespread  use  in  various  heavy-duty  vehicle  applications.
Although  both  are spark-ignited, homogeneous charge combustion
systems, their engine-out  emissions and catalyst  strategies are
different  and  result  in  characteristically different  tailpipe
emissions.  Since there are characteristic differences  in both
engine  hardware and emissions  performance,  the  two designs will
be  evaluated  separately  in  this  report.   Also,   since  the
lean-burn  combustion   system  is  derived   from   and  will  most
likely   replace  diesels   in   future   uses,   diesel   engine
performance  will  be   the  baseline  for  comparison  with  CNG
lean-burn  combustion  engines.    Similarly,  evaluation  of  CNG
stoichiometric • combustion  engines  relative  to  current   fuels
will  be based upon gasoline-fueled  designs.   Even  though this
convention will  be carried through the rest of  this report, it
should  be born  in mind  that  there  is  no fundamental  reason
preventing  the  eventual  crossover  of  future   applications  of
lean-burn and stoichiometric combustion designs.

      It  is  also  worth noting  at  this  point   that  there are
different emissions  testing  cycles   for  current  diesel  and
gasoline-fueled heavy-duty engines.   These  cycles  are  used to
reflect the  characteristic differences  between typical  usage
patterns  for these engines.   When comparing emissions  between
lean-burn and stoichiometric  CNG combustion designs,  a single
cycle will be used for  both,  to insure compatible results.  The
diesel  cycle has  been  chosen for this  purpose because  it is the
only  cycle  for  which  data   on both  designs  exists.   The
evaluation of  environmental  benefits  in  Chapter  5,  however,
will  be  based  upon  the  diesel  cycle   when  comparing  to
conventional   diesel   engines,   and   the  gasoline  cycle  when
comparing to conventional  gasoline-fueled engines.

      As  just   noted,   the   lean-burn  dedicated  CNG   engines
 currently  being   developed  are  based   on   existing  diesel

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                              3-5

engines.   They are usually  run  at  a relative fuel-air  ratio  of
around 0.7,  although  fuel-air ratios well below  this  have been
demonstrated by Southwest Research  Institute  using  a prechamber
with  a  stoichiometric   spark-ignited  charge  and  a very  lean
mixture  in  the main  chamber.[2]   This  latter   approach  holds
promise to  improve  the  efficiency/NOx  tradeoff  associated with
lean-burn  engines.    In general  the  open  combustion  chamber
approach  utilizes   intake   throttling  for power  control,   The
prechamber  approach  is  unthrottled except  at  idle and  relies
upon  control  of  the  amount of  fuel  being metered   into  the
chamber through check valves for power  control.   In either case
the  low  cetane number  of  natural  gas  means a strong  spar!<  is
needed for ignition in dedicated CNG engines.

     Although  the  antiknock qualities of  natural gas  are very
good,  the   compression  ratios   of  current   diesel  engines
(approximately 19 to 1)  are generally too  high  to prevent knock
in  a  dedicated  CNG  engine.   Thus,  the  compression  ratios  in
lean-burn CNG  engines are  generally  reduced to  below 15  to  1
and  can  be as low  as 10 to  1.   The lowest  compression  ratios
are  conservative designs to account for the wide range of fuel
composition which the engine might see in-use.

     In  contrast  to   lean-burn   engines,  stoichiometric  CNG
engines  are  generally  based on   gasoline  engine  technology.
They have  used a 3-way catalyst  based control  system, with or
without  the  closed  loop   controls characteristic  of  current-
gasoline engines.   The  relative fuel-air  ratio  is  ideally held
extremely   close  to   one   so  that  the  3-way   catalyst  can
simultaneously   oxidize  THC  and   CO  while   reducing  NOx.
Throttling  of  the  fuel-air  mixture is used to  control  power.
As  was previously  mentioned,  the  antiknock characteristics  of
natural  gas are much better than gasoline.   As a  result the
compression  ratio  of a  gasoline engine can  be boosted somewhat
when converted to dedicated CNG use.

     The  fuel-air  mixer is  a  central  component   in any  CNG
engine configuration.   The mixer serves  the same function on a
CNG  engine  as a carburetor or  fuel injectors  on  conventional
engines,  i.e., metering the fuel  into the  air  at  the  proper
fuel-air  ratio for  proper  operation.  Precise  control  of the
fuel-air  ratio  over  the  entire  speed and  load range  of the
engine is essential  in  achieving  good performance,  fuel economy
and  emission characteristics.

     Common mixer  designs  today   are  generally mechanical  in
nature   and meter  the  fuel  as   a  function  of   intake  air
pressure.   A common mixer design is  shown  in Figure 3-1.   In
addition to  this spring-loaded  diaphragm  variety, venturi based
mixers are  also  available.   These current technology mixers are

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                               3-6
                           Figure 3-1
             INTAKf (OOWMt
                Diaphragm Operated Air-Gas Mixer
Source:  IMPCO Master Catalog,  1987

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                              3-7

not nearly  as  sophisticated as  the  fuel metering  equipment  on
today's conventional engines.   The less  developed  nature  of CNG
technology  in  conjunction  with  the  difficulty  in general  of
precisely metering a gas comoine to produce  current CNG engines
which do not have  the  precise  control of fuel-air ratios needed
to  truly optimize  performance  and  emissions.   Although  much
progress  is  being  made  in the  area of fuel metering  for CNG
engines,  including  the  introduction  of  simple  closed  loop
control systems, which utilize  an exhaust oxygen sensor (lambda
sensor)  and  feedback  controls  to   maintain  proper  fuel-air
ratios,  and  the  development   of  fuel  injection  to  replace
mixers, there is still a great  deal of work  to be  done to bring
the level  of  CNG fuel metering  technology  in line with current
gasoline and diesel technology.

     B.    Fuel Storage

     The   onboard   storage  of   fuel   for   CNG   vehicles  is
significantly  different  than for  vehicles  operating  on liquid
fuels.   The natural  gas  is  stored  in  gaseous   form in  high
pressure  cylinders  at  pressures up  to  3,000 psi.   The number
and size of the cylinders mounted on  the vehicle  determines the
amount  of fuel  stored  on the vehicle  and thus  its  range.  On
heavy-duty  trucks  the cylinders  are  often  mounted underneath,
as shown in Figure 3-2.

     Currently,  the  weight and bulk   of  CNG  cylinders  is  a
concern with CNG vehicles, both  in terms of  available space for
storage   and   reduced   performance   and  fuel  economy  due  to
increased   weight.    However,    this   concern  appears   to  be
primarily  related  to  light-duty  applications  and  does  not
appear  to  present  as  much difficulty  in the  heavy-duty  area.
Conventional  steel  cylinders   filled  with  natural   gas  weigh
roughly  five times  more than diesel  or  gasoline  tanks and fuel
on  an  energy  equivalent  basis.   Also, a given volume  of natural
gas at  3000  psi contains only about one fifth  of  the energy of
the  same volume of diesel fuel. [3]  The effect  this  has on the
weight and range of heavy-duty vehicles  depends  a great deal on
the application.   As  will be shown in Chapter 5, putting enough
storage  capacity   on  a  heavy-duty  vehicle  for  it  to  have
equivalent  range on CNG  as  its gasoline  or diesel counterpart
increases vehicle weight  6.5-9.4 percent.   In many applications
this  is not  a problem.   However,  for  transit  buses increased
weight  may  result in  a  de-rating  of  the  passenger carrying
capacity  of  the bus.   Thus, there is a  trade-off between weight
penalty  and vehicle  range  which must  be  examined   for  each
application.   Finally,  CNG fuel  tanks are presently constrained
to   cylindrical   shapes   and    do   not  offer   the   packaging
flexibility that is available with liquid fuels.

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                               3-8.

                           Figure 3-2
                                       ITjl   ',
                                      •AT
                                                          MUCH
          Typical Mounting Locations For CNG Cylinders
             on School Buses and Medium-Duty Trucks
Source:   Nu-Fuels, Inc.

-------
                              3-9

     Although most  cylinders currently  being used  are of  the
plain   steel  variety,   lighter   weight   designs   including
fiber-wrapped steel  and fiber-wrapped aluminum are  beginning to
see some  commercial  use.   Advanced  all-composite  cylinders  are
also being developed.  These designs  offer much  improvement in
the weight  to energy  ratio over  plain steel  cylinders  as is
shown in Figure 3-3.   However,  Figure  3-3  shows that  even  the
best designs  still  have a  2:1  weight disadvantage  compared to
gasoline  and diesel fuel.   Since  this weight  penalty  arises
from the storage cylinders  and  not the fuel itself, the area of
fuel storage system weight  and  bulk reduction,  through  the  use
of   increasingly   lighter   materials   to  reduce   weight  and
adsorbent  technology to  reduce needed storage  volume  and/or
pressure,  is  an  area where  additional  work can yet be done to
improve  the  attractiveness  of   CNG  as  an   alternative   to
conventional motor fuels.

     Natural  gas  may  also  be  stored  onboard  as   a  cryogenic
liquid  (LNG).   The resultant  improvement  in  the  volumetric
energy content of LNG  as  compared  to CNG may result in improved
range or performance due to  lower  storage  weight  and volume  per
unit of  energy  stored.  However,  because  natural  gas liquifies
at -259aF, the onboard storage  vessels must be  well insulated.
Even   then   an   unused   vehicle   must   vent   boiloff   gas
periodically.  While this has  been  reported to be  as  frequent
as  every 7-8 days[4],  the  American Gas  Association indicates
that current LNG  vehicles can  hold  LNG without boiloff  for up
to  three  weeks.    The   costs   associated  with  liquifaction,
refueling, and storage hardware for  LNG  make  it  less attractive
at  present,  although   this  could  change  with   advances  in
technology and changes in fuel  economics.   The  vast majority of
natural   gas powered   vehicles   today   have  fuel  stored  in
compressed,  rather than  liquified  form.   Therefore,  this report
will only address CNG.

     C.    Emissions From Current Technology CNG Vehicles

     1.    Dual-Fuel Vehicles

     The  actual  emissions  of  current  technology  CNG-powered
heavy-duty engines are not well characterized and there is  only
an  extremely limited amount of transient  emission  test results
available.*   Turning first  to  dual-fuel  vehicles,  EPA  is  not
     The  EPA  heavy-duty transient test (as defined  in the U.S.
     Code  of  Federal  Regulations,  Title 40,  part 86)  is the
     standard  engine   test   for  heavy-duty  engine  emissions
     certification.    The  engine   is  placed   on  an  engine
     dynamometer  and  run  through  a   standardized  test   cycle
     which simulates in-use engine operation.

-------
                                 3-LO

                             Figure 3-3
            Comparison of Fuel and Storage System Weights
                           For Various Fuels
                          Compressed natural gas i Other fuels
           Plain steel      Wrapped aluminum       Methane!
                  Wrapped steel       All-composite
                      Gasoline
               Diesel
                              Fuel
Storage
Source:     "Natural  Gas Vehicles:   A  Review of  the State of  the
            Art",  Sierra Research  Report  No.   SR89-04-01,  April
            1989.

-------
                              3-11

aware of  any  heavy-duty transient  testing  ever performed  en  a
dual-fuel  retrofit  engine.    Presumably,   conclusions  can  be
drawn,   at  least  for  stoichiometric  heavy-duty  gasoline/CNG
dual-fuel  engines,   based  on   limited  testing   of  similar
light-duty dual-fuel  vehicles.   These  results shew  that  -he
vehicle's operation on  CNG can yield large reductions in CO and
the  same  or  somewhat    lower   non-methane   KG   compared  to
operation  on  gasoline.   However,   CNG  also  yields  somewhat
higher  NOx  and  much  higher  methane  emissions  compared  to
gasoline.[5]    Also,  the  emissions   can   vary  a  great  deal
depending  on  the  quality  of  the  conversion  and  the  state  of
tune  the vehicle  is  in.   Reductions  in  CO  are  by  no  means
guaranteed,  as   is  evidenced  by testing  of   three   light-duty
dual-fuel  retrofit  vehicles  done   at  the   EPA  Motor  Vehicle
Emission Laboratory.   Two  of  the  three vehicles  tested  in  as
received condition  had CO emissions  two to  eight  times higher
on natural gas than on gasoline.   Subsequent  recalibration and
maintenance,  however, yielded a substantial CO benefit.[5]

     Recently,  the United  Parcel  Service  (UPS)  converted ten
parcel  delivery  vehicles  to  dual-fuel  gasoline/natural  gas
operation.  A diagram  of  the vehicle is  shown  in  Figure 3-4.
During  the two  year  UPS  program  the vehicles  will  be  run
exclusively  on  CNG  when  possible   and  information  on  fuel
economy and maintenance will be collected.

     Emission  testing   results   of   one   pf  these  converted
vehicles  are shown in  Figure  3-5.   Although  the  data  shows
emissions  to  be lower  on  natural  gas'  than gasoline,  these
results  are of  limited use in the context of  this study for two
reasons.   First, the  test  cycle used  was  a  chassis dynamometer
test  in  which the  whole vehicle was run over  a cycle  simulating
New York  City driving conditions.   This means that comparisons
between  the  UPS  vehicle  and  engines  tested  over  the  EPA
heavy-duty engine transient test cycle cannot be made.  Second,
no   baseline   data   of  the   vehicle  before   conversion  is
available.   Thus,   although  the data  shows  the  vehicle  to  be
cleaner  on CNG  than gasoline,  it  is  unknown what  effect,  if
any,  conversion had  on gasoline  emissions.   Past  testing  on
light-duty vehicles has shown that  the addition of a dual-fuel
CNG conversion kit to a gasoline vehicle  can degrade emissions
performance   on   gasoline.[6]    Nevertheless,   the   data  is
interesting  as  a   reference  and the UPS  program  promises  to
provide  some  much  needed  data  on  the  durability,  maintenance
requirements  and emissions deterioration of CNG vehicles.

     Turning  to  lean-burn dual-fuel  CNG  engines   with  diesel
fuel  pilot  ignition,   the  emission  benefits  of  this  type  of
system  are also unclear.   Steady-state testing of  a converted
Caterpillar  3406  large  truck engine  with electronic  injection

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                              3-14

of both CNG and the  diesel pilot  showed  mixed results,  with  a
reduction  in  NOx,  little  or no  effect  on HC  and  particulate,
and  a  significant  increase  in  CO.[l]    With  little  useful
emissions data on  these types  of conversions, it  is  difficult
to predict any emissions  benefit with any assurance and  it  is
clear  that  more  testing  of  heavy-duty  dual  fuel  conversions
must take place before any emission  benefits  attributed to them
can be accurately quantified.

     2.    Dedicated CNG Vehicles

     As was  previously discussed,  the properties  of  CNG  as  a
vehicle  fuel  are  quite  different  from  those  of  gasoline  or
diesel fuel.   Because of this, dual-fuel  CNG engines  cannot be
fully optimized for  natural  gas  operation,  because compromises
in engine  design  have to  be made in  order to operate on both
fuels.  At  the same time,  dual-fuel  vehicles  clearly  have  a
place in the  transition  to clean alternative fuels because they
offer  the  flexibility  to  operate  on  conventional  and  CNG
fuels.  This  ability will be especially  valuable  in easing the
transition to dedicated CNG use while  the CNG refueling station
infrastructure is  further developed.  However,  due to  the lack
of substantial emissions data on  dual-fuel engines, this report
is  not  in  a  position to  quantify the  benefits  of   dual-fuel
vehicles (other than to  say that  they will be  less  than those
of dedicated  vehicles) and will  focus principally on  dedicated
vehicles.   Since  lean-burn  and  stoichiometric  engines  have
somewhat   different   emission  characteristics  they   will  be
treated separately, beginning with lean-burn.

     a.    Lean-Burn Combustion

     Cummins Engine  Company  is  presently  developing a  dedicated
CNG  version of  its L-10 heavy-duty diesel  engine for commercial
introduction  into  the bus engine market   in  1991.   To  date, two
different   configurations   have   been    tested.    The    first
configuration    utilizes    a    mechanical    diaphragm   mixer
manufactured  by  IMPCO  Carburetion,  Inc.   similar  to   the one
shown  in Figure 3-1.   This  system has  separate idle  and full
power  adjustments  but is an open-loop (i.e.,  one which does not
utilize feedback from  an  exhaust  sensor  for automatic  fuel-air
ratio  control)  mechanical  system.    The  second   configuration
utilizes  an  open-loop  electronically controlled  venturi-type
mixer  manufactured  by TNO Road  Vehicles  Research  Institute in
Holland.   A  diagram of  a  similar  system  is  shown  in Figure
3-6.   Both configurations  utilize  intake  throttling   for  power
control  and  an  oxidation catalyst  for   HC control.    Transient
test results  are  not publicly available for  the  two  CNG L-10
configurations.   However,  Cummins  has  released  its   emissions
design targets,  and based upon EPA's  review of  the confidential

-------
                                 3-15
                             Figure  3-6
                Catalyst
0
o
                                          Speed control
                                         Ignition control
                                                    Gas
                            Engine speed
                                         Valve
                               A  ^--  ^ 1
                          Engine
                                          Mixing unit
      Open Loop Electronically Controlled Air/Fuel  System
                      With Venturi-Type Mixer
Source:   TNO Road-Vehicles  Research  Institute

-------
                              3-16

transient test data  EPA is confident these  targets  will  likely
be  met  or  exceeded.    Cummins's  desian  targets  are  shown  in
Table 3-2.[7]

     Also  included  in  Table  3-2 are  emission  results  for  a
natural  gas-fueled Caterpillar 3406.  This  engine also utilized
an  open-loop   IMPCO   system  and  had  no  catalyst.[8]   It  was
developed for steady-state electrical cogeneration purposes and
was  not  optimized  for  transient operation.[9]   Nevertheless,
these data are useful in analyzing emission  trends of  lean burn
heavy-duty CNG engines,  especially given  the limited  amount of
data available.

     Using  the   information   on   the   Cummins   L-10   and   the
Caterpillar 3406,  emission  levels  which,  for  the  purposes  of
this  report,  will   represent   current   lean-burn  combustion
technology  emissions  were  developed.    For  the  most  part,
Cummins'    design  targets   are   assumed   to    represent   the
capabilities  of   current  technology.   Some modifications  and
additions were made  to  the  design targets  as  Cummins did not
specify  CC>2,   non-methane  HC  or  formaldehyde  levels  in  its
targets.

     The  C02  and formaldehyde values  shown in  Table 3-2  for
current  technology  were  chosen  to to   be representative  of
current  technology  based  on  energy consumption  data  on  the
Caterpillar  3406 . as  well  as  confidential data  on  the  L-10
engine emissions  and  catalyst  efficiencies.   The non-methane HC
was  assumed  to be ten percent  of total hydrocarbon, based upon
the  Caterpillar  data.   The use of a catalyst could affect this
number  somewhat, but to  an  unknown  degree.   Experience  with
conventional catalysts  on  light-duty CNG vehicles suggests that
the  non-methane  fraction would be decreased.   However, the L-10
CNG  catalyst  has been optimized  for total hydrocarbon control,
meaning  that  it  has  significantly better  methane control than
conventional   catalysts.    It   would   therefore   affect   the
non-methane fraction  much  less, if at all.

     Concerning   the   particulate  emission  level  chosen  to
represent  current technology,  the  Cummins  design target  was
used even  thought  the Caterpillar  engine  showed particulate
emissions  ten times  higher than this.   There  is  no inherent
reason  why CNG  should  have  high particulate  levels  in  and of
itself,   given  that   fuel-derived  particulate  is  generally
attributed  to  heavy,  long  chain hydrocarbons  which  are not
found  in  natural gas.   The  higher  particulate  rate  on the
Caterpillar   engine   has   been  attributed  to   lubricating  oil
passing   the  piston   ring  pack   and   entering  the  combustion
chamber.   This  assumption was supported  by the fact that the
particulate   was   90   percent   soluble   organics,  generally

-------






Pollutant
THC
xmc
MOx
Part .
CO
CO?
Formaldehyde


Summary of Low
Lean-3urr. CNG I:
EPA HDDS Trans ie
Cummins CNG L-10
Design Targets
0.9
	
4.5
0.06
4.0
	

3-L7
:abie 3-2
Mileage Emissions From
i7ir.es Coe raced 3ver "r.e
nc Test; Cvcle ( 5 ' 3H? -r.r '
Cat. 3405 1991 Diesel "arr^^- "V(G
w/o catalyst Standards Techno Iccv
9.2 1.3 0.9
0.34 --- 3.C9
4.1 5.0 4.5
0.50 0.25/0.10* 3.C5
3.2 15.5 4.0
575
0.34 • -— 0.05
The  0.10  standard applies  to  urban  bus  engines  only.
standard applies to all other heavy-duty diesel engines.
The  0.25

-------
                              3-18

associated  with   lube  oil.    Improved  oil   control   is   a
significant  part  of  current  efforts   to   reduce  engine-out
particulate levels  for  1991  diesels,  and  low oil  consumption
designs have been successfully  incorporated  into  diesel engines
in  these  attempts.   Thus,  there is no  reason to  believe'than
lean-burn  combustion CNG  engines  will  have  high  particulate
levels.

     b.     Stoichiometric Combustion

     The  only  available transient  engine emissions data  for  a
dedicated  Stoichiometric  heavy-duty CNG  engine   is  shown  in
Table 3-3.  This  engine  was  built from a Chevrolet 454 gasoline
engine and was converted to dedicated CNG use  by  Brooklyn Union
Gas  (BUG).   This engine was  tested at  the  EPA  Motor Vehicle
Emission  Laboratory (MVEL)  in  Ann Arbor,  MI.   Although  the
results from  two configurations are shown here,  only the IMPCO
mixer conf igruration was  introduced  into  the  two buses  which are
presently  operating in  New  York  and,  thus,  this engine  was
chosen  to  represent  current   Stoichiometric  technology.   The
hardware  for  the BUG  engines  is similar to  that described for
the Cummins L-10/ with the addition of  a lambda  sensor  in the
exhaust stream of the  TNO  configuration for closed loop control
of the Stoichiometric fuel-air mixtures, as shown  in Figure 3-7.

     During the  testing of the BUG engine  at MVEL  tests were
run  over  both the  diesel  engine  test  cycle and the gasoline
engine  test cycle.   Although these cycles  are  different,  the
results from  both are useful  and  are  shown  in  Table 3-3.  The
results from the diesel  test cycle  will  be  used  to compare this
engine  to  the   lean  burn  CNG engine  while  the  gasoline test
cycle  results  will   allow  a direct  comparison to  the gasoline
engine  from which  it  was  derived.   This latter  comparison will
be  discussed  in  Chapter  5  while  the   comparison  of  the
Stoichiometric  and  lean  burn  CNG engines  will  be   discussed
shortly.

     One  other  point  that  should be mentioned with  respect to
the BUG data  in  Table 3-3 is  that  the with-catalyst results for
the  gasoline  test  cycle   were  hot start  only,  rather  than
composite   results.   Official   transient   test   results  are
weighted  six hot  start  tests to  one  cold  start  test  and   a
composite   result  is  arrived  at.   No  cold  start  tests were
performed  on  the gasoline test  cycle  with  a  catalyst and only
the  hot  start results  are shown  in Table 3-3.  However, the hot
and  cold  start  data for the without-catalyst  tests were  nearly
identical.   Given this  fact together with the six to  one  hot to
cold  start ratio,  the hot start results  shown  are  assumed to
represent  composite results.

-------
                                        3-19
                                     Table 3-3

                          Summary of  Low Mileage Emissions
                         From the Brooklyn Union Gas  (BUG)
                        Stoiehiometric  CNG Engine (g/BHR-hr)
Pollutant
THC
NMHC
NOx
Part.
CO
C02
Formaldehyde
THC
NMHC
NOx
Part.
CO
C02
Formaldehyde
IMPCO
w/o Catalyst

3.6
0.32
6.62
0.01
31.9
529
0.03
Mixer
w/catalyst
Diesel
1.03
0.15
1.33
0.01
10.3
557
0.0008
TNO
w/o catalyst
Test Cycle
3.57 .
0.33
6.37
0.01
23.3
520
0.02
Mixer
w/cataivst

1.01
0.17
1.16
0.01
6.64
540
0.0007
Gasoline Test Cycle**
1.7
0.40
5.75
0.01
23.5
474
0.02
0.72
0.09
0.51
0.01
10.6
500
0.0001
1.47
0.34
6.57
0.05
3.16
471
0.02
0.34
0.12
1.46
0.01
4. 54
504
0.0007
Applicable
Standards
  1.3

  5.0
0.25/0.13
   15.5
1.1/1.9***

   5.0

14.4/37.1***
*    The  0.10 standard  applies  to  urban  bus  engines  only.   The  0.25  standard
     applies to all other heavy-duty diesel engines.
**   The without catalyst results are hot start results only, not  composite.
***  The  less  stringent  standards apply  to engines  used in trucics  greater  than
     14,000 Ib. GVWB.

-------
                                  3-20


                              Figure'3-7
                Catalyst
               X Sensor
                                          Speed control
                                         Ignitwn control
                                                     Gas
                            Engine speed
                  0
                  D
                                       X Valve
Engine
                                                    Air
                Mixing unit
          Closed  Loop Electronically Controlled  Air/Fuel
                   System With Venturi-Type Mixer
Source:   TNO Road-Vehicles  Research  Institute

-------
                              3-21

     The  assumed  emissions  from  current  technology  lean-burn
and stoichiometric combustion  engines  are  shown sid~e by side in
Table 3-4  and  graphically for  THC,  CO and  NOx in  Figure  3-3.
As can be  seen, the  HC emissions of the two engines are similar
while the  NOx  and  formaldehyde levels for the  lean-burn engine
are  higher  than   the  stoichiometric  engine.    It  should  be
recognized that data on  formaldehyde  emissions  from  lean-burn
CNG engines  is very preliminary,  with  data  available  from only
one engine test.   The lean-burn  engine  particulate  emissions
are  also  higher   than those  from  the stoichiometric  engine,
although   both are   very   low.    On  the   other  hand,   the
stoichiometric  engine  has  substantially  higher  CO  emissions
than the  lean-burn  engine.  Finally, the  stoichiometric engine
has  somewhat  lower  C02  emissions  due  primarily  to  a higher
amount  of   the   fuel   carbon  being  emitted   as   CO  (energy
consumption  from  the  two engines  being  nearly  equivalent,  as
will  be  discussed  below).   A   complete   discussion  of  the
environmental  impacts  of  CNG emissions including  a comparison
with gasoline  and diesel can be found in Chapter 5.

     D.    Engine Efficiency

     The  efficiency  of CNG  engines  is an  important discussion
as  it  relates  to  both fuel economy  and cost,  as  well as total
C02  emissions  and   relative  global  warming  effects.   Fuel
efficiency  in  particular  is  very important in  the  heavy truck
market,  especially  among  fleet  and  line-haul  operators,  as   a
large  truck  operator may  spend as much as  $10,000 annually for
fuel alone.

     A  very  large  determinant  of   engine  efficiency  is  the
compression  ratio,  which  is  in a large part  determined by the
anti-knock  g/uality  (octane)  of  the  fuel  for  a stoichiometric
engine.   For a diesel  engine the compression  ratio is  high to
initiate  spontaneous  (compression)   ignition.   Thus,  a diesel
engine  has  higher  thermal  efficiency than   a  stoichiometric
gasoline  engine.   When  deriving a  CNG  engine  from a diesel
engine,  compression  ratio must  be  reduced to  control knock,
reducing  efficiency.   Conversely, when using  a gasoline engine
base  for  a  dedicated  CNG engine, the  compression ratio can be
raised  due  to  the  superior  knock   characteristics   of  CNG,
raising   the  thermal  efficiency.   Experiments  with  a single
cylinder  spark ignition  engine  showed  an  ir.-rease  in  thermal
efficiency  of  22 percent through  both  increased  compression
ratio optimization and a  lean fuel mixture.[10]  However,  it is
difficult  to  predict from  this  type  of  work  exactly  what
efficency   improvements   can   be   made    on   a   full   size
multi-cylinder engine.

     The   energy   consumption   rates  for  the  various  engines
previously  discussed  are  shown  in Table  3-5.    The  current
technology lean-burn CNG engine  and the 1990 diesel L-10 values
were  estimated using a carbon balance  procedure  on  the exhaust

-------
                             3-22
                          Table  3-4

        Current  Technology  Heavy-Duty CNG Lean-Burr.
       and Stoichiotnetric Engine Emissions (q/3K?-hr)
Pollutant             Lean Burn            Stoichiometric

  THC                    0.9                     1.03
  NWHC                   0.09                    0.15
  NOX                    4.5                     1.33
  Part.                  0.06                    0.01
  CO                     4.0                    10.8
  C02                      575                     557
  Formaldehyde           0.05                    0.0008

-------
                               3-23
                            Table 3-5

                  Energy Consumption of Current
               Heavy-Duty CNG Sr.air.es ( 5TTJ/5H?-'r.r )
     Encine           Diesel Fuel         CNG          Gasoline
LlO/Curren- lean-burn    7180            10,000

Cat.  3406                7430            10,800

BUG (Chevy 454)
 diesel test cycle        —              9,950

Gasoline test cycle       —              8,950         10,036

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                              3-25

components.    The  Caterpillar  CNG  3406  is  the  steady  state
ccgeneration engine while the  diesel  3406  value is typical of a
1988 3406 engine.[8]  The Chevrolet  454  gasoline value  is also
typical of a current engine.

     As  can  be  seen  from Table  3-5  the lean-burn  CNG  engines
consume  39   to  45  percent  more  energy   than  their   diesel
counterparts.    This  is  not  surprising given that  the  peak
thermal efficiency of the diesel engines is  higher  than  the CNG
engines  and  the  presence  of  throttling  losses  with  the  CNG
engines at low loads.   Conversely, as was expected,  the  BUG CNG
engine  consumes  less  energy  than  the  Chevrolet  454 gasoline
engine from which  it  was derived.   In  fact, an eleven  percent
reduction in  energy consumption was  achieved.   Over the diesel
test  cycle,   however,  the  CNG  lean-burn  and  stoichiometric
engines  consumed  almost the same amount of  energy.   As  will be
discussed  in  the  section  on   optimized   engine  efficiency,
optimized lean-burn CNG  engines  are expected  to  consume less
energy than optimized stoichiometric CNG engines.

     E.    In-Use Performance

     The  in-use emissions  performance  and durability  of  CNG
engines   is   an   area   where   little   information   has   been
collected.  Generally,  in-use emissions deterioration would be
a  function  of  engine-out emissions  deterioration  and catalyst
deterioration.  Since the engine  technology used on CNG  engines
is  not  fundamentally 'different  in  characteristics  affecting
likely   durability   than   that   currently   being  used   on
conventional engines, there  is no reason to expect  that,  as CNG
technology  matures, the  engine-out  emissions  deterioration of
CNG  engines  would  be significantly  different  than  for gasoline
or  diesel  engines.    It  is  true  that  the   BUG  engine  has
experienced  some   difficulty  maintaining  proper   calibrations
during routine service, causing dramatic  increases  in HC and CO
emissions.[11]    Valve  seat  recession  has  also been  a  concern
on  some  configurations.   However,  EPA expects  such problems to
be  readily solvable on  future versions.

     As  for  in-use catalyst performance,   it  is again unlikely
that,  given  the   similarity  of  CNG  catalyst  technology  to
current   in-use   technology,   there  will  be  a  significant
difference  in  deterioration.   Catalyst  use  on  future   diesel
engines  may  present added durability issues due to  the presence
of  particulate in  diesel  exhaust  and  the potential  for catalyst
plugging.   However,  the  actual  emissions  impact  of  catalyst
deterioration  or of  catalyst  failure on a  diesel  engine would
be  much  less  than  for  the CNG engine,  due  to  the fact  that the
emission reductions being provided by the diesel catalyst would
only  be  relatively modest to begin with, in the range of 20 to

-------
                              3-26

30 percent.   A  failed catalyst  on  a  lean-burn  CNG engine,  on
the other hand, would  dramatically  increase HC,  CO and "aldehyde
emissions .

IV.  Optimized Vehicle Projeccions

     A.     Emissions

     Because  the   development   of  dedicated   heavy-duty  CNG
technology  is  still in its  early stages there  is every reason
to believe  that  significant  improvements  can be  made  in both
emissions  and efficiency.   Past experience  with  gasoline  and
diesel  technology  would  support this.  The  use  of  electronic
controls,    fast-burn    combustion    technology,    increased
compression ratios, and general  engine optimization would seem
likely  candidates  for improving CNG engines.   Since,  as noted
earlier,  dual-fuel  engines  offer  limited  opportunities  for
optimizing  CNG  combustion,   the projections  of  optimized  CNG
engine  emissions will  be  focused  on dedicated   engines.   It
should  be  noted  that the projections  derived here are based on
limited   current   data   and   assumptions   about   projected
improvements  over current  designs.   Actual  improvements made in
optimizing  CNG  engines may  yield different  results than those
projected here.

     It   appears   that  little   of  the  work  on  CNG   engine
development  thus far  has  been  directed  at  emissions,  as  is
evidenced  by  the lack of data.   In  this  section projections of
the potential  emissions of optimized CNG heavy-duty  engines of
the mid-1990s will  be made.   It  should be noted that no data on
advance  concepts  for  CNG  were available   and   the  projected
improvements  were   based  largely  on  assumptions  about  the
potential   of  different   technologies  and   extrapolation  of
gasoline and  diesel vehicle  experience.

     Based  on confidential  prototype  test  data  on the Cummins
CNG  L-10,   as   well  as   evaluations  of   the   potential  for
improvements  in  engine-out emissions and catalyst  efficiencies,
the  emissions of  an  optimized,  dedicated   lean-burn  CNG  engine
were projected.   These projections are shown  in Table 3-6 along
with the current technology emissions.  An  improvement  in fuel
consumption was  also  projected, as will be  discussed  shortly.
This    improvement   results   primarily   in   CC>2  reductions,
although  its  effects   are  felt  on  all  emissions   through
engine-out  reductions.

     Given the relative  infancy of  CNG technology as  compared
to diesel  technology  and  the limited  amount of work which has
been  done on   catalyst   optimization  for   methane   and  other
hydrocarbon components of CNG  exhaust,  significant  reductions

-------
    3-27
Table 3-6
CNG

Diesel
Pollutant
THC
mac
NOx
Part.
CO
C02
Formaldehyde
Estimates of Optimized
Lean-Burn Incline Emiss

Current
Technology
0.9
0 .09
4.5
0.06
4.0
575
0.05
Heavy-Duty
icr.s (q/3H?-r.r)
1994
Diesel
Standards
1.3
	
5. 0
0 . 10
15. 5
	
— _ _


Projected
Optimized
0 . 6
0 , 06
4 . 0
0 . 05
1 . 5
525
0. 03

-------
                              3-28

in future HC emissions should be possible.  For  this  analysis a
one-third reduction  in  total HC was  projected  through improved
combustion  and   catalyst   optimization.    The   percentage   of
non-methane HC was held  unchanged at 10 percent  of total  HC as
with  the  current technology  numbers.  A  similar reduction in
formaldehyde was  projected.

     Smaller percentage  reductions  in NOx  and  particulate were
projected.   In  the  case of  NOx,  there  is  little  basis  for
improvement and,  if  anything, future efforts to improve engine
efficiency and performance could put  upward pressure  on NOx.
In the  case of  particulate, any reduction  would likely come as
a  result of  improved  oil   controls.   Since  1991-type  diesel
engines  such  as  that  from which  the lean-burn  CNG  engine  was
derived  have  already  introduced  many  advanced  oil  control
features, there is little improvement projected.

     Finally,  there  are   reasons   to  believe that  future
lean-burn  CNG  engines   will  have  significantly   lower  CO.
Although  the current L-10 CNG engine is meeting  a design target
of 4.0  g/BHP-hr,  there  is  evidence  that  CNG engines  can reach
CO  levels much  closer  to  current  diesels  (1.5-2.5  g/BHP-hr).
This  fact,  along with  the  high  CO  conversion  rate  available
from  oxidation  catalysts,  should  allow  a significant reduction
in future engine CO  emissions.

     Turning  to  stoichiometric  combustion engines,  engine-out
emission  and catalyst  efficiency  improvements  were  projected
separately  due  to   the  availability  of  data   on  the  current
stoichiometric engine  both  with  and without  the catalyst.  The
current  technology  and  projected optimized emission  levels   for
stoichiometric heavy-duty CNG engines  are  shown in  Figure  3-7.
Considering   first   engine-out  emissions,   for  all  emission
components  except  C02»  which  is  largely  a  function  of  fuel
consumption,  it  was  assumed that levels could be reduced by 20
percent   over   current   levels.    This    seems   a    reasonable
assumption  given  the  relatively  young   nature of  dedicated
heavy-duty   CNG   technology  in  comparison  to  the   level  of
sophistication of gasoline  engines.  Such  things as  electronic
controls,   more  precise   fuel  metering,   and  spark   timing
optimization  offer  means  of  improvement  to  current systems.
Additionally,  improved  fuel efficiency,  as  will be discussed
shortly,  would also  serve to  reduce  engine-out emissions.

      For  the stoichiometric  three-way  catalyst  it  was  assumed
that  a five percentage point  improvement  could be  made for  all
pollutant  conversion  efficiencies   except   particulate   and
gasoline test  cycle NOx.   No  improvement  in  particulate   was
projected because future improvements in catalyst HC  efficiency
will   likely  not   affect  the  heavy  hydrocarbons   which   are

-------
                              3-29

characteristic of the particulate.  Also, no  improvement  in NOx
efficiency was projected  for the gasoline  test  cycle catalyse.
This  is   because the  catalyst  already  showed  a  91  percent
efficiency  and  an   efficiency  much  higher  than  this   is  not
likely,  at  least in-use.   The  projected catalyst  efficiencies
for  the   future  optimized stoichiometric   engine  are shown in
Table 3-7, along with the resultant tailpipe emissions.

     B.     Future Optimized Engine Efficiency

     The  last topic  of  discussion  for  future  optimized  CNG
engines  is  that  of  fuel  efficiency improvements.  there  are a
number of reasons to believe that fuel efficiency  of dedicated
CNG  heavy-duty  engines   can  improve  over  current  technology,
Natural   gas  as   a   fuel  has  a  relatively  slow  flame  speed
compared  to  gasoline and diesel  fuel.  The development  of  fast
burn  combustion  chambers  would  result  in  an increase -in  peak
pressure  and  higher  engine thermal  efficiency.   This  is an  area
that can apply equally to lean-burn and stoichiometric engines.

     For  lean-burn engines, the  current technology  has a rather
low  compression  ratio compared to  the  BUG engine  and  is  a
rather conservative  design.   Thus,  an  increase  in compression
ratio and a resultant increase  in thermal efficiency could be
expected.   Also, the  prechamber approach  previously discussed
has  potential for  higher cylinder  pressures  and  better  fuel
economy.  For  these  reasons  it seems reasonable  to  assume  that
a  significant improvement in fuel  efficiency  can be made for
lean-burn  engines.-  Thus, it was  assumed  that  a ten  percent
decrease  in  brake-specific   fuel  consumption  (from  10,000 to
9,000 BTU/BHP-hr)  could  be   achieved  in  future  optimized  lean
burn  CNG  engines   over  current   technology.    This   is  a
significant  improvement,  but  would still  result in  an  energy
consumption  for   the optimized  lean-burn  CNG  engine  about 25
percent higher than the diesel.

     For  stoichiometric  engines, the  compression ratio  of the
current  technology  CNG  engine  is  already  higher  than  that of
the  current  lean-burn  and   there   is  not   as  much  room  for
compression  ratio increase.    Also,  the prechamber  approach is
not  likely  to  be  useful  in  stoichiometric  engines,   and no
efficiency  increase  is  expected here for it.  For these reasons
it  is reasonable to  assume  that, while further  improvement in
stoichiometric  CNG   engine efficiency can  be expected,  it  will
not  be  as  large as  that for  lean  burn.    Thus,  it  was  assumed
that  only  a  five  percent  reduction in brake-specific  energy
consumption  would  be  achieved  on  future  stoichiometric  CNG
engines  over  current technology.   This  results   in  a  fifteen
percent   lower   fuel   consumption   rate   for    the   optimized
stoichiometric   engine  compared  to  the  current Chevrolet 454

-------
                                           3-30
                                         able 3-7


Pollutant

THC
MMHC
MOx
Part.
CO
C02
Formaldehyde
CNG

W/O
Cataivst

3.50
0.32
6.62
0.01
31.9
529
0.03
ata icniometr ic
Current
Catalyst
Efficiency

72%
32%
80%
0%
66%
97%
mains L.TUSS

With
Catalyst
Diesel
1.00
0.15
1.32
0.01
10.3
557
0.0003
ions ( cr 3.-.?-
Pro
w/o
Cataivst
Test Cvcle
2.33
0 . 56
5.30
0.01
25.5
499
0.02
"* ^ '
iected Catin
Catalyst

77%
37S
35S
OS
7 IS
37S

i zed
'•**' 1 1 r.
C a t a 1 v 3 1

3 •• ^
3 . 39
0.79
3.31
7.40
534
3.33C5
                                          Gasoline Test Cycle
THC
NMHC
NOx
Part.
CO
C02
Formaldehyde
1.7
0.40
5.75
0.01
23.5
474
0.02
53%
73"S
91%
' 0%
63%
	
99.5%
0.72
0.09
0.51
0.01
10.5
500
0.0001
1.36
0.32
4.60
0.01
22.3
453
0.02
53S
33S
9 IS
OS
53S
	
99. 5S
0.50
0.05
0.41
0/31
7 .2?
430
3. 3CC

-------
                              3-31

gasoline  engine.    A  similar  comparison   to   other  gasoline
engines would be expected.

     For  an  optimized  stoichiometric  combustion  CNG  engine
operating over  the  diesel  cycle  test cycle  this  five  percent
reduction in  energy consumption  over current  technology would
result  in  a brake-specific  energy consumption of  about 9,450
BTU/Bhp-hr.   Comparing this  value to  the  optimized lean-burn
combustion engine value of  9,000 BTU/Bhp-hr  it  can be seen that
optimized lean-burn CNG  engines  are expected to  be  somewhat
more   fuel-efficient   than   their   stoichiometric   combustion
counterparts.

V.   Safety Issues for CNG Use in HP Applications

     A.    Introduction

     For  certain  heavy-duty  applications,   CNG  is  likely  to
become a viable alternative in the near  future.  However, there
are  several  issues  which should be analyzed  to insure  that the
relative safety risks of using compressed natural  gas as a fuel
for  heavy-duty  applications   are  known  and addressed.   These
issues  are  discussed  in  these  sections.   The  specific issues
looked  at  include  flammability,  explosivity,  toxicity,  and
special safety  issues  of  concern to the use  of CNG  as  well as
the  implications  of these issues  when using CNG  as a fuel for
heavy-duty  vehicles-.   The. other   fuels  looked   at  are  gasoline
and diesel fuel for comparison purposes.

     For conventional  fuels,  there is a broad  base of tabulated
in-use  experience and,  as a result,  detailed  comparisons  of
many  aspects  of  the  safety  issues  can  be made empirically.
However,  in the  case  of  CNG  used  as  a  motor  vehicle fuel,
experience  is  only just  now being  built  up   and,  therefore,
there  remains  an  insufficient supply of in-use data to  produce
meaningful   statistical   results.*   Therefore,    the   safety
analysis contained here will  focus mainly on expert  projections
of  the safety-related  issues involved in  the  use of  CNG  as  a
heavy-duty  motor   vehicle   fuel.   This   analysis   is  mainly
qualitative  in  nature,  although  a  limited  amount of actual
in-use data is  included.
     CNG  fueled vehicles have been in use  for  as much as thirty
     years  in  other  countries  with no reports of unusual safety
     problems, however.

-------
                              3-32

     B.     Fuel Properties and General Considerations

     1.     Toxicity

     The  issue  of   toxicity  will  receive   only   peripheral
attention  in  this  report.  Natural  gas,  being  mostly  methane,
is not toxic.   However,  because of  its  gaseous nature,  it  can
act  as  an asphyxiant.   Therefore,   from  a toxicity viewpoint,
the  only  concern  in handling  methane  is  to  ensure  proper
ventilation   so  that   concentrations   sufficient   to   cause
asphyxiation do not form.

     In  contrast  to   CNG,  diesel fuel  and gasoline both  pose
significant toxicological  risks.   Gasoline and  diesel  fuel"are
toxic if  ingested, inhaled, or even absorbed through  the skin.
The  potential  effects  of acute  or  prolonged  exposures  are
nausea,   vomiting,   cramping,   liver  and  kidney  damage,  lung
irritation, and central  nervous  system  depression  ranging from
mild  headaches  to  coma  or   even  death.[12,13,14,15,16 ]   In
addition,  both  gasoline  and  diesel  fuels  have  carcinogenic
risks associated with them.    While  diesel fuel vapors  are  not
known to be  carcinogenic,  skin  tests  show  diesel  fuel  to  be
weakly to moderately  carcinogenic.   On the other hand,  gasoline
is believed  to pose  carcinogenic  risks  from vapors  as well as
from direct contact with the fuel.[17]

     2.     Flammability

     With  an   estimated  15,000  to  20,000   fires  resulting
annually  from  motor  vehicle  accidents,   the  risk  of  fire  is
probably  the greatest  safety risk  associated   with fuels.[18]
Associated with these fires  are approximately 1700 deaths, 3700
serious  injuries,  3600  moderate injuries  and   property damage
costs of well  over three  billion dollars.[18,19]   There are an
additional   2553  reported   service  station   fires    annually
resulting  in approximately four deaths and 115 injuries.[18 ]

     While  all fuels  are flammable, the  conditions needed for
ignition and  the severity of   the results  of  an ignition depend
on   the   properties   of  the  fuel.    Table 3-8  summarizes  the
properties  thought   to  be  most   important  in  assessing  the
hazards  of different  fuels as well  as other key physicochemical
properties.

      Because  of the  varying properties of  the  different fuels,
the  probability of an  actual  ignition is strongly dependent on
the  conditions  under  which a  fuel spill  or leakage occurs.  The
analysis begins with a  look  at the hazards associated with  a
fuel escape  under  conditions  with  good  ventilation  (such as
outdoors)  and then reviews conditions with poor ventilation and
the  potential  results should  a fire  occur.

-------
                                          3-33
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                              3-34

     a.     Ease o£ Ignition Under Conditions  of  Good Ventilation

     Under conditions of good ventilation, accumulation  of  fuel
vapor  in  flammable  concentrations   is   likely  only  in  close
proximity to the vapor source.  Any fuel  release  outdoors  would
likely be  well-ventilated.   Available studies  also show  that
even fuel releases  due  to collisions  in  areas  such  as  tunnels
should be  sufficiently ventilated to  prevent the  formation of
hazardous   concentrations   of   fuel   vapor.[20]    In   these
situations,  the  volatility  of  the liquid  fuels combines  with
the  lower  flammability limits,  vapor densities  and  diffusion
coefficients of the  vapors  to form the most  critical factors in
assessing   ignition  probabilities.    EPA's   methanol   safety
analysis  rated  diesel fuel  as  the safest material  under  these
conditions  and  gasoline  as   the most dangerous  with  methanol
fuels being an  intermediate risk.[13]   These  rankings  follow
the relative volatilities  of the fuels.   A study of  the safety
issues of  CNG  use  done by  the Los  Alamos  National Laboratory
for  DOE  in  1983  concluded  that  CNG fuels  were  safer  than
gasoline due  to the high rate  of  dispersion  and the relatively
higher lower  flammability  concentration.[21]   Diesel  fuel  was
once  again  considered  to  be the  safest  fuel  due  to  the fact
that  it  would take  a relatively  high temperature  to  allow an
ignitable mixture of vapor to form (flash point 125°F) .

     b.     Ease of  lo^iition Under Conditions of Poor Ventilation

     Under  conditions of  poor  ventilation,   the  probability of
an  ignition  is  greatly  different  than under  conditions  of good
ventilation.   Since  the  fuel vapors  are confined  to  a closed
space, the  likelihood of  ignition is  greater for  ail  types of
fuels  than under  conditions where  the  vapors  are  allowed to
disperse  readily.   These are  the  types of  conditions  which
might be found  in  a  storage or  repair  garage or  in  a poorly
ventilated covered  parking area.

     Again,  under these conditions,  diesel  fuel is  the safest
of   the   three   alternatives   due  its   very  low  volatility.
Temperatures   high   enough   to   generate   the   needed  vapor
concentrations  are  rarely   found unless  an   ignition  source
strong enough to heat  the  fuel  is  nearby.   Therefore, diesel
fuel   under  most   conditions   is   relatively  safe   from   a
flammability  standpoint.

      Gasoline   can   easily   form   flammable  or  even   explosive
mixtures  under  poorly ventilated conditions since the vapors
cannot  disperse  as  readily.   As  with diesel  fuels, vapors  are
heavier  than air  so  that the greatest risk of fire  or  explosion
would be near  the ground.   For  this  reason,  existing repair  and
 indoor  refueling  stations  often  put electrical  equipment  and
other possible  sources  of  ignition up  in  the  ceiling.

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                              3-35

     CNG,   as  a  lighter than  air  gaseous  fuel  also  poses  a
serious flammability and  explosion risk under  conditions  which
do not allow the  vapors to dissipate.   These hazards would  be
similar to  the  types  of hazards often faced  by  residential  and
commercial  users  of  natural   gas  for  heating  or  other  uses.
Furthermore, the  most  likely  place  for  formation  of  flammable
or  explosive  mixtures  of  natural  gas  would  be   near  the
ceiling.    Therefore,  facilities which  would handle  repairs  or
refueling  of  CNG  vehicles  indoors  may  not  want   to  place
ignition sources  close to the  ceiling.  Proper  ventilation and
gas sensors  would likely be  utilized  in  any such  facility  to
eliminate  the possibility  of   a  combustible  mixture collecting.
In  some   cases   seperate   facilities   for   CNG  vehicles  and
liquid-fueled vehicles may be  required.

     3.    Hazards Associated  With a Fire

     The  most  telling  distinctions between  liquid  fuel  fires
and natural gas fires  is  the  rate of combustion  and  the amount
of smoke produced by combustion.   Both conventional liquid fuel
vapors  and natural gas  will   rapidly  combust  any  fuel  vapors
present  in flammable  concentrations once  a  source of ignition
is   introduced.     The  major  differences   in   flammability
characteristics  are   in   sustained,   severe  fire   properties.
Pools  of  gasoline or  diesel  fuel  burn with  a defined  rate  of
heat  release based on the heat of  combustion and the  rate of
combustion  of  the  fuel  and  produce a.  large amount  of smoke.
Rather than burning from  a pool at a  fairly well  defined rate
natural gas  will  burn with a torch type  flame at the  site of
the  leak.   The  rate of  heat  release will be controlled by the
rate of fuel  release  and various safeguards  can  be implemented
to control the  fuel release  rate.   These  safety strategies are
discussed   in   a   later  section.    Furthermore,   natural  gas
produces very little smoke while burning.

     Diesel  fuel  fires  tend  to  start  slowly  but  progress
violently.    High  heat   release   rates   result  in   a   high
probability  of  spreading  the  fire to  other  nearby flammable
materials  or causing serious  burns to  exposed individuals.C13 ]
Gasoline,  due to  its  high volatility,  immediately erupts  into  a
fully developed fire.  Because  of  the  rapid  burn rate, gasoline
has  an  even higher  heat  release  rate  than  diesel fuel  and
therefore  poses an  even greater risk for  spreading the  fire or
for  serious burns to  individuals.[13]   Also, both  gasoline and
diesel will  produce  a  large amount of  smoke  during combustion.
This  smoke itself can pose serious  risks  of injuries to nearby
individuals.

     Natural  gas,  on  the other  hand,  burns  at a  lower  flame
temperature  than  gasoline  or diesel  fuel.   Since the  rate of
combustion would be defined by the  rate  of release  of the fuel,
it  is more  difficult  to quantify  a rate of  heat  release.   It

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                              3-36

is, however, estimated by  the Los Alamos  report that  the  rate
of heat  release  for CNG is  less  than that of burning  pools  of
liquid fuel.[21]   A more recent study by EBASCO  indicates  that
the heat release rate would  be  less  than 40 percent of  the hear
release rate of gasoline fires.[20]   Furthermore  the  burning  is
more  likely  to  be  confined  to  a  small   area  immediately
surrounding  the  release  point  than  for  liquid  fuel  fires.
Consequently the likelihood  of spreading the  fire is  less for a
CNG torch fire than for  a  liquid  fuel pool fire.   In turn,  the
likelihood  of  an   individual  receiving  a serious  burn  would
probably be  less  than for  liquid fuel  fires.    Finally,  since
natural  gas burns  without  producing   sizeable  quantities  of
smoke  and  toxic  materials,  toxic risks  from exposure to  the
products of  combustion  would be  less compared to  gasoline  and
diesel fuel fires.

     4.    Issues of Special Concern in the Use of CNG

     Several  researchers  have   identified  issues  of   special
concern    to    CNG   and    pressurized    gaseous    fuels   in
general.[21,22]  First of  all, there is the  inherent danger  of
storing  and  handling  a compressed gas.   If  a fuel  line should
rupture,  particularly  at   a refueling  station,  injury  could
result from the  flailing hose.  Another concern with compressed
gases  is  the  possibility  of frostbite  resulting  from someone
being  exposed  to gases  cooled by rapid expansion  or  fixtures
cooled  by  these gases.   These   issues  are  dealt  with  more
thoroughly in section C below.

     Second, there  is the concern  of  the fuel  cylinder being
improperly   restrained.    Cylinders   generally    have   greater
structural  integrity  than  the surrounding vehicle and therefore
have  the  potential  to  penetrate into  parts of the  vehicles
where  they were  not  intended  to  be  or  to  break loose  from the
vehicle   and  become  a   potentially   dangerous   projectile,
especially  in  collisions.    However,  with  proper placement and
restraint  of  CNG  cylinders,  risks  posed by   fuel  cylinders
breaking  loose should be  no greater compared  to conventional
fuel tanks.

     The  Department  of Transportation  (DOT)   has   established
standards   for   the  safe   transport  of   hazardous  materials.
Natural  gas   falls   into  this  designation.   These   standards
establish   both   maximum  operating   and  burst  pressure  for
cylinders,   the   type  of   testing   required  before  use  and
periodically   during   the   cylinder   lifetime,   and   allowable
contaminants   in   the   compresed  natural   gas  to   prevent
corrosion.[23]   While the  cylinders  used on  CNG  vehicles  in the
U.S.  meet  these  specifications,   it  should  be  noted  that  the
specifications were designed for  the transport   of  CNG and not

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                              3-37 •

specifically for  the use  of these  cylinders  as  vehicle  fuel
tanks.   DOT has not  yet  set  standards for cylinders used in CNG
vehicles.

     There is also concern about  these cylinders being  able  ro
withstand corrosion  from external sources and from contaminants
in the gas itself.    Indeed, the National  Fire Protection Agency
has  established purity  standards  for  compressed  natural  gas
which  limit  the  water  content  to  less  than  0.5  pounds  per
million  cubic  feet,  0.1  grains  of  hydrogen  sulfide  per  100
cubic feet, and no  more than three  percent  carbon  dioxide for
gas to be  transported  in steel  cylinders.  Pipeline quality gas
specifications    are   generally   less   stringent.    However,
cylinders thus far have  shown excellent resistance to corrosion
from current gas supplies.

     Furthermore,   the  cylinders  must  be  able  to  withstand
physical  strain.  Cylinders  will be  subjected  to  the strain of
repeated  cycles  of pressurization   and  temperature  swings.
Rigorous  standards  have been set for  structural  resistance  to
such  conditions  and currently   available  CNG  cylinders  have
routinely been certified to meet these standards.

     Finally,  there  is  the  question of  the integrity  of the
fuel cylinders  and  their  ability to  withstand  physical abuse.
Severe  abuse tests  have  been  performed on  CNG  cylinders  to
demonstrate their fundamental durability.   These cylinders have
survived  being  dropped in a  car  from  a  height  of  more than 60
feet,  having sticks of  dynamite strapped to  the  side  of the
cylinder  and detonated, and being shot  at  with  small caliber
bullets.[24]

     C.    Implications for Vehicle Safety

     1.    Refueling

     The hazards posed in  refueling with  compressed natural gas
are  different  from  those  posed in refueling with conventional
liquid   fuels.    As   long   as   normal,   properly   functioning
equipment  is being used,  CNG refueling should be generally less
hazardous  than refueling  with  gasoline  or  -diesel  fuel  since
there  will be  no  toxic or  flammable vapors  escaping from CNG
refueling  equipment,  as  there  often  is  with  conventional.
refueling  equipment.  In  the event  of  equipment  failure,  CNG
systems   would  offer   a  significant  advantage   compared  to
gasoline   or   diesel  systems  in  the   area  of  environmental
exposure.   While  gasoline  and  diesel  fuel  storage   tank  or
dispensing  equipment leaks  could  lead to  contamination of the
surrounding environment  with toxics,  CNG  leaks  would  introduce
no  such  toxic  materials  into  the environment.   On  the other

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                              3-38

hand,  CNG  equipment  failure  could  pose  a  greater  risk  of
physical injury to the  operator  compared to gasoline and diesel
fuel systems.  These injuries  could  take the form  of  cryogenic
burns from  gas  cooled  by rapid  expansion  or  injuries resulting
from being  struck  by  a  flailing  hose.[4]   Both of  these  risks
can  be  minimized  by designing  the  equipment  to  both  resist
catastrophic failures  and so  that  if  such  a  failure were  to
occur,   it  would occur  at a  point  which  is  already  anchored,
contains a valve to shut  off  the flow of  fuel,  and/or  is  in =an
area where people are not likely to be exposed to the leak.

     In  cases   where  the  refueling  is  being  done  indoors,
flammable  concentrations  of  fuel  vapors   of   any  type   can
build-up.   For  CNG  systems,   very  little  vapor  should  be
released  during  normal   operation.   However,  in  the  event  of
fuel leakage from  malfunctioning equipment,  large quantities of
vapor  could rapidly  escape.    The  risks  of  vapor  build-up,
however,  can  be  minimized  by  enclosing the  fuel  line   in a
ventilation  line.    Since fuel   leakage   cannot  be  completely
prevented,  however,  the  best  strategy for minimizing  risks  is
building design.    Proper  ventilation and placement  of equipment
which could  serve  as  ignition sources should greatly reduce the
risks of fire or explosion posed by any fuel source.

     2.    Vehicle Operation and Crashes

     During  normal  operation  of   existing  fleets   of   CNG
vehicles/  it appears  that small leaks have  been  observed  with
greater  frequency  than  in conventionally  fueled vehicles.[21]
It  would be reasonable  to conclude  that the highly-pressurized
nature of the CNG  fuel system could  make  it  more prone to  small
leaks and  to more  fuel  being released from a given leak.   Small
leaks  pose  concerns  about   vapors  accumulating  to  flammable
concentrations   either   in  vehicle   compartments   or  vehicle
storage  enclosures.   Incorporation  of  design features  such as
vents   in   the  vehicle  body  and ventilation  of  garages  can
mitigate  the  risks  of  fuel   leakage.   Currently,  there  is
insufficient data  to determine  to what  extent small fuel  leaks
actually pose any  hazards.

     In vehicle collision scenarios, CNG  would appear to pose  a
level  of risk  somewhere  between diesel fuel  and  gasoline.  To
analyze  collision  hazards it  is  necessary to evaluate the  risks
of,  and the likely extent of, fuel  leaks.  It is also necessary
to  examine the ease with which  such problems  can  be dealt  with
in  the  event of combustion of  leaked or  leaking  fuel.

     In  the   absence   of extensive  data  on  the  use of  CNG
vehicles,  it is difficult to  make an accurate assessment of  the
relative risks of fuel  release.  However, some assessments  can

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                              3-39

be made  as to  estimates  of  relative  risks.    Because  of  the
structural  integrity  needed by  the  fuel  storage  cylinders  to
hold  compressed  natural  gas,  these  cylinders   are  much  more
likely than gasoline or  diesel  fuel tanks to survive collisions
without release  of  fuel from the storage  tank.   On  the  other
hand,   fuel  lines,  valves  and fittings  would  be more  prone  to
severe leaks  than gasoline  or   diesel  systems  because  of  the
pressurized nature  of the  fuel.   However,  safety  devices such
as fuel release regulators and solenoid valves to shut off fuel
flow when  the  engine  stops can  be  built  onto  the fuel cylinder
to lessen the severity of any release of natural  gas  from  a CNG
vehicle,  and minimize the significance of this risk.

     In the event of  a fuel release  resulting   in a  fire,  the
resulting problems from  a  CNG  fire are  likely to  be  easier  to
deal with.  First of  all,  fires  from liquid fuels are difficult
to  extinguish "and  difficult  to  control   if   they  are  not
extinguished.    On  the  other hand,  natural  gas  torch  fires
(which  are  the  only  kind  likely to  be  sustained)  can  be
extinguished  by  shutting  off the fuel  source,  which is  not
generally possible in liquid fuel  fires.   Some currently  in-use
CNG vehicles  have a  readily accessible quarter  turn shut-off
valve for this purpose.  Should it  be difficult  to  shut off the
flow  of CNG,  it  should still be  possible  to control  the  damage
from  the   fire  because  of  its  localized   nature.    Severe
explosions  are  unlikely in  any case  since CNG  cylinders  are
designed to handle conditions  likely to lead  to  explosions and
will  eventually  vent  off  gas which  will burn  in  a  relatively
controlled manner rather than rupturing  to produce an explosive
release.

     Despite  the lack  of  a database of CNG  use sufficient  to
draw definitive conclusions, there  are  some surveys and studies
which  do   present   some  in-use   information.   In   1987,  the
American  Gas  Association  completed  a  survey of  fleet use  of
dual-fueled  vehicles.    Their  survey,   covering  434.1  million
miles   of   accumulated   use   with  dual-fueled  CNG/gasoline
vehicles,  claimed that  the  injury  rate  for  CNG  vehicles  was
significantly  less  than for  all  U.S. vehicles  and fewer  fires
were  attributed  to  the CNG  fuel  system  than to  the gasoline
fuel  system.[24]  Furthermore,  data  from  New  Zealand indicated
that  CNG  vehicles  caught  fire  much  less  frequently  than did
conventionally  fueled  vehicles.[20]   However,  the information
about  vehicle  operation characteristics  in these reports  lacked
information on number  of miles  travelled or. CNG  and  on  injury
causes   was   therefore   insufficient   to  enable   any  solid
conclusions to be drawn  from the  data reported.

      In cases  where  the fuel release occurs before a  source  of
ignition  is  available,  both gasoline and CNG have the potential

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                              3-40

to form a flammable cloud of vapor which  could  burn explosively
when   an  ignition   source   is   introduced.    However,   such
conditions   are   only   likely   under   conditions   of   poor
ventilation.   Tunnels  and   lower  decks of multi-decked  bridges
have attracted attention as a  possible  location where explosive
conditions  might   occur  following  an   accident.    Both  the
Triborough Bridge  and  Tunnel   Authority  and  the Port  Authority
of  New   York  and  New  Jersey,  as  part  of  their  general
restrictions on compressed  gas transportation  (stemming  from a
severe LP-gas accident  in  the  first half of  this century in the
Holland  Tunnel),  limit  the amount  of  compressed natural  gas
which can be  transported in these areas to less than 100 pounds
of gross weight with each   cylinder  being less  than  ten pounds
of  gross   weight.    This   same   restriction   applies  to  all
flammable compressed gases.[25,26]

     In spite of these  concerns,  available studies suggest that
tunnels  generally have sufficient  ventilation to prevent  gas
vapors   from   building   up   large   volumes   of   flammable
concentrations,  so  that the relative  flammability risks of the
fuels  should  be  similar   to  those  for  conditions  of  good
ventilation.[21]   Both  the Los  Alamos report  and the recently
completed EBASCO  report for the New York  State Energy Research
and  Development  Authority,   Brooklyn  Union  Gas  Company  and
Consolidated  Edison Company  on the  safety  of  CNG  in  tunnels
concluded  that  CNG  is  safer  than  gasoline  for  use  as  a
transportation  fuel  in  tunnels,  although  neither  study claimed
CNG   would   be   safer   than   diesel   for    use   in   these
environments.[20,21]   The  •EBASCO report  in particular  did  a
detailed  modeling study of likely  fuel  concentrations  in the
Holland  tunnel  which would result from a  rapid release  of fuel
from  a  bus  and   concluded that  there  would   be  only  a  very
limited  time and location where  there  would  be a flammable
concentration of  natural gas,  and  that  the  only scenario under
which  natural  gas vehicles   might  pose  a  greater   risk  than
gasoline  vehicles would be if the  fuel  line ruptured under the
vehicle  and the natural gas   became  trapped under  the vehicle
with no  safety  devices  to   stop the  flow  of  gas or to ventilate
the undercarriage  of the bus.

     3.    Maintenance

     The issue  of  maintenance might  also pose some concerns.
On  CNG  equipped   vehicles,  it  is  possible  that  a  maintenance
worker  could release a large  amount  of  fuel  by  inadvertently
creating a  vent in the  fuel system.   In  such a case,  the worker
could  also  face   the   risk  of  a  cryogenic  burn   as described
earlier.    If  the  fuel  release  occurred  outdoors,  no  other
hazards  should be posed  unless  there  was  an  ignition source
 immediately present, due  to  the rapid  dispersion of the gas.
On  the  other  hand,  if the  maintenance were  being performed

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                              3-41

indoors,   there  is  a  potential   for  the  rapid  formation  of  a
flammable or explosive cloud.

     The  same suggestions  for garage  design  described  earlier
would  reduce  the  hazards   associated  with  a  fuel  leak  during
maintenance.   In   particular,  well  ventilated  buildings  will
greatly  reduce  the  risks   of  a  flammable  or  explosive  cloud
forming.   Furthermore, it  should  be  pointed out that  a  vehicle
with a properly functioning solenoid valve to shut off the fuel
flow  when  the  engine  is  not   running  could  minimize  the
possibility of  the  maintenance  crew  accidentally  releasing  a
large  volume  of   fuel.    With   the  establishment  of   proper
maintenance procedures (such  as  shutting  the fuel  flow  valve
during  maintenance)   and  the  accumulation  of  experience  in
maintaining CNG vehicles,  the  risks  associated with maintaining
CNG  vehicles   should  not   necessarily  be  different  than  for
maintaining conventionally fueled vehicles.

     D.    Summary

     Evaluating   CNG  as   an   alternative  fuel   for   use  in
heavy-duty  vehicles  from a  safety standpoint,  it  can  be  seen
that CNG poses  some  unique safety concerns due to its  gaseous
and  pressurized  properties.   On  the  other hand,  it has  some
properties  which  are  superior   (from  a  safety viewpoint)  to
those of conventional  liquid fuels,  including  the  fact that the
material  itself is  not toxic (a  significant safety benefit, over
conventional  liquid  fuels).   .Owing to   the  lack  of  extensive
experience  with this fuel  in'the United  States, it is difficult
to  draw definitive  conclusions   regarding the  safety  of  this
fuel relative to  fuels with a broader base of use.  However, it
would  appear  that   the  area  of  most  concern  and uncertainty
would  be flammability.  Studies  have suggested that  CNG would
be  safer than  gasoline in  well ventilated  areas but  could pose
a greater  risk  of explosion in areas with poor ventilation.  In
the  event  of  a  fuel  leak or spill,  therefore,  the  risk  of fire
from  CNG  could  be  comparable to the  risk  from  gasoline and
somewhat greater  than the  risk from diesel  fuel.   However, the
consequences  of  a  CNG  fire  should be   less  severe than for
gasoline  or   diesel   fuel  fires.    Although   the   relative
likelihood  of  fuel  spills  or leaks  from  CNG as  compared to
conventional  fuels  is yet to  be determined,  it  would  appear
that  CNG   would  not  pose  a  greater   f lammability   risk  than
gasoline.   Despite the need  for  further work  in  ensuring that
fuel  leaks are  minimized  and controlled,  in  developing  safe
maintenance practices and  in  assuring   that  refueling   can  be
done  safely,  it  would appear that  there  are  no  safety issues
which cannot be dealt  with  which  would preclude the development
of  CNG  as  an alternative  fuel  for heavy-duty vehicles.   Taking
all  factors into  consideration,   it  would appear  that  CNG  is
certainly no more dangerous than  gasoline as a vehicle fuel.

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                              3-42

                      References Chapter 3
     1.     L.  E.  Gettel, G. C.  Perry,  "Performance  and Emission
Characteristics  of  a  Dual   Fuel   Caterpillar  3406  Engine,"
presented at  the  Fifth Windsor  Workshop on Alternative  Fuels,
Ontario,  Canada,  June 13,  1989.

     2     G.   O'Neal,   Southwest  Research  Institute,  telephone
communication, November 1988.

     3.     "Natural Gas Vehicles:  A Review of  the  State  of the
Art," prepared by  Sierra  Research Inc.  for  the  Natural  Gas
vehicle Coalition, Report No.  SR 89-04-01, April 1989.

     4.     M.  A.  Deluchi,  R.A.  Johnston,  D.  Sperling, "Methanol
vs.  Natural   Gas  Vehicles:   A  Comparison  of  Resource  Supply,
Performance,    Emissions,   Fuel  Storage,  Safety,   Costs,   and
Transitions,"  SAE  Paper  No.  881656,  Society  of  Automotive
Engineers, Warrendale,  PA  1988.

     5.     R.   I.   Bruetsch,    "Emissions,  Fuel   Economy,   and
Performance   of   Light-Duty   CNG   and  Dual-Fuel   Vehicles,"
EPA/AA/CTAB-88-05, U.S. EPA MVEL  Ann Arbor, MI June 1988.

     6.     J.   Alson,  J.  Adler, T.  Baines,  U.S.   EPA.,  "Motor
Vehicle  Emission Characteristics  and  Air  Quality  Benefits of
Methanol  and  Compressed  Natural   Gas,"   presented   at   the
Symposium  on  Transportation  Fuels  in  the  1990 's  and  Beyond,
Monterey, CA.   July 1988.

     7.    Telefax from V. K.  Duggal,  Cummins  Engine Co., to J.
W. Mueller, U.S. EPA, October  12, 1989.


     8.    J.R. Gladden,  J.C.  Kline,  "Development  of  Fast   Burn
Combustion With Elevated  Coolant  Temperatures  for Natural Gas
Engines,"  report  to the   Gas  Research  Institute,  Caterpillar,
Inc., Peoria,  IL, July  1988.

     9.    D.  J.  Waldman,  J.  R. Gladden, D. L. Endicott, and B.
A.  Cull, "Caterpillar  3406  Spark   Ignited  Natural  Gas  Engine
Emissions  on  EPA  Heavy-Duty  Transient  Test  Cycle,"  report to
the  Gas Research   Institute,   Caterpillar  Inc.,  Peoria,  IL,
January  1989.

      10.   R.   D.   Fleming,   G.   3.   O'Neal,   "Potential   for
 Improving the Efficiency  of  a Spark Ignition  Engine for  Natural
Gas   Fuel,"    SAE  Paper   No.  852073,  Society  of   Automotive
Engineers, Warrendale,  PA 1985.

      11.   c.   Spielberg,   "Compressed   Natural   Gas   Program
Monthly  Report *2,  January-May 1989," New York City  Department
 of Transportation, September  12, 1989.

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                              3-43

                 References Chapter 3  (cont'd)

     12.    "Material  Safety  Data  Sheet:   Diesel  Fuel  Oil  Mo.
2-D," Occupational  Health Services,  Inc.,  September,  1985.

     13.    "A  Perspective  on  the Flammability,  Toxicity,  and
Environmental   Safety   Distinctions    Between   Methanol   and
Conventional  Fuels,"  Paul  A.   Machiele,   Standards  Development
and Support Branch,  U.S.  EPA,  August,  1989.

     14.    "Material  Safety  Data Sheet:   Gasoline/Automotive,"
Occupational Health Services,  Inc.,  September, 1985.

     15.    "Hydrocarbon  Contact Injuries," J.F.  Hansbrough et.
al., The  Journal  of  Trauma, Vol.  25, No.  3,  March,  1985.   As
cited in reference 13.

     16.    "Gasoline  Intoxication,"   W.  Machle,   J.  Amer.  Med.
Assoc., [1]:  1967-  1971,  1941.   Cited in  "A Perspective  on the
Flammability,  Toxicity,  and  Environmental Safety  Distinctions
Between  Methanol  and Conventional Fuels," Paul  A.  Machiele,
Standards  Development  and Support  Branch,  U.S.  EPA,  August,
1989.

     17.    "Summary  and  Analysis  of  Comments  Regarding  the
Potential   Safety   Implications  of   Onboard  Vapor   Recovery
Systems," Office-of Mobile Sources,  U.S. EPA,   August 1988.

     18.    "Methanol Fuel  Safety:  A  Comparative  study fo M100,
M85,  Gasoline, and Diesel  as   Motor Vehicle  Fuels," Paul  A.
Machiele, SDSB, U.S. EPA, August 1989.

     19.    Memorandum:   "Analysis  of  Fuel  Tank-Related Fires,"
from  Kathleen  A.   Steilen,  Standards  Development   and Support
Branch,  to Charles  L.   Gray,   Jr.,  Director,  Emission Control
Technology Division, U.S. EPA,   April,  1987.

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                              3-44

                 References Chapter 3_£cont'd)

     20,    "Safety Analysis of  Natural Gas Vehicles
Highway  Tunnels,"  E3ASCO Services  Incorporated,  August,  1989.
Prepared  for  New  York  State  Energy  Research and  Development
Authority,  The  Brooklyn  Union  Gas  Company,   and  Consolidated
Edison Company of New York,  Inc.

     21.    "Gaseous   Fuel   Safety  Assessment  for   Light-Duty
Automotive  Vehicles,"  M.C.   Krupka,   A.T.   Peaslee,  and  H.L.
Laguer,   Los   Alamos   National   Laboratory,   November   1983.
Prepared for the Department of Energy.

     22.    "Identification  of   Safety  Related  Research   and
Development Needs  for CNG Vehicle Fuel Systems,"  Gas  Research
Institute, March, 1983. (GRI-82/0061)

     23.    Code  of   Federal   Regulations,  Title  49,   Section
173.34.

     24.    "Severe Abuse Testing"  video tape  distributed by the
CNG Cylinder Corporation

     25.    Rules  and  Regulations  Governing  the  Use  of  the
Triborough  Bridge  and  Tunnel  Authority  Facilities  and  the
Transportation of Hazardous Material as in effect September 30,
1984.

     26.   Hazardous' Material:  Transportation  Regulations   at
Tunnel and  Bridge Facilities, The Port Authority  of NY  & NJ,
revised  June 1, 1976.

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                              3-45

      References For Further Information on CNG Technology

     "Assessment: of  Methane-Related Fuels for  Automotive Fleet
Vehicles," U.S.  Department  of Energy,  DOE/CE/50179-1,  February
1982.

     W.  A.  Goetz,  D.  Petherick  and T.  Topaloglu,  "Performance
and Emissions of Propane, Natural Gas,  and Methanol Fuelled Bus
Engines,"    SAE  paper   no.   880494,   Society   of  Automotive
Engineers, Warrandale, PA,  1988.

     R.    R.   Raine,   J.   Stephenson   and   S.   T.   Elder,
"Characteristics of  Diesel  Engines Converted  to Spark Ignition
Operation  Fuelled  with  Natural  Gas,"  SAE  paper  No.  880149,
Society of Automotive Engineers,  Warrendale, PA, 1988,

     "Assessment   of  Costs   and  Benefits  of   Flexible  and
Alternative  Fuel  Use   in   the   U.S.   Transportation  Sector,"
Progress  Report  Number One:   Context  and Analytical Framework,
DOE/PE-0080, January 1988.

     "Dual-Fuel  School   Bus   Demonstration,"   New  York  State
Energy  Research  and Development  Authority,  Albany, NY, October
1986.

     Vice President's  Task  Force on Alternaitve Fuels," Report
of  the  Alternative  Fuels Working  Group."  Office  of  the Vice
President, Washington, DC, July  1987.

     L.   E.  Gettel,  G.  L.   Perry,   J.   Boisvert  and  P.  J,
0'Sullivan,  "Microprocessor   Dual-Fuel  Diesel  Engine  Control
System," SAE Paper No.  861577, Society of Automotive Engineers,
Warrendale, PA,  1986.

     N.  J.  Beck,  et.  al,  "Electronic Fuel  Injection  for Dual
Fuel  Diesel   Methane,"   SAE  Paper   No.   891652,   Society  of
Automotive Engineers, Warrendale, PA,  1989.

     J.    Heenan   and  L.   Gettel,   "Dual-Fueling  Diesel/NGV
Technology,"   SAE  paper  No.   881655,  Society  of  Automotive
Engineers, Warrendale, PA,  1988.

     P.  C.  Few  and P.  Sardari, "Dual  Fuel Control  of  a High
Speed   Turbocharged   Diesel   Engine,"   SAR  Paper  No,  871670,
Society of Automotive Engineers, Warrendale, PA", 1987.

     P.   C.  Few and H.  A.Newlyn,  "Dual  Fuel   Combustion  in  a
Turbocharged  Diesel  Engine,"  SAE  paper No.  871671,  Society of
Automotive Engineers, Warrendale, PA,  1987.

     T.  Adams,  'The Development of Ford's  Natural Gas Powered
Ranger,"  SAS paper No.  852277, Society of Automotive Engineers,
Warrendale, PA,"1985.

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                              3-46

    References For Further Information on CNG Safety (con't)

     American Gas Association,  "A.G.A.  Requirements  for  Natural
Gas Vehicle (CNG) Conversion Kits," No.  1-895,  August 1985.

     G.  A.  Karim and I.  Wierzba,  "Experimental  and  Analytical
Studies of  the  Lean Burn Operational Limits  in Methane  Fuelled
Spark Ignition and Compression  Ignition  Engines,"  SAE  paper  No.
891637,  Society of Automotive Engineers, Warrendale, PA",  1989.

     D.   Micle,   T.  Krekee  and  T.  Giannacopoulos,  "Electronic
Injection System for Natural Gas  in  a Diesel Engine-Development
and  Testing,"  SAE  paper  No.   890852,  Society  of  Automotive
Engineers, Warrendale, PA, 1989.

     J..  van der  Weide,  et.   al.,  "Experiences  with CNG  and  LPG
Operated  Heavy  Duty  Vehicles  With Emphasis  on U.S.  HD Diesel
Emission   Standards,"   SAE   paper   No.   881657,   Society   of
Automotive Engineers, Warrendale, PA, 1988.

     "Assessment  of  Methane-Related Fuels  for  Automotive Fleet
Vehicles,"  Volumes   1-3,   The   Aerospace  Corporation,   Energy
Conservation  Directorate, DOE/CS/50179-1,  Office of Vehicle and
Engine  R  &   D,  U.S.  Department  of  Energy,  Washington,  B.C.,
February,  1982.

     "The  Practical  and  Economic Considerations  of  Converting
Highway  Vehicles to  Use Natural  Gas  as   a  Fuel," Richard L.
Bechtold,  et  al., SAS Technical Paper 831071, 1983.

     "Some  Considerations of the Safety of Methane (CNG), as an
Automotive  Fuel  -   Comparison  with  Gasoline,  Propane,   and
Hydrogen  Operation," G.A.   Karim,  SAE  Technical  Paper  830267,
1983.

     "High  Speed  Collision   and   Severe  Abuse   Testing   of
Composite   Reinforced  Aluminum  CNG  Vehicle   Fuel  Cylinders,"
Norman  C.  Fawley,   Symposium  Papers,  Nonpetroleum  Vehicular
Fuels   IV,  Institute  of   Gas  Technology,  Chicago,  Illinois,
October,  1984.

     "Environmental  and  Safety  Aspects of  Natural  Gas-Fueled
Vehicles,"    J.W.    Porter,   Symposium   Papers,   Nonpetroleum
Vehicular   Fuels II,   Institute   of  Gas   Technology,   Detroit,
Michigan,  June,  1981.

     "The San  Antonio   Story," J.W.  Brooks,  Symposium  Papers,
Nonpetroleum  Vehicular  Fuels  II,  Institute of Gas Technology,
Detroit,  Michigan, June,  1981.

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                              3-47

    References For Further Information on CNG Safety (con't)

     "Safety  Testing  of  LPG  and  Gas-Fueled  Vehicles,1'  J.  van
der  Weide,   Symposium   Papers,   Nonpetroleum  Vehicular  Fuels
Symposium,  Institute  of  Gas  Technology,  Arlington,  Virginia,
February, 1980.

     "A  Fleet of Twenty CNG  Cars and  Trucks  in Upstate  New
York: Ten  Months Experience," D.J. Whitlock, Symposium Papers,
Nonpetroleum Vehicular Fuels  III,  Institute  of  Gas Technology,
Arlington, Virginia, October, 1982.

     "Analysis of the Pneumatic  Burst of a Large Seamless Steel
Vessel in Natural Gas Service,"  B.W.  Christ,  U.S.  Department of
Transportation report DOT/MTB/OHMR-78-4,  March 1979.

     Gas-Powered  Vehicle Evaluation Program,  TES,  Ltd., report
TES C372, for the Road and Motor Vehicle Traffic Safety Branch,
Transport Canada, March,  1982.

     "Dual-Fuel  Motor  Vehicle  Safety  Impact  Testing,"  U.S.
Department  of  Transportation report  DOT/HS-800622,  November,
1971.

     "Assessment  of Research and  Development Needs for Methane
Fueled Engine Systems," T. J.  Joyce,  Final  Report, Gas  Research
Institute report GRI 81/0046, March 1982.

     "The  Benefits  and  Risks  Associated  with Gaseous-Fueled
Vehicles,"   D.    Shooter   and   A.  Kalelkar,   Report   to   the
Massachusetts Turnpike  Authority,  Case No.  74400-2, prepared by
A.D. Little,  Inc.,  May 1972.

     "CNG   Reports,  No.   1,"   T.J.   Joyce   Associates,   Inc.,
October,  1982.

     "Report   of  Overseas   Visit  to  Investigate  Compressed
Natural  Gas in  Italy,"  R.N.  Abrams,  A.L.  Titchener,  and  J.?.
West,  Liquid  Fuels   Trust  Board,  Wellington,   New   Zealand,
February,  1980.

     "Preliminary Analysis of the  Safety History of Natural Gas
Fueled  Transportation  Vehicles,"  J.  Winston  Porter,   American
Gas  Association,  Arlington, Virginia,  December,  1979.

     Gas  Powered Vehicle Evaluation  Program, Transport  Canada,
Road Safety  Branch,  TES Report CS  372, Ottawa,  Canada, March
1982.

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                              3-48

   References For Further  Information on CNG Safety  (cont'd)

     "The  Benefits   and  Risks  Associated  with  Gaseous-Fueled
Vehicles," D. Shooter  and A.  Kalelkar,  Arthur D.  Little  Inc.,
May,  1972.

     "Installation of  Compressed  Natural Gas  Fuel  Systems  and
Containers on  Highway Vehicles  and Requirements  for  Refueling
Stations," Canadian  Gas  Association,  CAN 1-B149.1-M80,  October,
1982.

     Code  of   Practice   for   CNG  Compressor   and  Refueling
Stations, NES 5425 Part  1, 1980.

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                           CHAPTER  4

                     ECONOMICS OF USING CNG
                   IN HEAVY-DUTY APPLICATIONS
I .    Introduction

     In this chapter the economics of using  CNG  as  a heavy-duty
vehicle  fuel  will  be  presented.  First,  the domestic  natural
gas  supply will  be discussed  followed  by  a presentation  of
natural  gas  prices.    Next,   CNG  refueling  station costs  and
hardware will be discussed.  Following  this,  overall fuel costs
for  current  and future natural  gas,  gasoline and diesel fueled
heavy-duty  vehicles will  be  compared.   Finally,  vehicle  and
engine   costs   associated  with  dedicated   CNG   use  will  be
discussed.

II.  Domestic Natural Gas Supply and Price

     The total United States proved  reserves  of  dry natural gas
in  1987  were  187.2 trillion  cubic  feet (TCF).Cl]   At  current
domestic usage rates this is enough  to  supply the United States
for  over  nine  years.[2]   These proved  reserves  only  include
identified  sources whose  quantity,  quality  and  location  are
known  and  which  can  be  economically  extracted  with  existing
technology.   Addition   of  estimates  of  conventional  resources
which  have been identified and  are  estimated to be potentially
recoverable  economically  bring  the  total  U.S.  conventional
resource  base  total  to  about  900  TCF.C3]   Large  amounts  of
natural  gas  from unconventional  resources  such  as  coal seams,
Devonian  shale,  geopressurized brines  and  tight gas reservoirs
are  also  available but would  only  be economical  to  extract at
somewhat   higher,   though  unclear,   increases  in  natural  gas
prices. [4]  For  the purposes  of this report  it will  be  assumed
that  the  use of  CNG   as  a  heavy-duty  vehicle  fuel will not
impact  the  domestic  demand  or  price  of  natural  gas  to  any
significant degree.  This  is  a reasonable assumption given that
a  significant  penetration  of  CNG  into  the   total  domestic
heavy-duty fleet   (i.e.,   ten  percent)   would  result  in  an
increase of natural gas use of  just  two  to  three percent, still
well below total domestic usage  rates of  the  early  1980s.

     The  United  States  currently  has   a  massive  natural  gas
transmission  and distribution pipeline  network   in  place which
serves  a  large  portion of  the country.   Also,  in  most major
cities and many other  areas there is  an  extensive distribution
network  in place which can be easily tapped.  For the purposes
of  this  study  it  will  be assumed  that  CNG  would  generally be
used for  heavy-duty vehicles  in  areas  which  already  have  a
distribution  infrastructure  in place and thus,  no  new capacity

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                               4-2
would need to be installed.   Also, it  is  likely  that  heavy-duty
CNG applications  would  generally be centrally*fueled  fleets  in
urbanized  areas   and  would  have ready  access  to  natural  gas
distribution lines.

     The  American   Gas   Association  has   recently   published
projected natural gas prices  for vehicle refueling stations  to
the year  2005.[5]   These  projections  for 1995 (in 1989 dollars)
range  from  $2.59/mmBTU  to  $5.39/mmBTU   (lower  heating  value)
depending on the region of the  country  being considered,   This
range of  prices  will  be  used in this  report  to  develop natural
gas vehicle  fuel cost estimates,

     No  attempt  will be made  in this report  to predict future
natural gas  price trends.   Unlike some  other  alternative  fuels
such  as  methane-1, where  a  whole new  market  must be  developed,
the use of natural gas as a heavy-duty vehicle fuel  is expected
to  result  in  little perturbation  in the  natural gas market.
Thus,  future predictions  of  price  trends  are not as  critical
here  as in the analysis of other alternative fuels.

Ill.  CNG Refueling Station Cost

      In  this  section the  cost  of  CNG  refueling  stations  for
heavy-duty applications will  be  examined.   First, a description
of  CNG refueling station operation,   hardware and  some factors
influencing  station  design  will  be. presented.   Next,   some
typical  • current   prices   for   different    refueling   station
components will  be  shown.   Finally,  the range of total station
costs will  be  discussed.   Due to the  wide variety of heavy-duty
vehicle  fleet  sizes  and  applications,  the  purpose  of  this
section  is  to give  the  reader  some idea of  the range of  costs
involved  in  a  CNG  refueling  station  rather than  to  define and
cost   out   a   "model  station"   to   represent   the    "typical"
heavy-duty station.

      A.    CNG Refueling Station Hardware

      In  contrast to  liquid  fuels,  CNG   is  a gas  and  must  be
compressed  for storage onboard  a vehicle.   Thus, the  refueling
station  equipment  needed  for CNG  is  different  than  that  for
gasoline  or diesel  fuel.   Generally,  there  are  two  methods of
refueling a  CNG vehicle,  slow-fill  and  fast-fill.   Although
these two methods  are similar in some respects, they  are quite
different  in  others.   A  diagram  of  a  typical  CNG fueling
station  utilizing both fill methods  is shown  in  Figure  4-1.

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                               i-3

                           Figure 4-1
     COM'MSOft
                 Typical  CNG  Refueling Station
Source:   "Assessment  of  Methane-Related  Fuels  for
          Fleet Vehicles", DOE/CE/50179-1, February  1982

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                              4-4


     With the  slow-fill method  there  is  a  direct  connection
between  the vehicle  and  the  natural  gas  compressor.   Thus,
refueling time  is  directly related  to the total  fuel  capacity
of the  vehicle  or  vehicles being fueled  and the  discharge rate
of the  compressor,  which  is  generally given in  standard  cubic
per minute  (SCFM).   Slow-fill can take  less than  one  half hour
for a single vehicle  fueled  on a  larger compressor,  as  is  the
case,   for   example,  with  the Brooklyn  Union  Gas (BUG)  buses
discussed   in   Chapter  3. [6]    However,  slow-fill   stations
generally refuel more than one vehicle at a time  and  often use
smaller  compressors  than  the  BUG  program  station.   Thus,  a
typical  slow-fill  operation  will  generally take  several  hours
and is usually performed overnight.

     In  contrast  to   slow-fill,   fast-fill  systems  utilize  a
large-volume  compressed   gas   storage  system,   or  "cascade",
between the compressor  and the  vehicle.   Generally,  the cascade
is divided  into three  banks  of cylinders, with  each  bank at a
different pressure.  The lowest pressure  bank  is  used  first and
an automatic or  manual  sequential valve system switches to the
higher  pressure  banks  as  the pressure between the  cascade and
the fuel  tank  is  equalized.   Through the successive connection
with  higher pressure  banks  the  vehicle can  be  refueled  in a
matter  of several  minutes  with fast-fill, as opposed to several
hours with  slow-fill.   Similarly to  fast-fill  vehicle fueling,
the  fast-fill  cascade  banks  are  filled selectively by  the
compressor  through  a  priority valving system,  usually  with the
highest pressure bank being filled first.

     Station  design   including  choice   of   slow-fill  versus
fast-fill,  compressor  sizing,  cascade  sizing  (if  fast-fill),
and  the number  of fuel  hoses is  determined  by  the  number of
vehicles to be  fueled,  their  onboard fuel storage capacity, and
their demand pattern.   A small captive  fleet may only  require  a
small   compressor   and  slow-fill   capability   to  refuel  all
vehicles  together  overnight.   A  larger  fleet will  require  a
larger  compressor,  more fuel  hoses  and  may even  utilize  some
fast-fill   capabilities.    With  this   system  vehicles  could
generally   be   slow-filled together   overnight,  but   fast-fill
would  be available for special-purpose filling during the  day.
Finally,  a  commercial  fueling  station  such   as  a truck  stop
would  be  exclusively  fast-fill  and  would have  a very  large
compressor  and  cascade capacity,  as well  as  fuel  dispensers
with metering capability rather than simple fuel hoses.

     Natural  gas  compressors are generally  two  to  four  stage
compressors  with  a  discharge  pressure   of   3,600   psig.
Compressors for   small installations  such  as  fleet  stations
generally  have  a discharge  rate  of well  below  100  SCFM.  The
gas  supply for the  compressor  is  taken from  an underground
natural gas transmission  line,  much  like  a  residential  home

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                              4-5
hook-up.   The natural gas  pressure  in these lines  is  generally
one  to   five   psig  for  distribution   lines,   although  main
transmission lines can have  pressures  of several  hundred psig.
The line  pressure  to the compressor affects cost  and  sizing as
less compression  is  needed  when starting  with  a higher  gas
inlet pressure.

     In  addition  to  the  refueling  hardware  its-elf  there  are
other components to the CNG  refueling  station.   These  include a
concrete  pad  to  mount  the  compressor  on,   as   well   as  any
enclosure  that  may  be  used to  protect  the  compressor  and
cascades  (if any)   from  the  elements.   There  may  also be  a
significant  piping  link  to  the gas  source  depending  on  the
proximity of the station  to  the underground gas line.   However,
for most applications in any  sizeable  city  this  would  likely be
a fairly short line.

     B.    CNG Refueling Station Hardware Cost

     In  order to  understand  the  total cost of  a CNG refueling
station  it  is useful to first look briefly  at  typical costs of
the  various  components of   a  station.   At  the  heart  of  any
refueling  station  is  the  compressor.   This  is  usually  the
single most  expensive component of the  station.   The  cost of a
variety of compressors from  various sources  was  compiled and is
presented  in Table  4-1.   As  can  be  seen there is  a fairly
linear   relationship  between  the  compressor   cost   and  its
capacity.   Generally,  a small  fleet  would use  a  compressor on
the lower  end of  the capacity scale while  a larger fleet would
likely  need a  compressor  on the higher end of  this  scale.  A
high  volume  commercial  truck   stop  would  likely   require  a
compressor sized larger than  anything  on this table.

     Table  4-2  shows  typical  costs  for  other  CNG  refueling
station   components.    The   fueling   post   for   slow-fill   or
fast-fill  application would  typically cost $500  to  $1,000  per
hose   (vehicle).    In  contrast,   a   two   hose   dispenser  for
commercial   fast-fill   use,   which   may   include   metering
capabilities  and   a sequential  valve  system,   can   cost   over
$35,000.

     If  fast-fill  capability is desired,  a small  cascade  and
associated  priority  and  sequential   valving  will cost  under
$15,000.   This  type of  cascade system would  generally be  used
to  fast-fill only  a  few trucks during  the day.   If  the fleet
were   large  or   utilized  fast-fill  exclusively,  several small
cascades  or  one or more large cascades may be required.

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                                *-6
                             Table  4-1

          TVoical CNG Refuel ir.a Station Compressor Costs
Source

  A
  B
  A
  C
  B
  B
  A
  C
  C
  A
  D
  A

Capacity (SCFM)
4.5
25
30
30
30
50
57.8
63-85
100
130
130
155
Inlet
Pressure (osiq)
0-5
—
0-5
5-15
—
—
0-5
40-60
150
0-5
15
0-5

Cost
$ 12,800
30,000
37,000
39,500
40,000
46,000
50,000
41,500
45,000
86,000
126,000
90 ,000
 Sources:   References  6  through 9.

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                                4-7

                            Table 4-2
              CNG Refueling Station  Ccrr.ccr.er.r  Cc3~3

  Component                                       Cost
Fuel Post                                      $500-1,ooo/vehicls
Dispenser (2 nozzle)                             25,000-35,000
Cascade (20 cylinder-9,200 SC?)                  8,500-9,300
Cascade (3 20" x 22'  tubes, 27,000 SCF)           35,000
Sequential & priority valve systems             2,600-5,000(for  both)
 Sources:  References 6 through  8

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                              4-8
     C.     Total CNG Refueling Station Cost

     The  total  cost  of  a  CNG  refueling  station  is  obviously
dependent a great deal on its total capacity  and  whether  or not
it is a  fast-fill  system.   To give some idea of the price range
of CNG  refueling  stations,   Table  4-3  shows  estimates for  the
total cost of  a  small  light-duty  fleet  station  and a large
truck stop,  as  well  as  the  actual  costs  of  the  refueling
station  for  the  BUG  bus  program  in  Brooklyn,   NY,  and  a
refueling  station  used for  a  small  (10  vehicle) school  bus
fleet in  Syracuse,  NY.  The  small fleet  station  estimate shows
that  it  is  possible  for  a  small  fleet  (17-35  light-duty
vehicles,  significantly  fewer  for   heavy-duty)  to  utilize  a
slow-fill   refueling    station   which    costs    well   under
$100,000.[10]    Conversely,  a commercial truck  stop which would
require  a  large compressor  and cascade  capacity,   as  well  as'
meter-type  fuel  dispensers  is projected  to  cost  well  over
$600,000. [11]   Although this  report is  intended to  cover  issues
specific   to   CNG  use  in  heavy-duty   applications,   it  is
reasonable  to  assume  that  the  lightest  of  heavy-duty  CNG
vehicles  (pick-up  and  delivery vans,  for example) may refuel at
public    stations     intended    primarily     for    light-duty
applications.    Cost  estimates for these  types  of stations were
developed  in  Volume   I  of   this report  and  are  shown  to  be
$225,000 to $396,000.

     Table  4-4  shows  the cost  breakdown of the CNG refueling
station  used   for  the   school  bus   fleet  in  Syracuse,  New
York.[12]   This station utilized two  30  SCFM compressors for a
total capacity  of  60  SCFM.    It also had  a small  cascade for
fast-fill  purposes.    Although  this   station  was  only  used for
ten  buses  its capacity would allow  a  fleet significantly larger
than this,  as  the  compressors were only  required  to run about
five hours a  day.   This station is an  excellent  example  of the
type of  station a  small captive  fleet could use.

     From  the  perspective  of a somewhat  larger  station, Table
4-5  shows  the cost breakdown of the  BUG  bus  refueling station.
This station   is  a simple  slow-fill  system  with  a  130  SCFM
compressor  and  a  single  two-hose  metered-type dispenser.   The
total  cost  for this   system is  rather  high  for  the  initial
application (two buses)  for a  couple  of  reasons.   First,  the
compressor is a very  large  one  for a  slow-fill system  servicing
a two vehicle  fleet.   However, this  allows slow-filling  of  a
single  bus  in   less   than  thirty  minutes.   In  fact,  when the
compressor is operating 20  hours  a  day this  system is  capable
or  refueling  45 buses  in a  day.   Second,  Brooklyn Union Gas is
 authorized to sell CNG from this station commercially,  thus the
expensive dispenser rather  than an inexpensive fill post.  This
 fact  would  also   justify   the  large   compressor   capacity.
Finally, the installation  and materials  cost  is somewhat high
 as the original concrete slab  and support  pilings  were  placed,
 unbeknownst  to the  designers,  on  an abandoned' landfill with
 poor soil conditions.   Thus,  following  initial   startup  the
 system had to  be  disconnected  so that  a new  concrete slab and
 additional support could be added,   adding three  weeks  to the
 initial five-week  installation time.

-------
                                        4-9
                                     Taole  4-3
                Total Costs  for  Heavy-Duty CNG Rsfuelir.r Sta-.cr.s

      	Station	                        	Cost
      Small fleet station                                 $ 81,500
      Syracuse school bus station                         101,933
      BUG bus station                                     273,108
      Commercial truck stop                               641,000
I
      Sources:  References, 6,10,11,12

-------
                              *-10
                           Tabie 4-4


       Cost  Breakdown for  Syracuse 3us Refueling 5ta~icr.


          Equipment  (incl. two  30
            SCFM compressors)              $ 78,133
          Compressor pad             .        3,500
          Installation                        1,OOP

          Total (1982 dollars)             $ 82,633

          Total (Current dollars) .         5101,933
Source:     "Dual-Fuel  School Bus Demonstration", Mew  York  State
           Energy Research  and Development  Authority,  Albany,
           NY,  October 1986.

-------
                              +-L1
                           Table 4-5
           Cost  Breakdown  for  3UG Refuel ir.a Station
           Compressor  (130  SCFtt)             $126,28-6
           Dispenser  (two hose)                25,000
           Other  Materials                      9,653
           Installation                       117,169

           Total                             $278,108
Source:     C.    Spielberg,   "Compressed   Natural   Gas   Proaram
           Monthly Report *2,  January-May 1989," New  York City
           Department of Transportation,  September 12,  1989.

-------
                              4-12
     With this perspective  on the BUG station,  it  appears  that
a  slow-fill   system  designed  for  overnight  fill   of  several
vehicles  could be  substantially  cheaper,   Use of  a  smaller
compressor  and simple  fill  posts  rather  than  a  meter-tyne
dispenser, together with the  elimination  of  the unexpected soil
condition problem, could easily  cut  the  cost of this  system in
half.   A  larger  compressor  and  the  addition of  some fast-fill
capacity  (at  $15-20,000  total  additional  cost  for  cascade,
priority and  sequencing valves,  additional fueling  hoses, etc.)
would  still   likely  keep  total  system  cost  under  $200,000.
Thus,  for the purposes of  a  small to moderate  sized fleet the
total  refueling  station  cost  could  range  from  $80-200,000
depending  on  total   capacity  and  whether  or  not  fast-fill
capacity is  included.    Conversely, the worst  case  would likely
be  the  large  fast-fill  truck  stop  at  over  $600,000.[11]
However, for the purpose of  this report it will  be  assumed that
most heavy-duty CNG applications  will  utilize a smaller station
in a fleet setting.

     It should be noted  that  the cost of land  is  not included
in this  discussion  as part of the total  CNG refueling station
cost.  For  the purposes of  this  report  it  is  assumed  that  a
fleet CNG refueling station would be constructed on the site of
the  fleet's  current  refueling station and  no  additional  land
costs would  be incurred.  However, EPA recognizes  that,  due to
the physical size of the the  compressor and  cascades, placing a
CNG  refueling station within  the  physical confines  of  an area
designed  for a diesel or gasoline  refueling  station may present
a problem at some facilities,  resulting in some  added cost.

     Finally,  in  the  case  of  truck stops,  which  are generally
located away  from urban areas,  there  may be  a  cost associated
with  the  pipeline  required  to  connect  the  station   to  the
natural   gas  distribution   pipeline.    This   connection  was
estimated by the Department of Energy to average between l and
5  miles  long  depending on the area  of  the  country,  and was
estimated to  cost $200,000/mile.C11]   However,  for the purposes
of this  report it was assumed the most  heavy-duty applications
of  CNG,  at   least  in  the  near  term,    would  be  in centrally
fueled  fleet  settings  in  urban  areas,  which  would  not  incur
this  cost.   Thus, it was not  included  in the economic analysis
contained in  this chapter.

 IV.  Heavy-Duty CNG, Gasoline  and Diesel Vehicle Fuel Cost

     This section will present  an  estimate of the relative  fuel
 costs  for  both  current  and  future CNG,  gasoline  and diesel
 fueled  heavy-duty vehicles.   First,  an   "equivalent  gallon" of
 natural  gas  in  relation to  gasoline and  diesel  fuel  will be
 established  so the  fuel costs  can be  projected  on an energy

-------
                              4-13
basis for  direct  comparison,   Second,  the  cost of  compressing
the  natural  gas   for  refueling  will  be   calculated.    The
capitalized refueling station cost will  then be calculated and
all  of  the factors  will be  combined  for  a comparison  cf per
equivalent gallon fuel costs.  Finally,  the total  vehicle fuel
costs  for  both  current  and  future  CNG,  gasoline and  diesel
fueled heavy-duty vehicles  will  be estimated  using these fuel
prices,  and factoring  in relative  engine efficiencies  and other
relevant factors.

     A.     Basis of  Comparison

     Although natural  gas   is  stored  and burned  in  a  gaseous
form,  it  is  easiest  to  compare  natural   gas consumption  to
consumption of  gasoline  or  diesel   fuel  on   an  energy  basis
'(i.e., "equivalent gallon").  The  energy density of natural sas
is  typically  1,030  BTU/SCF  higher  heating  value  (HHV).[13]
However,  for  purposes of  comparison  with  gasoline and  diesel
fuel,  the  lower heating  value  (LHV)  of  natural   gas must  be
used.   This  value  is  not  usually guoted,  but an  earlier EPA
technical  report  determined  that the  LHV of  natural  gas  is
typically  around  90  percent  of the  HHV.[14]   Thus,  an  energy
density  of  930  BTU/SCF  will  be used  for  natural  gas in this
report.   Comparing  this  to the BTU/gal value for  diesel fuel
from  Table 3-1  and  114,132  BTU/gal   for  9   RVP  gasoline[13]
yields an  energy equivalence  of  122.7 SCF of natural gas to one
gallon of  gasoline,  and  139.6 SCF of  natural gas  to one  gallon
of  diesel  fuel.    These relationships  will  be  used for the
comparison of relative fuel costs between the fuels.

     B.    Compression and Station Maintenance Costs

     The  natural  gas  prices  just presented  are  not  the only
factor  in  the cost  of natural gas to  the heavy-duty CNG vehicle
operator.   As   will  be  seen, there   is  a   cost   of   energy  to
compress  the  natural  gas  during refueling, which  is  dependent
on  the  efficiency of the compressor  and  the price  of  energy to
power  the compressor  motor.  Also,   station  maintenance  costs
are  significant  enough  to  consider.   These  costs  must  be
factored into the cost of natural gas  as a vehicle fuel.

     Data  on   the   energy   used  per  volume  of  natural  gas
compressed are  available for  four  different compressors  in both
public  and fleet use in Canada.[15]   These  compressors  varied
in  output  delivery   capacity from 20  to 178  SCFM.   The  energy
used to  power  the  compressors  ranged  from  0.0075  to   0.0099
KW-hr/SCF, depending on compressor efficiency.

     Compressors currently  used in  CNG refueling  stations are
generally  powered  by electric  motors.    It  is  reasonable  to
assume that in  the future compressors  may be  powered by natural

-------
                              4-14


gas  engines   at   a  significant  energy   cost  savings   over
electricity.   This  is  especially likely in  the case  or  large,
commercial   station  compressors.    However,   since   current
compressors  generally  use  electricity   and  most  heavy-duty
applications    of    CNG   are   expected   to   utilize   smaller
compressors,  electricity  costs will  be  used here  to  calculate
compression  costs  while recognizing  that  these costs  may  be
significantly reduced in some future  cases.

     The 1988 national  average commercial  price for electricity
was 7. 01<£/KW-hr . [ 13]  Using the  equivalent gallon relationships
previously derived along  with  this  current electricity  price
yields  current natural  gas compression  costs of 6.4-9.0  cents
per  equivalent  gallon  of  gasoline,  and  7.3-9.7  cents  per
equivalent gallon of diesel fuel.

     Based on  actual  maintenance cost  data  from  a variety  of
actual   CNG  refueling  stations,  DeLuchi  et.al.  estimated  CNG
station    operation    and    maintenance   costs    to     be
$0.25-0.50/mmBtu.[16]     Using    the    energy    equivalences
established  earlier  results  in  CNG   station  operation  and
maintenance  costs  of  $0.03-0.06 per  equivalent gallon for both
gasoline and diesel fuel.

     C.    Capitalized Service Station Cost

     In order to  include the  capitalized refueling  station cost
in  the   cost  of  natural  gas  a  typical  refueling  station
configuration  was  assumed.   In  general,   station  cost  and
station capacity  are somewhat linear  [17],  and the capitalized
refueling  station cost is probably  much more  dependent  on the
utilization  rate   of  the  station  than  on it's  capacity.   For
example,  an  urban  transit bus  fleet would  have   a  fairly low
utilization  rate  as it's  vehicles would be  on  the  road all day
and  could only be  refueled  at night.   In contrast to  this,   a
fleet  of  delivery vehicles which are in and  out of the facility
throughout  the day could much  more  effectively  utilize  some
fast-fill  capacity during the day  in addition  to  refueling at
night.   Thus,  for the purposes of this  report  a single station
design  will be  used  and capitalized   refueling  station  costs
will be derived using high and low utilization  rate scenarios.

     The   refueling  station   chosen   is  similar   to  the  one
previously discussed  for  the  Syracuse  school  district.   This
station  has  two  cascade banks for fast-fill capability and 60
SCFM compressor  capacity.   Further,  this  station is  assumed to
cost about $100,000.   The  per-equivalent-gallon costs  will  be
calculated based  on a  10  year payback period with  a  10 percent
rate of return.

-------
                              4-15
     For the  low  utilization rate scenario  it  was  assumed that
the vehicles  would  only be  available  for refueling  8 hours  a
day.    In  addition to  operating the compressor for 8  hours  in
slow-fill  operation,  the cascades  could be  used  initially  -a
fast-fill   additional  vehicles  during   this   8  hour  period,
utilizing   the  compressor  an  additional  4  hours  a  day  for
cascade filling.   This  means  that  this  station  could  refuel
about 11 urban  transit  buses, each  using the  equivalent  of  27
gallons of diesel fuel  a day (similar to the  BUG buses).   This
scenario  yields   capitalized   refueling   station   costs   of
12.8<£/gal  for gasoline and 14.60/gal for diesel fuel.

     For the  high utilization scenario  is was  assumed that the
compressor could  be  operated 20  hours  a day,  making efficient
use  of  both  slow-  and  fast-fill  capacity.   Although  this
scenario  is  probably  not  representative  of  an urban  transit
operation, for  purposes of  comparison  to  the  low utilization
scenario this  type  of  operation  could  refuel  around  20  urban
transit  buses  daily.    This  high  utilization  scenario  yields
capitalized  refueling  station  costs  of  7.50/gal  for  gasoline
and 8.2<£/gal for diesel fuel.

     D-    Relative Fuel Prices

     Tables  4-6  and 4-7 shows  the relative comparison  of fuel
prices  between gasoline and  CNG,  and  diesel  fuel  and  CNG,
respectively.   The  gasoline  prices  were taken  from  a previous
EPA  report. [18]   The diesel  cost  is the  average  of  the  first
seven  months  of 1989,  without  taxes.[13]   Although state taxes
for  gasoline and diesel fuel  are similar,  Federal   taxes  for
diesel  fuel  are six cents/gallon  higher  than  for gasoline.  In
both  cases,  however,  the same  gasoline tax was assumed  to  be
applied to  CNG  on  an  energy-equivalent basis.  This was done
because it is likely that a single taxation  strategy would be
used   for  CNG,   and  given   the  potential   for   light-duty
applications,  equivalent  gasoline  taxes seem  most  probable.
The  tax on  a diesel-eguivalent gallon of CNG is  somewhat higher
than  for  a  gasoline-equivalent   gallon of  gasoline,  due  to
diesel fuel's higher energy density.

     E.    Relative Vehicle Fuel Costs

     The  total  vehicle fuel  cost  comparison  between  CNG and
gasoline  or  diesel fuel must take into  consideration not only
relative  engine energy use,  but  also  the  fuel economy effects
of  increased fuel storage  weight and,  as  was  just   discussed,
the  cost  of  natural  gas   compression.   As  was  discussed  in
Chapter 3,  a current  stoichiometric  CNG engine  uses  almost 11
percent  less energy  than its  gasoline counterpart,  while the
future  optimized  CNG  engine  uses  some  fifteen  percent  less

-------
                               4-16
                           Table  4-6
      Gasoline and CNG Energy  Equivalent Price Comparison

Cost Classification                    Gasoline     Natural Gas
Extraction,  refining,  other             $0.69
Long-range and local distribution        0.06
Current natural gas end user
  delivered price range*                  —        $0.30-0.67
Service station markup**                 0.09
Capitalized refueling station cost        —        $0.08-0.13
Compression cost                          —        $0.06-0.09
Operation and maintenance cost            —        $0.03-0.06
Profit markup***                          —        $0.00-0.01
Taxes                                    0.24       	0.24
Total                                  $ 1.08       $0.71-1.20
*    The  end  user  price  range  for  natural  gas  includes • all
     distribution costs
**   The  service  station  markup  for  gasoline   includes  all
     overhead  and operating costs as well as profit markup
***  The  $0.00  profit markup  applies  to  fleet-owned stations
     while  a profit of $0.01 was  assumed for commercial stations

-------
                           Table 4-7

     Diesel Fuel and CNG Energy Scruivalent Price


Cost Classification.                    Diesel, Fuel _  Natural Gas

End user diesel fuel price*             SO.56

Current natural gas end user
  delivered price range                   —        $0.34-0.76

Service station markup**                $0.09

Capitalized refueling station cost        —        $0.08-0.15

Compression cost                          —        $0.07-0,10

Operation and maintenance cost            —        $0.03-0.06

Profit markup***                          —        $0.00-0.01

Taxes                                    0.30            0.27
Total                                  $ 0.95       $0.79-1.35
*    The  end user  diesel  fuel  price is  the- price  charged to
     fleet   operators   rather   than   for  resale   (i.e.,   to
     commercial retail outlets)
**   The  service  station  markup for  diesel fuel  includes   all
     overhead and operating costs as well  as  profit  markup
***  The  $0.00  profit  markup  applies to  fleet-owned stations
     while a profit of $0.01 was assumed for commercial stations

-------
                              4-18
energy.   Conversely, a current  technology  lean-burn engine uses
39 percent  more  energy than  its  diesel counterpart,  while  the
future  optimized  lean-burn  CNG  engine is  assumed  to  use  25
percent  more energy.

     Fuel  storage  weight  is  an  important  part  of  the  total
vehicle  fuel cost as  it  affects total vehicle weight and,  thus,
fuel economy.   For  the purpose of this  study  it  will be assumed
that  the vehicles  being  compared  will  have equivalent  range
(i.e.,  equivalent   fuel  capacity).    This  assumption  is  being
made  in order to  allow a  direct  comparison between  the fuel
types.  This may be somewhat of a worst case  approach,  as many
heavy-duty  vehicles could  likely  operate satisfactorily with
less  fuel storage capacity  than they currently  have.   However,
as  will  be   described  below,  the  overall  impact  on  fuel
consumption of maintaining equivalent range is small.

     For the purpose of calculated  fuel  storage  weight and fuel
economy   penalties   two  model  vehicles  were   chosen  which
represent  excellent  candidates  for  dedicated  CNG use  in  a
captive  fleet setting.   The  first  is  a  UPS parcel  delivery
truck similar to the  ones  discussed  in  Chapter  3,  with  the
exception that this vehicle  will  be assumed  to  be a dedicated
CNG vehicle and will  not have  the  30 gallon  gasoline tank and
fuel  weights  included  in the weight calculation.  The second is
an urban  transit bus  with the  CNG  fuel  storage  equivalent of a
100  gallon diesel  fuel  tank.   The actual weight  calculations
and resultant  fuel  economy penalties are derived  in Chapter 5.
However,  the   results  of  these calculations  show  that bringing
the UPS delivery truck  to  the  equivalent range  on CNG  as it
would have  as  a  gasoline vehicle would  result in  a  five percent
fuel  consumption   increase  on   CNG   compared   to  gasoline.
Similarly,  the transit bus  would have  a seven percent increase
in fuel consumption.

      Combining the  relative  engine  efficiencies,  the   fuel
storage  weight impacts and the fuel prices previously presented
yields  the relative  total vehicle  fuel  costs  shown  in  Table
4-8.    For  the   reasons  given  in  Chapter  3,   the  gasoline
comparison  is  based  upon  the  stoichiometric  combustion  CNG
engine  performance while  the  diesel  fuel comparison  is  based
upon  the lean-burn  combustion CNG engine.  As can be seen  from
the   table,  the  stoichiometric  combustion  CNG  engine   offers
significant  potential  fuel  cost  savings over   an  equivalent
gasoline vehicle.   Conversely, the  fuel economics  of replacing
a diesel engine with  a dedicated  CNG  engine are  not  as  good,
especially  with   current   CNG  technology.   This  was   to  be
expected given the  fact  that diesel engines already represent  a
very  efficient form of  fuel combustion and  the  relatively  low
cost  of diesel fuel compared to gasoline.

-------
                              4-L9

                           Table 4-8

             Vehicle Fuel Costs  (Gallon Equivalent)


                      Gasoline Comparison*


                                    Cur r_ent       Optimized

Gasoline                              $1.08

CNG - Stoichiometric Combustion     $0.67-1.12    $0.63-1.07


                       Diesel  Comparison**

                                    Current       Optimized

Diesel Fuel                           $0.95

CNG-Lean Burn Combustion            $1.18-2.01    $1.06-1.81
*    Cost per gallon, or equivalent gallon, of gasoline.
**   Cost per gallon, or equivalent gallon, of diesel fuel

-------
                              4-20


V.   Heavy-Duty CNG Engine and Vehicle Costs

     A.     Engine Costs

     There  is  currently  very  little  information available  on
the  cost  differential  between  a heavy-duty  CNG engine  and  a
heavy-duty gasoline  or diesel-fueled engine.   What  information
is available characterizes  the  costs associated with converting
a diesel-fueled  engine to  either  dual-fuel or  dedicated spark
ignition  CNG  operation.    Estimates  show  that  conversions" of
diesel engines to  dedicated spark ignition CNG operation range
in cost  from $3,100 to  $5,600.   [11]  These  cost estimates are
for   engine  conversion   only   and  do   not  include   such
vehicle-related components as CNG fuel cylinders.

     Although  conversion  has historically been  the  method for
obtaining heavy-duty CNG  engines,   there   is  a  clear  movement
within  the  heavy-duty   industry  toward   the  introduction  of
dedicated  CNG   engines   by  original  equipment  manufacturers
(OEM).  This  is  evidenced  by  Cummins'  commitment to  offer its
CNG  L-10  engine  starting  in 1991.  Also,  most major  heavy-duty
diesel  manufacturers are  currently  involved  to  some  degree in
the  development and  assessment of dedicated CNG engines.

     It is difficult to predict  with  any accuracy what the cost
of an  OEM-supplied  dedicated CNG engine will  be.  -In general,
though,  the bulk  of the  costs  associated with conversion to
dedicated  CNG  operation  are  for  new  CNG-optimized  'parts  to
replace the diesel-optimized parts,  such as the pistons, piston
rings,  cylinder  heads, camshaft and intake  manifold.   For OEM
engines these  parts  would likely be  similar  in cost  to  current
parts.  The  addition of a spark  ignition  system  would  be needed
at   some  cost.   Some  savings  may be  possible  with   the  fuel
system  depending on the  type of system  used  (i.e.,  mixer, port
injection,  direct  injection,  prechamber)  since the  diesel fuel
injection  system  which  it would replace  is  generally regarded
as one  of the more  expensive systems on a diesel engine.  Some
initial  recovery of research  and development costs is expected
to  result in  a  higher engine  introduction cost than  would be
expected  in  the  long run.  In general,   however,  EPA  expects
that an  OEM  mass-produced,  dedicated  CNG   heavy-duty  engine
would   have   at  most   only  a   modest   price   increase  over  a
comparable  diesel  or  gasoline  engine and  that this  cost would
not'be  large  in  comparison with  the  total  engine  cost.

      B.     Vehicle Costs

      As with CNG  engines,  there are some  vehicle modifications
which would require the  engineering and  developments  costs to
be   recovered in initial  vehicle offerings without resulting in
 a  significant   long  term  price  increase   for  the   affected

-------
                              4-21
components.   These would primarily  include  modifications  to the
frame  and  engine compartment  to  accomodate  the  CNG  fueling
system (i.e., storage cylinders,  pressure  regulators,  refueling
interface).   Also,  the  exhaust  catalysts  to  be  used  on  CNG
vehicles are not  expected  to  differ significantly  in  cose  from
those currently  used  on gasoline vehicles  or  those expected to
be used  on  diesels.   If CNG  were  replacing  a diesel  equipped
with a  trap oxidizer  for  particulate  control,  significant  cost
saving would result.  However,  the  degree  of trap  oxidizer use
to be  expected  for diesels is unclear  and  difficult  to  predict
at the present time.

     The only area which  is  expected  to  increase  the cost  of
heavy-duty  CNG  vehicles to  any  significant  degree is that  of
fuel storage.   As was  discussed  in  Chapter  3,  fiber-wrapped
steel  tanks  are  currently  the most  likely  choice  for "CNG
vehicle use.  Fiber-wrapped  aluminum cylinders also  offer  some
weight advantages  at  an increased price.  The current prices of
some typical  cylinders  are  shown in  Table 4-9.[19,20~]   It  is
apparent  from   the   table  that  several   cylinders  would  be
required to have any  reasonable  amount  of  storage onboard.   For
reference,  The  Flxible  Corporation (transit  bus manufacturer)
quotes  a  typical transit  bus  diesel fuel  tank at  $1,134.[21]
It  is  readily  apparent  that  the  fuel  storage  cost  would
increase several times when moving to CNG,  as  is  shown in Table
4-10.  However,  this  is still a  modest  price increase when one
considers  the  base  price  of  a  diese-1-powered  Flxible  bus
($173,120).   This cost,  however,  would be  more  difficult  to
absorb in  a much smaller vehicle.  It  is  also  apparent  that in
some   applications   the   available   space  may   make   adding
sufficient  fuel  storage  capacity for equivalent range difficult
or impractical.

-------




















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-------
                           Table 4-10

                     CNG Fuel Storage Costs


                             U?S Vehicle           Urban 5us

Number of CNG cylinders           3                     11
     (16.3x53.Sin)

CNG cylinder cost*            $2,330-3,000         $8,500-11,000

Fuel tank savings               $340                  $1,134

Net fuel storage cost**      $1,990-2,660         $7,366-9,866
*    Price range is result of purchase volume range.
**   This  is  a worst  case  approach,  as  CNG vehicles  may not
     require equivalent energy  storage  capacity as conventional
     vehicles  (for  example:    the   BUG  bus  uses  less  than
     one-third of the equivalent diesel capacity a day).

-------
                              4-24
                      Chapter 4 References

     1.     "Natural    Gas    Monthly,"    Energy    Information
Administration, DOE/SIA-0130(89/07).

     2.     "Assessment  of  Costs  and  Benefits  of  Flexible  and
Alternative  Fuel   Use  in  the   U.S   Transportation  Sector",
Progress Report Number  One:  Context  and Analytical  Framework,
DOE/PE-0080, January 1988.

     3.     "Assessment  of  the  Natural Gas  Resource  Base of the
United States", DOE/W/31109-H1, May 1988.

     4,     "Unconventional  Gas  Sources",  Vols  I-v,  National
Petroleum Council, December 1980.

     5.     "Natural  Gas Prices  for  the  Vehicle  Market,"  Issue
Brief 89-19, American  Gas Association,  Arlington, VA,  November
22, 1989.

     6.     C.   Spielberg,   "Compressed   Natural  Gas   Program
Monthly Report 82, January-May 1989",  New  York City Department
of Transportation, September 1989.

     7.     G.   Barker,   Norwalk   Company,   Inc.,    telephone
communication, November 1989.

     8.     D.   Leivestad,   Carburetion   and  Turbo   Systems,
telephone communication, November  1989.

     9.     M.     Garret,     Bauer     Compressors,     telephone
communication, November 1989.

     10.   R.L.  Bechtold,  G.   Wilcox,  "An  Assessment  of  the
Infrastructure Required to Refuel a Large  Population of Natural
Gas  Vehicles",  SAE paper  No.  892066,   Society  of  Automotive
Engineers, Warrendale,  PA,  1989.

     11.    "Assessment of  Costs  and  Benefits of Flexible and
Alternative  Fuel   Use  In  the  U.S.    Transportation  Sector,"
Progress   Report    Three,   Vehicle   and    Fuel   Distribution
Requirements   (draft),  U.S.  Department   of  Energy,  Office   of
Policy,  Planning  and Analysis,  July 1989.

     12.    "Dual-Fuel   School   Bus  Demonstration,"    New  York
State  Energy  Research and  Development  Authority,   Albany,  NY,
October  1986.

     13.    "Monthly   Energy   Review   -  July   1989",   Energy
 Information Administration,  DOE/EIA-0035 (39/07).

     14.    R.I.   Bruetsch,    "Emissions,   Fuel  Economy,   and
 Performance   of   Light-Duty   CNG  and   Dual-Fuel   Vehicles",
 EPA/AA/CTAB-38-05,  U.S. EPA, MVEL, Ann Arbor,  June 1988.

     15.    "CNG   Compressor   Operations   Monitoring",   Canada
 Department  of  Energy,  Mines and Resources,  January 1986.

-------
                              4-25
                 Chapter  4 References  (cont'd)

     16.    M.A.  DeLuchi,  L.A.  Johnston,  D.  Sperling,  "Methane 1
vs.   Natural  Gas  Vehicles:    A comparison  of Resource  Supply,
Performance,   Emissions,  Fuel  Storage,  Safety,   Costs,   and
Transitions",   SAE  Paper  No.  881656,  Society  of  Automotive
Engineers, Warrendale, PA, 1988.

     17.    "Assessment of Methane-Related  Fuels  for  Automotive
Fleet Vehicles," DOE/CE/50179-1, February 1982.

     18.    "Analysis  of  the  Economic  and  Environmental  Effects
of  Methanol  as  an  Automotive  Fuel,"  Special   Report  of  the
Office of Mobile Sources, OAR, EPA, September 1989.

     19.    Pressed Steel  Tank  Co,,  Inc., Price  List, Milwaukee,
Wl,  October 1989.

     20.    S.    Anthony,   Structural   Composites   Industries,
personal communication, November 1989.

     21.    Letter from T. Leedy,  The Flxible Corporation,  -o P.
Como, Southern California Rapid Transit District, January 1989,

-------
                              4-26
       References For Further Information On CNG Economics

     R.   L.  Bechtold,  et   al.,   "The  Practical   and  Economic
Considerations of Converting Highway Vehicles to Use  Natural  Gas
as  a  Fuel,"  SAE   Paper   No.   831071,   Society  of   Automotive
Engineers,  Warrendale, PA,  1983.

     K.  G.  Darrow,   "Economic  Assessment of  Compressed  Natural
Gas  Vehicles  for  fleet Applications,"  Gas Research  Institute,
Chicago,  IL, September 1983.

     "The  Gas  Energy  Supply  Outlook   Through  2010,"   Policy
Evaluation  and   Analysis   Group,   American  Gas   Association,
Arlington,  VA, October 1985.

     "The   Economics   of   Alternative   Fuels   and  Conventional
Fuels," SRI Internatinal,   presented  to  the  Economics Board  on
Air Quality and Fuels, February 1989.

     Vice  President's  Task Force  on  Alternative  Fuels,  "Report
of  the Alternative  Fuels  Working Group,"  Office of the  Vice
President,  Washington, DC, July 1987.

     R.  D. Fleming,  R.  L.   Bechtold,   "Natural  Gas,  Synthetic
Natural  Gas   and   Liquified   Petroleum  Gases   as   Fuels   For
Transportations," SAS paper  No.  820959,  Society   of  Automotive
Engineers,  Warrendale, PA, 1982.

-------
                           CHAPTER  5

     AIR QUALITY  IMPACTS OF  CNG  USE IN  HEAVY-DUTY


I .    Introduction.

     Compressed natural  gas  (CNG)  is an  aiterr.ative  fuel that
has the potential to provide  significant  benefits  for  urban air
quality."  This  chapter  will  provide  an  analysis  of the overall
air  quality  impact  of  using  CNG  as   a  fuel  for  heavy-duty
vehicles,   focusing especially  on  its   impacts  on  urban  ozone
levels, air  toxics,  and global  warming.   Emissions of  both CO
and NOx will also be briefly  touched upon.

     As was  noted earlier,  CNG technology  is  still undergoing
develocrr.ent   and   optimization.     As    such,   it   would   be
inaoprooriate to  discuss the impacts of CNG vehicles based only
on "current  emission levels.  Thus,  this  analysis  will  compare
both  current and advanced  CNG vehicles  to  the  corresponding
cetroleum-fueled vehicles.    Also,  the analysis  is  structured to
allow  comparison  of  lean-burn CNG technology   to  comparable
diesels,  and stoichicmetric CNG  emissions  to  gasoline-fueled
engines.  This  is appropriate because  the lean-burn engine was
derived  from a  dies"el  engine  and will  likely replace  diesel
engines, while  the  stoichicmetric  engine was derived  from and
will   likely  be   used   primarily  as   a  replacement   for,
gasoline-fueled  engines.   As was noted  in Chapter 3,  there are
currently different  test  cycles for  diesel  and gasoline-fueled
(or  Otto-cycle)  engines.    To- allow  for proper  comparison to
gasoline-fueled   vehicles,   the   stoichiometric   CNG  emissions
listed here  are from the Otto-cycle testing.
r
 I.  Urban JDzone Levels
     One  of  the  most notable air quality benefits of CNG use  is
its ability  to reduce the  contribution  of  mobile sources to the
urban  ozone problem.   This benefit  could be  significant  with
respect  to  heavy-duty  vehicles,  since traditional  heavy-duty
vehicles  account  for   about  one-fifth  of  mobile  source ozone
precursor  emissions.   The  effect  of  mobile   source  control
programs  on urban ozone  levels  is  generally estimated from the
change  in non-methane  hydrocarbon  (NWHC)  emissions  alone,  even
though  a given  change  in  emissins does  not produce  a linear
change  in  ozone  levels  because  urban  ozone  formation   is  a
complex  process   involving  the  chemical  reactions of  NWHC and
NOx"   (The  effect  of   CNG  use  on  NOx  emissions  is  discussed
further   below.)    Futhermore,    when   comparing   completely
different  fuel-types (such  as  CNG  and  petroleum)  the  issue  of
photochemical  reactivity  can  become  equally  important.   Most
NlttHC  emissions  from  CNG vehicles are very  light paraffins such
as  ethane  or  propane.[1]  These  species  are  generally  less

-------
                              5-2
reactive   than   the   NMHC   emissions   from   petroleum-fueled
vehicles.[2,3]   Thus,  the benefit from NMHC  reductions  achieved
by using CNG would  be  expected to be enhanced by a reduction in
the reactivity  of  the emissions  as  well.   On  the other  hand,
however, CNG vehicles  also emit  such highly  reactive compounds
as formaldehyde  and propylene.[1]   These  emissions  may  offset
this benefit  to some  extent.  Unfortunately,  due both  to  the
complexity of the photochemistry  of  urban ozone  formation,  and
the  fact  that  only  a  limited amount  of  speciated  exhaust
hydrocarbon  data for  CNG  vehicles   is  available,   it  is  not
possible to quantify this  additional effect  at this time.  Thus
in this  analysis,  only the  relative NMHC emissions  are  shown,
but the reader  is  reminded that  the actual ozone benefit may be
be greater than that  estimated  simply  from NMHC  reductions.
This is different   from  how  the  Agency has  been  handling  the
impacts of methanol-fueled  vehicles; which emit  primarily only
two components  (methanol  and  formaldehyde).   The photochemistry
of both of these compounds  is, however,  fairly well understood,
partly  because  they  are  both one  carbon  molecules that  have
simpler chemistry than  larger molecules.   It  is  clear,  though,
that further  information  on  the speciation  of  NMHC emissions
from heavy-duty CNG vehicles is needed  before  their  relative
impacts can be fully quantified.

     The estimates  of  emissions  from  petroleum-fueled  vehicles
which  will be  used here  correspond to  engines   in  a condition
similar  to  the CNG  engines  that were  used  as the basis  for
estimates   of    the   emissions   from   CNG   vehicles   (i.e.,
well-maintained  low-mileage  test engines).   As  was rioted  in
Chapter 3, this condition was used  as  the basis for emissions
estimates  because  it  is  the only condition for which sufficient
data  are  available  for  CNG  vehicles.   Some  CNG  engines  have
shown  a tendency toward high in-use emissions,  but EPA expects
these  difficulties  to  be dealt with as the  technology  matures.
Thus,  it  must  be  emphasized  that the actual  in-use effects of
CNG  vehicles  relative  to  petroleum-fueled   vehicles could  be
different  from  those  described  here,  either better or  worse,
depending  on in-use emission  variability and deterioration.

     The estimates  of  exhaust emissions  from  1991  diesels have
been   selected   to  represent  expected  emissions  of 1991  bus
engines.   They were calculated using manufacturers'  test data,
and  scaling  the emissions  to meet  newer  standards for  NOx and
particulate.   Bus  engines  were  used for the  current estimates
since  urban buses  represent  a prime  application for lean-burn
CNG  engines in the near  term.   Farther  into  the  future  CNG
engines   could   be   used   more   broadly   in   other   diesel
applications,  since all diesels  will need to meet the stringent
particulate  standards  after   1994.   The 1994  diesel estimates
are    the   results   from   testing   of   a    prototype    of    a
advanced-technology 1994  diesel (non-bus)  engine  produced by
Navistar.[4]   Diesel  exhaust emissions are  summarized  in Table
5-1.

-------
                                   5-3


                               Table 5-1

          Petroleum-Fueled Diesel Exhaust Emissions (~/3H?-hr)
              Current Bus Enair.es
                                       1991 ?ro_j_e_c19d  1_994 Navistar

                                                             1.4
                                                             574
                                                             4 . 44
                                                             0 .08
                                                             0 . 296
                                                             0 .30
Pollutant
CO
CO 2
NOx
PM
JJMHC
Total HC
1989 6V92
1.5
640
8.2
0.32
0.636
0.66
1990 L10
2.5
549
5.01
0.37
0.466
0 .48
1991 :
1.72
6223
...
0.225
0 ,40s
0.427
1.    Emissions after catalytic treatment.
2.    Average of 6V92  and L10 CO  emissions  weighted by  relative sales
     volumes (80%/20%).
3.    Average  of   6V92  and  L10  C02  emissions  weighted  by  relative
     sales volumes.
4.    Needed to meet 1991 standards.
5.    Needed to meet  1991  heavy-duty diesel  standards.   3us standard  is
     0.1 g/bhp-hr.
6.    95  percent of total HC for diesel engines (EPA-450l2-33-003a).
7.    Average  of  ratios   of  total HC to  PM for  6V92  and  L10  engines
     weighted  by  relative  sales volumes and multiplied by  PM level
     need to reach 1991  standard.

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                               5-4


      In   addition   to   exhaust,   estimates   of   evaporative
 emissions,  running   losses  and  refueling  emission  are   also
 needed.   To date there  is no  data  suggesting that diesels  have
 significant evaporative  or running  loss  emissions,  so these  are
 assumed   to  be  zero.   Diesels  also  have  minimal  refueling
 emissions   due  to   the   low   volatility   of   diesel   fuel.
 Preliminary EPA data have shown  such emissions from dieseis  to
 be  on the  order  of  0.05 g/gal,  which on  a g/BHP-hr  basis  is
 somewhat  less  than 0.Olg/BHP-hr.[5]

      The  estimates  of  current  gasoline exhaust  emissions  are
 based on  data  from  manufacturer  testing  of   certification
 engines.   The  exhaust  HC and CO  emission  factors (Table  5-2)
 were   calculated  by   averaging   emissions   of  1989  heavy-duty
 gasoline   engines   which  were   actually   certified   to   the
'heavy-duty  standards.  This means that  all  engines that did  not
 meet    the    standards,   but   were   certified   by    paying
 non-conformance penalties or  through use of  the  "five-percent
 option,"*  were  excluded.    This   average   total  hydrocarbon
 emission  factor was  multiplied by  0.75  to  convert it   to  NMHC.
 (Heavy-Duty gasoline-fueled engine  emissions  are  assumed  to  be
 about 25  percent methane.   There  is some  uncertainty  in  this
 figure,  but it  represents  a  reasonable assumption, and using a
 somewhat  different  value would  not  significantly  impact  the
 analysis.)   The  C02  emission factor  comes  from  data on  the
 base  GM 454 'engine from which the  CNG engine was  derived.   NOx
 levels are  those needed  to meet the 1991  NOx standard.

      Non-exhaust NMHC   emissions  (see  Table 5-4)  for  current
 heavy-duty   gasoline  engines  are  based   on   MOBILE4.    The
 evaporative emission factor came directly from MOBILS4, and the
 running loss  emission factor  is an  average  of MOBILE4  estimates
 of  running losses at  87 and  95°F.   These temperatures were used
 because they  bracket  the temperatures typical of days with high
 ozone levels  in major  cities.   The  refueling  emission  factor
 comes from MOBILE4,   assuming  a brake specific fuel  consumption
 of  0.531  Ib/BHP-hr.,  as  was done  in Chapter  3.

      The    emissions   from  future   heavy-duty   gasoline-fueled
 engines  also   need   to  be  adjusted  for the  impacts   of  the
 President's proposed clean air  act amendments.    The  estimates
 of   gasoline-fueled   vehicle   emissions   under  the  President's
 proposal   are  the  current  emissions  corrected  for   enhanced
      This  option  allows  a  manufacturer to  certify up  to  five
      percent of its engines to  the  non-catalyst  standards.   The
      same   option   would  be    available   to   CNG   engines.
      Non-conformance  penalties,  on the  other hand,  are not  a
      viable long term strategy for either engine type.

-------
                           Table 5-2

      Gasoline-Fueled  Vehicle  Exhaust  Emissions  (g/BHr-r.r)
                 CO                      9.2*
                 C02                     753-*
                 NOx                     4.5***
                 NWHC                   0.45****
                 Total  HC               0.60*
*    Average  of   1989   certification  results   for   engines
     certified to  the  14.4  g/BHP-hr  CO  and  1.1  g/8H?-hr  HC
     standards.
**   Certification C02  emissions  from GM  454  gasoline-fueled
     engines.
***  Needed to meet 1991 standard.
**** 75 percent of total HC.

-------
                              5-6
evaporative  emissions   controls,   lower  gasoline   volatility
limits,   and  for  the  implementation  of  Stage  II   refueling
controls in non-attainment areas.  EPA  estimates that -he lower
volatility  limits  would result  in a  43  percent  reduction  in
evaporative  emissions  and  a  16  percent  reduction  in  running
loss  emissions.   In  its  analysis  supporting  its  evaporative
emissions    rulemaking,     EPA     estimated     that,"    when
consideringnon-tampered    heavy-duty    vehicles-,     enhanced
evaporative controls  would result in a  40 percent reduction in
evaporative  emissions  and an  80 percent  reduction  in  running
loss  emissions  when  using  a 9  psi  gasoline,[6]   Refueling
emissions would  be  affected by  two  aspects  of  the  President's
program:   reduction  of  fuel   volatility  to  9  RVP  and  the
implementation of Stage  II controls  in  selected non-attainment
areas.  The exact amount  of  control  to be realized by the State
II requirements  is  a  function  of the degree of  coverage  of  the
program  (station  size  exemption  level)  and  the  degree  of
enforcement exercised by the states.  Based  upon the  experience
of several  state programs, EPA has selected a station exemption
cutoff  of  10,000 gallons  per  month  for  these  estimates.   With
this  cutoff,  Stage  II efficiency has previously been estimated
to  lie  between  57  percent  and  79  percent.[7]   Using  the
mid-point  of  this   range  (68  percent),  and  recognizing  that
Stage II  controls  will not  significantly control  the spillage
portion  of  refueling emissions, an overall  Stage II  efficiency
of 64 percent results.

     The estimated  emissions of  non-methane  hydrocarbons (NMHC)
from  both   optimized  and  non-optimized  CNG  vehicles  (from
Chapter  3),  diesel  vehicles  and  gasoline-fueled  heavy-duty
vehicles are shown  in Tables 5-3 and 5-4.  Table  5-3 shows that
CNG  vehicle NMHC emissions  are 67-80  percent   less  than those
from  the   advanced  diesel.   While  this   is  a   significant
reduction  on a  percentage basis,  it  must  be  noted that  the
absolute   reductions  are   small,   because   diesel   emissions
themselves  are only a few  tenths of a gram per brake horsepower
hour.   It  should   also  be  re-emphasized that  this  comparison
does  not   account  for  in-use  performance  differences.   The
actual   reductions  will   likely  be  smaller   than   this  since
diesels  do not  rely  on exhaust aftertreatment  for  significant
control  of  NMHC emissions,  and thus  will  probably have  less
in-use  deterioration  than  CNG  vehicles.

      The    potential   reductions    are   much   greater   for
stoichiometric   CNG   vehicles   relative   to   gasoline-fueled
vehicles.    Table    5-4    shows    the   emissions   from   the
stoichiometric  CNG engine are 93-96  percent less than those of
gasoline-fueled  vehicles.   The  larger  reduction arises because
of   the    fact   that,   while  the   advanced   lean-burn   and
stoichiometric CNG  engines have similar emissions, the gasoline
vehicle NMHC emissions are  roughly four times  as high as those

-------
                                         5-7
                                      Table  5-3

               KMHC  Emissions  From  Diesel  and  Lean-Burr. CNG ir.air.es*
Exhaust
  (g/3HP-hr)

Evaporative
  (g/MI)

Running Loss
  (g/MI)

Refueling
  (q/BHP-hr)

Total (g/BHP-hr)

Corrected for
  r ange and
  performance
Percent Reduction
from 1994 + diesels
1391 1994+ Current
Diesel Diesel Lean-Burn CNG
0.40 0.29 0.09
00 0
00 0
0.01** 0.01** 0
1 0.41 0.30 0.09
Cot'."ni70d
Lean-3urr. CNG
0.06
0
0
0
0.05
0.10
67%
0 .06***
aos
*    Emission  factors  are  based  on  testing  of  well-maintained,   low-mileage
     test engines.  In-use emissions would be expected  to be higher.
**   Unpublished EPA data.
***  The  effect  of correctio  for  range  and performance was  small  enough  to
     dissappear in the  roundoff to  two significant digits.

-------
                               5-8
                      Table 5-4

                 NttHC Emissions From
   Gasoline-Fueled arid Stoichiometr ic CMG
                                                    .es
               Gasoline
                      Gasoline
                       under
                      President's
                      Proposal
 Current
Stoich CNG
Op-irnizec
 Stoich
   CNG
Exhaust          0.45
  (g/BHP-hr)

Evaporative**    1.10
  (g/mi)

Running Loss**   2.04
  (g/mi)

Refueling        0.45
  (q/3HP-hr)     	

Total            3.70
  (g/BHP-hr)

Corrected for
  Range and
  Performance

Percent
  Reduction
  from Gasoline
  under
  President's
  Proposal
                         0.45



                         0.34



                         0.34



                         0.18



                         1.24
   0.09



      0



      0
  0 .05
   0.09
                                       0 . 09***
  0 . 05
                  . 05**
                                       93%
                  96%
 * *
 * * *
Emission factors  are based on  testing of well-maintained,
low-mileage  test  engines.   In-use   emissions  would  be
expected to be higher.
Evaporative  and  running  loss  emissions   are   reported  as
g/mi  and  converted  to  g/BHP-hr  in  the  total  by  the
conversion factor .89.
The  effect  of  correction  for  range  and  performance was
small  enough   to dissappear  in  the   roundoff   to  -wo
significant digits.

-------
                              5-9
from diesels.   As noted  above,  this  means  that  CNG  vehicles
would  be  expected  to have  a  positive  impact  on urban  ozone
formation,   even  without  considering  the  possibility  of  any
reactivity benefit.

     It should  be  noted that  these  emission  factors  are  all
calculated  on  a  g/BHP-hr  basis.   Actual  on-road emissions  of
CNG  vehicles  relative  to current  vehicles  would be  slightly
higher  due to an increase in  fuel tank weight needed  to provide
equivalent  range  and performance.   It has been,  estimated that
to  achieve  equivalent range  CNG  vehicles would require  26.8
additional  pounds  (using wrapped  steel  tanks)  for each gallon
of  equivalent   petroleum-fueled  vehicle  tank  capacity.[8]   A
previous EPA analysis has  shown  that this weight increase would
be compounded by  a  factor of  approximately 1.3 to account  for
other  necessary  modifications  to  the   vehicle  to  carry  the
additional  weight.[9]   For   a   16,000   pound   gasoline-fueled
vehicle with a  30 gallon fuel  capacity,  and a 37,000  diesel bus
with a 100  gallon  fuel  capacity,  achieving equivalent  range
with a CNG vehicle would  require  increasing vehicle  weight  by
6.5  and 9.4 percent  respectively.  For  this  analysis it  was
also   assumed   that   to   achieve   equivalent   performance,  the
horsepower would  need to  be  increased  by the same percentages.
EPA  has previously  derived sensitivity  factors  for  the percent
change  in  fuel  economy  from  changes  in  vehicle  weight  and
horsepower.[9]   It  was  estimated that  for  light  duty vehicles,
fuel economy decreased  by 0.329  percent and 0.454 percent for
each percent change in weight  and  horsepower respectively.   In
the  absence  of  factors  for  heavy-duty vehicles,  it  is assumed
that  these  factors  can  be  applied to  heavy-duty vehicles  as
well.   Thus, the  range  and performance  adjustment  factors used
in   this   analysis   are   1.05   for  the  stoichiometric  engine
emissions  and   1.07  for   the  lean-burn  engine  emissions.   The
relative  NMHC  emissions  of  CNG  vehicles  are  calculated  by
multiplying  the ratio of g/BHP-hr  emission factors by 1.05  or
1.07.   These   relative   NMHC  emissions  are   summarized  in
Figure  5-1.

Ill. Air Toxics

     CNG  is  expected  to  provide  significant  benefits  with
respect to  air  toxics.    EPA  has  presented its  estimate of the
impact  of conventional  mobile sources  on  air  toxic   in  great
detail  previously.[10]   The analysis  in this chapter  uses  the
estimates   from  that  work  of   cancer   incidences   caused  by
heavy-duty  vehicle  emissions  to  provide  a perspective  on  the
toxics  impacts  due to heavy-duty vehicles.   It  then  develops  an
estimate  of the per-vehicle reductions  expected  to  result from
CNG  use.

-------
                     5-LO
                 FIGURE 5-1A
 RELATIVE DIESEL/CNG NMHC EMISSIONS
           Heavy-Duty Vehicles
         g/BHP-hr
     0.5
     0.4 -
           1991
           Diesel
 1994     CURRENT   ADVANCED
Diesel    Lean-Burn CNG  Lean-Burn CNG
                 FIGURE 5-1B
RELATIVE GASOLINE/CNG NMHC EMISSIONS
             Heavy-Duty Vehicles

        g/BHP-hr
NMHC
          Currant  President's Proposal  Current     Advanced
         Gasoline     Qaaollne   3tolcftiom«trtc CNQ stolcniometrie CNQ

-------
                              5-11
     The following mobile  source-related toxic  pollutants  were
examined:    benzene  (including  exhaust,  evaporative,  running
loss,   and   refueling   benzene),   gasoline   refueling  vapors,
exhaust   1,3-butadiene,   poiycyclic   organic   material   (POM.)
absorbed   onto   gasoline-derived   particulate   matter,   and
formaldehyde.   These  pollutants  are  emitted  by  diesel  and/or
gasoline-fueled vehicles  and  are  classified  by EPA  as  either
known  or  probable  human  carcinogens.   The  base  heavy-duty
vehicle cancer  cases in  the  year  2005  from petroleum  fueled
vehicles are  shown in Table 5-5.   Included  are  cases predicted
for  the  nine  cities affected by  the  light-duty  alternative
fuels  program  included  as  part  of  the President's  clean  air
program.[11]  These  nine  cities are  expected to  be  those most
likely to  use  alternative  fuels to  combat their  severe ozone
problems.  This choice was  also made to be  consistent with  the
other   EPA   reports   on   alternative   fuels.    Since   these
projections are directly proportional  to population,  they could
easily be extrapolated to larger areas.

     Formaldehyde  in ambient   air  includes  both "direct"  and
"indirect" formaldehyde.  Direct formaldehyde  is emitted in the
exhaust  of  vehicles, while  indirect formaldehyde is  formed in
the   atmosphere  from  the   reactions   of    various   reactive
hydrocarbons.   As   discussed   in  the   Final  Rulemaking  for
methanol-fueled vehicles,  indirect  formaldehyde  is  responsible
for  the  majority  of  the  formaldehyde  in  ambient  air.[12]
However, this  is  not true when dealing with  that portion from
heavy-duty  vehicles,  since  they  emit  a  higher fraction  of
formaldehyde directly.

     The estimate of the  impact of  diesel particulate emissions
was  done  under the  assumption that all  diesels will need trap
oxidizers   to   meet   the  stringent  post-1994   particulate
standards,  and includes  the  impacts  of traps  failing  in use.
More  recent developments,  however,  suggest  that trap  use  may
not  be so pervasive; other,  more reliable  technologies  may be
used.   Thus  the  actual   total risk  from  diesel  particulate
emissions  is expected to be somewhat  lower  than this estimate.
In  addition,  since in-use deterioration  is  a factor  in  all of
the  risk  estimates  in  Table  5-5,  the  per-vehicle   reductions
developed  below cannot properly be  applied  to  these numbers.
Comparisons  of  the two  sets of numbers  can, however,  be used to
provide a perspective on the potential benefits  available.

     As  can be seen from  Table 5-5,  heavy-duty vehicles  are
predicted  to contribute to about 35 annual  cancer incidences in
these  nine  major  cities.   This  is approximately  one-half  the
number  estimated  for  light-duty  vehicles  (69), [133  and  when
diesel   particulate  effects   are   excluded   the    heavy-duty
contribution drops to  about one-sixth (which  is  expected, given
the  fact  that  heavy-duty  vehicles account  for only one-fifth of
mobile source  VOC emissions  in  general).   Nevertheless,  on   a
per-vehicle  basis the  impact  of  CNG  use  on air  toxics  can be
significant.

-------
                                  5-L2

                              Table  5-5

              Air Toxics From Traditional Mobile Sources


                           2005  Base
                           Heavy-Duty      Cases Due     Cases Due
                          Cancer Cases     to Diesel     to Gasoline
Toxic                    In Nine Cities    Emissions     Emissions

Exhaust Benzene               1.47           0.53          0.94
Evaporative Benzene           0.20           0.00          0.20
Running Loss Benzene          0.05           0.00          0.05
Refueling Benzene             0.06           0.00          0.06
Gasoline Refueling            0.35           0.00          0.35
 Vapors (W/0 Benzene)
Exhaust 1,3-Butadiene         4.29           2.74          1.55
Exhaust Gasoline POM          3.09           0.00          3.09
Direct Formaldehyde           0.08           0.55          0.33
Indirect Formaldehyde         0.66           0.42          0.24
Diesel Particulate           24.10          24.10          Q . 00

Total                        35.15          28.34          6.80

-------
                              5-13
     The per-vehicle air toxic  reductions  for  CNG vehicles were
calculated by comparing the  toxic  emissions  of CNG  vehicles  to
those  of  petroleum-fueled vehicles.   This was  straightforward
for  the evaporative,  refueling,   and  running  loss  emissions,
since CNG vehicles would not normally  have such emissions.  The
exhaust benzene  emissions  for  CNG  were based  on  very limited
speciated  hydrocarbon   data   found  in  reference  l, where  the
emissions of  benzene were estimated to  be one  percent  of  the
NMHC  emissions.   (Benzene  emissions  from gasoline  and diesel
engines were  estimated  to be 3.5  percent  and  1.1  percent  of  HC
emissions  respectively.)[10]   The  presence of  benzene  in  CNG
exhaust  is  somewhat surprising,  since   CNG  itself  does  not
contain  any  benzene,   and  it  would   not  be  expected  to  form
during  combustion.   The only remaining source of  benzene would
be  the  lubricating  oil.  Clearly,  more data are  needed in this
area.   The   reduction   of   1,3-butadiene  emissions   is   very
difficult to  estimate  for CNG  vehicles,   since it  is difficult
to   distinguish   1,3-butadiene   from  butane   when  speciat ing
hydrocarbons.   It  seems unlikely  that CNG vehicles would have
large  emissions  of  1,3-butadiene,  but   they  may  have  some,
Therefore, for this  analysis,  it  was  assumed,  that  CNG vehicles
would   result  in  a  near   total   (99  percent)   reduction  in
1,3-butadiene.   The  direct  formaldehyde  impact   was  estimated
from the  emission factors  listed  in Chapter 3, and by assuming
(as  was done  in reference  10) that  formaldehyde accounts  for
3.1  percent  of  gasoline HC  emissions and  3.0  percent  of diesel
HC  emissions.   The  indirect   formaldehyde  impact  was  assumed
proportional  to NWHC  reductions,   and the  diesel  particulate
reductions  were  based  on   the particulate  emissions  factors
listed  earlier.

     Table 5-6 shows the relative  reductions  in toxic emissions
from CNG  vehicles compared  to  future petroleum-fueled vehicles,
for   both   current   technology   and   for    future  optimized
technology.   (These  estimates  were   adjusted  for  range  and
performance  in the  same  fashion  as  the  NMHC emissions.)   In
most  cases,  the CNG   vehicles show  major  reductions  in  air
toxics.   In  fact, the  only  toxics  in  which the  reductions  are
below   70   percent   are  the   direct  formaldehyde  and  diesel
particulate  impacts  for  the diesel/lean-burn  comparison.   The
direct   formaldehyde  emissions   may   very   well  be  lowered
significantly in the future  by technological  advances  such  as
catalysts  specifically  designed to  control  formaldehyde.   The
reduction in  diesel  particulate is below  70  percent because the
advanced  1994 diesel  engine already  has  very  low particulate
emissions  so as  to meet  the  stringent  particulate  standards
that  take  effect   in  1994,   When   all  the  emissions  are
considered,  the  lean-burn CNG engines  are  expected to result in
approximately a  19-35  percent reduction  in  per-vehicle toxic
emissions  compared  to  diesel  engines.   The  stoichiometric  CNG
engine  would be  expected  to  result  in approximately  a  99

-------
                           Table 5-6a

                Per-Vehicle Air Toxic Reductions
              of CNG Vehicles  Compared to Diesels
Toxic

Exhaust Benzene
Evaporative Benzene
Running Loss Benzene
Refueling Benzene
Gasoline Refueling
  Vapors (W/0 Benzene)
Exhaust 1,3-Butadiene
Exhaust Gasoline POM
Direct Formaldehyde
Indirect Formaldehyde
Diesel Particulate

Weighted Total**
                               Percent
                              Reduction
                               Current
                              Lean-Burn
   Percent
  Reduction
  Optimized
  Lean-Burn
                                 19
     35
**
Direct formaldehyde risk would increase
Weighted according to Table 5-5 impacts,
                           Table 5-6b

                Per-Vehicle Air Toxic Reductions
              of CNG Vehicles Corn-oared to Gasoline
Toxic

Exhaust Benzene
Evaporative Benzene
Running Loss Benzene
Refueling Benzene
Gasoline Refueling
  Vapors (W/o Benzene)
Exhaust l,3~Butadiene
Exhaust Gasoline POM
Direct Formaldehyde
Indirect Formaldehyde
Diesel Particulate

Weighted Total*
                                Percent
                               Reduction
                                Current
                             Stoichiometric
   Percent
  Reduction
  Optimized
Stoichiometric
                                   99
      99
      Weighted according to Table  5-5  impacts.

-------
                              5-15


percent  reduction  in  per-vehicle toxic  emissions  compared  to
gasoline engines.*  These overall  reductions  are  also  shown  in
Figure 5-2.

     One  might  expect  greater  overall  reductions  from  CNG
vehicles as compared to  diesels.   However, the  comparison here
is with  future "diesels  (i.e.,  1994)  which will  have extremely
low levels of  particulate  (see Table 5-1) in order  to  meet the
1994  diesel  particulate  standard.    Since  the  diesel  toxic
impact is  largely  dominated by  particulate,  the  total  percent
reduction  in  air toxics  from lean-burn  CNG  primarily reflects
the reduction in particulate.

     Of  course,  the   overall  impact   of   these  per-vehicle
reductions depends on  the  fraction of the heavy-duty fleet that
is eventually replaced by CNG.  Since, at  this  time,  it is very
difficult  to  predict  overall  penetration,  this  analysis  is
limited  to  per  vehicle   reductions.    Also,    it   should  be
reemphasized  that  these predictions  are based  on  low-mileage
emissions;  thus  the  actual  in-use  impacts  could  be  somewhat
different.   It would  be reasonable  to  assume  that since both
CNG  and  gasoline-fueled vehicles  use  catalysts  as  the primary
means  of  emission  control,  the  in-use  deterioration  will  be
similar.  However, this  is not  true  for diesels, which do not
rely  on  exhaust  aftertreatment  for  significant  hydrocarbon
control,  and would  be  less   affected  by  in-use deterioration
than  CNG'vehicles.   On  the other  hand,, .diesels are expected to
rely   on   aftertreatment  for  at  least   partial  control  of
particulate  emissions,  while  CNG  vehicles  will not; so in this
regard  CNG would be  expected to  have some  in-use   advantages.
Thus,  at  this time,  it is  not  possible  to  fully assess the
overall  in-use impact.  It  is,  however,  possible  to  say that
CNG  vehicles  should  offer  significant reductions of air  toxic
emissions  from heavy-duty vehicles.

IV.   Global Warming

      Recently,  the  greenhouse  effect   (i.e.,   the effect  of
emissions  of  certain  "greenhouse" gases, most  notably C02,  on
global   temperatures)  has   been  receiving  a   great  deal  of
attention.   Since  combustion of  different fossil  fuels can
result  in different C02 emissions,  it  is  appropriate that the
analysis  of the  environmental  impact  of CNG  vehicles  include
its  C02 impact.  Because  CNG has" a  higher  energy  density per
carbon  atom   than  traditional  petroleum  fuels  (about   20-30
percent  more),  there  is   a   potential  for  reductions  in the
      The  composite   weightings   are  based   upon   the   relative
      contributions   from  Table  5-5.    Based  upon  the   earlier
      cautions  about  comparing those values  to the  per-vehicle
      reductions,   these   composite   weightings   can  only   be
      considered  to be approximate values.

-------
                        5-16
                   FIGURE 5-2A
   Relative  Air  Toxics  Impacts Diesel/CNG
             Heavy-Duty Vehicles
           120 -'
           100 -
    relative
   percentages
                  1994
  Current      Advanced
Lean-Burn CNG   Lean-Sum CNG
                   FIGURE 5-2B
  Relative Air Toxics Impacts  Gasoline/CNG
             Heavy-Duty Vehicles
        120 "

        100 -

         80-
 relative
percentages60 _
              Gasoline      currant      advanced
           Pr««id«n?s Prooo.al Stoich.om.tnc CNG Sto.crnom.mc CNG

-------
                              5-17
global  warming   impact   of  motor   vehicles.    However,  -his
potential  benefit  can  be  offset  by  changes  in  the  energy
efficiency of the vehicle.

     The estimated  C02 emissions of  CNG vehicles  used in this
analysis are  those  described in  Chapter 3.  The  CC>2  emissions
of diesels  were  earlier  estimated  to be  622  g/GHP-hr for 1991
vehicles,  and  574  g/BHP-hr  for  1994 vehicles  using  the  same
approach used  to calculate  the  NMHC emissions.   The emission
factor  (753  g/BHP-hr)  for  gasoline-fueled vehicles  is  from
manufacturer test data.[14]

     In  addition   to  CC>2,  other   gases  can   have  a  very
significant  impact  on   global   warming  as  well.   The  most
important   of  these,   when  considering   the   effect  of  CNG
vehicles,  is methane.   Methane  is  much   more  effective  than
C02  at  absorbing  infrared  radiation;   in  fact   it   has  been
estimated  that each molecule of  methane in the  atmosphere has
an  effect  equivalent  to  approximately  25 molecules  of  CC>2
(approximately a  70:1 ratio on  a weight  basis).[15]   Thus any
CC>2 emission  benefit   from CNG  vehicles  will be  offset to some
extent by the increased  methane  emissions,  even  if the increase
in  terms  of  grains per   mile  is small.    Unfortunately  though,
there  is still  much   debate  about  other  complicating factors.
Most  significant  is  the fact that methane is known  to  have  a
shorter  atmospheric lifetime  than C02/  which  would  serve   to
decrease  its  impact   on  global  temperatures  to  some  extent.
There  is currently  no consensus on what the decrease  would be;
but  it clearly  cannot  be  ignored.   This  analysis will  use  a
factor  from an analysis  by  the University of  California which
accounted   for  both  the  absorption   and   atmospheric lifetime
effects  of methane. [16]   EPA  is  not  intending  to endorse this
study,  which  made  a  number  of  simplifying  assumptions   in
arriving  at  its  results.   Rather,  it  has  been  chosen  as   a
conservative  approach  so as  not to  over  emphasize  the still
uncertain  role of  methane in global  warming.  According  to the
University  of  California analysis,  each gram of methane  emitted
can be considered equivalent to  11.6  grams  of CC>2 •

     Finally, the total  effect of the use  of any fuel on global
warming  also  depends   on the  secondary  emissions,  both C02 and
methane, that  occur during the  production  and  distribution  of
the  fuel.   The energy consumption  at all  stages  of  production
and  distribution can  be converted  to equivalent  CC>2   emissions
and  added  to  the  vehicular  emissions  of CC>2   and methane.
These   effects   were  analyzed   previously  in   a   draft   EPA
report.[9]    There   it   was   calculated  that   the   ratios   of
secondary   C02  and  methane  to  vehicular CC>2   emissions  for
CNG   vehicles   using   domestic   gas   are  1:3.2   and   1:122
respectively.   A   similar  analysis  for   gasoline  showed  the
ratios  to  be 1:4.4  and  1:8720.   (The gasoline-based ratios were

-------
                              5-18
assumed to  be valid  for  diesels as  well.)   The fact  that  -he
ratios of secondary methane  emissions are so different  for  CNG
and  gasoline  is  not  surprising since  most  of  the  secondary
methane emissions for  CNG  occur  during the  distribution chain"
for which there  are no comparable emissions with gasoline.

     When considering  all  of  these  factors,  as is done in Table
5-7,  the global  warming  impact  of  heavy-duty  CNG  vehicles
appears to  be  in  the  same  range of that  of  petroleum-fueled
vehicles.   Compared to diesels,  lean-burn  CNG  vehicles would be
expected  to  result  in a  13-15  percent  increase in  the global
warming impact.   However,  compared  to gasoline-fueled vehicles,
stoichiometric  CNG  vehicles  would   be  expected  to   achieve   a
19-23  percent  reduction  in  the global  warming impact.   This
latter benefit  is  largely  related to the improved efficiency of
the CNG engine noted in Chapter 3.

     The reader  is cautioned that the two  sets  of figures  shown
in  Tables 5-7a  and  5-7b  are  not directly  comparable,  as they
are   derived  from  different   heavy-duty  test   cycles,    As
discussed earlier  in this report,  these  test  cycles  are  based
on  the usage  patterns  of  the   engines   they  represent.    The
gasoline  test cycle  (Table 5-7b) is based on the usage patterns
of  a  typical  gasoline  engine  whereas  the  diesel  test  cycle
(Table  5-7a)  is  representative  of  the  heavier  loads  and duty
cycles  a  diesel engine  encounters.   Earlier  discussions  (see,
for example, Table 3-7)  have shown that significantly different
emissions result from the  change  in test cycles.

     The  results   in  Table  5-7   show  that   vehicular  CC>2
accounts  for  about 70-80  percent of  the  global warming impact,
and much  of  the remainder is due to  the  energy consumed during
the   production  and   distribution  of   the   fuels.    Methane
emissions play  a relatively  small  role.   However, the role of
methane  estimated here  is very sensitive  to  two  assumptions.
First,  it  is  obviously  dependent   on  the  assumed  per  gram
conversion  factor used to  convert  the  methane  to  equivalent
C02-   Second,  it  is  also  dependent  on  levels  of  methane
control  assumed here  for  CNG  vehicles.   Both the lean-burn and
stoichiometric  engines  have  sufficient   catalytic   control  of
methane   to   be  able  to  meet  the heavy-duty  engine  total
hydrocarbon  standard.   This  would not  generally be  the  case,
for example, with current  light-duty  vehicle CNG  technology.

      It  should  be  noted  that  this  analysis  was done  assuming
that  CNG was  produced from  current  domestic production sources
of  natural  gas.   The conclusions  of  this  analysis  are also
sensitive to this  assumption.   If  the CNG was  imported from
overseas,  the  C02  emitted during production  would be  doubled,
primarily due  to  the  energy  consumed during  liguifaction and
ocean transport  of   the   natural  gas.   On  the  other  hand,  if
natural  gas  that  is  currently  being flared  or vented to the
atmosphere  were   used,  then  clearly  the  CNG  vehicles  would
provide a very  significant

-------
1
•
1
1
Vehicle
•Current Diesel
Advanced Diesel
•Lean-Burn CNG
(Current)
•Lean-Burn CNG
(Optimized)
1
_* g/BHP-hr.
5-L9
Table 5-7a
Global Warm i no; Imoact of Lean-Burn CNG*

CO 2 Methane
From From Equivalent
CC2 Methane Production Production 122 **
622 0.02 141 0.09 764
574 0.01 130 0.07 705
575 0.31 130 4.71 319

525 0.54 164 4.30 745






Rar.ge-
CC 2 * *
754
705
375

797



• ** Total impact in equivalent C02 emissions.
1

1

1

• Vehicle

Gasoline
• Stolen. CNG
(Current)
I Stolen. CNG
(Ootimized)
1
* g/3HP-hr.
• ** Total impact
Note: Due to

Table 5-7b

Global Warminq Imoact of Stoichiometric CNG*
C02 Methane
From From Equivalent
C02 Methane Production Production C02 **

753 0.15 171 0.09 927
500 0.63 156 4.10 711

430 0.45 150 3.93 631



in equivalent C02 emissions.
the fact that the lean-burn and Stoichiometric analyses




Range -
Adjusted
CG2**

927
747

715




are based
Ion different engine test cycles the results shown in Tables 5-7a and 5-7b
are not directly comparable and no comparisons should be made between the
two.
1
1







-------
                              5-20


global warming  benefit,   since  the emissions  from the  vehicle
would be replacing both the vented and  flared  emissions  and the
emissions  from  the  petroleum-fueled  vehicle  simultaneously.
Finally,  if natural  gas  were  to be produced from  coal  (as  in'a
large  scale  CNG   program)  the  C02  emitted  during  production
would more than double.

V.   Other Air Quality Impacts

     Emissions  of  CO  and  NOx  are  also  important  from  an
environmental  perspective;  both  can  result  in  adverse  health
effects.   NOx also  plays  an important role  in the  formation of
ozone  in urban areas  and  can  contribute to  acid  rain.   Using
the CNG  emission  projections from  Chapter 3  and  the petroleum
fuel numbers  presented at the beginning of this chapter, CO and
NOx  emissions  of  the various  engine  types  can  be  compared.
These data are given in Table 5-8.

     Turning  first to the  lean-burn  engine,  Table  5-8a  shows
little variation  in CO or NOx amongst  engine  types,  except for
the current technology lean-burn  engine CO value.  As described
in   Chapter  3,   EPA   expects   this  value   to  be   reduced
significantly  in  future designs.   As  for the  stoichiometric
engine,  Table  5-8b shows it  to  have a  significant NOx benefit
compared  to  its  gasoline-fueled  counterpart.  The  CNG engine
here shows  an approximately 90  percent NOx  reduction.   Low NOx
is  a  characteristic  of  stoichiometric heavy-duty CNG engines
with  a  three-way  catalyst  as  shown  in  Table  3-3.   Finally,
although  overall  CO  emissions  are  similar,  stoichiometric CNG
engines may have some  CO  advantage  over gasoline engines during
cold  start  as  the  gaseous  fuel overcomes  the need  for   cold
start  enrichment.

-------
                            Table 5-3a

                    CO and MOx Emissions of
            Heavy-Dutv Diesel/Lean Burn CNG Er.gir.es*


                            CO (q/3HP-hr)          MOx (q/3H?-hr)

Current Diesel                  1.7                    4.50

Advanced Diesel                 1.4                    4.44

Lean-Burn CNG                   4.0                    4.50
  (Current)

Lean-Burn CNG                   1.5                    4.00
  (Optimized)
     Emissions  are  based  on  testing; of  well-maintained,   low
     mileage test engines.   In-use emissions would be higher.
                           Table 5-8b
               CO and NOx Emissions of Heavy-Duty
              Gasoline/Stoichiometric CNG Engines*
                            CO (g/3HP-hr)         MOx  (c/3HP-hr)

Gasoline                         9.2                    4.5

Stoichiometric CNG              10.6                    0.51
   (current)

Stoichiometric CNG               7.3                    0.41
   (Optimized)
     Emissions  are based  on  testing  of  well-maintained,   low
     mileage test engines.  In-use emissions would be  higher,

-------
                              5-22
                      References Chapter 5


     1.     "Definition  of  a   Low-Emission   Motor   Vehicle  in
Compliance with the Mandates  of Health and Safety  Code  Section
39037.65  (Assembly Bill  234,  Leonard,  1337),"  California Air
Resources 3oard,  May 19, 1989.

     2.     Atkinson,  R. ,  "Kinetics  and Mechanisms  of  the Gas
Phase Reactions  of the  Hydroxy Radical with  Organic Compcur.ds
under  Atmospheric  Conditions,"  Chemical  Reviews,  1985"  35,
69-201.

     3.     "Reactivity/Volatility  Classification  of  Selected
Organic  Chemicals:   Existing  Date,"  US  EPA,  EPA-600/3-84-082,
August .1934.

     4.     Data supplied to EPA by Navistar,  September,  1989.

     5.     Hutchins,   F.P.,    "Gasoline,   Diesel   and  Methane 1
Refueling Emissions  - Data Collection,"  Memorandum  to  Charles
L.  Gray,  Jr., Director,  Emission Control  Technology Division,
August 24, 1989.

     6.     Stout,  A.,  "Reductions in  Evaporative Emissions and
Running  Losses  From  Enhanced  Vehicle-Based  Control,"   Draft
Memorandum to  Charles  L.  Gray, Jr.,  Director,  Emission Control
Tecnology Division, November  1989.

     7.     Draft    Regulatory   Impact    Analysis:     Proposed
Refueling  Emission  Regulations   for   Gasoline-Fueled    Motor
Vehicles-Volume  I  Analysis  of  Gasoline  Marketing  Regulatory
Strategies,"    EPA    Office    of     Air    and    Radiation,
EPA-450/3-37-001a, July 1987.

     8.    "Natural Gas  Vehicles:   A Review of the State of the
Art," Sierra Research, NO SR89-04-01, April 13, 1989.

     9.    Sprik,  T.  L., "Alternative  Transportation Fuels and
The Greenhouse Effect, U.S.EPA, Draft Report."

     10.   "Air  Toxics Emissions  and  Health  Risks From Mobile
Sources,"   Jonathan  M.   Adler   and   Penny  M.   Carey,    rJ.S.
Environmental  Protection  Agency,  APCA Paper No., 89-34A.6,  June
1989.

     11.   "Text  of the Remarks  by the President  on the  Clean
Air  Act Announcement," Office of  the Press  Secretary,  June 12,
1989.

     12.   "Standards  for Emissions  From  Methanol-Fueled  Motor
Vehicles and Motor Vehicle  Engines;  Final Rule,"  54 FR  14426,
April  11,  1989.

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                              5-23
     13.    "Analysis  of  the Economic  and  Environ
~i- Me~harol as  an  Automotive Fuel," Scecial  Reccrt,  E?-.  Dffic—
of Mobile Sources,  September 1989.

     14.    Data  from  General   Motors  certif i oat lor.   reccris,
:otooer 1939.

     15.    Rar.ar.athar.,  V.,  R.  J,  Cicerone,  H.  3.  Singh,   and
J.T.Kienl,  "Trace  Gas  Trends   and   Their   Potential   Role   in
Cl irate  Change,"   Journal  of  Geophysical  Research,   1985,   90,
5547-5555.

     15.    Deluchi,   M.   A.,   R.   A.   Johnston,    D.   Sperling,
 'Transportation .fuels  and  the  Greenhouse Effect,"  "Jniversity  of
California, "JZR-130,  December 1987.

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                              5-24
           References For Further  Information on CNG


     "National Air  Quality  and Emissions  Trends  Reoort,  1987,"
EPA 450/4-89-001, March  1989,  U.   S.  EPA Office of "Air  Quality
Planning & Standards.

     Alternative  Transportation  Fuels:    An  Environmental  and
Energy  Solution,   Daniel   Sperling,   Editor,   Quorum/Greenwood
Press (Westport, Connecticut), 1989.

     "The  Air  Quality  Benefits  of  Alternative  Fuels,"  EPA
Office  of  Mobile Sources,  prepared  for the Alternative  Fuels
Working  Group  of  the  President's  Task  Force  on  Regulatory
Relief, July 1987.

     "Guidance  on Estimating Motor Vehicle  Emission Reductions
From the  Use of Alternative Fuels  and Fuel  Blends,"  EPA Office
of  Mobile  Sources,  Technical  Report   No.  EPA-AA-TSS-PA-87-4,
January 29, 1988.

     "The  Emission  Characteristics  of   Methanol  and  Compressed
Natural  Gas  in  Light Vehicles,"  J.  A.  Alson,  APCA  pacer  No.
88-993, June 1988.

     Carter,  W.P.L.,  R.  Atkinson, "Computer Modeling Study of
Incremental  Hydrocarbon  Reactivity," Environmental  Science  and
Technology, 1989, 23, 864-880.

     Moulis,  C.  E.   "Formaldehyde  Emissions  from Mobile Sources
and  the Potential  Human Exposures,"  Air  and  Waste Management
Association Paper 89-34A.1, June  1989.

     "National  Emission  Standards for Hazardous Air Pollutants;
Benzene,"  Final Rule  and Proposed  Rule,  54 FR 38044.

     "Mutagenicity    and    Carcinogenicity    Assessment    of
1,3-Butadiene,"   EPA  Office   of  Research   and  Development,
EPA/600/8-85/004F,  September  1985.

     "Diesel  Particulate Study,"  EPA Office  of Mobile Sources,
November  1983  (Available  in Public Docket  8A-80-18.)

     Bolin,  B.,  Doos,  B.,  Jager,  J.,   and  Warrick,  R. ,  eds. ,
SCOPE   29:   The   Greenhouse   Effect,   Climatic   Change,   and
Ecosystems,  John  Wiley &  Sons,  New York, 1986.

     Eddy,  John  A. ,    "The   Solar   Constant   and  Surface
Temperature,"    Interpretation  of  Climate  and   Photochemical
Models,  Ozone  and Temperature  Measurements,    AIP   Conference
Proceedings  No,  82,  Ruth  A.  Reck  and  John  R.  Hummel,  eds.,
American Institute  of Physics,  New York, 1982,  pp.  247-262.

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u
                              5-25


       References For Further Information on CNG (cont'd)


     Ramanathan,V. ,  Call is,  L.B.Jr.,  Cess,  R.O.,  Hansen,  J.E.,
Isaksen,  I.S.A.,  Kuhn, W.R.,  Lacis,  A., Luther,  F.M.,  Mahlmar.,
 ,D.,  Reck,   R.A.,   Schlesinger,  M.S.,  "Trace  Gas  Effects  or.
Climate,"   Atmospheric   Ozone    1985    World   Meteorological
Organization,   Global  Ozone  Research   and  Monitoring  Project,
Report No.  16.

     World Meteorological  Organization (WMO)  (1986)  "Report  of
the  International Conference on  the  Assessment of the  Role  of
Carbon  Dioxide  and  of  Other   Greenhouse  Gases  in  Climate
Variations  and  Associated  Impacts,"  Villach,  Austria,  9-15
October 1985,  WMO No. 661.

     "Comparing the Impact  of  Different Transportation Fuels  on
the   Greenhouse   Effect,"   prepared   for    California   Energy
Commission by Michael D.   Jackson,  Douglas  D.  Lowell,  Carl  B.
Moyer, Stefan Unnasch, Acurex Corporation, October 1987.

     "Transportation  Fuels  and the Greenhouse  Effect,"  Mark  A.
Deluchi,  Robert  A.   Johnson,  Daniel   Sperling,  University  of
California, Davis, October  1, 1987.

     "Summary  of  Available  Research  on  Natural  Gas  Vehcile
Methane  Emissions   Contribution   to   the  Greenhouse  Effect,"
Natural Gas Vehicle Coalition,

     "Natuarl  Gas  and Climate Change:  The Greenmhouse Effect,"
American Gas Association,  Issue Brief  1989-7, June 14, 1989

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