DISC and Two-Stroke Engines

Direct Injection Stratified Charge (DISC) Engines are considered as the highest level of technology refinement for SI engines. These engines are almost completely unthrottled, and will require variable valve timing to reach their maximum potential fuel efficiency. Their high efficiency is associated with high compression ratio (up to 13), absence of throttling loss, and favorable characteristics of the products of combustion. Although DISC engines have been researched for decades (with some versions such as Ford’s PROCO almost entering production) there is renewed excitement about DISC owing to:

· Advancements in fuel injection technology (e.g., the air atomized injection system developed by Orbital, and new fast-response piezo-electric injectors developed by Toyota).

· Improved understanding and control of vortex flow in the combustion chamber (e.g., Mitsubishi’s vertical vortex system maintains charge stratification through the compression stroke over a wide speed/load range. Increased turbulence in the chamber can also be used to support combustion to very lean A/F ratios-as lean as 40: 1).

· Developments in lean NOX catalysts.

DISC engines still have problems associated with meeting future hydrocarbon (HC) and NOX standards. Manufacturers indicated that the HC problem was easier to solve than the NOX problem, and meeting a standard of 0.4 g/mi NOX or lower would require a “lean-NOX” catalyst capable of conversion efficiency over 60 percent. The development of the lean-NOx catalyst is discussed below, but several manufacturers appeared to be optimistic about the future prospects for the DISC.

Two-stroke engines

The two-stroke engine is a variant of the four-stroke DISC engine, with the potential to produce substantially higher specific power. The reduced engine weight provides fuel economy benefits in addition to those provided by the DISC design. The two-stroke design is thermodynamically less efficient than the four-stroke, however, because part of the gas expansion stroke cannot be used to generate power.

Two-stroke engine designs have been developed by various research groups and manufacturers, with Orbital, Toyota, and Chrysler publicly displaying alternative designs. The Orbital engine uses crankcase scavenging (like a traditional motorcycle two-stroke engine), with a specially developed direct injection system with air assisted atomizers. An Orbital engine installed in a European Ford Fiesta has achieved 44 mpg city, 61.3 mpg highway, for a composite fuel economy of 50.4 mpg on the EPA test cycle.52 Orbital claims a 22 percent benefit in fuel economy for this engine, 53 although it is difficult to verify this claim with available tests because the baseline vehicles have different performance.

The Orbital engine uses a very low-fiction design, with roller bearings for its crankshaft, but manufacturers doubt the durability of this system. Chrysler uses an externally scavenged design with an air compressor, so that crankcase induction and lubrication problems are avoided. Toyota uses an external induction system with exhaust valves in the cylinder head. These designs are likely to be more durable, but lose the fiction advantage, so that their fuel economy benefits are lower than the Orbital design. However, a four-stroke DISC will be more thermodynamically efficient than a two-stroke DISC, and the current opinion is that the four-stroke’s effect on fuel economy will be greater than the two stroke’s despite the latter’s weight advantage.

Summary of Engine Technology Benefits

Estimates of engine technology benefits are given in table 3-5, assuming that a lean-NOx catalyst is available for lean-bum and DISC engines. The mean for all manufacturers over the long term suggests that use of a DISC engine coupled with available friction reduction technologies can yield a 17 to 18 percent fuel consumption reduction, while an optimistic view suggests that as much as 25 percent may be available. These reductions can be achieved with no trade-off in performance although cost and complexity will increase.