E PA-420-S-88-101
88-99.3
THE EMISSION CHARACTERISTICS OF METHANOL AND
COMPRESSED NATURAL 6AS IN LIGHT VEHICLES
Jeffrey A. Alson
Environmental Protection Agency
Ann Arbor, Michigan
y4PGJ
PQUNOEO IN <007
Th* Aiioclatlon
Dedicated to
Air Pollution Control and
Hazardous Watt* Management
For Presentation at the
81st Annual Meeting of APCA
Dallas, Texas
June 19-24,1988
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INTRODUCTION
Research into non-petroleum fuels in the late 1960s was
motivated by the realization that combustion of gasoline and
diesel fuels was responsible for a large portion of urban air
pollution. Alternative fuels became an energy policy issue in
the 1970s for two reasons: the oil price fluctuations of that
decade emphasized the need for the nation to reduce its
dependence on imported petroleum, and emissions from new
gasoline-fueled vehicles were substantially reduced through the
application of progressively more sophisticated control
systems. Today, air quality concerns have renewed interest in
alternative fuels, with the Environmental Protection Agency
(EPA) and State regulatory agencies alike examining every
opportunity to reduce emissions.
EPA, under the auspices of the Alternative Fuels Working
Group that includes representatives from agencies and
departments throughout the executive branch, recently released
two documents that evaluate the potential of alternative fuels
to improve urban air quality. ^,2 The Department of Energy
(DOE) is currently undertaking a comprehensive 18-month study
of alternative transportation fuels.3 The States of
California, Colorado, Arizona, and New York either have or are
considering alternative fuels programs in selective urban
areas. Pending before Congress are several pieces of
legislation relating to alternative fuels, with provisions
ranging from corporate average fuel economy incentives to
automotive manufacturers for selling vehicles capable of
operation on non-petroleum fuels to Clean Air Act mandates that
centralized vehicle fleets in certain nonattainment areas begin
to purchase new vehicles that operate on alternative fuels.
Alternative fuels can be divided into two distinct
groups: those that could completely replace gasoline and those
that can be low-level additives to gasoline. Much -of the
current near-term interest, particularly in urban areas with
very high levels of carbon monoxide pollution, is in gasoline
additives such as ethanol, methanol, and methyl tertiary butyl
ether (MTBE), which can be added to the current gasoline pool
and provide immediate carbon monoxide reductions. This paper
will address two alternative fuels—methanol and compressed
natural gas (CNO)—that EPA believes have the potential to be
able to completely replace gasoline and/or diesel fuel, at
least in certain new vehicle applications, in the near term.
Liquefied petroleum gas (LPQ) and ethanol ace not addressed in
this paper because of long-term supply constraints. There are,
of course, other fuels that could provide very significant
urban emission reductions, such as electricity and hydrogen,
but there appears to be little likelihood that these fuels will
be feasible in the near term (except for extremely limited
applications).
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The primary purpose of this paper is to identify the
emissions reductions available through the use of methanol and
CNG in light-duty vehicles (i.e., passenger cars and light
trucks), by projecting realistic emission factors for these
fuels and comparing these to emission factors for gasoline
vehicles. Since publication of the most recent EPA report on
this subject (see reference l), we have performed additional
testing, particularly with CNG vehicles. A secondary purpose
of this paper is to outline for the reader some of the
non-emissions issues that are relevant in any overall
evaluation of the potential of methanol and/or CNG to become
primary motor vehicle fuels.
MOTOR VEHICLES AND AIR QUALITY
Ozone is our most serious long-term urban air quality
problem. The ozone National Ambient Air Quality Standard
(NAAQS) is a maximum one-hour level of 0.12 ppm, not to be
exceeded more than three times in a three-year period. Ozone
is not emitted directly, but is a product of a series of
complex atmospheric processes involving hydrocarbons (HC),
nitrogen oxides (NOx), and sunlight. EPA believes that for
most urban areas HC control is generally the most promising
strategy for reducing ozone levels. Motor vehicles are
typically responsible for 30 to 50 percent of urban HC
emissions. Based on data through 1986, there are 62 ozone
nonattainment areas. As new, cleaner vehicles continue to
displace older, more polluting vehicles and as EPA and States
implement other controls, we expect many of these areas to move
into compliance. Still, 20 to 30 of our largest cities will
require major HC emission reductions (on the order of 40
percent or more) to reach attainment, and such reductions will
be very difficult to achieve, especially since HC sources are a
very diverse group. It must also be noted that recent studies
have suggested that ozone levels near the NAAQS level of 0.12
ppm can affect otherwise healthy adults, and the Agency is
currently reviewing these new studies to see if the standard
ought to be lowered.
The NAAQS for nitrogen dioxide (NO2) is an annual mean
of 0.053 ppm. Motor vehicles generally emit about 40 to 60
percent of urban NOx (NOj and NO) emissions. While Los
Angeles is currently the only city in nonattainment for NO2.
NOx control is still very much a priority. First, because NOx
emissions have not been as tightly controlled as other
emissions, continued economic growth will likely mean that
overall NO? levels will begin to grow at some point in the
future ana some areas currently in attainment could te
threatened with nonattainment. Second, for certain
nonattainment areas, EPA encourages NOx reductions as ar.
additional ozone control strategy.
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Carbon monoxide (CO) is the one urban pollutant that is
almost exclusively a motor vehicle problem. It is a direct
product of incomplete fuel combustion, which is much more
prevalent with automotive engines than with stationary fuel
combustion. Vehicles are generally responsible for 80 to 90
percent of CO emissions in urban areas. The NAAQS for CO is a
9 ppm eight-hour average and a 35 ppm one-hour average not to
be exceeded more than once per year. There are currently 65
areas in nonattainment for CO, but the future situation is much
more promising than for ozone. Because new gasoline vehicles
emit much less CO than old vehicles, our projections show that
all but about 5 to 15 areas will move into attainment by the
late 1990s simply with the improvements brought about by our
existing motor vehicle standards.
The final NAAQS pollutant of concern is particulate
matter. Prior to 1987 the standard was expressed on a total
suspended particulate basis, and the majority of EPA's data is
still on this basis. Nov the standard is expressed on an
inhalable particulate basis, and considers only those particles
less than 10 micrometers in diameter. The new standards of 50
micrograms per cubic meter as an annual mean and 150 micrograms
per cubic meter for a 24-hour average are projected to be
approximately equivalent in stringency to the older total
suspended particulate standards.4 Thus, while the exact
number of nonattainment areas for the new standards is not
known at this time, it is expected to be significant. While
vehicles that use unleaded gasoline emit very low levels of
particulate, diesel trucks and buses are important sources of
inhalable particulate, particularly in central city areas.
Diesel particulate is a special concern because of both its
small size and hazardous composition. While this paper will
only address light-duty applications for alternative fuels, it
should be noted that EPA, bus engine manufacturers, and the
transit industry are considering both methanol and CNG as
possible fuels to meet much more stringent particulate emission
standards that take effect for new transit bus engines in 1991.
ALTERNATIVE FUELS IN LIGHT-DUTY APPLICATIONS
Gasoline Vehicle Emissions
Over 95 percent of all fuel consumed in passenger cars and
light trucks in the U.S. is gasoline.5 Accordingly,
gasoline-fueled applications are the appropriate context in
which to consider alternative fuels with passenger vehicles.
Any projection of the future potential of alternative
fuels to reduce gasoline passenger car emissions must recognize
the very large reductions that have occurred over the last two
decades. Congress and EPA established progressively more
stringent passenger car emission regulations that culminated
with the following standards that have been in effect since
1981 and which apply to EPA's Federal Test Procedure: 0.41
grams per mile
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exhaust NOx, and 2.0 grams per test evaporative HC (these
standards must be met by all new gasoline and diesel passenger
cars sold in the U.S., except for those sold in California,
which has somewhat more stringent standards). These standards
have provided the impetus for the development of sophisticated
emission control technologies which have greatly reduced
passenger car emissions, as shown in Tables I and IX (all
emissions data in this paper are over the Federal Test
Procedure).6 Table I shows that zero-mile emissions from new
gasoline passenger cars have been reduced by between 84 and 98
percent between the years 1966 and 19B6. It is well known that
emissions tend to increase with vehicle age, and of course
in-use emissions are the relevant issue. Table II shows that
while in-use emissions reductions have not been quite as great
as zero-mile emissions reductions, they have ranged from 62 to
88 percent, which is quite an achievement. These very large
per mile emission reductions have resulted in lower overall
motor vehicle pollutant burdens in the late 1970s and 1980s
even with growth in the economy, number of vehicles, vehicle
miles traveled per vehicle, etc.
It is essential to recognize that the regulatory program
currently in place, with very stringent emission standards for
gasoline passenger cars, provides a much more challenging
context in which to project emission reductions for alternative
fuels. When gasoline engines were basically uncontrolled in
the 1960s, emissions were nearly exclusively a function of fuel
type, and the substitution of fuels with inherently more benign
properties such as methanol and CNG would clearly reduce
vehicle emissions. Today, however, emissions are a primary
design concern and are a function not only of the fuel being
utilized, but of many other variables such as the level of the
applicable emissions standards, the type of engine design, the
specific calibration of various design parameters, etc. And,
of course, emissions are just-one of many items of interest to
automotive engineers, and must always be viewed in combination
with other important characteristics such as power,
driveability, reliability, fuel economy, and cost. The
relevant issue is not juBt whether emissions reductions are
possible with a given fuel. The answer to that question is
almost always yes, and in fact emissions from gasoline
passenger cars could probably be reduced further at greater
cost or with sacrifices in other aspects of vehicle
performance. The more relevant issue is whether alternative
fuels have properties that will inherently reduce emissions of
certain pollutants, while at the same time obviating the need
for more complex emission controls or sacrifices in other
performance characteristics.
Relative Reactivity of Hydrocarbon Compounds
One concept that is critical to the understanding of the
potential for methanol and CNG vehicles to reduce urban ozone
levels is that of photochemical reactivity of organic compounds
(organics is used here to refer to all unburned and partially
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combusted fuel compounds, i.e., HC and oxygenated ' HC).
Gasoline itself is a mixture of HC compounds, and the
combustion of gasoline results in a large number of individual
organic products. Although many of these organic compounds in
gasoline exhaust are considered to be toxic, the primary
justification for their regulation is their role in the
formation of ozone. Because there are far too many organic
compounds in gasoline exhaust to regulate individually, and
because almost all organic compounds participate in ozone
photochemistry, EPA regulates gasoline vehicle organics under
HC standards for exhaust and evaporative emissions.
It has long been recognized that different organic
compounds have different -photochemical reactivities, i.e., each
compound has a unique rate at which it reacts in the complex
photochemical reactions that lead to ozone formation. Our
present exhaust and evaporative HC emission standards
implicitly assume that the mix of individual HC constituents
remains fairly similar from one gasoline vehicle to the next,
which is probably a reasonable assumption. But in the context
of fuels that are considerably different than gasoline, it is
no longer valid to simply assume that unburned fuel-related
emissions will have the same overall photochemical reactivities
as gasoline vehicle HC emissions.
One important characteristic of ozone formation is the
reaction of organics with the hydroxyl radical (OH). Table III
lists the OH rate constants for a number of organics, all of
which are present in gasoline vehicle emissions except for
methanol, normalized so that the reaction rate of butane, one
common gasoline constituent, is unity.7 This table can be
used as one indicator of relative reactivity, while
acknowledging that other parameters such as maximum ozone
yield, NO2 oxidation rate, and HC/NOx ratio are al60
important.
Methane, the primary constituent of natural gas and the
dominant HC constituent in CNG vehicle exhaust (and which also
exists in gasoline and methanol exhaust as well), is the
simplest compound to address in this regard. As reflected by
the relative reaction rate shown in Table III, it is considered
to have such a negligible photochemical reactivity that EPA
recommends that methane be excluded from State Implementation
Plan emission inventories and regulatory controls.8 EPA's
current motor vehicle HC standards do in fact include methane,
but EPA proposed a non-methane HC standard in the early 1980s
and would likely reconsider this issue if certification of CNG
vehicles appeared imminent. Past practice has been to assume
that the methane component of CNG emissions has zero reactivity
while the remaining HC have an overall reactivity similar to
gasoline vehicle HC. This paper will follow that practice, and
thus will focus on non-methane HC from CNG vehicles.
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Methanol vehicle organics are a somewhat more difficult
issue to address. The fairly limited data base suggests that
when pure methanol is used as a fuel, the organic emissions are
largely unburned methanol, with a much lower percentage ~£
formaldehyde, and only trace amounts of a small number of KC
compounds. When gasoline is added to methanol fuel to aid cold
starting, then there is a higher percentage of HC emissions.
As with CNG vehicles, it has been commonplace to assume that
methanol vehicle HC have reactivity profiles similar to those
of gasoline vehicle HC. But what about the relative reactivity
of the unburned methanol and formaldehyde emissions?
Table III shows that methanol itself tends to have a
relatively low reaction rate with the hydroxy1 radical, while
formaldehyde has a relatively high reaction rate. In order to
assess the overall ozone impact of substituting methanol
vehicle organics for gasoline vehicle organics, a number of
computer simulation studies have been performed. These studies
simulated air chemistry and transport within certain urban
areas, and accounted for entrainment and dilution of local
pollutant inventories into the urban airsheds. Based on these
studies, EPA has developed a model that provides reactivity
factors for methanol and formaldehyde relative to typical
non-oxygenated HC from gasoline vehicles. This model is the
subject of a separate paper being presented at this APCA
session.9 Based on this model, the average reactivity
factors are projected to be 0.43 for methanol and 4.8 for
formaldehyde. That is, on an equivalent carbon basis, the
methanol molecule has only 43 percent of the potential to form
ozone as the typical gasoline HC molecule, while the
formaldehyde molecule has a 4.8 times higher potential. It
must be emphasized that this model simplifies a very complex
process that is best simulated by detailed computer programs.
There are a number of caveats pertaining to the studies upon
which the model was based as well as the model itself, which
limit the scope of its applicability. While EPA believes it is
useful as an analytical tool, EPA does not consider it to be
appropriate for use in formulating standards or other binding
regulatory decisions.
Methanol Vehicle Emissions
Methanol has long been considered to be an excellent motor
vehicle fuel. Its simple molecular structure, high octane,
wide flammability limits, high flame speed, and low flame
temperature result in a fuel that can potentially be burned in
a very clean and efficient way relative to petroleum fuels.
Because methanol is such a different fuel than gasoline, it is
helpful to distinguish between two types of methanol
vehicles—current technology and advanced technology methanol
vehicles. These two types of methanol vehicles would be
expected to differ with respect to both engine design and
vehicle emissions. Methanol is not considered to be a good
fuel for retrofit programs because of its corrosive effect on
many materials used in older vehicles.
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Current Tectoology Methanol Vehicles. Current technology
methanol vehicles utilize engines that are very similar to
engines used in today's gasoline vehicles, with modifications
to allow the engine to operate well, but not optimally, on a
blend o£ 85 percent methanol and 15 percent gasoline (M85).
These are the types of methanol vehicles currently involved in
demonstration programs, and would have emissions and efficiency
characteristics very similar to flexible fuel vehicles (FFVs)
or variable fuel vehicles (VFVs) operating on M8S.
There has been a very large amount of emissions data
generated from current technology methanol vehicles over the
last few years. EPA recently published a paper that summarized
the current data base, which has been computerized and is
available to interested parties.10 The data base currently
includes results culled from 13 different studies by EPA, the
California Air Resources Board, and other organizations
involving 10 different vehicle models and 64 different engine
and vehicle configurations. Since many of these first
generation methanol vehicle prototypes were not designed to
meet any specific emission requirements, some vehicles failed
either the CO or NOx federal emissions standards (both of which
have been proposed to apply for methanol vehicles as well).
The data base includes exhaust emission test data for 40
vehicle configurations that met the proposed methanol vehicle
standards and Table IV gives the average and range for exhaust
emissions over the Federal Test Procedure for these vehicles.
The average mileage of these vehicles was on the order of
10,000 miles, although individual vehicle mileage ranged from
zero to over 100,000 miles.
With respect to CO and NOx emissions, the data in Tables
I, II, and IV are very instructive. It is clear that average
current technology methanol vehicle emissions for CO and NOx
are somewhat higher than the zero-mile emissions for current
gasoline vehicles shown in Table I, but considerably lower than
the 50,000-mile emissions for these gasoline vehicles given in
Table II. Since the methanol vehicles had, on average,
accumulated around 10,000 miles, CO and NOx emissions appear to
be about the same for today's gasoline and methanol vehicles.
This is to be expected for CO emissions, as CO levels are a
strong function of air-to-fuel ratio and current gasoline and
methanol vehicles have all generally been designed to operate
at stoichiometric air-to-fuel ratios. Because methanol has a
relatively low flame temperature, it has been speculated by
some that methanol vehicles should yield lower NOx levels.
With current NOx standards, however, we believe that
manufacturers will likely trade off methanol's low-NOx
characteristic to gain other benefits such as fuel economy,
performance, or a less expensive catalytic converter.
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The analysis of the ozone Impact of organic emissions from
current technology methanol vehicles is more complex. Instead
of using organic emission factors from the data base (the data
are much more limited for organics than for CO and NOx because
of differences in measuring and reporting these emissions among
various organizations), EPA has assumed that the organic
emission levels of current technology methanol vehicles at zero
miles would be the maximum levels permitted under the proposed
methanol vehicle standards (which essentially require that
methanol vehicles emit no more than the amount of carbon
allowed from gasoline vehicles). The data base was utilized,
however, to project the proper methanol to hydrocarbon to
formaldehyde ratios. The projected total (exhaust plus
evaporative) organic emissions, at zero miles, are Q.71 gprn
methanol, 0.048 gpm formaldehyde, and 0.21 gpm HC.10 Further
assuming estimated in-use deterioration factors for organic
emissions from methanol vehicles based on HC deterioration from
current gasoline vehicles. Table V gives projected 50,000-mile
organic emissions from current technology methanol vehicles and
compares those levels to those given earlier for current
gasoline vehicles. It can be seen that methanol vehicles emit
greater amounts of methanol and formaldehyde, but less HC.
Utilizing the reactivity factors given earlier in the paper of
0.43 for methanol and 4.8 for formaldehyde, and assuming that
HC from methanol vehicles are similar to the HC from gasoline
vehicles, the projected in-use methanol vehicle emissions would
provide a 35 percent reduction in ozone producing potential
relative to current gasoline vehicles.
The EPA model used to generate the relative reactivities
is a simple generalization of the complex modeling that must be
performed to project ambient ozone impacts. Nevertheless, the
many studies that have been performed to date support the
contention that methanol is less reactive than typical gasoline
HC, and that therefore, as long as formaldehyde emissions are
not excessive, current technology methanol vehicles will
provide some ozone benefits. Our best estimates at this time
are 30 to 40 percent reduction in peak ozone levels for a
one-day episode. Research continues in many of these areas, in
particular with respect to whether the lower reactivity of
methanol will continue to provide ozone benefits in a multi-day
ozone episode.
Advanced Technology Methanol Vehicles. There are reasons to
believethat future engine and vehicle designs optimized to
take full advantage of the combustion properties of methanol
fuel could provide much larger emission benefits than those
discussed above for current methanol vehicles. Current
methanol vehicle prototypes are basically gasoline vehicles
with only the most rudimentary modifications to permit methanol
combustion. Gasoline cannot be combusted at a very high
air-to-fuel ratio because of engine misfire and the fact that
stoichiometric air-to-fuel ratios are necessary in order to
allow the NOx reduction function of the catalytic converter to
operate properly. But methanol's wide flamroability limits,
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high octane and higher flame speed allow it to maintain stable
combustion at much leaner air-to-fuel ratios than gasoline.
And methanol's low combustion temperature inherently produces
low engine NOx levels. These properties suggest the
possibility of an optimized lean burn, high compression
methanol engine with good driveability which would be able to
comply with NOx emission standards without the need for a
reduction catalyst. Such an engine would likely provide very
significant benefits in terms of low CO emissions and improved
energy efficiency. Once again, we would not expect NOx
emission reductions from such an engine design, although
methanol's low-NOx characteristic facilitates the application
of the lean burn concept.
One important issue with optimized methanol vehicles is
fuel specification. All of the potential improvements
discussed above could be achieved through the use of either M85
or pure methanol (M100) fuel. Almost all prototype and
demonstration vehicles to date have utilized M85, both to
improve cold startability and to provide a more luminous flame
in case of a fire. But there are several tradeoffs associated
with choosing between M85 and M100, including several with
safety ramifications (for example, the addition of
high-volatility gasoline makes M85 more likely to ignite and a
more severe burn relative to M100). But most important for
purposes of this paper, the use of M85 could significantly
reduce the potential ozone benefits available from the use of
methanol fuel. The use of M85 would increase total evaporative
HC emissions, and would increase the proportion of reactive HC
(as opposed to less reactive methanol) in both exhaust and
evaporative emissions. Thus, from an environmental
perspective, M100 is the preferred fuel. In order to utilize
M100 a breakthrough in cold starting will be necessary, which
has not been achieved to date, but is the focus of considerable
research.
Toyota Motor Corporation is the first automotive
manufacturer to make available vehicles utilizing many of the
concepts discussed above. Toyota has recently commercialized
the lean-burn concept on selected gasoline vehicle models in
the Japanese market. They adapted the lean-burn concept to
methanol by increasing the engine's compression ratio and
making other changes to the intake system, the control system,
and the exhaust catalyst. These modifications resulted in a
methanol engine which yielded stable combustion at leaner
air-to-fuel ratios with better energy efficiency, higher
torque, improved driveability, and lower engine-out NOx
emissions than the gasoline engine counterpart already
available in the Japanese market.11
The 1.6-liter, 4-cylinder engine was installed in two
Toyota Carina vehicles with inertia weights of 2250 lbs. The
vehicles can operate under Federal Test Procedure conditions on
both M100 and M85. These two vehicles have been loaned to EPA
and the California Air Resources Board (CARB). All of the
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emission data available for these two vehicles are given in
Table VI. 12,13 The methanol and HC data combine both the
evaporative and exhaust components, and so can be compared to
the total HC values given in earlier tables. Since all of the
Carina testing has been performed at low-mileage, it is
appropriate to compare Table VI with Table I. As with the
current technology methanol vehicle data, CO and NOx emissions
are fairly similar. The trend of somewhat lower CO emissions
and somewhat higher NOx emissions (though still below the EPA
standard) is to be expected with the lean burn calibration.
Once again, the largest emissions benefits would be with
respect to ozone producing potential. The data for methanol,
formaldehyde, and HC emissions in Table VI include both exhaust
and evaporative emissions expressed on a gram per mile basis.
Since we do not have emissions data from the methanol Carina at
high mileage, the only calculation that can be done is to
compare the ozone producing potential of the low-mileage Carina
to zero-mile gasoline vehicles. Using the reactivity factors
developed earlier, the emissions from the Carina fueled with
M85 would yield an 83 percent reduction in ozone producing
potential, while the emissions from the M100 testing suggest a
reduction of 86 percent compared to zero-mile gasoline vehicles.
It must be emphasized that even this Toyota Carina must be
viewed as only a partial step toward an optimized methanol
vehicle. It would be expected that future, more comprehensive
research programs will provide vehicles with more favorable
emission characteristics than those of the Carina. At this
time, EPA projects that future advanced technology methanol
vehicles could provide as much as a 80 to 90 percent reduction
in ozone producing potential and significantly lower CO
emissions, while simultaneously maintaining NOx emissions at
levels comparable to current gasoline vehicles.
Related Concerns with Methanol Vehicles- Are the types of
potential emission reductions discussed in the previous
sections realistic? will the automotive industry and consumers
accept methanol vehicles? These questions are critical in any
evaluation of the potential of alternative fuels to reduce
motor vehicle emissions absent a legislative fuel use mandate.
Methanol is considered to be an excellent vehicle fuel by
most of the automotive industry. It is a very efficient fuel,
with current technology vehicles generally exhibiting slightly
higher efficiencies and advanced technology vehicles projected
to be considerably more efficient. Methanol almost' always
provides a boost in power output, although this can be traded
off with efficiency. And being a liquid fuel that can be
produced from natural gas and coal, there are no barriers to
widespread supply and distribution of methanol.
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Potential drawbacks associated with methanol include
formaldehyde emissions, vehicle range, and cold startability.
Formaldehyde's role in ozone formation has been included in
earlier sections, but it is also of concern as a human toxin
and carcinogen. EPA has analyzed the potential formaldehyde
exposure scenarios in great detail, and has concluded that
formaldehyde levels from current technology vehicles do not
appear to present a serious public health concern.14
Formaldehyde emissions from advanced technology vehicles should
be much lower, possibly approaching the levels from gasoline
vehicles. Formaldehyde emission levels will clearly continue
to be a top priority for research by the industry and analysis
by EPA and other regulatory agencies.
Methanol has only half the volumetric energy content of
gasoline, so vehicle range is a serious concern. This debit is
partially offset by methanol's increased efficiency (a small
factor for current vehicles but more significant for advanced
vehicles) and also by the addition of gasoline in the case of
M85. Carrying approximately 50 percent more fuel on-board in
traditional tanks seems unlikely, so the alternatives would be
reduced range or the application of bladder tanks. The latter
might permit the additional fuel to be carried on-board with
acceptable safety characteristics.
Cold starting appears to be generally acceptable with M85,
so is primarily an issue with Mioo. A breakthrough is thus
necessary in order to be able to reap the maximum ozone
benefits of M100, but is not necessary in order to achieve the
significant benefits available from an advanced technology
vehicle fueled with M85.
Compressed Natural Gas Vehicle Emissions
CNG consists primarily of methane, but also contains
smaller quantities of other compounds such as ethane and
propane. Its characteristics as a fuel are dominated by those
of methane. Methane shares many of the same beneficial fuel
characteristics of methanol: simple molecular structure, high
octane, ability to combust under lean conditions, etc. The
most obvious difference between methane and methanol (and
current automotive fuels) is that methane is a gas, not a
liquid. This is an advantage in terms of cold startability and
cold start emissions. It is a disadvantage with respect to
on-board fuel storage.
Unlike methanol, CNG does not pose problems with
corrosion, and can be used in existing gasoline vehicles
retrofitted with CNG conversion kits. These kits typically
permit the vehicle owner to fuel with either CNG or gasoline,
and are referred to as "dual fuel" conversion kits. The
following section will discuss emissions from such vehicles.
The succeeding section will address the performance of
dedicated and optimized CNG vehicles.
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CNG Dual-Fuel Retrofit Vehicles. According to the American
Gas Association there are approximately 30,000 dual-fuel
vehicles operating in the United States. It has been
problematic for EPA to estimate the emissions impacts of such
vehicles for several reasons: 1} there has been little
reliable emission testing performed, particularly on
conversions of recent computer-controlled vehicles, 2) the
performance of conversion kits can vary greatly depending on
kit manufacturer, the expertise of the installer, the quality
of maintenance, etc., 3) the fact that the vehicle can operate
on either CNG or gasoline means that overall emissions depend
on the fuel that is actually used, and 4) the conversion
process itself sometimes interferes with the gasoline
combustion process and can lead to increased emissions on
gasoline. It should also be obvious that an engine that must
be able to operate on fuels as different as CNG and gasoline
cannot be optimized for either fuel (this is true with methanol
flexible or variable fuel vehicles as well).
There has. been considerable emissions testing of CNG
dual-fuel vehicles over the years by EPA, California Air
Resources Board, Colorado Department of Health, various
Canadian agencies, and others. Unfortunately, the great
majority of this work has involved gasoline vehicles that were
not designed to meet the emission standards that have been in
effect since 1981. It should be noted that the evidence is
clear that the bulk of CNG dual-fuel conversions of pre-1981
gasoline vehicles resulted in reduced emissions. But as noted
earlier, the relevant baseline is now a gasoline vehicle with
a more sophisticated emission control system meeting much more
stringent emission standards.
EPA has performed a comprehensive literature search in
order to compile a list of CNG dual-fuel retrofits involving
1981 and later model year - vehicles that have been emission
tested over the Federal Test Procedure on both gasoline and CNG
at recognized test laboratories. At this time we have only
identified four vehicles that fulfill these criteria and they
are listed in Table VII. Two of these vehicles were tested by
the California Air Resources Board.15'16 EPA is currently
carrying out a cooperative test program with the American Gas
Association and retrofit conversion kit research and marketing
companies. Two of the vehicles shown in Table VII were tested
by EPA earlier this year and additional testing of vehicles
from other companies is scheduled for later this year.
The data in Table VII clearly indicate that CNG dual-fuel
vehicles offer very significant CO emission benefits, ranging
up to 98 percent. This confirms both theoretical expectations
(better mixing of gaseous fuel, lean operation, lack of fuel
enrichment for starting) and the data from programs involving
pre-1981 vehicles. The data for nonmethane hydrocarbons (NMHC)
and NOx emissions are mixed. NMHC emissions with CNG operation
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were higher in one case, essentially equivalent in two others,
and significantly lower in the final case, compared to gasoline
operation. While the bulk of emissions data from tests of
pre-1981 dual-fuel vehicles suggests that NMHC are generally
lower with CNG use, it appears that this trend is not so clear
with recent conversions. NOx emissions were increased on CNG
operation with two of the four vehicles, and reduced slightly
in the other two, relative to the NOx emissions on gasoline.
This concern is heightened by the fact that spark timing is
sometimes advanced on CNG operation in order to compensate for
methane's lower flame speed and improve performance. CNG Fuel
Systems provided data to EPA that suggested that NOx emissions
would be increased further on its vehicle if timing were
advanced.
These data suggest the need for further work in this
area. Clearly, CNG dual-fuel vehicles can provide very large
CO emission reductions when operated on CNG, but the NMHC and
NOx emission impacts are far less clear. Related issues
include the impact of the conversion on emissions when the
vehicle is operated on gasoline, and the emission impacts of
calibration changes that may be attractive to vehicle owners
for improved efficiency and/or performance.
Advanced Technology CNG Vehicles. CNG is such a different fuel
than gasoline that, as with methanol, there is every reason to
expect that the optimum engine for CNG will be much different
than today's CNG dual-fuel engine, and that such an engine
would likely provide greater emission reductions and better
performance and efficiency than are available from dual-fuel
engines.
For purposes of efficiency and CO emissions, the optimum
CNG engine should be a high compression, lean burn engine. CNG
may have a slightly more difficult challenge in this regard,
relative to methanol, because of its relatively higher flame
temperature which increases NOx emissions. The issue is
whether it will be possible to reap the efficiency and CO
benefits of a high compression, lean burn design while
maintaining NOx emissions within acceptable levels.
The most complete attempt to date to design, build, and
evaluate an optimized CNG vehicle was undertaken by the Ford
Motor Company in 1983 and 1984. Ford built and leased 27
dedicated CNG Ranger pickup trucks in cooperation with the
American Gas Association and member utilities. These vehicles
have been in service since that time.
The 2.3-liter gasoline engine normally used in the Ford
Ranger was modified in several ways to improve it for CNG
utilization including higher compression ratio and advanced
timing. The final engine provided efficiency and performance
very similar to that of gasoline Rangers. Emissions data from
three low-mileage Rangers, two fueled with CNG and one fueled
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with gasoline, are given in Table VIII.17 These data
correlate fairly well with those for dual-fuel vehicles
discussed above. CO emissions for the two CNG Rangers
represented a 99 percent reduction relative to the gasoline
Ranger. NMHC emissions were 30 percent lower with CNG for both
Rangers. NOx emissions were higher in both cases, still below
the 2.3 gpm NOx standard that was in effect for light trucks
certified in 1984 but higher than the NOx standard of 1.2 gpm
that took effect beginning in 1988.
It is clear that the development of advanced technology
CNG vehicles is in its infancy and that the Ford Ranger was
simply the first attempt to try to optimize a CNG vehicle. It
is very likely that future research will yield improvements in
emissions, efficiency, and performance, but the question is
whether all can be improved simultaneously. At this time EPA
projects that future advanced technology CNG vehicles will
likely be able to provide reductions of over 90 percent for CO
emissions and should be able to provide large reductions in
NMHC emissions as well. The magnitude of the NMHC reductions,
and of any possible deleterious effects on NOx emissions are
not clear at this time.
Related Concerns with CNG Vehicles. Cold startability and
formaldehyde emissions, two of the major concerns with pure
methanol as a fuel, should not pose problems for CNG. As a
gaseous fuel it is generally considered to have good cold start
characteristics, and emission testing has shown that
formaldehyde levels from CNG vehicles are generally equivalent
to or less than levels from gasoline vehicles.
Efficiency and performance of CNG vehicles are significant
concerns, particularly with respect to dual-fuel retrofits.
Each of the dual-fuel vehicles shown in Table VII suffered a
major penalty either in terms of efficiency or acceleration
performance. It can be seen in Table VII that the two vehicles
tested by EPA had higher efficiencies on CNG than on gasoline
after conversion, but in both cases the CNG efficiency was
lower than the EPA certification fuel economy data for the
pre-conversion gasoline vehicle. This trend is also confirmed
by evaluations of pre-1981 vehicle conversions as well.
Decreases in efficiency and/or performance are especially
relevant with a dual-fuel vehicle because of the potential for
the user to be motivated to use gasoline fuel, or to tamper
with the CNG control system, both of which would likely
increase emissions. Fortunately, there is theoretical and
practical evidence (the Ford Ranger) that dedicated and
optimized CNG vehicles could have at least equivalent
efficiency and performance levels compared to gasoline vehicles.
Vehicle range is probably the issue that most concerns the
automotive industry. Estimates are that current CNG storage
tanks only provide one-sixth of the range of an equivalent size
gasoline tank. Thus, this is not just a problem with dual-fuel
vehicles where the CNG tanks must be added to a vehicle with an
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existing gasoline tank, but is also a design problem even for
dedicated and optimized vehicles. This is certainly one of the
major reasons (along with fuel supply and distribution issues
which cannot be addressed here) why many experts believe CNG is
better suited for centralized urban fleet applications than for
general automobile use.
CONCLUSIONS
Vehicle and engine designs available today that operate on
methanol and CNG fuels can provide emission benefits. Current
technology methanol vehicles (dedicated to operate only on M85
fuel and meeting proposed EPA emissions standards) are
projected to reduce the peak one-day ozone producing potential
of a motor vehicle by approximately 30 to 40 percent relative
to that of a current gasoline vehicle. CO and NOx emissions
would not be expected to be affected by the use of current
technology methanol vehicles. CNG dual-fuel retrofit vehicles
could provide very large CO reductions on the order of 80 to 9 5
percent compared to current gasoline vehicles. The NMHC and
NOx emission impacts can vary greatly depending on the
conversion. EPA believes it should be possible to ensure some
NMHC, and thus ozone, benefits with CNG dual-fuel retrofits.
The emission benefits available from both methanol and CNG
would be greater in dedicated vehicles optimized for the
individual alternative fuels. From an environmental
perspective, both fuels would be best utilized in high
compression, lean burn designs that should yield very large
NMHC/ozone and CO benefits. But with only very preliminary
designs and data at this time, it is impossible to project
specific emission reductions with any certainty. EPA believes
reductions of up to 90 percent for both pollutants with both
fuels may be achievable, although the validity of this
conclusion is dependent on the resolution of specific engine
design issues for both fuels.
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88-99.3
Table I. Average zero-mile emissions from gasoline cars
(grams per mile over the EPA federal test procedure)
Model Year Exhaust HC Evap HC Total HC CO
1966 7.2 4.5 11.7
1986 0.23 0.55 0.78
Reduction 97% 88% 93%
CO
NOx
78
3.4
1.2
0 . 54
98%
84%
Table II. Average 50,000-mile emissions from gasoline
cars (grams per mile)
Model Year
Exhaust HC
Evap HC
Total HC
CO
NOX
1966
8.1
4 . 5
12.6
89
3.4
1986
1.0
0.6
1. 6
13
1.3
Reduction
88%
87%
87%
85%
62%
Table III. Reaction rates with hydroxyl radicals relative to
butane
Compound Relative Reaction Rate
Methane 0.003
Methanol 0.40
Benzene 0.51
n-Butane 1.0
Toluene 2.4
Ethylene 3.4
Formaldehyde 3.6
Acetaldehyde 6.4
m-xylene 9 . 7
Propylene 10.4
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Table IV.
Pollutant
Summary of exhaust emissions from current technology
methanol vehicle data basea (grams per mile)
Averaae Emissions
Emissions Ranae
Methanol
Formaldehyde
CO
NOx
0 . 4 7
0. 035
1. 7
0.61
0 . 12-1 .00
0.00-0.17
0 . 43-3.2
0.04-0.88
Data are from the 40 vehicle configurations that
current federal CO and NOx emission standards.
met
Table V. Projected 50,000-mile organic emissions and ozone
producing potential from current technology gasoline
and methanol vehicles (grams per mile)
Fuel Methanol Formaldehyde Total HC Ozone Potential
Gasoline 0 0.007 1.60 100%
Methanol 1.57 0.106 0.48 65%
Table VI. Emissions from low-mileage methanol Toyota Carinas
(grams per mile)
Test Site
Fuel
Methanol4
Formaldehyde
HCa
CO
NOx
Toyota
M85
—
—
_
0 . 93
0 . 69
CARB
M85
-
0.014
-
0 . 70
1. 00
EPA
M85
0.23
0 . 007
0 . 07
1 . 07
0.75
EPA
M100&
0 .33
0.011
0 . 02
0 . 74
0 . 76
Methanol and HC emissions are total (exhaust plus
evaporative) emissions. Methanol and HC were not measured
separately, but were projected from FID readings based on
ratios determined from the EPA methanol vehicle emission
data base for both M85 and M100.
This vehicle cannot be started on M100 at low ambient
temperatures.
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Table VII. Exhaust emissions from CNG/gascline dual-fuel cars
(grains per mile)
Test
Site Vehicle Fuel N*MHC CO NOx Elf lb Eff 2C Accelc
EPA. CNG Fuel Sys. Gasoline 0.30a 9.8 0.40 -20% Base Base
1984 Delta 88 CNG 0.25a 1.7 1.18 -10% *10% -30%
EPA Total Fuels Gasoline 0.27a 1.4 1.07 -10% Base Base
1987 Crovn Vic. CNG 0.36a 0.45 0.93 -5% *10% -3S%
CARS Dual Fuel Sys. Gasoline 0.36 3.3 0.56 HA Base MA
1983 Ford LTD CNG 0.35 0.07 0.47 NA -20% NA
CARB Pacific Light. Gasoline 0.26 7.0 0.70 NA Base NA
1985 GMC Pickup CNG 0.05 0.20 1.06 NA -20% NA
Non-Methane HC was not measured, but was calculated assuming that
methane was equal to 25% of gasoline HC emissions and 90% of CNG HC
emissions.
Post-Conversion energy efficiency over the FTP relative to EPA
certification fuel economy data for the specific gasoline vehicle
model.
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).
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88-99.3
Table VIII. Exhaust emissions from low-mileage CNG and
gasoline Ford Rangersa (grams per mile)
Test Site
Fuel
NMHC
CO
NOx
Ford-1984
Gasoline
0 . 20
3.2
1.1
Ford-1984
CNG
0 . 14
0 . 03
1.9
EPA-1988
CNG
0 . 14b
0 . 04
2.0
The CNG Ranger was designed to have approximately the same
efficiency and acceleration characteristics as the gasoline
Ranger.
Non-Methane HC was not measured, but was calculated
assuming that methane was 90% of CNG HC emissions.
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88-99.3
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References
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from the Use of Alternative Fuels and Fuel Blends,"
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88-99.3
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13. "Alcohol-Fueled Fleet Test Program — Seventh Interim
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NOTE TO EDITORS
Under the new federal copyright law,
publication rights to this paper are
retained by the authors).
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