Spark Ignition and Diesel Engines

Spark 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:

· Advanced 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.

The latest designs of diesel engines recently unveiled in Europe are far superior to previous designs. Oxidation catalysts and better fuel control have substantially improved particulate emission performance. Four-valve per cylinder design and direct injection have separately led to better fuel economy, higher output per unit weight, and lower emissions—though NOX emissions are still too high. Compared with a current gasoline engine, the four valve indirect injection design will yield about a 25 percent mpg gain (about 12 percent gain on a fuel energy basis), while the direct injection (Dl) design may yield as much as a 40 percent gain (30 percent fuel energy gain).

The new diesels are likely to meet California’s LEV standards for HC, CO, and particulate, but will continue to require a NOX waiver to comply with emission requirements. Although the four-valve design and other innovations (e.g., improved exhaust gas recirculation and improved fuel injection) will improve emissions performance and may allow compliance with federal Tier 1 standards, LEV standards cannot be met without a NOX reduction catalyst. Although manufacturers are optimistic about such catalysts for gasoline engines, they consider a diesel catalyst to be a much more difficult challenge.