ENVIRONMENTAL PROTECTION
AGENCY
SPECIAL REPORT
OFFICE OF MOBILE SOURCES
Analysis of the Economic and
Environmental Effects of Compressed
Natural Gas as a Vehicle Fuel
Volume I
Passenger Cars and Light Trucks
April 1990
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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 I
Passenger Cars and Light Trucks
April 1990
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Table of Contents
Page
ECONOMIC ASPECTS OF COMPRESSED NATURAL GAS USE ...... 1
Current Natural Gas Prices . . 1
Dual-Fuel Conversion and Dedicated Vehicle Costs . . 6
Service Station Retrofit Costs 12
Pipeline Expansion Costs . . . . 16
Capitalized Distribution Costs ........... 17
Gasoline Equivalent CNG Retail Price 19
ENVIRONMENTAL IMPACTS OF CNG USE 25
.Urban Ozone Levels ........... 25
.Carbon Monoxide and Oxides of Nitrogen . . 28
Air Toxics and Global Warming 33
OTHER ISSUES ............... 39
Safety 39
Reduced Maintenance . . . . : . 40
Performance and Fuel Economy 40
Vehicle Range 43
Impact On Home Heating . 49
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PREFACE
In July 1989, the President submitted to Congress his
Administration's proposals for revising the Clean Air Act. One
major component of his plan is the Clean Alternative Fuels
Program. This program would replace a portion of the motor
vehicle fleet in certain cities with new vehicles that meet
stringent exhaust emission limits operating on clean-burning
fuels such as methanol, ethanol, compressed natural gas,
liquefied petroleum gas, electricity, and reformulated gasoline.
This report, released by EPA, is one in a series of
reports that discuss the economic and environmental issues
associated with each of these clean-burning alternative fuels.
This report discusses natural gas use in cars and light
trucks. This application is emphasized because the first
report in this series, the subject of which was methanol, also
concentrated its emphasis on cars and light trucks.
Because natural gas has special features that also make it
attractive for urban heavy-duty fleet applications, the Agency
has prepared another report specifically treating the use of
CNG in heavy-duty vehicles. This Volume II report is available
under separate cover.
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ECONOMIC ASPECTS OF COMPRESSED NATURAL GAS USE
Current Natural Gas Prices
The current U.S. highway vehicle fleet is almost totally
dependent" on petroleum-based fuels, [l.] Alternatives to
petroleum-based motor vehicle fuels (i.e., gasoline and Diesel
fuels) are of interest for two separate reasons. First,
alternative fuels may contribute to a solution to air quality
problems in the United States. A Vice Presidential Task Force
Report issued in 1987 [2] detailed how the use of various neat
and blended alternative fuels might contribute to a strategy
designed to achieve the air quality goals of the Clean Air
Act. President Bush's Clean Air Act proposals contain
provisions for widespread . utilization of alternate motor
vehicle fuels including natural gas. Second, fuel substitution
in the transportation sector could curb imported oil demand and
help to restrain price rises as well as assure a reliable
supply of oil from abroad.[3]
Natural gas, .a.;, gaseous .hydrocarbon ..fuel, is composed
.-primarily of methane (CH«), but it may contain up to 20
.•percent higher weight;/hydrocarbons. [4] These hydrocarbons are
primarily ethane, propane, and butane. Minor other constituents
include nitrogen, carbon monoacide, hydrogen sulfide and
helium. Natural gas in compressed form (CNG) as an automotive
fuel is the subject of this report.
One of the most important factors impacting the use of CNG
as an automotive fuel isu the price of natural gas fuel. A
nominal value for the .net heating value of Indolene fuel has
been reported as 114,132 BTTJ/gallon. [5] The price of a
gasoline energy equivalent of CNG at the retail price level,
even for competing uses in the absence of a significant CNG
motor fuel market, would be one indiccition of CNG's
attractiveness to the consumer as an alternate transportation
fuel.
Natural gas prices are typically presented in terms of a
price per unit ..volume, or I/cubic foot. [6] To convert, to
I/energy equivalent, .a factor for energy content per standard
volume of natural .gas must be used. The CNG fuel used in an
earlier EPA report [7] had a calculated higher heating value of
1010 to 1032 BTU/SCF. The American Gas Association in a recent
publication [8] listed 1031 BTU/SCF as a reference value for
natural gas passing through major interstate pipelines. The
heating value changes for CNG as its composition changes; for
ease of comparison, however, a value of 1030 BTU/SCF was used
here.
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One frequently quoted index of wholesale prices is the
price major interstate gas pipeline companies pay for gas.
This price -has two components:'- 1) the price of imported gas,
and 2) the price of gas purchased from domestic producers.
Table 1 contains a summary of price data for the years 1980-88
and the first eight months of 1989. This data, presented in
$/1000 SCF, was taken from the Monthly Energy Review, a
Department of Energy publication.[9] The conversion to $/mmBTU
was accomplished by using the conversion factor of about 1030
BTU/SCF. To obtain an energy equivalent price based on lower
heating value, the prices which used higher heating values were
divided by 0.90. An explanation of this calculation has been
given in an earlier EPA publication. [7] The data in Table 1 is
presented in terms of 1989 dollars.
At an average 930 BTU/SCF lower heating value of natural
gas, approximately 123 SCF of natural gas would provide an
energy equivalent of 1 gallon of gasoline. The current prices
in Table 1 may be considered a crude index of gas cost which a
wholesaler pipeline company would use to base his delivery
price to a retail operation. Using the average purchase prices
listed for 1989, .the ..current1 cost to a pipeline company for a
.natural:gas thermal equivalent of 1 gallon of gasoline would be
26 cents. This .estimate . ignores . cost changes .that might be
associated with increased-natural ;gas usage as a .transportation
fuel such as operating expenses, capital recovery, and
alternate sources of gas, however.
Another index of interest is the price of gas delivered to
various categories of end users. Table 2 contains the average
price of gas delivered to residential, commercial, industrial,
and electric utility users over the same time periods referred
to in Table 1. The data in $71000 SCF was taken from Monthly
Energy Review [9] and converted to $/mmBTU using the conversion
factor given in reference 8. The conversion to prices based on
the lower heating value for natural gas is explained in
reference 7.
The prices in Table 2 for 1989 range from $2.61/mmBTU for
delivery to electric utilities to $6.49/mmBTU, for residential
end users. .The residential delivered prices may be relevant
for a .scenario which involves refueling, vehicles with natural
gas overnight using a home-based compressor. Commercial or
industrial prices may be more relevant for customers who
purchase large volumes of natural gas, such as large fleets or-
refueling stations.
The prices for delivery to consumers vary considerably by
region within the United States. The American Gas Association
[8] has published average U.S. consumer gas prices by region
for the quarter ended December 31, 1988. For residential end
users, these prices, based on higher heating values, varied
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Table 1
Average Natural Gas Prices
Major Interstate Pipelines Average Purchase Price
Higher/Lower Heating Value Prices (1989 $)
Purchased From Producers
Year Imports ($/mmBTU) ($/mmBTU)
1980 6.39/7.10 2.36/2.62
1981 6.38/7.09 2.84/3.15
1982 6.12/6.80 3.37/3.75
1983 5.38/5.98 3.50/3.88
1984 4.70/5.22 3.35/3.72
1985 3.57/3.96 3.19/3.54
1986 2.75/3.06 2.60/2.89
1987 2.29/2.54 2.21/2.46
1988 2.06/2.29 2.17/2.41
1989 (Jan-Aug) 1.96/2.18 2.07/2.30
iranBTU = Million BTUs.
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Table 2
Average Price -of Gas Delivered To Consumers
Higher /Lower Heating Value Prices
Values Given In $/mmBTU, Adjusted
Year
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
(Jan-
Aug)
Resident rial
5.32/5.92
5,66/6.29
6.41/7.12
7.23/8.03
7.05/7.83
6.84/7.60
6.35/7.05
5.84/6.48
5.59/6.21
5.84/6.49
Commercial
4.90/5.45
5.28/5.86
5.98/6.64
6.67/7.41
6.39/7.10
6.15/6.83
5.53/6.14
5.02/5.58
4.73/5.25
4.61/5.12
Industrial
3.70/4.12
4.14/4.60
4.80/5.33
4.99/5.54
4.86/5.40
4.42/4.91
3.52/3.91
3.10/3.44
3.01/3.35
2.81/3.12
To 1989 $
Electric
Utilities
3.28/3.65
3.81/4.24
4.31/4.79
4.27/4.75
4.26/4.73
3.97/4.41
2.65/2.94
2.44/2.72
2.39/2.66
2.35/2-61*
Overall
Averages
4.21/4.68
4.63/5.14
5.36/5.95
5.75/6.39
5.58/6.20
5.28/5.86
4.50/5.00
4.27/4.74
4.18/4.64
4.01/4.45*
mitiBTU = Million BTUs.
* January - July 1989 only.
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from $4.49/rnmBTU to $7.04/mmBTU, with the higher prices found
in the northeast region of the U.S. It may be expected,
.therefore, that prices of natural -gas for vehicle fuel usage
could vary considerably by region because of differences in
prices of gas from various sources, pipeline distribution, and
operating costs. In addition, the spot market for natural gas
can be low, thus offering some price advantages, but by its
very nature the spot market price may not be a reliable
indicator of high continuous demand such as that provided by
vehicular use. ! .
An earlier U.S. EPA Special Report [10] has noted that the
price at which natural gas is available is dependent upon the
price of competing energy sources, the existence of alternative
markets for gas, and the cost of collecting and transporting
the gas. In more remote regions, such as Prudhoe, Bay, Alaska,
natural gas is coproduced with oil and is reinjected back into
the wells at a cost because at current and expected prices it
is not economic to produce and transport this "remote" natural
gas. A presentation by SRI International [11] quoted in the
aforementioned Special Report [10] assumed that natural gas
.could be-available .at Prudhoe Bay for less .than $0.50/mmBTU
over the next 20 -years. A recent U.S. DOE analysis [12]
.suggests that prices .ranging .from $0.50-1.00/mmBTU for remote
natural gas appear-reasonable.
The overseas transportation and port costs to receive
remote natural gas in the form of liquefied natural gas (LNG)
may be considerable, however. DeLuchi, et al., in a recent
comprehensive analysis of resource supply issues relating to
natural gas vehicles, [13] stated that the high capital cost of
liquefaction plants, competition for the gas between importing
nations, and uncertainty over the size of reserves have
increased the price of LNG from remote natural gas. .At the
domestic level, . natural gas prices necessary to make
substantial increases in the "amount of imported LNG attractive
might make the recovery of unconventional reserves of domestic
natural gas competitive in price. DeLuchi [13] states that
both the American Gas Association (AGA) [14] and the Energy
Information Administration [15] project negligible increases in
the amount of LNG imports over the next 10 to 25 years.
- Assuming that the importation of LNG was economically
desirable, the U.S. could not import more than 1 TCP/year at
present. [13] Only one U.S. LNG terminal is now in service, at
Everett, Massachusetts. Three other facilities, in Maryland,
South Carolina, and Louisiana, are currently on standby. DOE
has stated that a considerable expansion of terminal capacity
and corresponding infrastructure would be required for a major
expansion of LNG imports into the U.S. [12] DeLuchi [13] also
notes that the U.S. public generally opposes new LNG terminals
on safety grounds, though some experts argue that such risks
are very small.
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The U.S. Department of Energy (DOE) is studying natural
gas pricing with respect to a natural gas vehicle market, which
may expand significantly as a result of the Administrator's
proposed Clean Air Act Amendments. AGA has published projected
natural gas fuel prices to the year 2005; [16] a portion of
their projections are reproduced here as Table 3.
Table 3 projects the price of natural gas delivered to
service stations in the year 1995 in 1989 dollars. The prices
listed take into account distribution margins and incorporate
wellhead price projections. Price projections are given for
eight separate regions; these regions might benefit
substantially from the introduction of alternative fuels
because of higher ozone concentrations.[16]
No attempt is made here to weight price according to
projected sales volumes by region. The prices range from a low
of $2.59/mmBTU (lower heating value) in Texas to $5.89/mmBTU in
Connecticut. A simple average among the regions listed here is
$4.66/mmBTU. These prices are used later in this report to
develop a cost to consumers for natural gas vehicle fuel. The
.range..of .prices .above:,approximates ..the current low utility and
higher commercial . use price .;range given .in Table 2. The AGA
.has .also made an attempt -to.-predict differences in gas price
.based-.on - geographic location. :The -geographic locations are
major urban areas where concentrations of ozone in the ambient
air are of great concern.
In this report, estimates are made of the gasoline
equivalent price per gallon for natural gas as a vehicle fuel.
The price of natural gas, like that of all fuels, is determined
based on supply and demand for natural gas as well as the price
for competing fuels in the marketplace. For transportation
fuels, the dominant fuels are petroleum-based, and therefore
oil prices will have a large influence on the eventual price of
natural gas as a vehicle fuel. Since no consensus exists on
the most likely trend for future oil prices, no long-term
forecast of either future oil or natural gas prices is made
here. Instead, current or near-term estimates for prices are
used.
.Dual-Fuel Conversion and Dedicated Vehicle Costs
Two types of ' CNG vehicle configurations are considered in
this report: 1) a dual-fuel vehicle, originally configured to
operate on gasoline but converted to selective operation on CNG
or gasoline fuels, and 2) a vehicle dedicated to operation
solely on CNG arid optimized for its use.
The conversion of gasoline-fueled vehicles to dual-fuel
usage with CNG is an established technology; there are more
than 30,000 gasoline-fueled vehicles in the U.S. converted to
run on natural gas.[17] Several companies currently offer
conversion kits and the services to install the additional
hardware on the vehicle to be converted.
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Figuire 1
, NATURAL OAS CYUNOWW
(«) MtfOllUU. OA*AJ* MUIH
^ niit sei£cro« SWITCH
•1 NATUftAl. IAS
nu CONMCCTON
5 •
Additional Components And Systems
Necessary To Convert Gasoline Fuei
Vehicles To CNG (Dual Fuel) Usage
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Table 3
Projected Delivered Natural Gas Price
To Vehicle Refueling Stations (1989 $/mmBTU)
Year 1995 Higher/Lower
Region (State) • . ! Heating Value Prices
California $4.81/$5.34
Connecticut : $!5.30/$5.89
Illinois $3.46/$3.84
Maryland $4.88/$5.42
New York ; $4.66/$5.l8
Pennsylvania $3.98/$4.42
.Texas $2.33/$2.59
Wisconsin • $4.14/$4.60
AVERAGE $4.20/$4.66
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A dual-fuel vehicle must be set to operate on one of the
fuels prior to cold start. When the vehicle's fuel selector
switch-is set to the natural gas mode/ gas leaves, the-vehicle
storage cylinders and passes through steel tubing to a pressure
regulator where it is reduced to near atmospheric pressure.
The natural gas from the regulator flows into a gas/air mixer
where air and natural gas fuel are mixed arid drawn into the
engine. A spark advance control for natural gas operation is
added to the engine, and the fuel selector switch is mounted in
the passenger compartment. ..A diagram of some typical retrofit
components is given here as Figure l.[18]
There are three significant costs associated with
conversion of a gasoline-fueled vehicle to dual-fuel usage with
CNG. First, there are "underhood" modifications that must be
made to accommodate the introduction and mixing of the gaseous
fuel with air. These modifications include the installation of
shut-off valves, fuel selector switches, gauges,
pressure-reducing valves, and a carburetor designed for use
with natural gas.[19,20] Second, the labor costs associated
with the conversion are substantial. This is primarily due to
the. fact .-that.;current,, conversion efforts .. involve relatively
.small-vehicle 'fleets. Larger numbers of vehicle conversions
.may-involve significant economies of scale.
Finally, the use of CNG involves the pressurized storage
of natural gas onboard a vehicle. CNG's low energy density
relative to gasoline necessitates the use presently of large,
bulky storage vessels. Weaver [21] has noted that the weight
of a conventional steel cylinder containing the energy
equivalent of l gallon of diesel fuel weighs 41 Ibs; the weight
of the fuel and tank for a typical gallon equivalent of diesel
fuel is only 9.2 Ibs. Although promising research is underway
in the area of lighter weight and more compact storage media,
this technology is not yet commercial, [13,21] and the heavy
gas storage cylinders required to meet U.S. Department of
Transportation safety requirements are a significant additional
cost and vehicle weight consideration.
Table 4 contains a range of costs for these components for
three different .scenarios: . 1) light-duty automobiles at 3,000
psi .onboard fuel storage .pressure, 2) light-duty trucks at
3,000 psi gas storage .pressure, and 3) .light-duty automobiles
at 2,400 psi storage pressure. These costs were developed from
several conversion supplier quotes.
Generally, total conversion costs decrease with increasing
vehicle size for a particular application (e.g., light-duty
automobiles). This is primarily due to the larger engine
compartments and trunk space associated with larger vehicles
(e.g., full-size versus mid-size passenger cars). It is easier
to install the bulky storage vessels and underhood hardware on
vehicles with large engine compartments and roomier trunks. An
attempt 'to quantify this difference within a particular
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Table 4
: Costs to-Convert Light-Duty Vehicles to Dual-Fuel Status
Scenario - Component Cost Range
Light-duty automobile (3,000 psi storage):
Underhood equipment costs $650- 750
Labor to install components 700-900
Compressed gas storage bottles 1,200 - 1,600
Cost Range $2,550 - 3,250
Light-duty truck (3,000 psi storage);
Underhood equipment costs $ 650 - 750
Labor to install components 500 - 700
Compressed gas storage bottles 1,200 - 1,600
Cost Range $2,350 - 3,050
Light-duty automobile (2,400 psi storage):
Underhood equipment costs $ 650 - 750
Labor to install components 700 — 900
Compressed gas storage bottles 300 - 600
Cost Range $1,650 - 2,250
Cost estimates for CNG-fueled vehicles in mass production;
Dual-fuel (CNG plus gasoline) \ $1,600
Dedicated (CNG only) $ 900
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I
application is not made here; this, however, should explain the
difference in conversion costs between light-duty truck and
automobile applications in Table 4. .
The data in Table 4 assumes that fiber-wrapped steel or
aluminum tanks are used in place of conventional steel tanks
for 3,000 psi storage. Fiber-wrapped tanks offer a savings of
approximately 30 percent in cylinder weight over conventional
steel tanks, but are more expensive,, [21] Advanced
fiber-wrapped aluminum cylinders and all-composite cylinders
may offer even greater weight savings in the future.[13]
Storage costs are greatly reduced at 2,400 psi storage
pressure; however, driving range is reduced with 2,400 versus
3,000 psi storage bottle pressure.
The costs in Table 4 are presented in a range of values
for vehicles typically representative of those found in each
category; vehicles having either very large or very small
interior volumes are not represented here. (The conversion
cost of -a two-passenger vehicle may exceed $5,000, for
example.)[22] Any economies-of-scale .associated with larger
..scale., conversion...of. .vehicle ...fleets .to dual-fuel .usage are not
.discussed .\here...::Large-scale, production of dual-fueled vehicles
(CNG/gasoline) ;would .involve the.:efforts of one or more major
. automotive manufacturers.
Costs similar to these have been quoted in the
literature. A 1983 study [23] used a range of $1,200-1,900 for
the conversion cost of a light-duty vehicle to dual-fuel
capability. In 1988, the Gas Research Institute estimated the
cost of converting a gasoline vehicle to CNG dual-fuel
capability at $1,500-2,100 per vehicle.[24] Other authors have
quoted conversion costs at $1,200-1,500 per vehicle.[25,26]
Dual-fuel CNG-gasoline vehicles like other dual-fuel
vehicles suffer from several drawbacks associated with their
ability to run on gasoline fuel. The weight of the compressed
gas cylinders large enough to provide at least a 250-mile
driving range on CNG .alone is significant and results in a fuel
economy loss particularly if the vehicle is alsso operating with
a full tank of gasoline (gasoline fuel usage). Useful vehicle
space is reduced when compared to a gasoline vehicle due to the
volume-restrictions .posed by the inclusion of both liquid and
gaseous fueling components. Finally, .engine power and
efficiency on CNG fuel are not optimized if the base engine is
configured for gasoline-fueled operation.
If a CNG dual-fuel vehicle was to be mass-produced by a
vehicle manufacturer, costs may be reduced due to high
production volumes and more efficient labor utilization. We
estimate that such vehicles would still cost $1,600 more than
the baseline gasoline-fueled car.
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An optimized CNG vehicle (single-fuel only) would use a
higher compression ratio in order to take full advantage of
CNG's -higher--octane-.rating -(relative to gasoline). Ignition
timing and valve timing could also be optimized for CNG usage.
Gasoline fuel components such as the carburetor or fuel
injection system, fuel pump/ .and gasoline tank would be"
eliminated, offering weight, volume, and. abate the cost
increase. The new fuel tanks could also be tailored to the
.vehicle design,, making more efficient use of .existing space on
the vehicle.
Very few working examples of dedicated CNG vehicles exist,
however. Ford produced several light trucks during 1984
modified to operate exclusively on CNG.[27] These were not
built in substantial volume, however, so it is necessary to use
gas industry estimates for the cost of dedicated vehicle
production. Incremental costs per vehicle guoted for an
optimized CNG vehicle range from $500 to $1,100.[23,28,29]
Further refinement of these estimates depends on the decision
by major automakers to produce a dedicated CNG vehicle for
public sale and advancements in CNG-related engine technology.
: Our estimates . of <;.-.dedicated ...CNG-..vehicle costs are about $900
••more than its gasoline-fueled,counterpart in mass production.
: Service Station Retrofit-Costs
One of the key obstacles to penetration by CNG in the U.S.
transportation fuels market is the virtual nonexistence of a
CNG maintenance and refueling infrastructure, a critical
disadvantage relative to gasoline.[19] There are approximately
30,000 vehicles equipped to run on natural gas in commercial
use in the U.S. [13,17] supported by a network of 275 private
refueling stations. Only 15 of these stations offer natural
gas for sale to the public, however. The U.S. Department of
Energy has estimated, that displacement of 1 million bbl/day of
petroleum-based transportation fuel by CNG would involve
coverting 16,000 regular public service stations and 2,000
truck stops to distribute CNG fuel.[30] Similarly, Bechtold
[30] has estimated that replacement of 2 million bbl/day of
'petroleum transportation fuel demand with natural gas would
require approximately:35,900 .public light-duty filling stations
as part of.a CNG refueling infrastructure.
Both of the scenarios above concerning petroleum fuel
demand displacement by compressed natural gas fuel are
substantially greater than the- market penetration assumed in
the President's recent Clean Air Act revision proposals. No
attempt is made in this report to reconcile differences in
market displacement assumptions among various authors. The
published studies above may have assumed a particular
significant displacement scenario without taking into account
specific proposed legislation. All of these estimates are
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large enough, however, to ensure at least some realization of
economies of . scale associated with costs related with the
introduction of compressed natural gas as a serious alternative
to petroleum-based transportation fuel.
Three types of CNG vehicle refueling options are
available. First, slow-fill refueling stations use a
compressor with limited or no storage capability. The
compressor is sized to provide filling capacity for a given
number of vehicles over a predetermined downtime. One
potential use of the slow-fill concept is at a. refueling point
for a vehicle fleet where downtime occurs at regular,
predictable intervals.
Home refueling may be a refueling option for residences
served by residential gas lines. Small compressors could
operate from lower residential delivery pressures (near
atmospheric) and pressurize CNG fuel tanks to 1,000-3,000
psig.[13]
The third option involves fast-refueling from on-site
..storage ..capacity at ;-the .filling station. The compressor that
pumps pipeline gas to .storage'vessel pressures must be sized to
ensure that peak .demand . is satisfied. Currently all public
r-refueling stations .in the U.S. -use.the fast-fill method.
Assumptions concerning the characteristics of a network of
CNG public refueling stations must be made. For the DOE 1
million bbl/day gasoline demand replacement scenario, the
stations are assumed to be partial conversions of existing
gasoline vehicle refueling facilities; no land acquisition
costs for additional filling stations are contemplated. These
public-filling stations would be designed to service
approximately 300 vehicles a day, with a peak capacity of 30
vehicles per hour. Four fueling nozzles -per station and a
refueling time of 8 minutes per vehicle are assumed.
Deluchi,[13] citing Canadian experiences,[32] has estimated
that refueling time for a CNG vehicle with gasoline vehicle
range, to include waiting for the pump and paying for the fuel,
might approach .10 minutes. DOE has estimated that these
stations would dispense an average 220,000 cubic feet of gas
per .day. [30]
A network of public access, fast-fill service stations
will be necessary eventually to enable large-scale, replacement
of gasoline fuel demand with CNG. Conventional dual-fuel
gasoline/CNG conversions have a limited driving range on CNG
fuel. CNG fuel availability away from the vehicle's garaging
point is necessary to make possible CNG fuel usage during
extended, non-commuter driving". Widespread acceptance of CNG
vehicles by individuals, particularly individuals without the
desire or the means to implement home refueling, will require
an expanded network of public access filling stations. This
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. report therefore considers that a network of fast-fill public
access refueling points is vital to widespread public
> acceptance of CNG as a .petroleum fuel replacement. Costs of
partial conversion of a gasoline-fueled station to CNG use are
considered below. These costs do not include any truck stop
conversions to serve heavy-duty vehicles powered by CNG.
Table 5 provides- a range of costs to convert a gasoline
"island" of four pumping points to CNG usage. It is assumed
that the present station location will accommodate the island,
and that no acquisition of land will be necessary to facilitate
the retrofit. The costs are eszpressed in a range from lower
cost to higher cost options for most of the components.
The compressors considered range from 200 to 300 SCFM of
natural gas pumping capacity, for a final, storage pressure of
3,600 psi. Compressor costs include starting and cooling
systems. Compressor cost is very sensitive to inlet pressure;
an inlet pressure range of near atmospheric to 40 psi was
. assumed.
.The ..range ..of -costs.. for .the . compressors given in Table 5
include the . cost of a large electric compressor- motor.
. Typically, the work; of .-.compression at .a fast-fill refueling site
•is done with an -electric • motor. :An alternative to this method
might be the use of an industrial natural gas-fueled engine,
assuming that any safety or other considerations could be
overcome. A recent report prepared for the U.S. DOE, [33]
presents a sample calculation for the cost of an industrial
natural gas-fueled engine together with a calculation for a
comparably sized electric motor. A 200-horsepower engine was
estimated to be sufficient for the compressor application given
here. The cost of the industrial natural gas engine was
calculated to be approximately $21,000; the cost of a
150-kilowatt electric motor for the same application was
calculated at roughly $11,000. Subtracting this $11,000
estimated electric motor cost from the compressor cost in Table
5, and adding $21,000 estimated for the industrial natural gas
engine, would have the effect of increasing the upper bound on
the CNG refueling station costs from $396,000 per station to
$406,300 per station.
.The bottle storage cost was a function of the type of
storage vessels used and total storage capacity. At the lower
cost end of the spectrum, 20-bottle cascades of 35,000 SCF
capacity are assumed to be of sufficient size. The higher cost
figure is for 100,000 SCF capacity at 4,000 psi storage
pressure. These ASME cylinders are 23 feet in length, 2 feet-3
inches in diameter, and stacked three high. A total of nine
cylinders (three banks of three) are necessary to provide this
storage capacity. Manifolding expenses- for these storage
configurations range from $2,250" to $6,800.
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-15-
Tafole 5
Costs, to Partially Convert a Gasoline
Service Station to Fast-Fill CNG Refueling
Capitalized Items
Compressor*
Gas storage
Fast-fill gas metering and
dispensing system
Manifolding
Sequential, priority panels,
wiring, and installation
Safety valves and devices
Spare parts
Gas line..connect ion .fees -and
expenses
Shipping costs
Engineering, overhead, and
administration
Buildings
Land.
RANGE OF TOTAL COSTS
Range of Costs (dollars)
$107,000 - 200,000
25,500 - 99,000
24,000
2,250 - 6,800
7,500
12,000
6,000
2,000
7,000
22,000
10,000
$225,250 - 396,300
Includes electric compressor motor.
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-16-
The costs of the service station retrofit calculated here
ranged from $225,250 to $396,300 per station converted. These
costs are similar- in magnitude to costs in.. other published
reports. In a report prepared for the U.S. Department of
Energy, [34] E. A. Mueller costed a CNG fast-/slow-fill
refueling station with two nozzles serving 50-60 vehicles at
$75,000-125,000. These costs assume a small fleet operation;
Mueller's [31] estimated cost of a larger gasoline station
.retrofit as $321,500 per station. Biederman [29] estimates CNG
refueling station costs for medium/large urban public
quick-fill stations serving 150-225 vehicles per day over the
range of $339,900 to $406,260.
Biederman [29] provides a range of station conversion
costs of $175,000 to $193,000 for mixed fast-slow fill
conversions serving private fleets of 45-115 light-duty
vehicles. Darrow [23] provides a range of $60,000 to $255,000
for the cost of refueling equipment to serve a 100 vehicle
fleet. This equipment, however, provides for a mix of fast-
and slow-fill capability.
.Several ...potential ..home compressor packages .are mentioned
. in .reference 29, but are referred to as test market
/applications . only. No large-scale use of . home CNG refueling
..has-.-been -made and documented. Although -very speculative,
prices as low as $2,000 for a home compressor have been
mentioned.[29]
Advanced natural gas vehicles may have storage pressures
as low as 500 psi using adsorbent storage. This form of fuel
storage, potentially very advantageous, is not yet commercially
viable. Any improvements in natural gas storage technology
will have benefits both in vehicle storage and in fueling
station storage.
Pipeline Expansion Costs
The increased use of natural gas as a light-duty
automotive fuel may have the effect of requiring substantial
new investment in pipeline infrastructure. Areas not currently
served by a local pipeline distribution infrastructure may have
to.be supplied with one'if natural gas fueling.of a significant
-portion of the .locality's vehicles is . desired. New end-use
loading may impact the entire transmission system,
necessitating pipeline expansion and the addition of extra.
storage capacity.
The U.S. Department of Energy has examined the impact of
increased utilization of natural gas as an automotive fuel on
the present gas transmission and storage systems.[30] DOE has
estimated that the degree of under utilization in the current
U.S. gas pipeline transmission system is sufficient to permit
the displacement of 1 million bbl/day of petroleum fuel demand
in the automotive sector with CNG without the construction of
-------
-17-
additional transmission capacity. DOE also assumed that
additional storage capacity will not be required nationally.
The primary reason for this, assumption is that the peak
seasonal driving demand for CNG fuel should coincide with the
lowest demand period for other segments of the natural gas
market.
Local distribution infrastructures may have.to be expanded
in order to provide gas to public access sservice stations.
Excess capacity would be used and incremental additions to the
current distribution system would be made rather than
construction of a dedicated distribution network designed for
transportation fuel use. The total incremental cost of
additions to local infrastructures was estimated by DOE to be
upwards from $604 million.
E. A. Mueller has estimated incremental natural gas
transmission and storage - costs for the U.S. pipeline
distribution system assuming the replacement by CNG of 2
million bbl/day of petroleum transportation fuel.[31] This
analysis includes costs to service both light- and heavy-duty
.vehicle .demand. . .Two .separate .scenarios ace considered by
.Mueller. ..The first .scenario includes distribution system costs
to service a refueling:- network composed of public access
service, stations . and .truck -stops, as well as a number of
private fleet refueling points. The incremental distribution
system cost for this scenario was about $3 billion. The second
scenario assumed that all refueling occurred at public access
service stations and truck stops; increase!! in distribution
costs were estimated at $2.5 billion for this option, which
incorporates only the light vehicle portion.
DOE [30] has used E. A. Mueller studies to arrive at the
estimates of distribution system incremental costs presented in
references 30, 34, and 35. Mueller's higher costs given in
reference 31 are associated with expanded scenarios to include
heavy-duty truck stop service, involving the replacement of a
greater portion of petroleum-based fuel demand than the DOE
work.[30]
Capitalized Distribution Costs
. The;.difference1 .between a wholesale price of a fuel and its
retail .;price can be * divided into three main components: l)
distribution of the fuel to the service station, 2) service
station markup, and 3) taxes. A summary of estimates for
capitalized distribution costs relating to both gasoline and
CNG fuels is given in Table 6.
The distribution costs associated with gasoline fuel were
..taken from an earlier EPA position paper. [10] Table 6 presents
four separate scenarios for capitalized CNG distribution costs.
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•-la-
Table 6
CNG Fuel..Distribution Costs
10-Year Capital Cost Payback
(cents per gasoline gallon equivalent)
Scenario
Service
Local Distri- Service Station Station All
bution Costs* Conversion Costs**Markup Taxes
Lower CNG 1.2/ l.O/ 0.9 6.6/ 5.6/4.9
station cost
10 percent ROR
Higher CNG 1.2/ l.O/ 0.9 11.6/ 9.9/3.6
Station cost
10 percent ROR
Higher CNG 1.2/ l.O/ 0.9 11.9/10.1/8.8
Station cost
CNG compression
10 percent ROR
Gasoline 6
24
24
24
Totals
40.8/39.6/38.8
45.8/43.9/42.5
46.1/44.1/42.7
24
39
* CNG figures assume alternately 556/200. 654,300 and 752,500 gasoline
gallon equivalents of CNG pumped per station.
** CNG figures do not include operating expenses.
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-19-
all assuming a 10-year payback period. Local additional
distribution costs of $604 million to support a light-duty
service station infrastructure . are spread over 16,000 service
stations, for an approximate average cost of $40,000 per
station. A 10 percent rate of return is postulated and an
annuity assuming a 10-year payback period is calculated. The
annual annuity is divided by three different yearly gas sales
estimates: 556,200, 654,300, and 752,500 gasoline gallon
equivalents, respectively. These gas sales estimates include
the average yearly sales per station estimated by DOE to occur
at each of 16,000 light-duty service stations assuming a 1
million bbl/day displacement of petroleum fuel demand by
CNG.[30] The upper and lower sales bounds assume 15 percent
greater and 15 percent lower sales volumes than the DOE
estimate. It should be noted that these estimates are spread
over a much higher level of penetration than is required in the
President's Clean Air Act Amendments. To the extent that scale
is smaller, unit costs will rise.
Two separate service station conversion costs, a higher
estimate of $396,300 and a lower estimate of $225,250 have been
presented earlier . in. ..this .report. .These two estimates assume
•that an electric;motor is..used to.compress the natural gas fuel
at the service station. .If .a .natural - gas-fueled engine is used
for.'compression, : we .estimate that.:the higher service station
cost would be raised to $406,300. All three estimates are used
here; annual annuities assuming a 10 percent annual rate of
return are calculated for each scenario. These annual!zed
costs are then dividied by the three different CNG sales volume
figures presented earlier.
Taxes are assumed to be applied at the same rate that
gasoline taxes are applied, 24£ per gasoline gallon
equivalent. This assumption was made in an earlier EPA
position paper.[10]
It is important to note that Table 6 considers only
capitalized costs and taxes. Variable operating expenses such
as utilities expenses are discussed later in this report.
Service station markup includes profit; this amount is the same
on a gasoline gallon equivalent basis as reported in the
earlier methanol.study.[10]
The distribution cost estimates given here are very
sensitive to sales volumes of natural gas through the converted
stations. If sales volumes per station as large as these
postulated here fail to materialize, these costs per unit of
fuel delivered could rise in proportion to the decrease in
sales volumes.
Gasoline Equivalent CNG Retail Price
The method used here to determine the retail price for a
gasoline gallon equivalent of natural gas automotive fuel
begins with the determination of a range of prices of natural
-------
-20-
gas delivered to vehicle refueling stations. The pricing used
here was discussed earlier in this report; the delivered prices
include distribution costs to the service station. These costs
per gasoline-equivalent are added to the capitalized service
station costs developed earlier. Service station operating
costs for CNG are developed below. Finally, sales consumption
taxes are computed on an energy-equivalent basis with gasoline
for CNG fuel.
Table 7 presents a summary comparison of current gasoline
and projected natural gas prices.
The current gasoline price rollup was taken from the
recent EPA position paper addressing the use of methanol as an
alternative fuel.[10] The $1.08 per gallon price is taken from
that report; this is the current average retail price of 1
gallon of unleaded, regular gasoline. The current average
price of unleaded premium is $1.23 per gallon; weighted average
sales of these two gasolines gives a current average price of
$1.12 per gallon for unleaded gasoline. Distribution, service
station markup, and taxes were taken from Attachment 2 of the
aforementioned report.[10]
•-Using .-the $2.59 -.(.lower), $4.66 (average), and $5.89
.(higher) per •.million BTU .low€»r heating value prices given
earlier for the range of natural gas prices, the conversion to
cents per gallon was determin€>d using 114,132 BTU/gallon, a
nominal value for gasoline used previously by EPA.[5] This
results in values of 30^/53^/67^ per gasoline gallon thermal
equivalent.
Capitalized service station costs and taxes for CNG were
calculated earlier in this report. Operating and maintenance
expenses associated with service station operation were
sensitive to .the method used to compress the natural gas at the
service station (electric motor or natural gas engine). These
expenses included compression energy, maintenance,
administrative, and other general expenses over the range of
CNG sales volumes .of 556,200 to 752,500 gasoline gallon
equivalents referred to earlier.
The price : range .for CNG light-duty automotive fuel
calculated in .-this manner is $0.78-1.27 per, gasoline gallon
.thermal equivalent of -fuel assuming electric motor
compression. For a vehicle with a CNG fuel economy of 17 miles
per gasoline gallon equivalent, the fuel price range translates
into a cost of between $4.59-7.47 to travel 100 miles. These
costs are higher than a recently published estimate of
$3.00-5.00 fuel cost to travel 100 miles in a light-duty
vehicle equipped to operate on CNG.[36]
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-21-
Table 7
Gasoline and CNG Energy Equivalent Price Comparison
(dollars per gasoline gallon equivalent) .
Compressed Natural Gas
Cost Classification Gasoline
Extraction/ refining, $0.69
other
Natural gas delivered
to service station —
Long-range and local
distribution costs
Capitalized service
station conversion
costs
.Operating expenses
Maintenance, admin.,
and general expenses
Service station
markup
Taxes
TOTALS
0.06
0.09
0.24
Electric Motor
Compression
CNG Engine
Compression
$ 0.30/0.53/0.67 $0.30/0.53/0.67
0.05 - 0.12
0.08
0.02 - 0.07
0.09
0.24
0.09 - 0.12
0.02 - 0.04
0.03 - 0.08
0.09
0.24
$ 1.08 $0.78/1.07/1.27 $0.77/1.05/1.24
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-22-
ECONOMIC ASPECTS OF COMPRESSED NATURAL GAS USE
• References
1. "Natural Gas, Synthetic Natural Gas and Liquefied
Petroleum Gases As Fuels For Transportation," Fleming, R. D.
and R. L. Bechtold, SAE Paper 820959, August 1982.
2. "Report of the Alternative Fuels Working Group,"
Vice President's Task Force on Regulatory Relief," July 1987.
3. "Assessment of Costs and Benefits of Flexible and
Alternative Fuel Use In the U.S. Transportation Sector,"
DOE/PE-0080, January 1988.
i
4. Gas Engineers Handbook, First Edition, Industrial
Press, Inc., New York, NY> 1965.
5. Federal Register, Vol. 50, No. 126, Monday, July l,
1985, p. 27179.
6. Natural Gas AnnuaJL, Energy Administration
.Administration, DOE/EIA-0131<86)/1, October 1987.
7. "Emissions, Fuel Economy, and Performance of
Light-Duty CNG and Dual-Fuel Vehicles," Bruetsch, Robert I.,
U.S. EPA, EPA/AA/CTAB/88-05, June 1988.
8. "Executive Gas Industry Statistics," American Gas
Association, Arlington, VA, April 1989.
9. Monthly Enercry Review, Energy Information
Administration, DOE/EIA-0035 (89-08), November 1989.
10. "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.
11. "The Economics of Alternative Fuels and Conventional
Fuels," SRI International, presented to the Economics Board on
Air Quality and Fuels, February 1989.
12. "Assessment of Costs and Benefits of Flexible and
Alternative Fuel Use in the U.S. Transportation Sector,
Technical Report Two: Executive Summary - Methanol and LNG
Production and Transportation Costs," Office of Policy,
Planning and Analysis, U.S. DOE, May 1989.
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-23-
13. "Methanol Vs. Natural Gas Vehicles:: A Comparison of
Resource Supply, Performance, Emissions, Fuel storage Safety,
Costs, and Transitions," DeLuchi, et al., SAE Paper 881656,
October 1988.
14. "The Gas Energy Supply Outlook Through 2010,"" Policy
Evaluation and Analysis Group, American Gas Association,
Arlington, VA, October 1985. .
15. "Annual Energy Outlook 1985," Energy Information
Administration, U.S. Department of Energy, DOE/EIA-0383(85),
Washington, D.C., January 1986.
16. "Natural Gas Prices for the Vehicle Market," Issue
Brief 89-19, American Gas Association, Arlington, VA, November
22, 1989.
17. Gas Research Institute Digest, Vol. 11, No. 3, Fall
1988.
18. Reprinted from "Natural Gas Powered Vehicles,"
International .Literature .Published by Minnegasco, Plymouth, MN.
.19. "CNG: .The. Ideal .-.Transportation Fuel," Automotive
Fleet, Large, .-R...B., March .1988.
20. IMPCO Carburetion, Inc., Master Catalog, Cerritos,
CA, 90701, 1987.
21. "Natural Gas Vehicles - A Review of the state of the
Art," Weaver, C. S., Sierra Research, Sacramento, Ca, April
1989.
22. Personal Communication, Chris Bruch, General
Manager, Garretson Eguipment Company, Mt. Pleasant, IA, October
1989.
23. "Economic Assessment of Compressed Natural Gas
Vehilces for Fleet Applications," Darrow, K, G., Gas Research
Institute, Chicago, IL, September 1983.
24. "GRI's ..Research Initiatives In Natural. Gas
Vehicles," Ban, .S. D., Gas Research Institute, September 20,
1988.
25. "Cheap Gas, Clean Air," Sperry, S. R. in Seattle
Post-Intel1iqencer, July 30, 1989.
26. "The Practical and Economic Considerations of
Converting Highway Vehicles to Use Natural Gas as a Fuel,"
Bechtold, R. L., et al., SAE Paper 831071, 1983.
27. "The Development of Ford's Natural Gas Powered
Ranger," Adams, T., SAE Paper 852277, 1985.
-------
-24-
28. Planning and Analysis Issues, Issue Brief 1989-11,
American Gas Association, Arlington, VA, July 21. 1989.
29. "Dedicated Low-Pressure Natural Gas-Fueled Vehicle
Baseline Assessment, Task 3 Topical Report, Light-Duty
"Vehicles," Biederman, R. T., et al.. Institute of Gas
Technology, Chicago, IL, January 1989.
30. "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. Dept,, of Energy, Office of Policy,
Planning and Analysis, July 1989.
31. "An Assessment of the Infrastructure Required to
Refuel A Large Population of Natural Gas Vehicles," Bechtold,
R. L. and G. Wilcox, SAE Paper 892066, September 1989.
32. "Gaseous Alternative Fuels Field Trials in the
Canadian Environment," Heenan, J. S., et al., Nonpetroleum
Vehicular Fuels Symposium IV, Institute of Gas Technology,
Chicago,..IL, October 1984.
.33. "An Assessment of Farm-Based Approaches For the Use
of the.; Stirling .Engine," RCG/Hagler, Bailly, Inc., Washington,
B.C., prepared for the U.S. Department of Energy, June 1988.
34. "Gaseous Fuel Vehicle? Technology, State of the Art
Report," (Revised Draft), prepared for The U.S. Department of
Energy by E.A. Mueller, Inc., December 1985.
35. "Assessment of Natural Gas Infrastructure for
Transportation Use (Draft)," prepared for Oak Ridge National
Laboratory by E. A. Mueller, Inc., Baltimore, MD, November 1988.
36. "The Methanol Car In Your Future," Fortune, Kupfer,
Andrew, September 25, 1989.
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-25-
ENVIRONMENTAL IMPACTS OF CNG USE
Urban Ozone Levels
The primary environmental benefit associated with the
alternative fuels program will be significant improvements in
ozone levels in the most seriously polluted areas of the
country. The clean, alternative fuel advantage over gasoline
in terms of urban ozone formation is due to both lower levels
of vehicle emissions and the lower photochemical reactivity of
these emissions. In the case of CNG, it is the lower levels of
photochemically reactive emissions that are of particular
importance.
The photochemically reactive fuel-related emissions from
CNG-fueled vehicles, for example, consist of mostly nonmethane
hydrocarbons (NMHC), compounds with reactivities assumed to be
.the .same as nonmethane .hydrocarbon . emissions from
-gasoline-fueled vehicles. The difference is that CNG-fueled
vehicle exhaust hydrocarbons are typically 90-95 percent
nonreactive methane, emissions (5-10 percent NMHC), whereas
gasoline-fueled vehicletexhaust hydrocarbons are typically 5-35
percent nonreactive methane emissions (65-95 percent NMHC).
Because of this much higher percentage of nonreactive methane,
the NMHC emissions of CNG vehicles are much lower than gasoline
vehicle NMHC emissions.
Another photochemically reactive compound, formaldehyde,
is emitted at very low levels; (approximately the same amount is
emitted from both CNG-fueled and gasoline-fueled vehicles).
In a test program performed by EPA in 1988, a small fleet
of dual-fuel vehicles (vehicles which run on either CNG or
gasoline) and one dedicated CNG light truck were tested for
exhaust emissions, fuel economy, and performance.[1] In that
study, non-methane hydrocarbons were found to be much lower
from CNG-fueled vehicles compared to vehicles operating on
gasoline as the fuel. ,
The California Air Resources Board reported exhaust NMHC
emissions totaling- 9.1 percent of total hydrocarbons from a
vehicle with CNG fuel which contained 7.2 percent nonmethane
hydrocarbons by weight.[2] Exhaust NMHC is somewhat greater
than feedgas NMHC presumably because the exhaust may contain
some NMHC from burned lubricating oil. Nonmethane hydrocarbons
made up only 3.7 to 6.5 percent of total hydrocarbons in the
EPA-tested CNG fuels so the exhaust NMHC in the EPA study might
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-26-
be expected to be somewhat less than 9.1 percent. Applying the
same ratio as that observed in the GARB study yields exhaust
NMHC of 4.7 to 8.2 percent of total hydrocarbons. In a
subsequent EPA report using the same data, an exhaust NMHC
fraction of 10 percent of total hydrocabons was assumed.[3] In
the following analysis, the "NMHC =10 percent of total HC"
assumption is used as an upper bound or "worst case," and the
"fuel NMHC = exhaust NMHC" assumption is used as a lower bound
or "best case" for estimates of NMHC from CNG-fueled vehicles.
In a third EPA report, emission factors are projected for
"in-use" dedicated CNG vehicles. This study also uses the
previous EPA and CARB-generated data mentioned above. However,
since no in-use (high-mileage) data were available, the in-use
dedicated-CNG vehicle NMHC exhaust emissions were determined by
taking the average of dedicated and dual-fuel laboratory (low
mileage) vehicle NMHC data assuming that NMHC = (0.1) THC.C4]
The EPA laboratory data show NMHC exhaust emissions from a.
dedicated CNG light truck is 0.06 g/mile (assuming fuel NMHC =
exhaust NMHC) or 0.14 g/mile (assuming NMHC - (0.1) THC). This
.test .program also .shows .NMHC, exhaust .emissions from dual-fuel
vehicles operating, on CNG-average 0.12 g/mile and 0.23 g/mile
.with.the .same assumptions,-; respectively. The .projected in-use
dedicated'CNG vehicle .is assumed to have exhaust emissions of
0.186 g/mile NMHC (i.e., the average of 0.14 and 0.23 g/mile).
On a reactivity-equivalent: basis, CNG dual-fuel vehicles
are projected to emit 36-47 percent less than typical future
in-use gasoline vehicles, while optimized dedicated CNG
vehicles are projected to emit 80-93 percent less than future
in-use gasoline vehicles as shown in Table 8.[5-7]
Passenger cars and light-duty trucks now are typically
responsible for approximately 87 percent of all motor vehicle
related ozone precursor emissions. If all passenger cars and
light trucks in a given metropolitan area were optimized
dedicated CNG vehicles that emitted 80-93 percent less ozone
precursors than gasoline vehicles, these vehicles would reduce
the total ozone impact in that area by an average of 70-81
percent assuming no'change in stationary source contributions.
Assuming that."in the year 2015, gasoline fueled motor vehicles
will be responsible'for only 20 percent of all ozone precursors
in such an area, the replacement of all gasoline .fueled
vehicles by dedicated optimized CNG fueled vehicles would
reduce the total by about 16 to 19 percent.
The ozone precursor emission reductions achievable with
the clean, alternative fuels program are a significant portion
of what could be achieved by taking the same number of cars off
the road, and are much larger than the reductions that would be
available from any other motor vehicle control program absent
major vehicle use restrictions.
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-27-
Table 8
Projected la-Use-Gasoline VOC-Eguivalent
Emissions for Gasoline and Methanol and CNG Vehicles
(grains per mile)
NMHC Methanol Formaldehyde Gasoline
Reactivity Reactivity Reactivity VOC-
NMHC Factor Metb. Factor HCHO Factor Equivalent
Gasoline;
- Current Standards
- Proposed Standards
FFVs on M85s
- Readily Feasible
- Optimized
M100;
- Optimized
Dual-Fuel CNG;
- Worst Case
- Best Case
Dedicated CNG:
(1.73 x 1.00) + ( 0 x 0.19) + (.007 x 2.2)
(0.94 x 1.00) + ( 0 x 0.19) + (.005 x 2.2)
(0.350 x 1.00) + (0.950 x 0.19) + (.060 x 2.2)
(0.310 x 1.00) + (0.750 x 0.19) «• (.035 x 2.2)
(0.05 x 1.00) * (0.572 x 0.19) + (.015 x 2.2)
(0.604 x 1.00) + ( 0 x 0.19) + (.004 x 2.2)
(0.489 x 1.00) + ( .0 x 0.19) + (.004 x 2.2)
1.75
0.95
0.66
0.53
0.19
0.61
0.50
- Worst Case
- Best Case
(0.186 x 1.00) + ( 0 x 0.19) + (.004 x 2.2) - 0.19
(0.057 x 1.00) + ( 0 x 0.19) •»• (.005 x 2.2) = 0.07
All fuels except dedicated CNG have hot soak/diurnal, running losses, and
refueling losses included in addition to exhaust. NMHC emissions. Hot
soak/diurnal, running .losses/ and refueling losses for dual-fuel CNG vehicles
are assumed to be nearly equal to those of gasoline-fueled vehicles
corresponding to the standards, proposed by the President. These values of 0.13
grams per mile for hot soak/diurial, 0.16 grams per mile for running losses, and
0.07 grams per mile for refueling were used as 0.18, 0.16 and 0.03 for dual-fuel
CNG. The reduction in refuling losses reflects less fueling with gasoline.
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-28-
Carbon Monoxide and Oxides of Nitrogen
The exhaust -emissions of dual-fuel .and dedicated CNG
vehicles are difficult to characterize due to the lack of a
significant database for either type of vehicle. However, EPA
has tested three dual-fuel vehicles: a 1984 .Oldsmobile Delta
88, a 1987 Chevrolet Celebrity (2 versions), and a 1987 Ford
Crown Victoria (2 versions), and one dedicated CNG 1984 Ford
Ranger pickup truck. The exhaust emissions of CO.and NOx from
these vehicles are discussed .together in this section,' because
current strategies to reduce the emissions of one of these
pollutants, tends to increase the emissions of the other.
Dual-Fuel CNG Retrofit Vehicles
The emissions data from dual-fuel vehicles, shown in Table
9, suggest a clear trend with respect to CO emissions.[3] CNG
dual-fuel vehicles offer the. potential for very significant CO
emission reductions. CO. emissions from each of five vehicle
-configurations tested by EPA and GARB (see Table 9) were
significantly reduced with CNG,, with three of the vehicles
emitting near .zero CO.[1,8,9] These results confirm both
.theoretical ..expectations (better mixing of gaseous fuel, lean
.operation, lack .of fuel enrichment for cold-starting or full
power, .and better efficiency) and data from test programs
involving pre-1981 vehicles.
Another tendency observed in the EPA test program,
however, is the CO emission reductions with CNG are not an
inherent property of the fuel. [3] As with any fuel, vehicle
calibration, and how the vehicle maintains the calibration, are
important determinants of CNG vehicle emissions. While all
three dual-fuel vehicles tested by EPA ultimately gave very low
CO values, two of the three vehicles yielded much higher CO
levels when originally tested by EPA in the as-received
condition, and the third (a 1984 Delta 88) exceeded the CO
emission standard with gasoline. The Crown Victoria vehicle
supplier indicated that this car was originally calibrated
slightly rich of stoichiometric to achieve low NOx emissions,
and had to be recalibrated in order to bring the CO levels
down. CO emissions from the Crown Victoria before and after
recalibration were 4.3 .and 0.4 gpm, respectively. NOx
emissions on the Crown Victoria were 0.6 and 0.9 gpm,
.respectively. The Celebrity CO emissions were 2.6 and 0.0 gpm
before and after recalibration, respectively. The
corresponding NOx emissions were 1.6 and 1.2 gpm, indicating
that a different calibration problem other than air/fuel ratio
(e.g. spark timing) could have existed with this vehicle.
While the Celebrity exhibited some driveability problems in the
high CO mode, the Crown Victoria did not, raising the
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-29-
Table 9
Exhaust Emissions from1CNG/Gasoline Dual-Fuel Cars
(grams per mile over the EPA Federal Test Procedure)[1]
Site
EPA
EPA
EPA
CARS
CARS
Test Vehicle
CNG Fuel Sys .
1984 Delta 88
Total Fuels
1987 Crown Vic.
Wisconsin Gas
1987 Celebrity
Dual Fuel Sys.
1983 Ford LTD
Pacific Light
1985 GMC Pickup
Fuel
Gasoline
CNG
Gasoline
CNG
Gasoline
CNG
Gasoline
CNG
Gasoline
CNG
NMHC2
0
0
0
0
0
0
0
0
0
0.
.30
.25
.27
.36
.20
.16
.36
.35
.26
.05
CO
9.
1.
1.
0.
1.
0.
3.
0.
7.
0.
8
7
4
5
3
1
3
1
0
2
NOx
0.
1.
1.
0.
0.
1.
0.
0.
0.
1.
40
18
07
93
60
19
56
47
70
06
Eff.l* Eff.24 Accel4
-15.%
- 7%
-15%
-13%
-15%
- 8%
NA
NA
NA
NA
Base
+ 10%
Base
-1-4%
Base
+8%
Base
-20%
Base
-20%
Base
-30%
Base
-27%
Base
-31%
NA
NA
NA
NA
[1] Taken from references 1 and 3
[2] Non-methane HC was calculated assuming that methane was egxial
to 25 percent of gasoline HC emissions and 90 percent of CNG HC
emissions for EPA test vehicles.
[3] Post-conversion energy efficiency over the FTP relative to EPA
certification fuel economy data for the specific gasoline
vehicle model.
[4] These final two columns use the post-conversion gasoline , fuel
mode as a baseline for comparison with the CNG fuel mode
(negative numbers mean lower efficiency or less acceleration
.with CNG).
-------
-30-
possibility that CO emission increases could go undetected by
the vehicle operator in the field. Also, the Crown Victoria
was the only vehicle .with, lower NOx emissions with CNG than
with gasoline, but the gasoline NOx levels exceeded the 1.0
g/mile NOx standard. !
Another clear emissions trend indicated in Table 9 is that
NOx emissions can be .increased on CNG relative to gasoline.[3]
While two of the vehicles showed slight improvements with NOx
on CNG, three of the vehicles suffered increases in NOx. With
two of the three dual-fuel vehicles tested by EPA, the
increases were large enough to caiuse the vehicles to exceed the
1.0 gpm NOx passenger car standard on CNG. NOx emissions from
CNG vehicles are a concern for two reasons: the desirability
of burning CNG fuel at lean air-to-fuel ratios; and because
advanced spark timing is often used to compensate for methane's
lower flame speed and thereby improve performance. Both
conditions in principle will tend to c«mse higher NOx
emissions. CNG Fuel Systems provided data to EPA that
• suggested i that NOx emissions would be increased by 55 percent
on the Delta 88 if timing were advanced by a typical margin.[10]
. :These recent 'EPA -test "data :suggest the need for further
work :.in .this area. The .data .show that the promise for NOx
-levels, comparable .to rthose .from gasoline-fueled cars is there,
but clearly more work is needed. Clearly, the dual-fuel
vehicles have the potential to provide very large CO emission
reductions when operated on CNG. The specific magnitude of CO
emission reductions that are achievable will depend on the
ability of in-use vehicles to maintain low CO calibrations as
well as the need for any design tradeoffs to provide acceptable
NOx levels.
Dedicated CNG Vehicles
CNG is such a different fuel than gasoline, that there are
many reasons to expect that the optimum engine for CNG will be
much different than today's CNG dual-fuel ssngines. Such an
engine would likely provide greater emission reductions and
better performance and efficiency than are available from
;dual-fuel engines.
For improved efficiency and lower CO emissions, the
optimum CNG engine should be a high compression, lean burn
engine. CNG optimization may be more challenging in this
regard than methanol because of its relatively higher flame
temperature and slower flame speed. The key to such a design
is to reap the efficiency and CO benefits of a high
compressions lean burn engine while maintaining NOx emissions
within acceptable levels.[3]
-------
-31-
The only reported attempt to date to design, build, and
evaluate a vehicle which might be an example of an optimized
dedicated CNG vehicle was- .undertaken by Ford Motor Company in
1983. Ford built and leased 27 dedicated CNG Ranger pickup
trucks in cooperation with the American Gas Association and
member utilities. These trucks were designed to meet the 1984
LDT standards of 0.8 g/mile HC, 10.0 g/mile CO, and 2.3 g/mile
NOx. No dedicated CNG vehicles have ever been designed and
built to meet the current LDV standards of 0.41 g/mile HC, 3.4
g/mile CO, and 1.0 g/mile NOx or more stringent standards such
as are being considered for future vehicles.
The 2.3-liter gasoline engine used in the 1984 Ford Ranger
was modified to improve it for CNG utilization including higher
compression ratio (12.8:1 instead of 9.0:1) and advanced
timing. The original design objective was to match the power
output and efficiency of the gasoline Ranger. The final engine
met these goals, though with reduced vehicle range.til]
Emissions data from three low-mileage Rangers, two fueled with
CNG
-------
Table 10
Exhaust Emissions from Low-Mileage CNG and
Gasoline Ford Ranger [1]
(grams per mile over the EPA Federal test procedure)
Test Site
Ford-1984
Ford-1984
EPA-1988
„ Fuel
Gasoline
CNG
CNG
NMHC
0.20
0.14
0.14[2]
CO
3.2
0.03
0.04
NOx
1.1
1.9
2.0
[1] Taken from references 1 and 3.
[2] Non-methane HC was calculated assuming that methane was 90
percent.of CNG-HC .emissions.
-------
-33-
emissions is subject to uncertainty. Nevertheless, large
reductions (e.g. 80 percent) in the overall NMHC emissions from
dedicated CNG-fueled vehicles .appear possible since evaporative
emissions and running loss emissions are projected to be zero.
With careful development we also project that refueling losses
of NMHC can also be eliminated. Balancing the large CO and
NMHC reductions possible with a dedicated CNG-fueled vehicle
with the degree of NOx control necessary will be the most
difficult technical area and is clearly one in which more
research, development, and demonstration is necessary.
Air Toxics and Global Warming
Air toxic emissions from cars are a function of the fuel
type, engine type, and emission control system. For vehicles
fueled only with natural gas, several advantages are
immediately apparent. CNG contains no benzene, so benzene
emissions from refueling running losses and running losses .can
be assumed to be zero. Exhaust emissions of benzene are very
low. Similarly, 'the emissions of gasoline vapors are zero and
the emissions of polycyclic organic emissions are also very
low. When, considering .the toxic .emission .risk from CNG-fueled
vehicles, only -exhaust benzene, exhaust 1,3-butadiene, and
direct and .indirect formaldehyde emissions are . of real
.concern. When, -the .emissions and potency are considered
together, the exhaust emissions from CNG-fueled cars can be
more than 90 percent lower in toxic impact compared to
gasoline-fueled vehicles.[13]
For global warming concerns CNG has an apparent advantage
over gasoline due to its hydrogen-to-carbon ratio. However,
carbon dioxide is not the only pollutant of concern when global
warming is considered. For this analysis, we are assuming that
CFC emissions from motor vehicles will be controlled eventually
via material substitution. With this assumption, the only
pollutants of interest are COa, methane (CH4), and nitrous
oxide (NjO). This analysis is incomplete in that it does not
include the global warming impact of fuel production,
processing, transport and distribution.
To account for the global warming effects of pollutants
other than CO*, emission of these other pollutants can be
converted into CO* eguivalents by considering their .relative
global warming reactivity. Relative warming estimates for
methane (CH4) and nitrous oxide (N*0) appear in reference
14. On a mass basis these were determined to be approximately
50 for CH« and 230 for NaO.
-------
-34-
Another evaluation of the global warming impact of CH4
and NjO is contained in reference 15. Converting the
molecule values from Table 2 of that report, values from 16 to
116 can be associated, with CH4 and 286 to 449 can be
associated with N2O for relative global warming impact.
The estimates of the global warming impact of CH« and
N2O relative to CO* show a large range. It should be noted
that the value of the relative impact is an active area of
research and that assigning a, single value to the relative
impact of CH4 and N2O might be controversial at this time.
More work is clearly needed in this area to refine these
relative impact factors.
The overall global warming index, GWI, will be a
combination of the CO*, CH4 ,• and N2O emissions from the
vehicle and the relative global warming impact factors assigned
to CH« and N2O. In general:
GWI * CO* -I- a CH4 * b NzO - GWI
-------
-35-
Table 11
Calculated Global Warming Index,
Gasoline and CNG-Fueled Vehicles
Vehicle
Delta 88
Delta 88
Crown Victoria
Crown Victoria
Celebrity
Celebrity
Fuel
Gasoline
CNG
Gasoline
CNG
Gasoline
CNG
GWI for
CQ2 CH4
(q/mi) (q/mi)
632 0.145
464 2.456
582 0.103
429 3.164
435 0.024
354 1.478
N20*
(q/mi) GWI
0.015
0.015
0.015
0.015
0.015
0.015
(65,300)
646
628
593
639
441
455
Three Values of CH4
Relative Impact
.Vehicle
Delta 88
Delta 88
Crown Victoria
Crown Victoria
Celebrity
Celebrity
Fuel
Gasoline
CNG
Gasoline
GNG
Gasoline
CNG
GWI
(16,300) (
639
508
588
484
440
382
GWI
65,300)
646
628
593
639
441
455
GWI
(116,300)
653
753
598
801
442
530
NZO was not measured in this test program; the same
nominal value is used for all entries.
-------
-36-
In reference 18, a summary of NZO values is provided.
The values in Table 1 of that paper range from 4.8 to 101 mg/mi
NaO, with an average .value of 52 mg/mi. Using 52 mg/mi in
the calculations in the above table (instead of the 15 mg/mi
used) would increase the GWI (35,300) values by approximately
11 units, but it would not change the rankingT Given the
spread in the values summarized in reference 18, it would
appear, however, that measured N2O data is a desirable part
of any estimates of global warming impact, and measurements of
CO2, CH4, and NZO from a variety of engines and fuels
would be the minimum needed before a definitive ranking could
be attempted. Clearly more work is needed in this area.
-------
-37-
4
. ENVIRONMENTAL IMPACTS OF CNG USE
References
1. "Emissions, Fuel Economy, and Performance of
Light-Duty CNG and Dual-Fuel Vehicles," Bruetsch, Robert I.,
U.S. EPA, EPA/AA/CTAB/88-05, June 1988.
2. "Definition of a Low-Emission Motor Vehicle In
Compliance With the Mandates of Health and Safety Code Section
39037.05 (Assembly Bill 234, Leonard, 1987),," California Air
Resources Board, Mobile Source Division, El Monte, CA, May 19,
1989.
3. "Motor Vehicle Emission Characteristics and Air
Quality impacts of Methanol and Compressed Natural Gas," Alson,
Jeffrey A., Jpnathon M. Adler and Thomas M. Baines, U.S. EPA,
Office of Mobile Sources, January 1989.
.4. "Emission ..Factors .for SAI..Runs .With CNG and Neat
Methanol," U.S. EPA, Memorandum from Phil Lorang, Office of
Mobile Sources,.to .Gene .Durham, Air Economics Branch, September
19, .1989.
5. "Guidance on Estimating Motor Vehicle Emission
Reductions From the Use of Alternative Fuels and Fuel Blends,"
U.S. EPA, EPA/AA/TSS/PA/87-04, January 29, 1988.
6. "The Emission Characteristics of Methanol and
Compressed Natural Gas in Light Vehicles," Alson, Jeffrey A.,
U.S. EPA,'APCA Paper 88-99.3, June 1988.
7. "Effects" of Emission Standards On Methanol
Vehicle-Related Ozone, Formaldehyde, and Methanol Exposure,"
Gold, Michael D. and Charles E. Moulis, U.S. EPA, APCA Paper
88-41.4, June 1988.
8. "Evaluation of Dual Fuel Systems, Inc.'s Compressed
Natural Gas/Gasoline Dual Fuel Conversion System," California
Air Resources. Board, 1983.
9. Memorandum from Rod Summerfield, Chief, Standards
Development and Support Branch to K. D. Drachand, Chief, Mobile
Source Division, California Air Resources Board, October 29,
1986.
10. Letter from Stephen Carter, vice President, CNG Fuel
Systems to Richard Polich, Consumers Power, December 3, 1987.
-------
11. "The Development of Ford's Natural Gas Powered
Ranger," Adams, T., Ford Motor Company, SAE Paper 852277, March
1985.
12. Concerns for CNG Conversion Emissions Durability,
letter from James M. Lents, South Coast Air Quality Management
District, El Monte, CA, to the Honorable Henry Waxman,
Congressman, House of Representatives, Washington, DC, October
2, 1989.
13. Testimony of William G. Rosenberg, Assistant
Administrator of Air and Radiation U.S. Environmental
Protection Agency, before the Committee on Energy and Natural
Resources, United States Senate, October 17, 1989, page 4 and
Chart 6.
14. Future Atmospheric Carbon Dioxide Scenarios and
Limitation Strategies, J. " A. Edmonds, et al., Noyes
Publications, Park Ridge N.J., 1986.
15. "Comparing- the Impacts of Different Transportation
•Fuels On the Greenhouse Effect," a. Consultant Report to the
.California Energy Commission, by Acurex Corporation, Report
E500-89-001, April 1989.
16. Compilation of Air Pollutant Emission Factors Volume
II; Mobile Sources, AP-42, Fourth Edition, September 1985.
17. Regulated and Unregulated Exhaust Emissions From
Malfunctioning Three-Way Catalyst Gasoline Automobiles,
EPA-460/3-80-004, January 1980.
18. "Nitrous Oxide N20 In Engines Exhaust Gases - A
First Appraisal of Catalyst Impact," SAE Paper 890492, Prigent,
M. and G. DeSoete.
-------
-39-
OTHER ISSUES
Safety
Safety-related issues affect: three areas of. utilization of
compressed natural gas: Theses broad areas; encompass: l)
transport to and storage at the refueling site, 2) vehicle
refueling, and 3) onboard vehicle safety. These areas have
been addressed by guidelines, requirements, and standards set
by the following: U.S. Department of Transportation (DOT),
National Fire Protection (NFPA), Compressed Gas Association
(CGA), and the American Gas Association (AGA).
DOT is responsible for the safe transport of hazardous
materials. Natural gas falls into this designation. DOT
guidance establishes both maximum operating and burst pressure
'for cylinders; the type of testing [1] required before use and
periodically during the cylinder lifetime, and allowable
contaminants in . the . compressed natural gas to prevent
corrosion. It should be noted that the DOT guidance applies to
.commercial shipments of industrial gases, not: to vehicle fuel
containers.
The CGA has published a pamphlet S-l.l which is used by
DOT in its regulation of pressure safety devices. These
pressure safety devices are used to reduce cylinder pressures
in the event of overfilling or exposure to fire. They keep the
cylinders from bursting or exploding. These devices can be a
rupture disc, a fusible plug, or a combination of both.
The NFPA established NFPA52 which is a standard for "CNG
vehicle fuel . systems." This standard established design
criteria for gas quality, cylinder .location, venting, and
support systems in the vehicle. It defines, for example, the
level of force that the supporting structure mus.t be able to
withstand.
The AGA in their .publication No. 1-85, "AGA Requirements
for Natural .Gas'Vehicle (CNG) Conversion Kits," established
guidelines for the safe design and construction of CNG
vehicles.[2]
Natural gas vehicles have seen significant utilization in
fleet-type applications in various parts of the United States
and for that matter in various parts of the world. This
utilization has lead to a body of experience with vehicle
safety that would appear to give CNG a reasonably good safety
reputation. It should be considered though that much of the
experience in the U.S. is fleet-type with trained operators and
refueling attendants. Further studies should be pursued to
quantify the affects,, if any,, of moving from fleet type
applications to utilization by the general public.
-------
-40-
Reduced Maintenance
1
The potential of reducing consumer operational cost is
generally raised in discussions concerning the use of •
.compressed natural gas. The potential exists but more work is |
needed to define the benefits accurately; better data are
needed. -
Such data could be generated through a series of engine *
dynamometer and vehicle fleet tests each of which could be
specifically designed to obtain the necessary comparative |
data. For example, industry standard tests of oils could be a
used to compare gasoline only, CNG only, and alternating
gasoline and CNG use on properties important for oil quality,
lubrication, and wear. Additional engine dynamometer
evaluations of spark plug life and valvetrain wear could also
be conducted. For vehicle tests, fleets of matched
gasoline-only, CNG-only, and alternating CNG and f
gasoline-fueled vehicles could be run on t€>st tracks or in *
fleet use such as in taxicabs to generate comparative measured
data on fuel consumption, performance, range, oil contaminants, \
..driveability, and .engine wear. ^
Performance and Fuel Economy i
5
The properties of fuel economy and performance are closely '
related, especially when comparing vehicle operation on two
different fuels. It is known that fuel economy is inversely •
proportional to increases in vehicle weight.[3] Vehicle <
performance, in terms of the time it takes to accelerate from
zero to 60 miles per hour, is greatly influenced by the maximum <
horsepower of the engine. When comparing the fuel economy of ,
two distinct fuels, it is important to take into account any
differences in the power output, inertia weight, and
performance of the vehicle(s) used to make this comparison.
The parameters of fuel economy, performance, horsepower, and
inertia weight are related by the following two expressions
which were developed by regression of data measured from 1978
through 1987 model year gasoline-fueled vehicles.[4]
-------
-41-
T = 0.82(HP/1W) -°-'a (1)
%AMPG = (0.454)%AT (2)
Where:
T =• 0 to 60 MPH performance time (seconds)
HP = Horsepower
IW =» Inertia weiglat (Ibs)
%AMPG «• Percent change in fuel economy (%)
%AT a« Percent change in 0 to 60 MPH performance
time (%)
.Dual-fuel (CNG/gasoline) vehicles are;slower and less fuel
.efficient when operating on CNG than when using, gasoline due to
.CNG.'s poorer volumetric efficiency, and the additional weight
;of -multiple CNG fuel .tanks. Since CNG is a gaseous fuel, it
displaces air in the combustion chamber, which accounts for
lower volumetric efficiency of engines that use CNG. Most
dual-fuel vehicles carry at least two CNG cylinder tanks which
are usually made of fiber-reinforced steel, though they can be
made of aluminum and other composites. The added weight of
these tanks plus their fuel capacity must be supported by
additional vehicle structure, and secured to the vehicle by
brackets. This added structural weight may average roughly 70
percent of the weight of the CNG cylinders.[5]
In the EPA study of light-duty dedicated and dual-fuel CNG
vehicles, gasoline-equivalent city fuel economy was improved on
CNG relative to gasoline operation on 5 of 6 vehicle
configurations tested by amount!? ranging from 1 to 12 percent
(average of 5 percent).[6] Only the Chevrolet Celebrity in its
first calibration .test sequence showed a 2 percent decrease in
gasoline equivalent.fuel economy. These results, though on an
energy equivalent basis, are misleading since they do not
account for the added weight of the extra fuel systems, or the
degraded performance of these vehicles when using CNG.
The dynamometer 5 to 60 MPH acceleration performance
measured on the EPA tested dual-fuel vehicles was significantly
degraded by an average of about 29 percent on CNG relative to
gasoline operation at a constant vehicle weight. Using the
equation for fuel economy as a function of vehicle performance
-------
-42-
(equation 2), a 29 percent increase in performance time is
equivalent to a 13 percent decrease in fuel economy with CNG.
.Adjusting the average gasoline-equivalent city fuel economy
measured on the EPA test fleet for performance yields an
average 8 percent decrease in fuel economy for CNG relative to
gasoline operation at a constant vehicle weight. Equation 2
predicts the percent decrease in fuel economy is 45.4 percent
of the percent increase, in performance time since it is based
on gasoline vehicle data. Regression of. the EPA CNG
performance data show this decrease in fuel economy is close to
20 percent of the percent increase in performance time, or a 6
percent decrease in gasoline-equivalent , fuel economy for CNG,
i.e., 1 percent below gasoline MPG).
In addition to the fuel economy penalty for performance,
the CNG fuel economy numbers must be adjusted to account for
the higher inertia weight of these vehicles relative to their
gasoline-only counterparts. The most commonly used gas
cylinders for passenger car CNG fuel are either 14 inches x 40
•inches--fiber-reinforced steel or 13 inches x 42 inches aluminum
composite fuel tanks.[7] Both tanks have a service pressure of
3000 . psi. .Although the ...aluminum tanks ..are lighter weight
.(roughly .110 -Ibs. 'empty) and must meet the same safety
..standards, .the . more .commonly used production .units are the
•steel ..tanks . at an empty weight of 176 Ibs. apiece. The
dual-fuel vehicles tested by EPA were equipped with two such
tanks each, and the. dedicated CNG light-duty truck had three
CNG fuel tanks onboard.
As mentioned above, the addition of the CNG fuel tanks for
both dual-fuel and dedicated CNG vehicles propagates the need
for additional vehicle structural weight to support the weight
of the CNG fuel tanks when filled to capacity. The weight of
the CNG fuel used in the EPA test vehicles was 0.047 Ib/ft1
and the capacity of the steel containers is 670 ft3 of CNG.
Each tank of CNG has a capacity of slightly more than 30 pounds
of CNG fuel. Therefore, the additional weight: of just the full
steel fuel tanks of the EPA test vehicles was roughly 415 Ibs
for dual-fuel vehicles and 622 Ibs. for the dedicated CNG
light-duty truck. The average weight compounding factors, to
account for-.the added vehicle structural support, are 1.7 for
dual-fuel vehicles and.1.25 for dedicated CNG vehicles.[5,8,9]
Therefore, the total increase in weight of these vehicles over
the weight of their gasoline-only counterparts is estimated to
be 700 Ibs. for dual-fuel vehicles (480 Ibs with aluminum) and
775 Ibs for the dedicated. CNG light-duty truck (530 Ibs with
aluminum).
Knowing the baseline gasoline vehicle inertia weights and
the measured performance for both CNG and gasoline operation.
Equation 1 was used to determine the maximum horsepower of the
-------
-43-
vehicle engines on both fuels (see Table 12). The CNG engine
horsepower was then inserted back into Equation 1 using the
higher inertia weights as determined above to estimate the
effect on performance of the additional weight. The average
effect of additional weight was an increase in 0 to 60 MPH
times of 15 percent. Using Equation 2, these degraded
performance values translate to an additional fuel economy
penalty of roughly 7 percent (3 percent with the EPA regressed
CNG data). It should be noted that this increased weight also
reduces the fuel economy of the dual-fuel vehicle when
operating on gasoline.
Using equation 1 again with the adjusted CNG inertia
weights and performance" values, yields adjusted CNG engine
horsepower values which are on the average over 30 percent
lower than the engine horsepower obtained, when operating the
vehicles on gasoline.
Vehicle Range
The Alternative Motor Fuels Act of 1988, (Public Law
100-494, ..October 14, . 1988)., . Section 6(a) requires that the
Secretary of.^Transportation establish a minimum driving range
of no less than 200 miles for dual energy automobiles when
operating .on .alcohol and an unspecified minimum range for
natural gas. This minimum range requirement pertains only to
passenger cars and does not apply to light trucks. [10] At
present, the minimum range for natural gas dual energy
automobiles (those vehicles which operate on natural gas and
either gasoline or diesel fuel) being given the most
consideration is 100 miles.[11]
Vehicle driving range was not determined during the EPA
light-duty CNG and dual-fuel vehicle test program.[12]
Vehicles were simply, run on CNG over the FTP and HFET cycles
(plus 10 accelerations per test sequence) until they ran out of
CNG. Vehicles were then refueled if further CNG tests were
required, and the vehicle mileage? was recorded at the beginning
of each test cycle. The exact: mileage when vehicles ran out of
CNG was not determinable and some vehicles were returned to
vehicle suppliers., after .testing with an undetermined amount of
CNG fuel still in :their .tanks. From the test dates, number of
vehicle preps run, and'-mileage records, the following observed
minimum' driving range estimates were determined from the EPA
test program data.
Two values of range are listed below. The first value is
the observed range as discussed above. The second value is a
calculated range based on the Ml?G values from line 10 in Table
12 and an assumed effective tank capacity equivalant to 10
gallons of gasoline. This is about what one would project from
two CNG steel fuel tanks (each with the storage equivalent to
5.5 gallons of gasoline) if one would allow for a one-gallon
reserve when computing a vehicle's range.
-------
-44-
Table 12
Fuel Economy Adjustment for the Additional Weight
of CNG/Gasoline Dual-Fuel Vehicles
[1] Dual-Fuel
Vehicle
Crown
Crown
[2]
[3] Tct
[4]
[5] Tg
[6] HPHo
[7] GEFEi
C8] IW*
[9] TCa
[10] GEFE2
[11] (10)-(7)
(7)
Delta 88 Victorial Victoria2 Celebrityl Celebritv2
3250
[12]
[13] (12)/(6)
4000
14.8
117.4
11.4
161.5
15.3
4700
16.9
14.3
-6.4
143.9
1.12
4250
13.3
142.1
10.8
183.2
16.0
4950
15.1
15.0
6.1
143.0
1.28
4250
14.2
131.2
10.8
183.2
16.7
4950
16.1
15.7
-6.1
135.2
1.36
3250
13.7
104.8
10.6
143.4
19.8
3950
16.1
18.2
-7.9
104.7
1.37
14.0
102/1
10.5
145.0
21.9
3950
16.4
20.2
-7.9
102.3
1.42
[1] Dual-fuel vehicles tested by EPA.
[2] Inertia weight of the gasoline certification counterparts to
the dual-fuel vehicles in (1) (Ibs).
[3] Measured dynamometer 5 to 60 MPH performance of the dual-fuel
vehicles using CNG (seconds).
[4] Calculated CNG engine horsepower using (2) and (3) in
Equation 1 (HP).
[5] Measured dynamometer 5 to 60 MPH performance of the dual-fuel
. vehicles using gasoline (HO) (seconds).
[6] Calculated gasoline engine horsepower using (2) and (5) in
Equation 1 (HP).
[7] Measured gasoline equivalent fuel.economy of dual-fuel
vehicles using CNG (MPG).
[8] Adjusted inertia weight of dual-fuel vehicles to account for
added fuel tanks, support, etc. = (2) + 700 Ibs (Ibs).
[9] Calculated 0 to 60 MPH performance of the dual-fuel vehicles
using CNG. Uses (4) and (8) in Equation 1 (seconds).
[10] Calculated gasoline equivalent fuel economy of dual-fuel
vehicles using CNG showing effect of slower performance (MPG).
[11] Percent change in CNG gasoline equivalent fuel economy due to
slower performance (percent).
[12] Calculated CNG engine horsepower using (8) and (9) in
Equation 1 (HP).
[13] Ratio of calculated gasoline and adjusted CNG
engine horsepowers.
-------
-45-
Vehicle Range for Dual Fuel Vehicles
Calculated Range
Assuming 2 CNG
Observed Range During Cylinders and MPG
During Laboratory from line 10 Table 12
Vehicle Testing (miles) (1 gallon reserve)
Oldsmobile Delta 88 135 143
Ford Grown Victoria 143 154
Chevrolet Celebrity 85 163
The CNG driving ranges listed above reflect the maximum
number of continuous CNG-fueled miles obtainesd during the EPA
test program before a vehicle was either returned to its
supplier (Delta 88, Ranger) or refueled for further testing
(Crown Victoria, Celebrity). Because mileages were not
recorded at the start of vehicle preps, or during performance
testing, it is difficult to determine the maximum driving
ranges of these vehicles.
With the . exception, of . the ^Celebrity vehicle, all of the
other light vehicles :.tested by EPA would meet a minimum
.requirement of 100 .miles .now under consideration for dual
:energy natural gas .vehicles.
The National Highway Traffic Safety Administration (NHTSA)
is in the process of determining the minimum driving range for
dual-fuel passenger automobiles as required by the Alternative
Motor Fuels Act of 1988.[13] For reference, NHTSA conducted a
quantitative study of the driving range of conventional
gasoline-fueled passenger cars using the 25 top-selling cars of
model year 1988 (63 percent of total 1988 car sales). Their
results show an average minimum driving range is 405 miles,
with an average lowest combined EPA fuel economy rating of 25.4
MPG, and average fuel tank capacity of 16.1 gallons.
The Alternative Motor Fuels Act of 1988 also defines the
gasoline equivalency of natural gas in natural gas fuel tanks.
Section 6(c) states that "...100 cubic feet of natural gas
shall be considered to contain 0.823 gallons equivalent of
(gasoline) fuel...."[10] Since the steel cylinders normally
have a capacity of 670 ft" CNG (575 ftj CNG for aluminum),
the CNG fuel tanks when, filled to capacity contain 5.5
gasoline-equivalent gallons (4.7 gallons for aluminum).[14]
Assuming the ratio of combined EPA CNG gasoline-equivalent
fuel economy to combined EPA gasoline fuel economy observed in
the EPA test .program (= 1.04 without weight and performance
penalties) can be applied to the NHTSA derived 25.4 MPG average
1988 model year gasoline vehicle fuel economy, the combined CNG
gasoline-equivalent fuel economy would be 26.4 MPG. Applying
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the CNG fuel tank capacity determined above, yields a CNG fuel
tank range of 145 miles (124 miles for aluminum). This would
•mean a vehicle need only be .equipped with one large CNG fuel
.cylinder and one smaller CNG fuel cylinder to meet a minimum
driving range of 200 miles. In other words, multiple CNG tank
dual-fuel vehicles such as were tested by EPA, could possibly
meet a 200-mile range, without additional fuel tanks or other
added vehicle weight using the NHTSA methodology..
In a separate study, the Department of Energy has
estimated the range of CNG vehicles to be between 80 and 200
miles.[15]
Of course, trying to match, the average gasoline vehicle
driving range would be difficult for a natural gas dual energy
vehicle, and would require at least three (possibly four) CNG
fuel cylinders onboard. The doubling of fuel capacity would
constrain cargo space considerably, and would require a vehicle
inertia weight over 35 percent heavier than that of a
-comparable gasoline vehicle. Performance would be further
degraded and the fuel economy penalty would double that seen
.with conventional two-tank CNG dual-fuel vehicles.
The. setting of a minimum driving range for dual-energy
automobiles, must .balance the needs of the consumer with the
technical and economic considerations that are faced by the
manufacturers. A. low minimum driving range requirement might
encourage the production of dual-fueled cars, but lead to
dual-fueled cars being designed with such a low alternative
fuel driving range that consumers do not buy them or, even if
they buy them, infrequently operate them on the alternative
fuel. Conversely, an excessively stringent minimum driving
range requirement might discourage the production of
dual-fueled cars and unnecessarily compromise other vehicle
attributes and aspects of performance. Manufacturers would be
discouraged by overly stringent minimum range because a vehicle
which does not meet the minimum driving range for its type is
by definition excluded from the definition of dual-energy or
natural gas dual energy vehicle, and is thus unlikely to be
built since the manufacturer would not receive any of the
benefits or incentives .provided by the Act.
From the .viewpoint of the consumer, the necessary driving
range is dictated by considerations unrelated to AMFA, such as
the convenience of a range that permits a typical workweek
travel distance, or a daily travel distance for a fleet car.
If the majority of consumers would use a dual-energy vehicle in
an urban area with more refueling stations or in a fleet
application with a central refueling station, a large driving
range may be less critical.
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Performance and range for CNG-fueled vehicles are
important issues. Surveys of CNG-fueled vehicle operators have
indicated that the major problems reported were fueling
inconvenience, power and performance, and limited range.
Assuming the infrastructure can provide convenient. fast
fueling, only performance and range remain as major concerns:
The more competitive CNG-fueled vehicles are to gasoline-fueled
vehicles, the easier the transition to increased use of CNG
will be.
Problems with performance and range for today's CNG-fueled
cars are primarily attributable to dual-fuel CNG/gasoline
applications. If dedicated, optimized CNG vehicles are
considered, then there is no reason why such a dedicated,
optimized vehicle could not be competitive with today's
gasoline-fueled cars. Integration of the fuel tanks and
supporting structure into the initial vehicle design instead of
just adding the fuel tanks to an existing vehicle is bound to
have overall vehicle weight benefits. Current research into
advanced adsorbent storage mechanisms for natural gas [16]
could also provide substantial benefits in CNG-fueled vehicle
range.
One drawback .to dedicated CNG light-duty vehicles is the
availability of fuel since they require 100 percent access to
CNG. To the extent that dedicated CNG vehicles penetrate the
market, the volumes of fuel necessitated by demand could
increase, which could also increase the costs of infrastructure
and fuel delivery.
It appears that the optimized dedicated vehicle is the
most attractive use of CNG in the transportation sector. It
also appears that more efforts are needed to determine the
optimum engine configuration, fuel metering system, and fuel
storage -technology for this promising fuel. For example, the
work being sponsored by the Gas Research Institute in advanced
fuel metering, improved storage media, lean burn and fast burn
engines, prechamber engines, and other technological advances
is the type of work that is necessary to extract the best from
natural gas as a vehicle fuel.
The potential for increased efficiency and power for a
dedicated CNG application is great. Harnessing this potential
is of more than just academic interest since improved engine
power and increased engine efficiency will have positive
synergistic effects on the two areas shown to require
improvement based on dual-fuel results, vehicle performance.
and range. Improved engine power will allow the engine to
become smaller and lighter and therefore, with the weight
compounding effect working in reverse, make the vehicle
lighter. Increasing engine efficiency will allow a smaller and
lighter gas tank for the same range or more miles of range with
the same fuel tank. The synergism appears via the fact that
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the lighter weight vehicle, which can be projected from the
increased power potential will provide better fuel economy,
.thus adding to the . range, and the smaller and lighter fuel
tanks possible with increased efficiency will weigh less, thus
resulting in a lighter weight, better performing vehicle.
Fortunately, when freed from the necessity of having to
run on gasoline a dedicated CNG system can take, full advantage
of the attractive characteristics of natural gas. From the
standpoint of improving power and increasing efficiency, the
most important benefit of natural gas is its very high octane
number, well over 100. Values of 120 for both research and
motor octane numbers are mentioned in reference 17. Increasing
the engine's compression ratio is one method of improving power
and increasing efficiency. In reference 17, a compression
ratio of 15.5:1 was thought to be optimum for the single
combustion chamber geometry considered. This resulted in 15
and 22 percent increases in power and efficiency, respectively,
compared to the base case 8.4:1 compression ratio.
Boosting the engine can also improve power and, indirectly
via .lower idle fuel .consumption and lighter engine weight,
improve vehicle .fuel .economy. Boosting the engine can be
.accomplished via.supercharging or turbocharging.
Rapid expansion of a compressed gas provides a cooling
effect. The use of rapidly expanded natural gas to cool the
intake charge either via a separate heat exchanger or by
injecting it into the intake system could increase charge
density and offset the volumetric efficiency loss due to use of
a gaseous fuel in a premixed charge engine.
Future engine design for natural gas use will also
incorporate optimized combustion chamber designs. Designs
which provide for a stable reliable ignition of the natural
gas/air mixture, while also providing the degree of turbulence
needed to enhance flame propagation for efficient heat release
patterns, need to be identified.
Considering increased compression ratio, boost, and
combustion chamber design, there may be a combination of
approaches, that yields the best performance and efficiency
relationship. .This would appear to be a.fruitful and needed
research area.
In addition to attractive octane characteristics, natural
gas also can be operated with dilute air/fuel ratios foe
improved efficiency. Dilution of the intake charge can be
achieved by running a lean air/fuel mixture or by using EGR, or
a combination of both. The optimum strategy and calibration of
charge dilution for natural gas use of optimized, dedicated
engines for light vehicles has yet to be determined. As might
be expected, there is an interaction between the degree of
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charge dilution and the other engine parameters such as
compression ratio, boost, and combustion chamber design
discussed previously. Unconventional approaches toward
extracting the most from natural, gas can also be considered.
Storage of the gas at high pressure yields several different
fuel metering possibilities all the way from conventional
carburetion to direct cylinder injection of: the fuel. In
reference 18, eighteen different applications of .possible
natural gas fuel metering systems! .are listed. While not all of
the systems may be practical for light-duty vehicle use, the
number of unexplored or partially explored fuel metering
systems for natural gas indicates that the optimum system for
performance, fuel economy, and emissions has yet to be
determined.
Impact On Home Heating
There is some concern that the added usage of compressed
natural gas as a motor vehicle fuel could drive up the cost of
home heating. In accordance with the provisions of the
Alternate Motor Fuels Act of 1988, the Department of Energy, is
studying this .issue and should be publishing a report
addressing this subject later this year.
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OTHER ISSUES
References
1. 49 CFR 173.34.
2. "Gaseous Fuel Vehicle Technology State of the Art,
Draft Report," prepared by E. A. Mueller, Inc., for U.S.
Department of Energy.
3. "Light-Duty Automotive Technology and Fuel Economy
Trends Through 1989," Heavenrich, Robert M. and J. Dillard
Murrell, U.S. EPA, Office of Mobile Sources, EPA/AA/CTAB/89-04,
May 1989.
4. "Adjusting MPG for Constant Performance," memorandum
from Karl ..H. Hellman to Charles L. Gray, Jr., U.S. EPA, Office
of Mobile Sources, Ann Arbor, MI, May 19, 1986.
5. ..General Motors' Comment on Fuel Economy Impacts on
.Onboard .Regulation," memorandum from Chester J. France to
Richard D. Wilson, U.S. EPA, Office of Mobile Sources, July 23,
1986.
6. "Emissions, Fuel Economy, and Performance of
Light-Duty CNG and Dual-Fuel Vehicles," Bruetsch, Robert I.,
U.S. EPA, EPA/AA/CTAB/88-05, Jun€> 1988.
7. "CNG Cylinder Weights," letter from Chris Bruch,
Garretson Equipment Co., Inc.", to Robert Bruetsch, U.S. EPA,
Office of Mobile Sources, Ann Arbor, MI, October 18, 1989.
8. "Weight Propagation and Equivalent Horsepower for
Alternate-Engined Cars," SAE Paper 780348, Klose, Gerhard J and
Donald W. Kurtz, California Institute of Technology, March 1978.
9. Final Regulatory Impjict Analysis; Part 58' Bumper
Standard, National . .Highway.: Traffic Safety -Administration,
Office of Program.and Rulemaking Analysis, May.1982.
10. "Alternate Motor Fuels Act of 1988," Public Law
100-494, 100th Congress, 102 STAT.2441, U.S. Congress, October
14, 11988.
11. Telephone conversation with Orron Kee, Office of
Market Incentives, NRM-21, National Highway Traffice Safety
Administration, Washington, D.C., October 23, 1989.
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