BOX 1-2: Spark Ignition and Diesel EnginesSpark Ignition Engines
Although spark ignition (SI) engines have been the dominant passenger car and light truck powerplant in the
United States for many decades, there are several ways to achieve additional improvements in efficiency---either
through wider use of some existing technologies or by introduction of advanced technologies and engine concepts.
Some key examples of improved technology, most having some current application, are:
lllAdvanced electronic controls; improved understanding of combustion processes. Improved thermodynamic
efficiency through improved spark timing, increased compression ratios, and faster combustion.Use of lightweight materials in valves, valve springs, and pistons, advanced coatings on pistons and ring
surfaces, improved lubricants. Reduced mechanical friction.Increased number of valves per cylinder (up to five), variable timing for valve opening, deactivating cylinders at
light loads, variable tuning of intakes to increase intake pressure. Reduce “pumping losses” caused by throttling
the flow of intake air to reduce power output.Combining the full range of improvements in a conventional engine can yield fuel economy
improvements of up to 15 percent from a baseline four-valve engine.
Besides improvements in engine components, new engine concepts promise additional benefits. The highest
level of technology refinement for SI engines is the direct injection stratified charge (DISC) engine. DISC engines
inject fuel directly into the cylinder rather than premixing fuel and air, as conventional engines do; the term
“stratified charge” comes from the need to aim the injected fuel at the spark plug, so the fuel-air mixture in the
cylinder is highly nonuniform. DISC engines are almost unthrottled; power is reduced by reducing the amount of
fuel injected, not the amount of air. As a result, these engines have virtually no throttling loss and can operate at
high compression ratios (because not premixing the fuel and air avoids premature ignition). DISC engines have
been researched for decades without successful commercialization, but substantial improvements in fuel injection
technology and in the understanding and control of combustion, and a more optimistic outlook for nitrogen oxide
(NOX) catalysts that can operate in an oxygen-rich environment make the outlook for such engines promising. The
estimated fuel economy benefit of a DISC engine coupled with available friction-reduction technology and
variable valve timing ranges from 20 to 33 percent, compared to a baseline four-valve engine.
Diesel Engines
Automakers can achieve a substantial improvement in fuel economy by shifting to compression ignition (diesel)
engines. Diesels are more efficient than gasoline engines for two reasons. First, they use compression ratios of
16:1 to 24:1 versus the gasoline engine’s 10:1 or so, which allows a higher thermodynamic efficiency. Second,
diesels do not experience the pumping loss characteristic of gasoline engines because they do not throttle their
intake air; instead, power is controlled by regulating fuel flow alone. Diesels have much higher internal friction than
gasoline engines, however, and they are heavier for the same output.
Diesels are not popular in the U.S. market because they generally have been noisier, more prone to vibration,
more polluting, and costlier than comparable gasoline engines. Although they have low hydrocarbon (HC) and
carbon monoxide (CO) emissions, they have relatively high NOX and particulate emissions.| DISC engines’absence of throttling means that at low power, less fuel is injected with the same amount of air. This leaves a substantial
concentration of oxygen in the exhaust, whichcancels the effectiveness of conventional NOX catalysts.