CONVERSION OF MOTOR VEHICLES TO GASEOUS
FUEL TO REDUCE AIR POLLUTION
U. S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF AIR PROGRAMS
APRIL 1972
-------
42OR72O05
ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF AIR PROGRAMS
POSITION PAPER
CONVERSION OF MOTOR VEHICLES TO GASEOUS
FUEL TO REDUCE AIR POLLUTION
1. INTRODUCTION
This position paper sets forth, in summary form, the
position of the Environmental Protection Agency on the
conversion of existing vehicles to gaseous fuels to reduce
air pollution emissions from motor vehicles. Substantial
reduction in emissions from new motor vehicles powered by
gasoline engines is expected by the mid-1970's, but the urgent
need for improvement of air quality in certain metropolitan
areas has focused attention on the possibility of conversion to
gaseous fuels of motor vehicles already in use in such metro-
politan areas. The conclusions expressed herein are based
on presently available technology and on the probable avail-
ability of gaseous fuels at motor vehicle refueling stations
during the next five years.
2. SUMMARY
The conversion of existing vehicles to gaseous motor
fuels is recommended for fleet-operated vehicles in those
metropolitan areas in which (a) logistical and economic
considerations are favorable in terms of availability of
fuel and conversion equipment, and (b) where major air
pollution problems are attributable to the use of motor
vehicles. Those vehicles likely to receive maximum usage
prior to being phased out of use should be converted first.
Except on the above terms, conversion of vehicles to gaseous
fuel is not warranted at this time for purposes of air pollution
co'ntrol.
In developing implementation plans to achieve the ambient
air quality standards, each community must consider the most
efficacious use of potentially available natural gas. If diversion
of natural gas from electric power generation were required to
supply large quantities for automobiles, increased power plant
emission of SOx could more than offset the benefits from reduced CO
emissions. Also, conversion of space heating to natural gas
could produce important reductions in SOx, NOx and particulate.
emissions, and could be significantly more effective in improving
overall air quality than conversion of fleet vehicles.
-------
2.
However, LPG is becoming available in larger quantities
as a result of removal of lead from gasoline. LPG is less
attractive than natural gas for spvce heating in urban areas
because of complexities associated with storage and transfer.
Fueling of fleet vehicles with LPG, therefore, could produce
significant reductions in CO without concern for a tradeoff
in the control of other air contaminants.
3. WHAT GASEOUS FUELS CAN BE USED FOR AUTOMOBILES?
LPG (Liquefied Petroleum Gas) has been used as an automotive
fuel for many years, usually because it provided an economic
advantage. LPG is available in limited quantities in urban
areas across the Nation. About 300,000 LPG vehicles are estimated
to be in operation at this time.
Natural gas is also used as a motor fuel and has greater
capabilities for reducing emissions than LPG. Natural gas is
used in two forms, Liquefied Natural Gas (LNG) and Compressed
Natural Gas (CNG). More than 4000 natural gas fueled vehicles
are currently being operated experimentally throughout the
country, mostly using CNG.
4. ARE ADEQUATE QUANTITIES OF GASEOUS FUELS AVAILABLE?
A. LPG
LPG is a by-product in natural gas processing and petroleum
refining. In the past there have been wide swings in LPG prices
in response to supply conditions. Under present conditions it
is anticipated that the demand for LPG will exceed available
supplies and result in higher prices during the next few years.
The use of LPG for motor vehicle pollution control could hasten
this result.
With higher prices, however, it is possible that the
petroleum refinery industry may find it profitable to increase
the yield of LPG at the expense of gasoline. LPG from the refinery
is the only gaseous fuel that has a sufficiently large potential
source of supply to permit its use in the conversion of a large
portion of the motor vehicle population. In the process of
lowering lead levels in gasoline an increased supply of LPG will
result as a byproduct as the refining process is modified.
As regards natural gas, many marketing areas in the U.S.
currently have severe shortages of this fuel. While it has been
estimated that on a national basis a substantial number of
vehicles could be converted to natural gas usage without adversely
affecting the total supply of natural gas, such estimates are of
little significance to an individual community contemplating
such conversions to abate local automotive air pollution problems.
-------
3.
Thus, in considering the feasibility of such conversions, each
community must make an assessment of its own situation as regards
the availability of the natural gas fuel.
5. ARE ADEQUATE ENGINES AND FUEL SYSTEMS AVAILABLE FOR
GASEOUS FUELS1
Engines that are fully optimized to take advantage of the
full emission reduction capability of gaseous fuels are not
currently available from the automobile manufacturers. Vehicle
manufacturers limit their warranty on vehicles converted by the
buyer to gaseous fuel operation so that they are not liable for
repair of engines which suffer malfunctions due to operation on
the gaseous fuel. Since the 1975 production models of gasoline
engines are expected to have emission levels substantially lower
than levels obtainable through conversion of existing vehicles
to gaseous fuels, it does not appear likely that engine manufac-
turers will embark upoh.Van extensive design program to optimize
existing light duty engines for gaseous fuels and to retool to
produce large numbers of such engines.
Components for conversion to gaseous fuel operation are
produced by several manufacturers. Some systems are more
successful than others in lowering emissions while maintaining
acceptable vehicle driveability. While gasoline-gaseous dual-
fuel systems greatly increase the driving range of the vehicle
and provide a reserve fuel supply for emergencies, they require
compromises for either fuel from perforraarice, fuel consumption
and emission standpoints. The degree of compromise of one fuel
over the other depends on the utilization of the fleet vehicle.
The operating ranges of vehicles fueled by LPG or LNG are com-
parable to gasoline fueled vehicles, but CNG fueled vehicles
are usually limited in range to about 70 miles, although new
tank designs have increased the range to approximately 100 miles.
Heavy duty truck engines that have been modified to avoid
durability or performance penalties whiJLe operating on gaseous
fuel are presently available from manufacturers. Such engines
have not been optimized for minimum emissions. When converting
engines without such factory modification to gaseous fuel operation,
the fleet operator must evaluate whether the duty requirements
are severe enough to require special valve protection. Some
manufacturers offer high compression pistons that help in main-
taining performance under gaseous fuel operation.
-------
4.
6. WHAT ARE THE EMISSIONS FROM AUTOMOBILES USING
CASEOUS FUELS?
It is difficult to define accurately the average reduction
in emissions that may be expected.from a group of vehicles con-
verted to LPG, LNG, or CNG. Available data indicate that with
proper conversion a significant reduction in hydrocarbons and
carbon monoxide can be expected, and that there usually is some
reduction in oxides of nitrogen levels. The extent of emission
reduction shown in the data that follows is inconsistent since
there is some variation from one installation to the next due
to configuration of the carburetion and fuel system and the
vehicle's particular engine and drive train system. A portion
of the inconsistency, however, is due to difference in adjust-
ment of carburetion, spark timing, and engine condition at the
time the test data were taken. Further refinement to the
various components of gaseous fuel systems will enable better
adjustments to be made, thereby reducing inconsistencies of
the type illustrated by the following data:
-------
5.
1972 FEDERAL TEST PROCEDURE
(LA-4 cycle, constant volume sampler)
Emissions (grams per mile)
Vehicle HC CO NOx
Converted 1968 Buick 350
Stock 1968 Buick 350
Percent Reduction
Converted 1969 Ford 351
Stock 1969 Ford 351
Percent Reduction
Converted 1968 Ford 302
Stock 1968 Ford 302
Percent Reduction
4 Converted 1969 Chrysler 318's
Stock 1969 Chrysler 318
Percent Reduction
2 Converted Rambler 343's
Stock 1969 Rambler 343
Percent Reduction
Converted 1969 Ford 429 1.3 4.0 1.9
10 Converted 1970 Ford 250's 0.69 1.8 2.6
10 Stock 1970 Ford 250's 3.70 16.0 9.4
Percent Reduction 81 8> 72
10 Converted 1970 Rebel 232's .51 3.9 3.1
10 Stock 1970 Rebel 232's 2.7 22.1 6.9
Percent Reduction 81 82 55
3.5
1.9
(84)
3.1
7.4
58
2.4
3.1
23
2.4
3.4
29
3.0
3.0
0
4.7
29.6
* 84
7.3
17.8
59
4.2
28.5
85
7.2
30.5
76
15.4
31.5
51
8.9
4.0
(123)*
8.6
5.2
(65)*
1.8
3.6
50
2.9
3.6
19
2.6
3.1
16
Type Conversion
LPG
LPG dual fuel
LPG dual fuel
LPG dual fuel
LPG dual fuel
LPG
LPG
LPG
-------
6.
1970 FEDERAL TEST PROCEDURE
(Open 7-mode cycle, Continuous Analysis)
Emissions (grams per mile)***
Vehicle
HC** CO
NOx
2 Converted 1968 Chevrolet 230's 1.1
2 Stock 1968 Chevrolet 230's 3.7
Percent Reduction 70
2 Converted 1968 Ford 250's
2 Stock 1969 Ford 250's
Percent Reduction
10 California State Cars
10 California State Cars
Same Cars on Gasoline
Percent Reduction
5 Los Angeles City Cars
Same Cars on Gasoline
Percent Reduction
0.9
2.6
65
1.5
3.1
52
1.4
2.9
52
9.5
58.2
84
7.8
25.3
69
1.5 10.5
6.7
42.9
84
5.0
31.0
84
1.4
1.2
3.2
63
3.0
3.5
14
Type Conversion
CNG dual fuel
CNG dual fuel
LPG
CNG dual fuel
CNG dual fuel
**
Figures in parentheses () reflect increases in emissions.
Although it is generally agreed that hydrocarbon emissions
from gaseous fueled vehicles are less photochemically reactive
than those from gasoline fueled vehicles, a Federal reactivity
scale has not been defined which would1 allow quantitative
correction for this factor. Therefore, all hydrocarbon values
are reported on the same mass basis as gasoline.
*** Data from Reference 2 and 3.
-------
Although these data do not represent a large population
oŁ vehicles „ they do illustrate the magnitude of emission
reductions that can be reasonably expected from various
types of gaseous fuel conversion. The UoSo General Services
Administration's experience with dual fuel (CNG-gasoline)
conversions coupled with spark advance vacuum line disconnected
on four cars (1968 and 1969 models) in California yielded
total hydrocarbon reductions (when operating on CNG) of 68%,
carbon monoxide reductions of 79%, and oxides of nitrogen
reductions of 651 in comparison with gasoline-fueled counter-
parts. Fifteen California State emission-controlled vehicles
with similar conversions gave reductions of 52% hydrocarbons,
84% carbon monoxide, and 47% oxides of nitrogen from emissions
before conversion. These conversions, which employed removal
of vacuum spark advance, resulted in some degradation in the
driveability that would be expected from a comparable vehicle
fueled with gasoline. The time to accelerate from 15 to 50
mph for the four GSA sedans increased by 50, 18, 48, and 80
percent respectively over comparable gasoline fueled control
vehicles.2 jhe California Air Resources Board also reports
poor driver acceptance of gaseous fueled vehicles.3
Data are available from eight 1968-69 model vehicles con-
verted to LPG-dual fuel usage and the average reduction from
these vehicles compared to standard gasoline counterparts is
25% for hydrocarbons 5, 69% for carbon monoxide and 13% for
oxides of nitrogen. Driveability impairment was noticeable
but not critical.
Twenty 1970 model GSA vehicles were converted to full-time
LPG operation and these vehicles approached 1975 Federal emission
standard levels. The vehicles were tested before and after
conversion, and emission reductions were 81% for hydrocarbons,
86% for carbon monoxide and 64% for oxides of nitrogen. Drive-
ability effects ranged from barely noticeable to hazardous.
Gaseous fuels have an additional advantage over a gasoline
fuel in the form of significantly less photochemical reactivity
of the hydrocarbons in the exhaust gases. Natural gas consists
primarily of methane and produces less reactive exhaust gases
than does LPG which is largely composed of propane.
-------
8.
If proper fuel transfer procedures are used, there are
few evaporative hydrocarbon emissions when using CNG or LPG.
"Boil-off" emissions may be encountered in using LNG, but
these emissions are largely methane and thus are more of a
safety consideration than an air quality problem. It is
recommended that fleet operators contemplating the usage of
LNG control "boil-off" emissions. Approaches that are
presently being investigated include piping from LNG storage
to a natural gas utility system and the use of catalytic
burners on vehicle vent lines.
7. WHAT ARE THE OPERATIONAL, HANDLING, AND SAFETY
PROBLEMS WITH GASEOUS FUELS'?
The range capabilities of vehicles converted for LPG
or LNG operation are typically 220 miles and 240 miles
respectively. These distances are comparable to ranges
experienced with gasoline fueled passenger cars. The range
of vehicles fueled with CNG is approximately 70 miles. In
most automobile installations the gaseous fuel tank occupies
about one-third of the trunk space. Liquefied petroleum gas
is presently available in most parts of the country from dis-
tributors which supply it for heating requirements. However,
local safety regulations often force the distributor to out-
lying areas, away from urban centers, so that refueling becomes
inconvenient.
Compressed and liquefied natural gases are currently
available at only a limited number of locations in the country.
This is largely due to the expense of liquefaction or high
pressure compressing plants. For example, a compressor of
sufficient size to service a fleet of eight (8) CNG vehicles
costs approximately $4000. Liquefaction plants are only
economical for very large scale usage. The transportation
of LNG by trucks for storage in remote tanks is becoming more
common. However, LNG is still primarily available at the few
locations where reserve capacity equipment has been built by
gas companies or at points where foreign shipments of LNG can
be stored. LPG and LNG are normally transferred as liquids
from the distributor's fixed tanks into the vehicles' tanks.
Refueling of CNG is accomplished in two ways; one, slow filling
directly from compressors to vehicle, usually overnight; or,
two, from a high pressure storage module which is kept
pressurized by a compressor at all times and requires from
two to five minutes.
-------
9.
A comparison of safety between gasoline and gaseous
fuels is necessary. Gasoline is a hazardous product; yet,
in its wide usage as a motor fuel by the public the number
of accidents has been relatively low. This is partly due to
gasoline's distinctive odor and usually visible evidence of
leakage. Gaseous fuels are odorless, but odorants usually
can and should be added to both LPG and CNG. A promising
odorant for use in LNG is under development, but until such
a material is available for routine usage, leakage can occur
without odor or visible evidence. This significantly increases
the care necessary to avoid accidents with LNG. All of the
gaseous fuels including LNG are stored under pressure. Extra
care must be exercised to assure that the entire fuel system
is leak-tight especially if the vehicles are to be parked in
confined areas. Recent impact barrier tests conducted by
GSA/DOT have shown that properly installed CNG and LNG
vehicle storage tank systems have survived intact a crash
at 30 mph into a concrete barrier, resulting in accelerometer
reading in excess of 40 G's.
Stringent local safety laws now in effect reflect a
general view that there is a safety problem with gaseous
fuels. Such laws can influence the cost of fueling stations
and storage equipment by requiring greater complexities in
the systems involved.
In summary, thousands of LPG fueled vehicles have been
in operation for many years and recently more than 4000
natural gas fueled vehicles have been placed in operation
in this country. Enough experience has now been accumulated
with gaseous fuel vehicles to demonstrate that under closely
controlled fleet-operation, the fuels.can be used safely.
8. HOW DO COSTS COMPARE AMONG GASEOUS FUELS AND OTHER
FUELS?
The average market price of gaseous fuel in terms of cost
per operating mile is somewhat lower than the cost of gasoline
and more expensive than diesel oil.l However, one investigator
reports no difference in fuel cost among gasoline and the
gaseous fuels. Vehicle conversion costs are approximately
$300 for LPG and CNG kits and $700 for LNG due to the high cost
of the cryogenic tank.
-------
] 0.
In the past, many LPG vehicles have been converted on the
basis of economics alone. In some cases a favorable tax
situation has been involved. In fleet applications, operating
costsj with natural gas can be competitive with gasoline even
including the added liquefaction or compression cost; this
is largely due to maintenance advantages including increased
life of spark plugs, exhaust systems and lubricating oil,
and increased time between overhauls.
9. WHAT AIR POLLUTION GAINS ARE LIKELY THROUGH FLEET
USAGE OF GASEOUS FUELS IN A MAJOR METROPOLITAN AREA?
No general statement is possible. In situations in which
supplying gaseous fuels to the converted vehicles can be done
without diverting natural gas from other uses, a rough
estimate of the potential benefits can be made by analyzing
the number of vehicle miles expected to be driven by the
converted vehicles, in terms of the emission reduction ranges
set forth above.
A different problem exists if the conversion of vehicles
to gaseous fuels would require the diversion of natural gas
from existing or potential space heating or power generating
uses. The type of analysis required is illustrated below.
A study by the Institute of Gas Technology indicated that
diverting one-half of the natural gas presently used for
generating electricity in New York City would furnish enough
fuel to operate all of the commercial fleet vehicles in the
City on natural gas. The IGT estimate of the resultant emission
reduction is shewn below.
COMMERCIAL VEHICLES* EMISSIONS
(Tons per Year)
Pbllutants Current Convert to Natural Gas Difference
HC 22,700 10,000 -12,700
CO 260,000 16,700 -243,300
NOx 16,700 10,000 -6,700
*Fleet of 172,000
-------
13.
REFERENCES
1. "Emission Reduction Using Gaseous Fuels for
Vehicular Propulsion", by Institute of Gas
Technology, IIT Center, Chicago, Illinois.
(Prepared under contract no. 70-69 for the
Environmental Protection Agency).
2. "Pollution Reduction with Cost Savings", a
report on the General Services Administration's
dual fuel vehicle experiment. GSA DC 71-10828.
3. "Emission Measurements from Vehicles Modified
to Operate on Natural Gas or Liquid Petroleum
Gas Fuel", State of California Air Research
Board, Staff Report, September 15, 1971.
------- |