EPA-420-R-75-100

FACTORS
AFFECTING
AUTOMOTIVE
FUEL ECONOMY
US. ENVIRONMENTAL PROTECTION AGENCY
I 1 OFFICE of AIR and WASTE MANAGEMENT
MOBILE SOURCE AIR POLLUTION CONTROL
M PR01 Emission Control Technology Division	September 1975

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FACTORS AFFECTING
AUTOMOTIVE FUEL ECONOMY
September 1975

£
55
\
%
111
o
SB.
U. S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Mobile Source Air Pollution Control
Emission Control Technology Division

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Page
CONTENTS	I. Introduction			1
II. Summary of Conclusions	2
III. Data Base and Test Procedures. . . 3
IV. Factors Affecting Auto Fuel
Economy			4
A.	Factors which affect engine
power load	4
B.	Factors which affect engine
efficiency. 		7
C.	Trends in car and engine
design	9
D.	Effects of vehicle
operation	12
V. Combining the Influencing
Factors	16
A. Emission controls 	 17
B.	Effects of tampering with
emission control systems ... 20
C.	Fuel economy of 1975 autos . . 21
D.	For information on specific
models	24
E. Alternative engines 		24
Bibliography 		26
Appendices			27

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This is the third EPA report on the sub-	I. INTRODUCTION
ject 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 manufacturers,
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 pat-
terns 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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II. 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 ac-
companied 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 lead-
ing to lower efficiency (Section IV-C).
3. Driving habits and trip character-
istics 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 con-
sume more than 30% of the nation's auto-
motive fuel, because autos operate so
inefficiently in short trips (Section IV-D)
5. There is no simple or inherent
relationship 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-G).
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,
depending on the manufacturer's over-
all 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 deter-
ioration 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).
2

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III. DATA BASE AND TEST PROCEDURES
The fuel economy data used in this report
came from tests made by EPA, auto manu-
facturers, the Department of Transporta-
tion 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 test-
ing 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 starttakes 23 minutes, and has
eighteen 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 ,(2) 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 stand-
still, the speed range is 28 to 60 MPH.
The cars are tested indoors by profes-
sional drivers on a chassis dynamometer,
a machine that reproduces the operation
of a vehicle under various driving con-
ditions. 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 pri-
marily on the carbon technique. (See
Appendix A).
These test procedures compare well with
driving patterns measured in actual traf-
fic; 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 statis-
tics 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, trans-
mission, axle ratio, accessories, etc.
(b)	reported test results are no
guarantee of the fuel economy a motorist
will get in actual driving. An indi-
vidual car operated by its owner can
deliver fuel economy different from
the official test values if the type
of driving he does differs signifi-
cantly from the city and highway cycles
used in the EPA tests.
^ Engine is started after vehicle has
been parked overnight.
f2}
Engine is started while still hot.
3

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FIGURE 1 EFFECT OF CAR SIZE ON
POWER REQUIREMENTS
A. 40 MPH Cruise
Subcompact
Compact
Standard Car
Luxury Car
1 I ril
Rolling Aerodynamic Drive Accessories*
Friction	Drag
Includes fan and alternator for all car sizes;
cars; does not include air conditioning.
Rolling Aerodynamic
Friction Drag
Drive
Train
Losses
Accessories'1
includes power steering for two largest
IV. FACTORS AFFECTING AUTO FUEL ECONOMY
Fuel Economy, expressed in miles per gal-
lon (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 re-
lated to engine power load, vehicle speed,
and engine efficiency. (See Appendix B
for more detailed explanation.)
For a given speed and engine efficiency,
fuel economy is high for low power re-
quirements and decreases as power goes
up. For a given speed and power load,
economy is directly proportional to
efficiency.
A. 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 contri-
bution of these loads for several car
sizes under steady speed cruise condi-
tions (where there is no inertia effect).
Figure 2 shows the effect of cruise
speed on these variables for a standard
size car. Note that rolling friction
predominates at low speed, while aero-
dynamic drag is the largest load at high
speeds.
FIGURE 2 EFFECT OF
SPEED ON POWER
REQUIREMENTS
Speed - MPH
4

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1. Rolling Friction
Rolling friction is the power lost in
tires and bearings. 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.
is not in direct proportion to weight,
i.e. 5000 lb. cars do not have twice the
frontal area of 2500 lb. cars. The four
car sizes that were used to calculate the
power requirements in Figure 1 show this
effect:
Table 1
Curb Weight, Frontal
pounds	 Area, Sq. Ft.
FIGURE 3 - ROLLING FRICTION
FOR THREE TIRE TYPES
(STANDARD SIZE CAR)
Subcompact
Compact
Standard
Luxury
2500
3200
4400
5300
17.5
19.0
21.5
22.5
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 pro-
vide reduced friction. However, infla-
tion of tires to pressures higher than
the manufacturer's recommendations can
cause increased tire wear as well as a
harder ride and increased suspension
system stresses.
2. 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
(31
The cross-section area of the car as
veiwed from the front.
The influence of vehicle shape is repre-
sented by a factor called "drag coeffi-
cient", which is lower for more stream-
lined shapes. The cars of the early
1930's, for example, had drag coeffi-
cients 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 gener-
ally 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. Head-
winds (and even crosswinds) increase air
drag, and tailwinds tend to decrease it.
3. 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 provided to "push" the
mass of the car to higher speeds.
Figure 4 shows this increase in power for
a 2 MPH/Second acceleration. Even this
mild acceleration can result in power
requirements more than triple that of
steady speed cruising.
5

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200
FIGURE 4 EFFECT OF
ACCELERATION ON POWER REQUIREMENTS
(STANDARD SIZE CAR)
4. 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 trans-
mission gear box 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:
20 MPH
40 MPH
60 MPH
80 MPH
Table 2
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%)
5 o Accessories
In this report, the term "accessories"
is used to describe both necessary engine
auxiliaries (fan, alternator) and con-
venience devices (power steering, air
conditioning).
Accessories can add to vehicle power re-
quirements 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:
Table 3
Increase over 30 mph cruise HP*
Due to	Due to
Accessory Accessory
Weight	Power
Fan	0.1%
Alternator	0.2%
Power steering	0.3%
Air conditioning	1.2%
2%-3%
5%-20%
5%-9%
30%-50% (85°F)
: The % decrease in fuel economy is about 2/3
of the % increase in HP.
Power-consuming accessories are more pre-
valent in large cars than small ones, as
shown in Figure 5.
FIGURE 5 - ACCESSORY INSTALLATION
VS. VEHICLE CURB WEIGHT
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40
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|' ^Power
Steering
I J?
N ^ Air
X 	 Conditioning
Model Year
1970
0
2000
3000	4000
Curb Weight - Pounds
5000
6

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FIGURE 6 HEAT ENERGY DISTRIB-
UTION, FULL SIZE AUTO ENGINE
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Efficiency
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15 17 19
Air-Fuel Ratio
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(A)
This value is a little less than 15
parts air to 1 part fuel, by weight.

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2. Compression Ratio
Higher compression ratios promote higher
peak temperatures and lower exhaust
temperatures, and hence greater con-
version of the fuel's heat energy into
mechanical work. The influence of com-
pression ratio on efficiency varies with
the engine's operating condition. At low
speeds, the compression ratio effect is
more pronounced than at high speed, as
shown in figure 8.
FIGURE 8 - EFFECT OF COMPRESSION
RATIO ON EFFICIENCY
Compression Ratio
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.
3. Engine Load Factor
As an engine's power level is reduced,
it operates less efficiently, as seen in
Figure 9. This occurs because a re-
latively closed throttle is a barrier
in the intake, and the piston has to
work harder to suck in the fuel and air
FIGURE 9 - EFFECT OF LOAD FACTOR
ON EFFICIENCY
% of Rated RPM
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
economy, 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.
4. Engine Speed
Figure 9 showed that efficiency depends
on engine RPM. This is more fully ^illus-
trated in Figure 10, which shows the
effect of speed on efficiency for several
fixed power levels.
A decrease in engine speed usually in-
creases 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.
8

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FIGURE 10 " EFFECT OF ENGINE
SPEED ON EFFICIENCY
60 HP
1000	2000
Engine Speed
4000
5. 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-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 mani-
fold 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 con-
fused 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.
FIGURE 12 - EFFECT OF SPARK
TIMING ON EFFICIENCY
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2 5 MPH
80	60
Spark Advance
40	20	0
Degrees Before TDC
C. TRENDS IN CAR AND ENGINE DESIGN
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90
80
70
60
50
FIGURE 11 - EFFECT OF SPARK
TIMING ON POWER
1500 RPM
1200 RPM
80	60	40	20	0
Spark Advance - Degrees Before TDC
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
automobile 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 re-
quirements will be; the automobiles in
any particular model year are always the
result of compromises, tradeoffs, and
design judgments.

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1. 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 weight of the compact and inter-
In every one of the eleven 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 in-
creased sales penetration of the light
weight classes.
FIGURE 13 - CURB WEIGHT TRENDS
BY MARKET CLASS, U.S. SALES
4500,—
Standard
Intermediate
Compact
Subcompact
1500
1958
1962 1966 1970 1973
Model Year
mediate classes dipped in the early 1960's
due to the introduction of new, lighter-
weight models (Falcon, Corvair, Chevy II),
but has increased steadily since then.
The subcompact 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
as heavy as 1962's compacts, 1973's com-
pacts are as heavy as 1962's inter-
mediates, etc.
Car dimensions have been increasing
too. Figures 14 and 15 illustrate the
trends in wheelbase and length for the
FIGURE 14 - TRENDS IN
AUTO WHEELBASE
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(1)
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XX
122
121
120
qj 119
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11*
117
225
Galaxie 500
Impala
_L
1958
1962
1966
1970
1974
FIGURE 15 TRENDS IN
AUTO LENGTH
1958 1962 1966 1970 1974
10

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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
crashworthiness was first required in
auto bumpers.
In addition to increases in car size,
there has been a rising demand for con-
venience items which increase both vehicle
weight and power consumption. Figure 16
shows this trend for those luxury items
best known for their high power require-
ments .
FIGURE 16- INCREASES IN USE OF
CONVENIENCE ITEMS (DOMESTIC MODELS)
100 _
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Transmission
Air
Conditioning
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2. Trends Affecting Engine Efficiency
Figure 17 illustrates how average engine
size has changed in the U.S. market since
1958. The drop in the early 1960's for
FIGURE 17 TRENDS IN
ENGINE DISPLACEMENT
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320 _
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FIGURE 18 - TRENDS IN ENGINE
SIZE VS, VEHICLE WEIGHT
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1962
1966	1970
1974
FIGURE 19 - TRENDS IN
COMPRESSION RATIO
Domestic
Models
1962	1966	1970 1973
FIGURE 20 - TRENDS IN
ENGINE BORE AND STROKE
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1947
1950
1953
1956
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.
D. 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
accelerating through the same speed:
Table 4
Accelerating
20-*-25 MPH
2 MPH/Sec 4 MPII/Sec
Cruise
22.5 MPH		
(3rd Gear) (3rd Gear) (2nd Gear)
Avg. MPH
22.5
22.5
22.5
Avg. RPM
1100
1100
1650
Avg. HP
8.5
40.5
67.9
Efficiency
16%
27%
27%
MPG
20.0
6.9
4.0
desirability of operating with lower
octane gasolines turned it back 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
This example shows how a large power in-
crease can significantly diminish fuel
economy, although engine efficiency nearly
doubles. It also shows how much accelera-
tion rate can affect fuel economy.
The example in Appendix B comparing a
50 MPH cruise and a 70 MPH cruise also
12
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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.
1. Characteristics of Trip Patterns
Trip patterns have been studied exten-
sively by the U.S. Department of Trans-
portation, the Environmental Protection
Agency, auto manufacturers, the Society
of Automotive Engineers, 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.
FIGURE 21 - AVERAGE TRIP SPEED
VS. TRIP DISTANCE
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Trip Length
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 measurements taken in actual traffic.
The figure also shows the average speed
and stop frequencies for test procedures
developed by EPA, the SAE, and a jnajor
auto manufacturer. While the average
speeds of these procedures vary, they
all correlate well with the traffic
measurements.
FIGURE 22-STOPPING FREQUENCY VS.
AVERAGE SPEED FOR CYCLIC TRIPS
10
GM
Interstate
30
Average Speed
MPH
13
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FIGURE 2 3 - INFLUENCE OF DRIVING PATTERN ON FUEL ECONOMY
30
It!
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20
10
30
20
10
Standard Size Car
Cruise
Cyclic
O EPA Test
9 Mfr Test
Compact Car
1
20	40	60
Average Speed-MPH
The fuel economy effects of these varying
trip characteristics appear in figure 23,
for a standard size 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
100
90
20	40	60
Average Speed-MPH
cruise MPG, but still drops off because
high speeds give lower economy than lower
speeds.
Another trip characteristic which influ-
ences fuel economy is the warmup effect
illustrated in figure 24. The data were
taken by driving over the same 1-mile
road course for varying distances, each
run being made from a cold start. Economy
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20
10
EFFECT OF TRIP LENGTH ON COLD-START
CITY FUEL ECONOMY
% Pre-Emission
Controlled Vehicles
xn A 1973 Vehicles
6 7 8 9 10 11
Trip Length -Miles
12 13 14 15
16
17 18
14
-------
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&
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3. 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 75c? per gallon,	the fuel costs would be:
4.7C per mile	on 40-mile trips, or
7.8c per mile	on 4-mile trips, or
10.4c per mile	on 2-mile trips, or
18.8c per mile	on 1-mile trips.
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)
18 MPH crosswind	2%
18 MPH headwind ....
50°F ambient temperature
20°F ambient temperature
Altitude (4000 ft). . .
State of Vehicle Maintenance:
17%
5%
11%
15%
One plug misfiring 50%
of time	7%
Tires underinflated 35% ... 7%
Front wheels 1/4 inch out of
alignment. ......... 2%
4. 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 in the table, with their economy
penalties based on steady speed cruising at
about 50 MPH.
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
ways these ingredients have been combined,
Figure 27 illustrates fuel economy trends
observed over the last quarter century.
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15
14
13
12
11
10
All Cars In U.S. (Actual Driving)
FIGURE 2 7 - TRENDS IN FUEL ECONOMY - ALL U.S. CARS
1950
1955
1960
1965
1970
1975
Year
16
-------
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" curve includes the fuel con-
sumption 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
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.
A. 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:
(a)	Evaporative losses - consisting
of raw fuel vapor escaping from the
fuel tank, carburetor, and any leaks
in the fuel system;
(b)	Crankcase vent gases - consisting
of blow-by combustion gases escaping
past the piston rings into the crank-
case.
(c)	Exhaust pollutants - consisting of
unburned hydrocarbons (HC), carbon mon-
odixe (CO), and nitrogen oxides (NOx),
along with a host of other compounds
emitted in smaller amounts.
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13
12
FIGURE 28
FUEL ECONOMY VS MODEL YEAR
(FIXED MODEL MIX, SALES WEIGHTED)
Economy Level
01
n 73
2 h
•H	rtf
CO r^-J
» a
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c	.p
H	CO
25
50
75
100
1967 1968 1969 1970 1971 1972 1973 1974 1975
Model Year
17
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1.	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 re-
lease it into the air cleaner for combus-
tion 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 by blow-by gases,
water vapor, and unburned fuel vapor.
Since the early 1960's, all new cars use
Positive Crankcase Ventilation (PCV)
systems which route crankcase vapors
into the engine's intake manifold.
2.	Exhaust Emissions
A variety of techniques are available for
controlling exhaust emissions. Modifica-
tions in the fuel and air intake systems
and within the combustion chamber reduce
the formation of pollutants in the com-
bustion process. Other techniques
function in the exhaust to clean up
pollutants which remain after combustion.
The most common of these techniques
are discussed below.
(a) 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 com-
bustion, which decreases HC and CO
emissions. Up to a point (Figure 7),
lean combustion also increases fuel
economy.
Standards for controlling evaporative
emissions were first put into effect in
1970 in California and 1971 nationwide.
Exhaust gas recirculation decreases NOx
formation by lowering peak flame tempera-
ture 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
controlled EGR systems can maintain or
even improve fuel economy by reducing
throttling and allowing higher com-
pression ratios and/or increased spark
advance with no change in fuel octane.
(b)	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 resulting
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 effi-
ciency 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.
(c)	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 con-
18
-------
version of exhaust pollutants to re-
latively 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, sys-
tems with thermal reaction manifolds use
either lean combustion or rich com-
bustion 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 3-way catalysts). Oxidation catalysts
are the only ones with proven durability
sufficient for incorporation in 1975 cars.
Unlike thermal reactors, oxidation cat-
alysts can reduce HC and CO emissions
effectively at normal exhaust tempera-
tures. These catalysts can make emission
control relatively independent from engine
operation, and permit 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 com-
bination 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 emis-
sions 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 control-
led emissions mainly through engine
modifications. This resulted in a re-
duction 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 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 re-
quire 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.
19
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B. 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 were
subjected to the following test sequence:
(a)	Tune to manufacturer's specifica-
tions and test for emissions and fuel
economy;
(b)	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 yet in Michigan; all modifica-
tions in this project were performed by
Michigan garages.)
(c) Test for changes in fuel economy and
emissions resulting from the garage
tampering.
Subcompact, compact, intermediate, and
full-size cars were included.
(d) Restore to manufacturer's specifica-
tions.
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 worse by the garage modifications,
as illustrated in figure 29.
About 2/3 of the cars lost fuel economy
and increased in emissions. Less than
10% improved in both emissions and econ-
omy.
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.
Of those shops which 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.
FIGURE 29 - EFFECTS OF PRIVATE GARAGE TAMPERING
2.0
1. 5
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w

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-p
rd
1. 0
A r
pi 0. 5
(% Of Cars Modified)


CO

15



NO
X
a 14






HC



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i—1
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rn
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1
1
1
fore
After

After

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W
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1 0
Fleet Emissions
Fleet Fuel Economy
20
-------
The results of the project lead to the
following conclusions:
(a)	An attempt to improve fuel economy
by tampering with emission controls is
more likely to fail than to succeed. Few
mechanics, even skilled ones, have the
information necessary to fully understand
emission control systems. (Even 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.)
(b)	Any massive effort to remove or
modify emission controls on existing
cars would result in no net gain, and
probably some deterioration, in nation-
wide fuel economy. The only certain
result of such an effort would be a
major increase in motor vehicle 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 cali-
brations 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 re-
circulation 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 pre-
dominantly unfavorable results of tamper-
ing 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. The results are shown
below:
Garage Tune-up
Emissions	Improved 9%
Fuel Economy	Improved 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.
C. 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.
21
-------
FIGURE 30 - FUEL ECONOMY OF 1975 MODELS
BY TEST WEIGHT CLASS
(SALES WEIGHTED)
4 0,	
O
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I
30
£
O
G
O 20
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0)
3
Pn
10
Highway
MPG

i
_L
2000 3000 4000	5000
Vehicle Weight Pounds
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.
FIGURE 31 - THE SEPARATE EFFECTS OF
VEHICLE WEIGHT AND ENGINE SIZE
ON 1975 VEHICLES' FUEL ECONOMY
30
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&<
S
>i
fd
5
JZ
tn
•H
EC
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u
20
10
Small V-8
Large V-
2000	3000	4000
Vehicle Weight - Pounds
5000
The composite MPG in figure 31 was cal-
culated 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.
Since the 1975 emission standards (and
control technologies used to meet the
standards) are different from previous
years, the difference in fuel economy
between 1975 cars and earlier models is
of considerable interest.
Figure 32 shows this comparison.
FIGURE 32 - LATE MODELS' ECONOMY COMPARED TO
PRE-EMISSION CONTROL MODELS
30
O
Pj
S
20
O
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o
o
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3
P4 10
-P
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u



v\
s
\ x
\
\
Pre-Cont
Model
1975
rol N
s
Models

1974
t
Models
0
2000
3000	4000
Vehicle Weight Pounds
5000
It is clear from the figure that 1975
fuel economy is better than both pre-
control and 1974 models.
Figure 32 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
achieved large gains in economy, while
others have lost ground. Since the
"gainers" outnumber the "losers" in
sales, the overall average has never-
22
-------
theless increased. Table 5 lists the
1974 to 1975 gains and losses for ten
individual manufacturers, as of October
1974.
Table 5
Overall Fuel Economy Change from 1974
FIGURE 34 - VARIATIONS IN FUEL ECONOMY
WITH 1975 CATALYST TECHNOLOGY
Manufacturer
Volkswagen
Nissan
Saab
Peugeot
American Motors
Volvo
BMW
Chrysler
General Motors
Ford
All Mfrs. together
(Sales weighted)
% Change in City MPG
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%
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 tech-
niques, as depicted in figure 33.
FIGURE 33 FUEL ECONOMY OF 1975
CATALYST AND NON-CATALYST CARS
30
20
w
o.
s
+J
10
\ v
\\


\ \
\ \
V— 	>
Non
Cataly
Cars
-Catalyst
st

Cdl s

0
2000
3000	4000
Vehicle Weight Pounds
5000
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.
30
u
CM
s
¦P
20
10
\


V
V
\
\
Best 1975
Catalyst Cars
	

Cc
/
Worst 1975
italyst Cars

0
2500
3500	4500
Vehicle Weight - Pounds
5500
Conversely, the absence of catalyst
usage does not necessarily mean poor
fuel economy: many of the better-fuel
economy 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.
Table 6
Relative 1975 Fuel Economy
Manufacturer
General Motors
Saab
American Motors
Peugeot
Nissan
Chrysler
Volkswagen
Ford
Volvo
BMW
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
(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
improvements during the 1975 and sub-
sequent model years.)
23
-------
From table 6 and figure 34 it is clear
that a manufacturer's ingenuity in optimi-
zing 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 com-
parison by manufacturer. Again, the 1975
California 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.
FIGURE 35 - ECONOMY OF 49-STATES
MODELS VS. CALIFORNIA MODELS
\


\\
\V
49-S
V Ca
tates
rs

i
Califor
Cars
lia
2000	3000	4000	5000
Vehicle Weight - Pounds
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. Higher*
Calif. Higher
Calif. Higher
No Difference
Calif. Lower*
Calif. Lower*
Calif. Lower*
Calif. Lower*
Calif. Lower*
In addition, EPA and the Federal Energy
Administration jointly publish each model
year a Gas Mileage Guide for New Car
Buyers 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
* '75 Calif, higher than '74 49-states.
E. ALTERNATIVE ENGINES
D. FOR INFORMATION ON SPECIFIC MODELS
Since the 1974 model year, EPA has
sponsored a voluntary fuel economy
labeling 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
manufacturers are participating in the
labeling program, and are using EPA fuel
economy test results in advertising as
well.
The conventional internal combustion
gasoline engine must feel like the pro-
verbial mousetrap: researchers are
always trying to replace it with a
better one.
Each proposed alternative engine concept
has its own advantages and its own parti-
cular disadvantages. Depending on the
type of alternative engine, fuel economy
can be "good" or "bad", compared to the
conventional engine.
24
-------
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 maintain
your car, optional equipment installed, and road and weather conditions.
The use of overdrive provides approximately a 3% highway 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.
FIGURE 36 - TYPICAL FUEL ECONOMY LABEL
25
-------
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 city or highway MPG's.
FIGURE 3 7 - RELATIVE FUEL ECONOMY OF
19 7 5 ALTERNATIVE ENGINES
0)
C
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CFi
C
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C
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+100%
-p
u
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+ 50%
^ Same
o
- 5 01
• Due To Higher•
Energy FuelC
22

Ga
4
in
k
(i)
(2)

Rotary
(1)


(2)
a)

CVCC Diesel


T
Loss
(1)	Compared To Conventionally Powered
Cars With Equivalent Power-To-Weight
Ratio
(2)	Compared To Cars With Different Power-
To-Weight Ratio
Other alternative engines being developed
are: advanced versions of the stratified
charge engine , and "continuous combustion"
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
development 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 al-
ternative engines; the only certainty is
that such engines will not see wide-
spread use before the 1980's.
R. A. Macula, "Consultant Report (to the
National Research Council) on Emissions
and Fuel Economy Test Methods and Pro-
cedures", Washington, D.C.; September,
1974.
L. E. Furlong, E. L. Holt, and L. S.
Bernstein, Esso Research and Engineering
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of Design Variables on Friction and
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Yorkj 1939.
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Massachusetts Institute of Technology,
"The Internal Combustion Engine,"
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Pa.; 1948.
BIBLIOGRAPHY
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Engine in Theory and Practice," Wiley
and Sons, New York; 1960.
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"Passenger Car Weight Trend Analysis,"
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Traffic", National Cooperative Highway
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26
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.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-l)
Gallon gms carbon in exhaust/mile
The carbon in the fuel is:
grams fuel
8rams	—
molecular wt. C	(A-2)
molecular wte fuel
= (2798) x (.866)
= 2423
where:
2798 is the mean density of EPA test
gasoline , in grams/gallon; 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 (C02),
as follows:
grams CHC gm HC x mol. wt> Hc
= gm HC x (.866)
mol. wt. C
(A-3)
mol. wt. C	/» /\
grams Cc0 = gm CO x mol- wt- c0	(A-4)
= gm CO x (.429)
	 n mol« wt. C	/a c\
grams - gm C02 x mol. wt. c02	
So fuel economy is:
Miles (Mi/Hr) x (lbs/gal)
Gallon * (HP) x (lb/HP-Hr)
MPG
MPH x Df
HP x SFC
(B-3)
Where Df fuel density, pounds per gallon
(approximately 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:
13.5
Efficiency
(B-4)
(An efficiency of 13.5% corresponds
to an SFC of 1.0 lb/HP-Hr)
Substituting equation B-4 into B-3,
MPH x Df	MPH x 6.2 xEff.
MPG =
HP x 13.5/Eff.
HP x 13.5
(B-5)
So we see that fuel economy is a function
of speed (MPH), engine load (HP), and
engine efficiency according to:
MPG = .46
MPH
HP
Efficiency
(B-6)
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:
MPG .46 x x 22.0
/o
19.5 MPG
To cruise at 70 MPH, the same car requires
51 HP, and the engine efficiency is 25.4%
(SFC = 0.532). The fuel economy is 2
.46
70
51
x 25.4 16.0 MPG
nno
-Appendix C
Suppose a motorist takes the follow-
ing trips:
200 miles, using 15..0 gallons;
100 miles, using 9.4 gallons;
140 miles, using 11.8 gallons.
The fuel economies of these trips are:
200 miles
15.0 gal.
100 miles
9.4 gal.
140 miles
11.8 gal.
= 13.3 MPG;
= 10.6 MPG;
= 11.9 MPG.
If he merely averages the trip MPG's,
he gets:
(13.3 + 10.6 + 11.9) t3= 11.9 MPG
But this is incorrect. The motorist
traveled 440 miles and used 36.2 gal-
lons, so his overall fuel economy was:
440 4-36.2 12.2 MPG
To get the correct fuel economy for
multiple trips, the following equa-
tion must be used:
Miles
total miles traveled
Gallon total gallons used
(C-l)
If the individual trip lengths and fuel
economy values are known, but the gallons
used are not known, the proper equation
is:
milesi + miles2 + ... + milesN
MPG « 	
miles! miles2	milesK
MPG,
MPG2
MPGW
(C-2)
where milesx length of trip "x,?;
MPGX gas mileage for trip rV'
and
N = number of trips.
For a number of test trips of the same
length, equation C-2 is equivalent to:
27
MPG
milest x N
milest
(mPG! + MPG
> C A IN	
+-M
ipg2 • • • mpgnJ
(C-3)
where milest the standard test length and
N the number of tests.
Equation C-3 simplifies to:
MPG
»n («k)
(C-4)
which is the "harmonic average11 of the
MPG's from the tests.
To calculate the composite MPG from
known city and highway MPG's, the apportion-
ment 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:
MPG
total miles
.55(total miles)
City MPG
.55
HPGr
.45
MPGh
+ .45 (total miles)
Highway MPG
(C-5)
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