United States
Environmental Protection
Agency
Office of Air
and Waste Management
Washington. D C 20460
October 1976
&EPA
Factors
Affecting
Automotive
Fuel Economy
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First printing Ma\ 1976
U.S. Environmental Protection Agency
Office of Air and Waste Management
Mobile Source Air Pollution Control
Emission Control Technology Division
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Introduction
This is the third EPA report on the subject
of automobile fuel economy. The two
previous reports were published in
November 1972 and October 1973.
The previous EPA reports have been
studied and commented upon by other
government agencies, the Congress, State
and local governments, private citizens,
fleet operators, motor vehicle manu-
facturers, and fuel producers. This report
is intended for the same broad audience.
This report contains new information on
emission controls and tampering, and the
average fuel economy of the 1975 cars. It
also includes information on driving
patterns and their effect on fuel economy.
Thus it should aid drivers as well as car
buyers in making choices which can affect
their gas mileage.
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Summary of Conclusions
1. The most important vehicle design
features affecting fuel economy are vehicle
weight and engine displacement. A 10%
change in either weight or displacement
causes a fuel economy change of 3 to 6%.
Since weight changes are usually
accompanied by displacement changes, the
fuel economy effect of both of these
changes has sometimes been attributed to
weight alone (Section V-C).
2. Vehicle size and weight and the use of
power-consuming convenience devices have.
all been increasing steadily for more than
10 years. Parameters which affect engine
efficiency have also been changing,
sometimes in directions leading to lower
efficiency (Section I V-C).
3. Driving habits and trip characteristics
can have more effect on fuel economy than
any vehicle design feature. A standard
size car can get over 20 miles per gallon
under favorable conditions; it can also get
less than 2 miles per gallon under poor
conditions (Section IV-D).
4. Travel habits in the U.S. lean heavily
toward driving conditions which give poor
fuel economy. U.S. autos accumulate about
15% of their mileage in trips of 5 miles
or less; however, these trips consume more
than 30% of the Nation's automotive
fuel, because autos operate so inefficiently
in short trips (Section IV-D).
5. There is no simple or inherent relation-
ship between fuel economy and the
emissions standards that new cars are
required to meet; especially misleading is
the contention that fuel economy always
becomes poorer as emissions standards are
made more stringent. With the use of
catalyst technology, the average fuel
economy of 1975 cars is nearly 14% better
than the 1974 models, although their
emissions are lower than the 1974's. In fact,
fuel economy of the 1975's is as good as
cars built before emission controls were
introduced (Section V-C).
6. Technology is under development in the
laboratory to further reduce emissions
without sacrifice in fuel economy from
1975 levels. Considerable engineering
development remains before these new
technologies are ready for production
(Section V-A).
7. There is no guarantee of superior fuel
economy through the use of catalytic
converters. 1975 cars using catalysts can
give excellent or poor fuel economy, de-
pending on the manufacturer's overall
design. Cars which do not use catalysts can
also give excellent or poor economy, again
depending on the overall design (Section
V-C).
8. Emission control system tampering by
garage mechanics is more likely to hurt
fuel economy than to improve it. Such
tampering virtually always makes emissions
worse, and can cause deterioration in
engine durability. Regular maintenance
according to manufacturer specifications
improves both emissions and fuel economy
(Section V-B).
9. Of the many types of alternative
engines developed and under development,
three types are available in mass-produced
1975 vehicles. When compared to
conventionally-powered 1975 cars with
similar power-to-weight ratios, present
rotary engines suffer a 30% penalty in
fuel economy, CVCC engines give about
the same economy, and Diesels provide a
35% improvement (Section V-E).
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Data Base and Test Procedures
The fuel economy data used in this report
came from tests made by EPA, auto
manufacturers, the Department of Trans-
portation and other researchers. Each
year, auto manufacturers demonstrate that
their next model year's vehicles comply
with Federal emission standards; they do
this by running their own tests and also by
submitting pre-production cars for testing
by EPA.
In the EPA tests, two separate fuel
economy values are determined for each
car—one for city and one for highway
driving, so each motorist can evaluate fuel
economy potential according to his own
mix of urban and highway travel. The
average U.S. motorist travels 55% of his
mileage under urban conditions and 45%
on the highway.
The city test procedure is a 7.5 mile
stop-and-go driving cycle with a speed range
from zero to 57 MPH and an average
speed of 20 MPH. The trip begins with a
cold start,1 takes 23 minutes, and has
18 stops. 18% of the trip time is spent
idling during these stops. The first 8-minute
segment of the trip is then repeated from
a hot start,' and test data are combined
to represent a realistic mixture of hot and
cold start urban driving.
The highway test procedure simulates a
10 mile non-stop trip with an average speed
of 48 MPH. The trip begins with a hot start
and lasts 13 minutes. Except for starting
and finishing at a standstill, the speed
range is 28 to 60 MPH.
The cars are tested indoors by professional
drivers on a chassis dynamometer, a
machine that reproduces the operation of
a vehicle under various driving conditions.
Use of a dynamometer, rather than road
tests, allows the tests to be conducted in
exactly the same way each time.
Fuel economy is calculated from measure-
ments of the amount of fuel consumed
during a test of known length (miles). This
measurement can be done by before-and-
after fuel weighing, by using flow meters
in the fuel line, or by measuring the
amount of carbon in the exhaust (since
exhaust carbon originates in the gasoline,
the amount of fuel used can be computed).
When performed correctly, any of these
techniques are acceptably accurate. EPA
uses all three methods, but relies primarily
on the carbon technique. (See Appendix
A).
These test procedures compare well with
driving patterns measured in actual traffic;
they also compare well with gas mileage
tests used by the auto industry. The fuel
economy values from these tests are in
reasonable agreement with statistics on
national fuel consumption.
Two notes of caution:
(a) Many of the fuel economy values in
this report are average values for a
number of vehicles in a given class:
An individual car run through the same
tests might give fuel economy results
above or below the average for its class,
depending on its engine, transmission,
axle ratio, accessories, etc.
(b) Reported test results are no
guarantee of the fuel economy a motorist
will get in actual driving. An individual
car operated by its owner can deliver
fuel economy different from the official
test values if the type of driving he does
differs significantly from the city and
highway cycles used in the EPA tests.
1 Engine is started after vehicle has been parked
overnight.
- Engine is started while still hot.
Ill
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Contents
Introduction i
Summary of Conclusions ii
Data Uase and Test Procedures iii
Factors Affecting Auto Fuel
Economy 1
Factors which affect engine power
load 1
Factors which affect engine
efficiency 3
Trends in car and engine design. . 6
Effects of vehicle operation 8
Combining the Influencing Factors. . 11
Emission controls 12
Effects of tampering with emis-
sion control systems 15
Fuel economy of 1975 autos .... 17
For information on specific
models 19
Alternative engines 20
Bibliography 20
Appendices 21
IV
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Factors Affecting Auto Fuel Economy
Fuel Economy, expressed in miles per
gallon (MPG), is an index of the overall
"effectiveness" achieved with a motor
vehicle which consumes fuel. It measures
what you get (miles traveled) versus what
you put in (gallons of fuel). It is related to
engine power load, vehicle speed, and
engine efficiency. (See Appendix B for a
more detailed explanation.)
For a given speed and engine efficiency,
fuel economy is high for low power
requirements and decreases as power goes
up. For a given speed and power load,
economy is directly proportional to
efficiency.
Factors Which Affect Engine
Power Load
To move a car, an engine must provide
power to overcome the following vehicle
loads:
• Rolling friction
• Aerodynamic drag
• Inertia (resistance to speed changes)
• Drive train losses
• Accessories
Figure 1 illustrates the relative contrihution
of these loads for several car si/es under
steady speed cruise conditions (where there
is no inertia effect).
Figure 2 shows the effect of cruise speed
on these variables for a standard si/e car.
Note that rolling friction predominates
at low speed, while aerodynamic drag is the
largest load at high speeds.
Rolling Friction
Rolling friction is the power lost in tires
and hearings. It depends on vehicle weight.
speed, and tire characteristics.
At any speed, doubling the weight will
double the rolling friction. For any weight,
doubling the speed will increase the
friction a little more than double, as shown
in Figures 2 and 3.
As seen in Figure 3, radial tires can have
up to 20% less rolling friction than bias-
belted tires when tested on typical road
surfaces. Tire pressure can also affect
rolling friction: higher pressures provide
reduced friction. However, inflation of tires
to pressure higher than the manufacturer's
recommendations can cause increased tire
Figure 1—Effect of Car Size on Power Requirements
40 MPH Cruise
II
10
70 MPH Cruise
1LJI
Rolling Aerody- Drive Train Accessories
Friction namic Drag Losses
Rolling Aerody- Drive Tram Accessories
Friction namic Drag Losses
Includes fan and alternator for all car sizes, power steering for two largest cars: does not include air conditioning
1
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Figure 2—Effect of Speed on Power
Requirements (Standard Size Car)
40 50
Speed-MPH
Figure 3—Rolling Friction for Three
Tire Types (Standard Size Car)
20
Figure
Power
200
30
60
40 50
Speed-MPH
4—Effect of Acceleration on
Requirements (Standard Size Car)
wear as well as a harder ride and increased
suspension system stresses.
Aerodynamic Drag
The power needed to force an automobile
through the air is a function of speed, and
the size and shape of the vehicle. The
effect of speed is quite pronounced, as
shown in Figures 1 and 2. The most
significant size factor is vehicle frontal
area.' The frontal area of modern cars
is not in direct proportion to weight, i.e.
5000 Ib. cars do not have twice the frontal
area of 2500 Ib. cars. The four car sizes
that were used to calculate the power
requirements in Figure 1 show this effect:
Table 1
Subcompact
Compact
Standard
Luxury
Curb Weight,
pounds
2500
3200
4400
5300
Frontal
area, Sq. Ft.
17.5
19.0
21.5
22.5
The influence of vehicle shape is repre-
sented by a factor called "drag coefficient",
which is lower for more streamlined
shapes. The cars of the early 1930's, for
example, had drag coefficients of about
0.70, which means the air drag on these
cars was 70% of the drag on a rectangular
box of the same overall dimensions.
Today's cars generally have drag
coefficients of less than 0.50.
Surface irregularities such as outside
mirrors, sun roofs, open windows, campers,
etc cause increased drag. In addition to
depending on vehicle speed, air drag is a
function of the direction and velocity of the
surrounding air. Headwinds (and even
crosswinds) increase air drag, and tailwinds
tend to decrease it.
Inertia
When cruising at steady speed, the only
forces acting on the car are rolling friction
and aerodynamic drag, but if one wishes
to accelerate, additional power must be
The cross-section area of (he car as viewed from
i he from
30
40 50 60
Speed—MPH
70
8C
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provided to "push" the mass of the car to
higher speeds.
Figure 4 shows this increase in power for
a two MPH/Second acceleration. Even this
mild acceleration can result in power
requirements more than triple that of
steady speed cruising.
Drive Train Losses
The power required to overcome rolling,
aerodynamic, and inertial loads must be
transmitted from the engine to the drive
wheels by the drive train (transmission and
differential/axle). Inefficiencies in the
drive train components represent a power
loss which must be made up by the engine.
The differential/axle and the transmission
gearbox each contribute about a 3% loss.
Additional losses occur with automatic
transmissions due to the torque converter
and transmission oil pump. The total losses
for automatic transmission drive trains are
approximately as follows for a standard
size car:
Table 3
Table 2
20 MPH
40 MPH
60 MPH
80 MPH
Wheel HP
4.97
14.4
31.7
60.0
Drive Train
Loss, HP
1.24 (25%)
3.17 (22%)
5.09 (16%)
7.99 (13%)
Accessories
In this report, the term "accessories" is
used to describe both necessary engine
auxiliaries (fan, alternator) and conveni-
ence devices (power steering, air
conditioning).
Accessories can add to vehicle power
requirements in two ways: by consuming
power themselves, and by adding weight.
For four particular accessories, power
consumption outweighs the weight effect:
these are the alternator, engine fan, power
steering and air conditioning. This is
illustrated in the table below for a 30 MPH
cruise:
Power-consuming accessories are more
prevalent in large cars than small ones, as
shown in Ficure 5.
Increase over 30 mph
Fan
Alternator
Power steering
Due to
Accessory
Weight
0.1%
0.2%
0.3%
cruise HP*
Due to
Accessory
Power
2%-3%
5%-20%
5%-9%
Air conditioning 1.2% 30%-50% (85°F)
* The f"c decrease in fuel economy is about 2/3 of
(he f"o increase in HP
Factors Which Affect Engine
Efficiency
As shown in the previous section, the
mechanical output of an engine must
overcome vehicle power loads such as
rolling friction, aerodv namic drag, etc. But
when fuel is burned in the cylinders of
an engine, only part of the combustion heat
energy is converted to mechanical power;
the rest of the heat is carried away by the
cooling water and the hot exhaust.
Figure 6 shows how the total combustion
energy splits between mechanical power
and waste heat under cruise conditions.
'1 he "efficiency" of an engine defines the
fraction of the combustion heat which ends
up as mechanical power output Modern
Figure 5—Accessory
Vehicle Curb Weight
100
Installation vs.
o
Q
3000 4000
Curb Weight—Pounds
5000
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Figure 6—Heat Energy
Full Size Auto Engine
200
Distribution,
Figure 7—Effect of Air-Fuel Ratio on
Efficiency and Emissions
150
100
50
auto engines operate at efficiencies from
about 10% to 30%. depending primarily
on the following factors:
• Air-fuel ratio (carburetion)
• Compression ratio
• Engine load factor
• Engine speed (RPM)
• Spark timing
Many other design features influence
efficiency, such as number of cylinders, bore
and stroke dimensions, number of rings,
valve size, etc. but these are generally
overshadowed by the above variables.
Air-Fuel Ratio
Gasoline engines are usually most efficient
at air-fuel ratios slightly above the
stoichiometric (chemically balanced)
value,' shown in figure 7. In non-stoichi-
ometric mixtures, there is either excess fuel
'(rich) or excess air (lean) present in the
combustion chamber but not entering into
the combustion reaction, and efficiency
is lowered.
The peak of the efficiency curve can be
shifted to leaner air/fuel ratios with engine
modifications.
The figure also shows the effect of air-fuel
ratio on emissions. Hydrocarbons and
carbon monoxide emissions generally
decrease with leaner mixtures due to the
increased availability of oxygen. Nitrogen
oxide emissions peak where temperature and
100
a:
06
13
15 17
Air-Fuel Ratio
oxygen concentration are both relatively
high. This is somewhat leaner than where
peak temperature occurs ( — 13:1) and
somewhat richer than the leanest mixtures
attainable.
Compression Ratio
Higher compression ratios promote higher
peak temperatures and lower exhaust
temperatures, and hence greater conversion
of the fuel's heat energy into mechanical
work. The influence of compression ratio
on efficiency varies with the engine's
operating condition. At low speeds, the
Figure 8—Effect of Compression Ratio
on Efficiency
* This value is a little less than 15 parts air to 1
part fuel, by weight.
100
Compression Ratio
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Figure 9—Effect of Load Factor on
Efficiency
Figure 10—Effect of Engine Speed on
Efficiency
100
90
LU 80
70
60
1 /4 Load
25
50
of Rated RPM
75
100
compression ratio effect is more pronounced
than at high speed, as shown in Figure 8.
Since higher compression ratios increase
engine power capability, an additional
efficiency benefit can be achieved by using
a lower-displacement engine; this holds
vehicle performance constant. Figure 8
includes this effect.
Engine Load Factor
As an engine's power level is reduced, it
operates less efficiently, as seen in Figure 9.
This occurs because a relatively closed
throttle is a barrier in the intake, and the
piston has to work harder to suck in the fuel
and air past this obstruction.
Also, when an engine is operated at low
power, it wastes a higher fraction of its
total power on internal friction, which is
essentially constant for a given RPM.
Of course, running an engine at a higher
power level will not produce better fuel
economv, even though it may make it oper-
ate more efficiently; a power increase
always overshadows the efficiency gain it
produces. For a given speed and load, a
small engine operating at a high load factor
will have higher efficiency and better
fuel economy than a larger engine running
at a low load factor.
Engine Speed
Figure 9 shows that efficiency depends on
engine RPM. This is more fully illustrated
in Figure 10, which shows the effect of
speed on efficiency for several fixed power
levels.
50
1000 2000 3000
Engine Speed—RPM
4000
A decrease in engine speed usually increases
efficiency. This occurs because lower speeds
give lower internal engine friction and
lower throttling losses.
The fuel economy effect of this can be
seen in actual practice when an engine is
run slower by means of lower axle ratios
and/or overdrives.
Spark Timing
An engine operates most efficiently when
peak combustion pressure is reached just
after the piston passes top dead center
(TDC) position. To achieve this, the fuel-
Figure 11—Effect of Spark Timing on
Power
1001
60
50
80 60 40 20
Spark Advance—Degrees Before TDC
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Figure 12—Effect of Spark Timing on
Efficiency
60 40 20
Spark Advance—Degrees Before TDC
Figure
Market
4500
4000
3500
13—Curb Weight
Class, U.S. Sales
Trends by
Figure
122
1958 1962 1966 1970 1973
Model Year
—Trends in Auto Wheelbase
8
f, 120
118
pala
1958
1962 1966
Model Year
air mixture must be ignited before TDC.
The proper ignition time is a function of
the flame speed in the combustible gas, and
must be varied with engine RPM and
engine load. Spark timing in today's engines
is controlled by manifold vacuum, an
indication of load; and by centrifugal
advance, related to RPM.
Figure 11 shows that, for a given RPM,
there is a spark advance setting which
maximizes power output.
The effect of spark timing on efficiency
appears in Figure 12 for two cruise speeds.
As with power output, maximum efficiency
occurs at some particular advance setting.
The amount of advance shown for these
driving conditions should not be confused
with idle spark advance, which is normally
just a few degrees.
Spark timing is similar to compression
ratio: too much spark advance will cause
knocking unless higher octane fuel is used.
Trends in Car and Engine Design
Many vehicle design factors have changed
notably in the last several years. Many of
these changes have had adverse effects on
fuel economy. Fuel economy is not ignored
by car designers, but many changes which
reduce gas mileage have been made in
response to requirements (real or
anticipated) in other areas. Because auto-
mobile design and development decisions
must be made several years prior to actual
production, the designer has to guess far in
advance what the consumer, economic and
government requirements will be; the
automobiles in any particular model year
are always the result of compromises,
tradeoffs, and design judgments.
Trends Affecting Power Requirements
In the past 15 years or so, most car lines
have changed in ways which increase power
requirements. The most obvious of these
is vehicle weight. As shown in Figure 13,
vehicles offered for sale in the U.S. have
been generally gaining weight at a rate of
50 to 100 pounds per year. The average
1970 1974
-------
weight of the compact and intermediate
classes dipped in the early 1960's due to the
introduction of new, lighter-weight models
(Falcon, Corvair. Chevy TI). but has
increased steadily since then. The sub-
compact class was at its lightest in 1964
due to the high sales fraction of imports,
but this class has grown in weight since the
introduction in 1970 of heavier U.S. built
models (Gremlin, Pinto, and Vega).
Note that 1973's subcompacts are nearly
Figure 15—Trends in Auto Length
225
220
205
1958
Figure 16—Increases in Figure 17—Trends
Use of Convenience Items in Engine
(Domestic Models) Displacement
o
1958 '62 '66 70
Model Year
Domestic Models
Imports
1958
as heavy as 1962's compacts, 1973's com-
pacts are as heavy as I962's intermediates,
etc.
In every one of the 11 years from 1958
to 1968, the best-selling test weight class
was 4000 pounds: in 1968 it jumped to
4500 pounds and for 1975 the projected
best-selling class is 5000 pounds. Fortun-
ately, average weight for the whole U.S.
market has gained only about 25 pounds per
year through 1973, because of the increased
sales penetration of the light weight
classes.
Car dimensions have been increasing too.
Figures 14 and 15 illustrate the trends in
wheelbase and length for the best-selling
standard size cars. This rate of increase in
dimensions is typical of all market classes.
Since some observers have linked growth in
vehicle size to government safety
regulations, note in Figure 15 that these
cars grew 14 to 15 inches longer from 1958
to 1972—presumably due to styling choices
—but they grew only 3 to 4 inches from
1972 to 1974—when increased crash-
worthiness was first required in auto
bumpers.
In addition to increases in car size, there
has been a rising demand for convenience
items which increase both vehicle weight
and power consumption Figure 16 shows
this trend for those luxury items best
known for their high power requirements.
Trends Affecting Engine Efficiency
Figure 17 illustrates how average engine
si/e has changed in the U.S. market since
1958. The drop in the early 196()'s for
domestic models resulted from the intro-
duction of new compacts and intermediates
From 1962 to 1970. average engine
displacement rose faster than average
vehicle weight a reflection of a general
trend toward higher performance. Figure 18
shows that there is a large disparity between
the way domestic cars are powered.
compared with imports: this disparitv has
not changed much in 15 years.
Detailed engine design features have been
changing also As seen in Figure 19, the
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Figure 18—Trend in Engine Size vs.
Vehicle \\eight
o 6
Q_
Q
O
Domestic Models
1958
1962
1966
Model Year
1970
1974
Figure 19—Trends in Compression Ratio
9.5
90
ra 85
cr
E 80
7.5
70
Domestic Models
Imports
1958
1962
1966
Model Year
1970
1973
Figure 20—Trends in Engine Bore and
Stroke
average compression ratio of domestic cars
declined due to high volume economy car
sales, then rose sharply until the desirability
of operating with lower octane gasolines
turned it hack downward. Compression
ratios of the imports, for a long time lower
than the domestics, have now risen to
comparable values.
As an illustration that auto engine design
changes have been going on for years,
consider Figure 20 which shows the sudden
mid-50's change in the average bore-to-
stroke ratio for domestic engines. The old
long-strokers gave way to higher RPM
short-stroke machines, to provide better
breathing, less engine friction, and snappier
vehicle performance.
Effects of Vehicle Operation
The foregoing section discussed separately
the variables affecting load and efficiency;
it is useful now to examine vehicle
operating situations wherein power level
and efficiency interact. Consider a car
cruising at constant speed versus accelerat-
ing through the same speed:
This example shows how a large power
increase can significantly diminish fuel
economy, although engine efficiency nearly
doubles. It also shows how much
acceleration rate can affect fuel economy.
Figure 21—Average Trip Speed vs. Trip
Distance
30
1950 1953
Model Year
20 40
Trip Length—Miles
-------
Table 4
Accelerating
20-»25 MPH
Cruise 2 MPH/ 4 MPH/
22.5 MPH Sec Sec
(3rd (3rd (2nd
Gear) Gear) Gear)
Avg. MPH
Avg. RPM
Avg. HP
Efficiency
MPG
22.5
1100
8.5
16%
20.0
22.5
1100
40.5
27%
6.9
22.5
1650
67.9
27%
4.0
The example in Appendix B comparing a
50 MPH cruise and a 70 MPH cruise also
illustrates the tradeoff between power and
efficiency, with the higher power case again
coming out second best in fuel economy.
Since the way a car is driven can make
such differences in fuel economy, it is
worthwhile to look at the driving patterns
actually used by motorists in the U.S.
Characteristics of Trip Patterns
Trip patterns have been studied extensively
by the U.S. Department of Transportation,
the U.S. Environmental Protection Agency
(EPA), auto manufacturers, the Society of
Automotive Engineers (SAE), and others.
These studies have found that trip length
has a large effect on average trip speed, as
shown in Figure 21.
This occurs because most long trips are
usually taken on the highway while shorter
trips tend to involve more urban travel.
with a higher frequency of stops.
In fact, there is a direct correlation between
frequency of stops and the average speed
of the trip, as illustrated in Figure 22. The
line in this figure comes from measure-
ments taken in actual traffic. The figure
also shows the average speed and stop
frequencies for test procedures developed
by EPA, the SAE. and a major auto
manufacturer. While the average speeds
of these procedures vary, they all correlate
well with the traffic measurements.
The fuel economy effects of these varying
trip characteristics appear in Figure 23, for
a standard si/e car and a compact.
Economy under cruise conditions for the
same cars is also shown for comparison.
Low speed cyclic (stop-and-go) driving
gives lower economy than steady speed
driving, because of all the accelerations in
these driving patterns. At higher speed, the
cyclic MPG is closer to the cruise MPG, but
still drops off because high speeds give
less economy than lower speeds.
Another trip characteristic which influences
fuel economy is the warmup effect
illustrated in Figure 24. The data were taken
by driving over the same one-mile road
course for varving distances, each run being
made from a cold start. Economy improves
Figure 22—Stopping Frequency vs. Average Speed for Cyclic Trips
4 I
GM Urban and SAE Urban
I
EPA Urban
GM Suburban
SAE Suburban
GM Highway
EPA Highway
Interstate
GM Interstate
10
20
30 40
Average Speed—MP
50
60
70
-------
Figure 23—Influence of Driving Pattern on Fuel Economy
Standard Size Car Compact
30
20 40 60
Average Speed—MPH
20
40
Average Speed—MPH
24—Effect of Trip Length on Cold-start City Fuel Economy
12
6 8 10
Trip Length— Miles
Figure 25 — Distribution of Auto Trips and Vehicle Miles Traveled (1970)
16
18
% of Total Trips 1 Division = 2.5%
% of Total Miles
1 Division = 1%
10
20
30 40
Trip Length—Miles
10
50
60
70
-------
Figure 26—Distributions of Trip Mile-
age and Fuel Consumption
with distance traveled because:
(a) rolling friction decreases as tires warm
up and inflation pressures rise;
(b) lubricants warm up and friction
decreases in the engine and transmission;
(c) carburetion gets leaner (less choke)
as the engine becomes hotter; and
(d) less combustion heat is lost to the
combustion chamber walls and coolant
after they warm up.
U.S. Travel Habits
In Figure 25 we see the results of some
of the trip pattern studies mentioned earlier.
Short trips overwhelmingly outnumber long
ones; the most frequently made car trip
is about one mile long. Because the most
frequent trips are so short, the distribution
of miles traveled peaks at a higher trip
distance (5 miles) than the trip
distribution, as shown by the dashed curve.
This means that more total mileage is
accumulated with 5-mile trips although
twice as many one-mile trips are taken.
Applying the data on economy vs. speed
and trip length to the distribution of trips,
we find that a significant amount of travel
in the U.S. is made under poor fuel
economy conditions.
The mileage and fuel consumption effects
of this are summarized in Figure 26, which
shows that trips of 5 miles or less make up
15% of miles driven but consume more
than 30% of all auto fuel.
Cost to the Individual Motorist
Due to the fuel economy effects of trip
length, a typical family car can take the
following trips on 25 gallons of gas:
Ten 40-mile trips, or
Sixty 4-mile trips, or
Ninety 2-mile trips, or
One hundred 1-mile trips.
At 75<1 per gallon, the fuel costs would be:
4.7o per mile on 40-mile trips, or
7.8e per mile on 4-mile trips, or
10.4* per mile on 2-mile trips, or
18.8' per mile on 1-mile trips.
30°
25°.
rt
% of Miles Traveled
% of Fuel Consumed
20%
10".
5%
0
h
0-5 6-10 11-15 16-20 21-25 26-40 Over 40
Miles Miles
Weather and Road Conditions
The section on engine power loads
mentioned how wind conditions affect
vehicle power requirements by changing air
drag. Other conditions which can influence
fuel economy are listed below, with
their economy penalties based on steady
speed cruising at about 50 MPH.
Road Conditions: MPG loss
Broken & patched asphalt 15%.
Gravel 35%
Dry sand 45%,
3% Grade 32%,
7% Grade 55%,
Environment:
18 MPH tailwind . ... .. (19% gain)
IX MPH crosswmd 2%
18 MPH headwind 17%,
50 F ambient temperature 5%
20"F ambient temperature 1 1%
Altitude (4000 ft) 15%
State of Vehicle Maintenance:
One plug misfiring 50% of time 7%,
Tires underinflated 35", 7%
Front wheels 1/4 inch out of alignment . . 2°V
Combining The
Influencing Factors
Power loads . . . efficiency . . . driving
patterns . . . these are the ingredients of
which fuel economy is made This section
will discuss the results of the \\a\s these
ingredients have been combined.
II
-------
Figure 27 illustrates fuel economy trends
observed o\er the last quarter century.
The solid curve shows the Federal Highway
Administration's estimate of total miles
driven by U.S. autos. divided by the total
fuel-they consumed. Thus the "actual
driving" cur\e includes the fuel consump-
tion of all the old and new cars on the
road, and all the driving conditions they
encounter: city and highway trips, weather
and road conditions, etc. National gas
mileage dropped about 9.4% in the 22 years
shown.
Figure 28 shows the city fuel economy of
new cars for the last nine model years,
as determined from EPA city mileage tests.
From 1967 to 1974, new cars lost 11.2%
in economy, while average new-car weight
climbed (7% for domestics, 17% for
imports), displacement rose (domestics
6%, imports 29%), air conditioning usage
increased sharply (38% of domestic 1967's
vs. 70% of 1974's), and emission standards
were legislated by Congress. More often
than not. the fuel economy loss has been
attributed to the emission standards.
Indeed, the economy trend seemed to
parallel the gradually-tightening emission
standards, as shown in Figure 28.
But in 1975 the pattern was broken: the
Figure 27—Trends in Fuel Economy—
All U.S. Cars
1960 1965
Calendar Year
1970 1975
1975 emission standards are the toughest
ever, but fuel economy has never been
better.
This calls for a closer look at recent
developments related to emissions and fuel
economy.
Emission Controls
While much has been said about the effect
of emission controls on automobile fuel
economy, a review of the available control
techniques shows that some can improve
economy, some can degrade it, and some
have no effect. Whenever fuel-efficient
techniques are chosen, emission control
need not result in fuel economy losses.
There are three types of automotive
emissions:
• Evaporative losses—consisting of raw
fuel vapor escaping from the fuel tank,
carburetor, and any leaks in the fuel
system;
• Crankcase vent gases—consisting of
blow-by combustion gases escaping past the
piston rings into the crankcase.
• Exhaust pollutants—consisting of un-
burned hydrocarbons (HC). carbon
monoxide (CO), and nitrogen oxides
(NOx), along with a host of other com-
pounds emitted in smaller amounts.
Evaporative and Crankcase Emissions
Evaporative emissions have been controlled
since 1971 " through the use of modified
gas tanks, sealed gas tank caps, and
activated charcoal canisters which store
fuel vapor during engine shutdown and
release it into the air cleaner for combustion
when the engine is operating.
Crankcase ventilation has always been
required in internal combustion engines,
to relieve pressure build-up in the crank-
case and reduce the sludge-forming and
oil dilution effects of blow-by gases, water
vapor, and unhurned fuel vapor.
•' Standards lor controlling e\aporative emissions were
lirst put into effect in 1970 in California and 1971
nationu idc
12
-------
Figure 28—Fuel Economy vs. Model Year (Fixed Model Mix, Sales Weighted)
Economy Level Before Emission Controls
Fuel Economy
S 501
HC
1001
1967
1968
1969
1970
1971
Model Year
1972
1973
1974
1975
Since the early 1960's, all new cars use
positive crankcase ventilation (PCV)
systems which route crankcase vapors into
the engine's intake manifold.
Exhaust Emissions
A variety of techniques are available for
controlling exhaust emissions. Modifications
in the fuel and air intake systems and
within the combustion chamber reduce the
formation of pollutants in the combustion
process. Other techniques function in the
exhaust to clean up pollutants which
remain after combustion. The most com-
mon of these techniques are discussed
below.
Fuel/Air Induction Modifications
Techniques frequently used upstream of
the combustion chamber include intake
manifold modifications to promote better
fuel/air distribution, intake air heating,
early fuel evaporation, improved chokes,
improved carburetion, and exhaust gas
recirculation (EGR). The first five are
usually combined to permit leaner combus-
tion, which decreases HC and CO
emissions. Up to a point (Figure 7), lean
combustion also increases fuel economy.
Exhaust gas recirculation decreases NOx
formation by lowering peak flame
temperature during combustion. EGR can
either improve or degrade fuel economy:
if excessive exhaust gas is recycled at light
loads, flame speed slows down and engine
efficiency is impaired; misfire can also
result. On the other hand, properly con-
trolled EGR systems can maintain or even
improve fuel economy by reducing
throttling and allowing higher compression
ratios and'or increased spark advance with
no change in fuel octane.
13
-------
Combustion Modifications
Emission control techniques used in the
combustion chamber include revised
chamber shapes and lower compression
ratios. The latter permits use of lower
octane, low-lead or no-lead fuels; the result-
ing lower peak flame temperatures and
higher exhaust temperatures reduce HC
and NOx emissions, but efficiency suffers.
Spark retard is often used to control
emissions. Late combustion reduces peak
temperatures, and hence NOx formation,
and increases exhaust temperature. The
hotter exhaust promotes oxidation of HC
and CO downstream of the cylinders. As
with lower compression ratios, less
mechanical work is extracted and efficiency
drops. A well-controlled spark retard
system can cut the fuel economy loss by
operating only during speed changes or
when driving in the lower transmission
gears. This reduces the economy loss in
the city and eliminates the loss on the
highway.
High-energy ignition has no direct economy
effect, but can promote leaner A/F ratios
and improve engine reliability.
Post-Combustion Chamber Modifications
Techniques used downstream of the com-
bustion chamber include enlarged exhaust
manifolds, thermal reactors, catalytic
converters, and air injection. All of these
techniques promote chemical conversion of
exhaust pollutants to relatively harmless
compounds; the devices themselves do not
affect fuel economy.
Revised manifolds and thermal reactors
both provide increased residence time of
the hot exhaust gases, promoting further
oxidation of the HC and CO leaving the
combustion chambers.
Because excess oxygen is required, systems
\\ith thermal reaction manifolds use either
lean combustion or rich combustion plus
air injection. Since thermal reaction
efficiency depends on temperature, timing
and carburetion calibrations may be used to
get high exhaust temperatures, and the
fuel economy effects of these may show up
in connection with thermal reaction
emission controls.
Catalytic converters can be used to reduce
HC and CO emissions (oxidation catalysts),
NOx emissions (reduction catalysts), or all
three (dual catalysts or three-way catalysts).
Oxidation catalysts are the only ones with
proven durability sufficient for incorpora-
tion in 1975 cars.
Unlike thermal reactors, oxidation catalysts
can reduce HC and CO emissions
effectively at normal exhaust temperatures.
These catalysts can make emission control
relatively independent from engine
operation, and permits tuning of the engine
for better efficiency.
There is no simple relation between fuel
economy and the emission levels that cars
are designed for.
It is possible to utilize any given combi-
nation of emission control techniques and
engine design specifications over a broad
range of emission levels, through
adjustment of design features and operat-
ing conditions (e.g. spark timing and
carburetor adjustments). Fuel economy
will also vary with the adjustments made
for emissions control.
Unfortunately the optimum fuel economy
is usually reached before the full emissions
control potential of the technology is
realized, so that pushing a given technology
to its ultimate emissions control potential
will result in a fuel economy lower than
optimum. For example, prior to 1975, auto
manufacturers controlled emissions mainly
through engine modifications. This resulted
in a reduction in fuel economy—especially
for large cars—from the optimum fuel
economy achievable with that particular
technology.
However, with the effective use of 1975
catalytic emission control technology, fuel
economy can be at its optimum for the
1975 Federal emission levels; hence a roll-
back of emission standards to pre-1975
Federal levels would not improve fuel
economy. At best, it might make it easier
(or cheaper) for those manufacturers with
14
-------
lower-than-average economy to come
up to par.
To achieve emission levels significantly
lower than the 1975 Federal standards, fuel
economy penalties can be incurred if only
the present technology is used; preservation
of current optimum fuel economy levels
requires the use of improved technology.
Many suitable technologies have been
demonstrated in the laboratory, but require
further development to assure adequate
durability and reliability, and to permit
production application of these technologies
to the many different types of vehicles
and engines that are produced. Some of
these technologies have fuel economy
potential that is at least as good as
provided by the best 1975 emission control
technology.
Effects of Tampering with
Emission Control Systems
It is widely believed that fuel economy can
be improved by tampering with emission
controls on today's engines.
A research project was conducted to test
this theory in actual practice; A number of
1973 and 1974 cars'1 were subjected to the
following test sequence:
11 Subconipact, compact, intermediate, and full-size
cars were included.
• Tune to manufacturer's specifications
and test for emissions and fuel economy;
• Consign to a private service garage with
a request to "do whatever they could to
improve fuel economy".
(Note: Emission control system tampering
is illegal in more than half the States, but
not as vet in Michigan: all modifications in
this project were performed by Michigan
garages.)
• Test for changes in fuel economy and
emissions resulting from the garage
tampering.
• Restore to manufacturer's specifications.
The garage modifications were performed
by a variety of shops, including service
stations, neighborhood garages, "speed
shops", nationally-franchised tune-up
centers, and foreign car specialists.
The main result of the project was that
both emissions and fuel economy were
made iror.sr by the garage modifications.
as illustrated in Figure 29.
About two-thirds of the cars lost fuel econ-
omy and increased in emissions. Less than
10% improved in both emissions and
economy. It is interesting that no
modification which improved emissions
resulted in poorer fuel economy.
The effectiveness of the garage modifica-
tions did not vary much from shop to shop.
Figure 29—Effects of Private Garage Tampering
(°o of Cars Modified)
ui 1.0
05
Fleet Emissions
15
Q. 12
10
Fleet Fuel Economy
-------
Of those shops \\hieh modified only one
car, two out of every three made fuel
economy worse: of those shops performing
more than one modification, every one
degraded the fuel economy of at least
one of the cars it worked on.
The results of the project lead to the
following conclusions:
• An attempt to improve fuel economy
by tampering with emission controls is more
likely to fail than to succeed. Few
mechanics. e\en skilled ones, have the
information necessary to fully understand
emission control systems. (Hven highly-
skilled engineers trained in emission control
technology and equipped with sophisticated
instruments sometimes make fuel economy
worse when they attempt to modify those
parts that are adjustable.)
• Any massive elTort to remove or modify
emission controls on existing cars would
result in n<> net gain, and probably some
deterioration, in nationwide fuel economy.
The only certain result of such an effort
would he a major increase in motor
\ehicle emissions.
In addition to the specific conclusions
above, these factors must be kept in mind:
• Today's auto engines have undergone
changes in design to incorporate emission
control systems. These changes are not
readily reversible on existing engines.
• Where emission reductions have been
achieved with specific devices or calibra-
tions which could be "reversed", these
modifications are so closely related to the
basic changes in engine design that they
cannot be varied independently.
• Some emission control systems and
adjustments used on late-model engines
improve fuel economy; removal or readjust-
ment of such items can only result in
simultaneous degradation of both
emissions and economy.
• Carburetor settings, ignition timing,
compression ratio, and exhaust gas
recirculation all affect engine durability.
Changes in these parameters to specifica-
tions other than those the engine was
designed for can result in mechanical
durability problems, performance problems,
or both.
Benefits of Regular Tuneups
The discussion above dealt with the prc-
dominantlv unfavorable results of
Figure 30—Fuel Economy of 1975
Models by Test Weight Class
(Sales Weighted)
Figure 31—The Separate Effects of Ve-
hicle Weight and Engine Size on 1975
Vehicles' Fuel Economy
3000 4000
Vehicle Weight-Pounds
5000
16
-------
tampering with properly-tuned engines.
The other side of the coin involves the
effects of performing manufacturer-
recommended tuneups on as-received
vehicles.
The tampering study also evaluated this
aspect of engine adjustment on a small
sample of cars.
Garage tune-ups improved both emissions
(9%) and fuel economy (8%).
Thus, it appears on the whole that well-
tuned cars are more likely to consume less
fuel and emit less pollution than either
untuned cars or cars with their emission
controls deactivated.
Fuel Economy of 1975 Autos
As discussed in earlier sections of this
report, fuel economy is determined by
many factors. This section will illustrate
1975 fuel economy as a function of vehicle
test weight class. Remember that vehicles
in these weight classes differ from each
other in more ways than weight: the
heavier cars are larger in size, they use
larger engines, and more of them use
power-consuming accessories. So weight is
not the only factor which causes the fuel
economy differences between weight
classes.
Figure 30 shows 1975 EPA city and
highway fuel economy versus vehicle test
weight. The values for each weight class
are averaged in proportion to projected
sales.
The average highway MPG for all weight
classes is nearly 50% higher than the city
MPG. This is consistent with the experience
of many motorists in actual driving.
Figure 31 shows the separate influences of
weight and engine size, when both are
varied independently.
A change to a lower engine size in the
same car can give a bigger fuel economy
improvement than a 25% weight reduction
with the same engine; a conservatively-
powered standard size car can have fuel
economy as good as, or better than, a
high-performance compact car.
The composite MPG in Figure 31 was
calculated for 55% city and 45% highway
driving. (See Appendix C.)
For each engine size, the RPM character-
istics as determined by axle ratio and tire
size are held constant over the vehicle
weight range used for that engine.
Figure 32—Late Models' Economy
Compared to Pre-emission Control
Models
Figure 33—Fuel Economy of 1975
Catalyst and Non-catalyst Cars
2000
3000 4000
Vehicle Weight—Pounds
3000 4000
Vehicle Weight—Pounds
17
-------
Figure 34—Variations in Fuel Economy
with 1975 Catalyst Technology
Figure 35—Economy of 49-Slates Mod-
els \s. California Models
3500 4500 r'"0i
Vehicle Weight—Pounds
Since the 1975 emission standards (and
control technologies used lo meet the
standards) are different from pre\ions
\cars. the difference in fuel economy
between 1975 cars anil earlier models is
of considerable interest.
Figure .12 shows that 1975 fuel
economy is belter than both pre-control
and 1974 models.
The figure is based on the economy of
individual makes and models averaged in
proportion to projected sales. Changes
from 1974 to 1975 are not the same for all
manufacturers: some automakers have
achie\ed large gains in economy, while
others ha\e lost ground. Since the
"gainers" outnumber the "losers" in sales.
the overall average has nevertheless
Table 5
Overall Fuel Economv Chance from 1974
Manufacturer
Volkswagen
Nissan
Saab
Peugeot
American Motors
Volvo
BMW
Chrysler
General Motors
Ford
All Mfrs. together
(Sales ueiehted)
Chance in City MFC
1974 to 1975
Up 4%
up 8%
up 24%
up 10%
up 21%
down 6%
down 11%
up 12%
up 28%,
down 2%
up 13.8%
2000 3000 4000
Vehicle Weight—Pounds
increased. Table 5 lists the 1974 to 1975
gains and losses for ten individual manu-
facturers, as of October 1974.
If the data is subdivided according to the
emission control approach used, it seems
that catalyst-equipped cars, as a group,
deliver better fuel economy than cars
which use other control techniques, as
depicted in Figure 33.
However, the mere presence of catalysts
in a car line does not guarantee good fuel
economy: As shown in Figure 34, the
average of the best economy 1975 catalyst
cars is significantly higher than the average
of the worst-economy catalyst cars.
Conversely, the abxcncc of catalyst usage
does not necessarily mean poor fuel
economy: many of the better fuel economy
Table 6
Relative 1975
Manufacturer
General
Motors
Saab
American
Motors
Peugeot
Nissan
Chrysler
Volkswagen
Ford
Volvo
BMW
Fuel Economy
1975 MPG
Relative to
Similar Cars
High
High
High
High
High
Average
Average
Low
Low
Low
Overall Car
Line Emission
Control
100% Catalyst
Engine Mods
15%, Catalyst
Thermal Reactor
50% Catalyst
95% Catalyst
65% Catalyst
75% Catalyst
15%, Catalyst
Thermal Reactor
18
-------
Figure 36—Typical Fuel Economy Label
Based on the results of tests conducted
or certified by the
U.S. ENVIRONMENTAL PROTECTION AGENCY,
the typical gas mileage of
this car is estimated to be:
Vehicle: Aardvark, 10 cylinder, 436 cubic inch displacement, 5 barrel
carburetor, automatic transmission, catalyst equipped, air
conditioning equipped.
10 MILES PER GALLON FOR CITY DRIVING
and
16 MILES PER GALLON FOR HIGHWAY DRIVING
These estimates are based on tests of vehicles equipped with frequently
purchased optional equipment.
Reminder: The actual fuel economy of this car will vary depending
on the type of driving you do, your driving habits, how well you main-
tain your car, optional equipment installed, and road and weather
conditions. The use of overdrive provides approximately a 3% high-
way fuel economy improvement.
To compare the fuel economy of this car with other 1976 cars, and to
learn how the tests were conducted, write for the EPA/FEA 1976
Gas Mileage Guide for New Car Buyers, to Fuel Economy, Pueblo,
Colorado 81009.
1975 models (particularly in the lighter
weight classes) achieved their optimum
fuel economy without catalysts.
Table 6 compares the relative fuel economy
and emission control approach for the same
manufacturers listed earlier.
(Tables 5 and 6 are based on data that
reflect fuel economy as of the date of the
introduction of the 1975 models. These
data can be expected to change as the
manufacturers make technology improve-
Table 7
Economy of California Models vs.
49-States Models
Manufacturer
Saab
Volkswagen
Volvo
Ford
Nissan
Peugeot
Chrysler
General Motors
American Motors
1975 MPG.
California
vs. 49-States
Calif.
Calif.
Calif.
Higher*
Higher
Higher
No Difference
Calif.
Calif.
Calif.
Calif.
Calif.
Lower"
Lower*
Lower*
Lower*
Lower*
'75 Calif, higher than '74 49-slates.
ments during the 1975 and subsequent
model years.)
From table 6 and Figure 34 it is clear that
a manufacturer's ingenuity in optimizing
his overall engine system has more effect
on fuel economy than the building blocks
he chooses.
The data base was also divided into
"California" and "49-States" groups.
Many manufacturers produce two versions
of their models—one version for sale in
California, which has stricter 1975 emission
standards, and another version for sale
elsewhere in the U.S. Figure 35 shows
the comparison between these groups.
Table 7 presents the California comparison
by manufacturer. Again, the 1975 Cali-
fornia versions tend toward lower economy
than the 1975 49-States versions, but
seven of the firms' 1975 California cars
were as good or better than their 1974's.
For Information on Specific Models
Since the 1974 model year, EPA has
sponsored a voluntary fuel economy label-
ing program, wherein manufacturers
post on each new car the EPA-determined
fuel economy of that model. Figure 36 is
a typical fuel economy label. Most manu-
facturers are participating in the labeling
19
-------
program, and arc using EPA fuel economy
test results in advertising as well.
In addition. I PA and the Federal Fnergy
\dniinistration jointly publish each model
vear a Cias Mileage (iuiiie for /Vor Car
Buyer* which lists the fuel economy of
each model of passenger cars and light duty
trucks eligible for sale in the U.S.
A separate guide is published for Cali-
fornia models.
For a copy of the Mileage Guide, write to:
Fuel Economy
Pueblo, Colorado 81009
Alternative Engines
The conventional internal combustion
gasoline engine must feel like the prover-
bial mousetrap: researchers are always
trying to replace it with a better one.
Each proposed alternative engine concept
has its own advantages and its own
particular disadvantages. Depending on the
type-of alternative engine, fuel economy
can be "good" or "bad", compared to the
conventional engine.
Three such engines are available in today's
production automobiles: the rotary
(Wankel). the CVCC stratified charge.
and the diesel. The fuel economy of these
engines is compared with conventionally-
powered cars' economy in Figure 37, which
shows that the rotary's fuel economy is
worse, the CVCC's is about the same,
and the diesel's is better.
For these engines, the comparison at
equivalent power-to-weight ratios is valid
for either citv or highway MPG's.
Figure 37—Relative Fuel Economy of
1975 Alternative Engines
-50°,
Due to Higher Energy Fuel
Same
Rotary
CVCC
Diesel
<
o
-50°,
, Loss
Compared to Conventionally
Powered Cars with Equivalent
A Power-to-Weight Ratio
Compared to Cars with Different
Power-to-Weight Ratio
Other alternative engines being developed
are: advanced versions of the stratified
charge engine, and "continuous combus-
tion" engines including the Rankine
(vapor cycle) engine, gas turbines, Stirling
engines, and others. Early experimental
versions of these have frequently shown
inferior fuel economy test results, but their
proponents believe that further develop-
ment will significantly improve these
engines' fuel economy capabilities.
It is too early to reach firm conclusions
about the feasibility of mass production
of cars powered by these advanced alterna-
tive engines; the only certainty is that
such engines will not see widespread use
before the 1980's.
Bibliography
R. A. Matula. "Consultant Report
(lo the National Research Council)
on Emissions and Fuel Economy Test
Methods and Procedures" Washing-
ton. [> C , September. 1974
L. E Furlong. E. L. Holt, and
L. S. Bernstein, ESSO Research and
Engineering Co . "Emission Control
and Fuel Economy" paper presented
at American Chemical Society meet-
ing. 1 os Angeles. Calif . April. 1974
I' S Ottice of Education. Division of
Manpower Development Training.
"Vehicle Emission Control", undated.
1. N. Bishop. Ford Motor Company.
"Effect of Design Variables on Fric-
lion and Economy." SAE paper H12
-\ . Januar> . 11>M
L E. Lirhty. Yale l"ni\ersit>. "In-
ternal C"omhust ion Engines,"
MU.rau-Hill. New York. 1939
C. F. Taylor and E S. Taylor.
Mass.ichusiMis Institute of Tech-
nnU>y>. "The I n ternal Combustion
Lr.yi:-;c." International Textbook Co ,
S^rjmon. Pa , 194X
C. F. Taylor, Massachusetts, Institute
of Technology, "The Internal Com-
bustion Engine in Theory and Prac-
tice " Wiley and Sons, New York,
1960
W. Smalley, Aerospace Corporation,
"Passenger Car Weight Trend
Analysis." report EPA-460/3-73-
006a; January, 1974.
P, J. Claffey, "Running Costs of
Motor Vehicles as Affected by Road
Design and Traffic". National" Co-
operative Highway Research Program
Report 111; 1971.
T. C. Austin and K. H. Bellman,
U. S. Environmental Protection
Agency, "Passenger Car Fuel Econ-
omy as Influenced by Trip Length."
SAE paper 750004, February. 1975.
U S. Environmental Protection
Agency. "A Report on Automotive
Fuel Economy." October, 1973.
Motor Vehicle Manufacturers Asso-
ciation. "Automobile Fuel Econ-
omy". September, 1973.
H A. Ashby and R. C. Stahman,
U.S. Environmental Protection
Agency, B. H. Eucleston and R. W.
Hum, U.S Department of the In-
terior, "Vehicle Emissions—Summer
to Winter," SAE paper 741053.
October. 1974.
J. P. DeKany and T. C. Austin,
U.S. Environmental Protection,
Agency, "Automotive Emissions and
Energy Consumption," paper pre-
sented at Air Pollution Control
Association Spring Seminar, Monte-
hello. Quebec; May, 1974.
U.S. Environmental Protection
Agency, "A Study of Fuel Economy
Changes Resulting from Tampering
with Emission Controls," Test and
Evaluation Branch Report 74-21
DWP, January, 1974
U. S Department of Transportation
and U. S Environmental Protection
Agency. "Potential for Motor Vehi-
cle Fuel Economy Improvement—
Report (o the Congress." October
1974
J. J. Gumbleton, R. A. Bolton, and
H. W Lanfc, General Motors Corp..
"Optimi/iny Engine Parameters with
Exhaust Gas Recirculation," SAE
paper 740104; February. 1974.
-------
Appendix A
To calculate fuel economy, in miles per gallon (MPG),
from an emission test, the following equation applies:
Miles gms carbon/gal of fuel
= (A-I)
Gallon gms carbon in exhaust/mile
The carbon in the fuel is:
grams fuel
grams Cm,.i = x
gallon
molecular wt. C
molecular wt. fuel
= (2798) XC.866)
= 2423
(A-2)
where:
2798 is the mean density of EPA test gasoline, in
grams/gallons; and
.866 is the weight fraction of carbon in the fuel.
The carbon in the exhaust is contained in the
unburned fuel hydrocarbons (HC). carbon monoxide
(CO), and carbon dioxide (Co^), as follows:
mol. wt. C
grams Cue = gmHC X (A-3)
mol. wt HC
= gm HC X (.866)
mol. wt. C
grams Cm =gmCO X
mol. wt. CO
= gm CO X (.429)
mol. wt. C
grams Ceo: =gm CO; X
So we have:
Miles
mol. wt. COa
= gm COs X (.273)
2423
(A-4)
(A-5)
Gallon (.866gmHC + ,429gmCO + .273gmCO2) /miles
MPG = -
2423 X miles traveled
.866gmHC + ,429gmCO + .273gmCO:>
Example: In a 10-mile test, a car's exhaust emission
measurements show the following amounts of carbon
compounds:
HC —9 grams
CO — 124 grams
COi—3641 grams
'sing equation A-6, the fuel economy is:
2423 X 10
MPG =
.866(9) +.429(124) +.273(3641)
24,230
- = 23.0 MPG
(A-6)
7.8 + 53.1 + 993.7
Appendix B
Fuel economy can be expressed in terms of speed and
fuel consumption rate, as follows:
Miles Miles/Hour
= (B-l)
Gallon Gallons/Hour
But fuel consumption rate is related to the engine
power output (not the power rating) by the
expression:
Gallons Ibs(fuel) Gals
= HPX X (B-2)
Hour HP-Hr Ib
So fuel economy is:
Miles (Mi/Hr) X (Ibs/gal) MPHXDf
-or MPG = -
Gallon (HP) X (Ib/HP-Hr) HP X SFC
(B-3)
Where DF = fuel density, pounds per gallon
(approxi mately 6.2 for gasoline); and
SFC = specific fuel consumption, pounds per hour
per horsepower output.
SFC is a commonly-used engineering term directly
related to engine efficiency. The more efficient an
engine is, the less fuel it needs to deliver a given power
output. For a typical gasoline fuel, the relationship
between SFC and engine efficiency is:
SFC =
- - (B-4)
Efficiency
(An efficiency of 13.5% corresponds to an SFC of 1.0
1.0 Ib/HP-Hr)
Substituting equation B-4 into B-3,
MPHXDf MPHX6.2XEIT.
MPG =
HPXl3.5/Eff.
HPX 13 5
So we see that fuel economy is a function of speed
(MPH), engine load (HP), and engine efficiency
according to:
MPH
MPG = . 46 X Efficiency (B-6)
HP
for a typical gasoline fuel.
Example: An intermediate size car requires an engine
output of 26 HP to cruise at 50 MPH. The engine
efficiency for this condition is 22.0% (SFC = 0.614)
Using equation B-6, the fuel economy is:
50
MPG = .46X —X22.0=19.5 MPG
26
To cruise at 70 MPH. the same car requires SfT'HP.
and the engine efficiency is 25.4% (SFC = 0.532),;,The
fuel economy is: -n;
70 hi
MPG = .46X —X25.4=16.0 MPG <^f
51 *f-
Appendix C 5 • j '.-'•
Suppose a motorist takes the following trips: ;'^ '
200 miles, using 15.0 gallons; -j ; .
100 miles, using 9.4 gallons; j
140 miles, using 11.8 gallons. _i
The fuel economies of these trips are: ' '"'
200 miles
-=13.3 MPG;
150. gal.
100 miles
9.4 gal.
140 miles
-=10.6 MPG;
-=11.9 MPG.
11.8 gal.
If he merely averages the trip MPG's, he gets: * -; .".
(13.3+10.6+11.9)-r-3=ll.9 MPG ' '- '• C, •-(
But this is incorrect. The motorist traveled 440 miles - "
and used 36.2 gallons, so his overall fuel economy was: "^ •
440-H 36.2 =12.2 MPG
To get the correct fuel economy for multiple trips, the
following equation must be used:
Miles total miles traveled
= (C-l)
Gallon total gallons used
If the individual trip lengths and fuel economy values
are known, but the gallons used are not known, the
proper equation is:
MPG = -
. . .+milesN
milesi miles2 milesN
-H -- + • • .+-
(C-2)
MPGi MPGi. MPGN
where mileS]c = length of trip 'V;
MPGi=gas mileage for trip 'V; and
N = number of trips.
For a number of test trips of the same length,
equation C-2 is equivalent to:
milest XN
MPG =
/ 1 I
nilest 1 K • +
\ MPGi MPG2 MPG
where milest = the standard test length and
N = the number of. tests.
Equation C-3 simplifies to:
(C-3)
V MPGx/
(C-4)
which is the "harmonic average" of the MPG's from
the test.
To calculate the composite MPG from known city
and highway MPG's, the apportionment of total
mileage between city and highway driving must be
used. If a motorist drives 55% of his mileage in the
city and 45% on the highway, his composite fuel
economy is:
total miles
MPG =
.55 (total miles) .45 (total miles)
City MPG
1
Highway MPG
.55
.45
(C-5)
MPGc MPGH
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