. 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 peformance.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.
52 U.S. Environmental Protection Agency, "Evaluation of Research Prototype Vehicles Equipped with DI Two-Stroke Engines” EPA Report No.
EPA/AA/CTAB/92-01, January 1992.
53 Orbital Engine Co., “OCP Technical Presentation,” December 1990.