PERFORMANCE, EMISSIONS, AND FUEL ECONOMY
The previous section described energy use over a prescribed driving cycle, and treated the
variable of average engine brake specific fuel consumption, bsfc, as constant. The value of bsfc is
dependent on the size of the engine, the gear ratios and final drive ratio, as well as the engine’s
emission calibration. The size of the engine and the transmission/axle ratios have an impact upon
vehicle performance capability and affect bsfc, although the driving cycle over which fuel
economy is measured remains constant. These issues and the resultant tradeoffs with fuel
economy are discussed below.
Different levels of performance can be attained most simply be varying axle ratio, which
determines the engine rpm to vehicle speed ratio in any particular gear. Increased numerical values
of axle ratio imply higher rpm at a given speed and increased performance. The tradeoff of fuel
economy with axle ratio is nonlinear, however; fuel economy increases with decreasing axle ratio
up to a point, but decreases beyond this maximum level at even lower axle ratios. The reason is
that, at very low axle ratios, gear shifts must be delayed owing to insufficient torque at low speed
to follow the driving cycle. Figure A-3 provides an illustration of the tradeoff between fuel
economy and performance with changing axle ratio, holding all else constants As can be seem
axle ratios below 3:1 (in this example) make both performance and fuel economy worse, and
would make no sense for a manufacturer to employ. The tradeoff between axle ratio,
performance, and fuel economy is defined to the right of the fuel economy maximum point in the
figure. Statistical analysis of data from EPA tests indicates that a linear approximation of the
effect of a 10 percent increase in axle ratio is a 2.0 percent decrease in fuel economy, and a 5
percent decrease in O to 60 mph time.
6
The next option is to increase engine size, and figure A-4 shows the family of tradeoff curves of
fuel economy and performance with axle ratio for different engine sizes.^7 Larger engines obtain
worse fuel economy than smaller engines for two reasons:
l
l
increased fuel consumption during braking and idling, when the fuel consumption rate is largely a
fiction of engine size, andlower average load relative to themaximum which requires more throttling and higher pumping loss.Of course, a larger engine could be utilized with a lower axle ratio that changes the performance
and fuel economy tradeoffs. As can be seen in the figure, for some combinations of axle ratios and
engine size, different engine sizes have nearly identical fuel economy and only slightly different
performance. Statistical analysis has shown that increasing engine size by 10 percent, while
keeping all other factors constant (including weight and axle ratio), leads to approximately a 3.6
percent increase in fuel consumption.
5Fwd M@w q p=~on to he Department of Energy on fiv=p=d aut~tictrammiasions, September 1992.
6H.T Mck, “S~tjti~ Projection of FuelEconomy to the Year 2000,” presentation at the SAE Government Industry Meeting 1992.
7FWd Mot~ &., = footnote 5.