Transportation - Energy and the Environment - page 207

Rolling Resistance. The rolling resistance coefficient C R can be lowered to 0.005 from current
values of 0.010-0.014 by improvements in tire design, but it is difficult to maintain durability,
performance, and safety (traction) while reducing rolling resistance. Light alloy wheels reduce
sprung mass, which is desirable, but more complex suspension systems will be needed to recover
normal performance with low-resistance tires.
8.5.2.2 Improving Engine Performance
There are several paths to improving the efficiency of engines while supplying the requisite power
needed by the vehicle. One is to improve the fuel (or thermal) efficiency of the engine, especially at
off-optimum conditions where the engine is usually operated. A second is to utilize transmissions
that permit the engine to maximize its efficiency for a required power output. A third is to reduce
engine mass, for a given power, so as to improve vehicle efficiency. However, there are serious
constraints imposed on engine efficiency improvements by the requirements for limiting exhaust
pollutant emissions. These may limit the possible efficiency improvements.
Reducing Intake Stroke Losses in SI Engines. As explained in Section 8.3, at partial load the cylinder
pressure during the intake stroke is lowered to reduce the fuel amount in the cylinder, resulting
in a loss of efficiency. This loss may be offset by varying the timing of the inlet and exhaust
valves with the engine load. Variable valve timing (VVT) systems are currently used in four-valve
engines, adding to the peak power output and engine mass reduction benefit. Another alternative is
direct fuel injection (DI) into the cylinder during the intake stroke, forming a nonuniform fuel-air
mixture at higher overall pressure and lower intake stroke power loss, albeit at some penalty in
NO x emissions. The recirculation of exhaust gas into the cylinder during the intake stroke so as
to reduce exhaust pollutant emissions can be arranged to reduce intake losses and improve engine
efficiency at part load.
Replacing SI Engines by CI Engines. The indirect injection CI engine enjoys about a 25% advantage
in fuel economy over the SI engine and does not suffer as much efficiency loss at part load as does the
SI engine. Direct injection CI engines have even higher fuel economy advantage, about 30-40%. 17
On the other hand, the CI engine is heavier, for a given power, thereby incurring a vehicle mass
efficiency penalty, and is more expensive. Also, it is more difficult to reduce NO x and particulate
emissions in the CI engine. If the latter can be overcome, the CI engine provides significant better
fuel economy.
Supercharging. Both SI and CI engines can be supercharged (or turbocharged) to increase maxi-
mum engine power for a given displacement and engine mass, with some improvement in engine
efficiency at higher loads. This provides a vehicle mass efficiency benefit.
Continuously Variable Transmission. As explained in Section 8.4.1, the traditional multistep trans-
mission does not permit the engine to operate at maximum thermal efficiency over the entire
17 In terms of thermal efficiency, the advantage is lower because of the difference in fuel heating value per unit
volume of fuel between gasoline and diesel fuel.