(DOHC) type, the spark plug is placed at the center of the combustion chamber, and the chamber
can be made very compact by having a nearly hemispherical shape. Engines incorporating these
designs have compression ratios up to 10:1, while still allowing the use of regular 87 octane
gasoline. High compression ratios also can increase hydrocarbon emissions from the engines,
although this is becoming less of a concern with newer combustion chamber designs.
Manufacturers indicated that increases beyond 10:1 are expected to have diminishing benefits in
efficiency and fuel economy and compression ratios beyond 12:1 are probably not beneficial,
unless fuel octane is raised simultaneously. The use of oxygenates in reformulated gasoline could,
however, allow the octane number of regular gasoline to increase in the future.
Reducing Mechanical Friction
Mechanical friction losses can be reduced by converting sliding metal contacts to rolling
contacts, reducing the weight of moving parts, reducing production tolerances to improve the fit
between pistons and bore, and improving the lubrication between sliding or rolling parts. Friction
reduction has focused on the valvetrain, pistons, rings, crankshaft, crankpin bearings, and the oil
pump. This is an area where OTA found considerable disagreement among manufacturers
interviewed.
Rolling contacts and lighter valvetrain.
Roller cam followers to reduce valvetrain friction are already widely used in most U.S. engines.
In OTA interviews, some manufacturers claimed that once roller cams are adopted, there is very
little fiction left in the valvetrain. Other manufacturers are pursuing the use of lightweight valves
made of ceramics or titanium. The lightweight valves reduce valvetrain inertia and also permit the
use of lighter springs with lower tension. Titanium alloys are also being considered for valve
springs. A secondary benefit associated with lighter valves and springs is that the erratic valve
motion at high rpm is reduced, allowing increased engine rpm range and power output.
Fewer rings
Pistons and rings contribute to approximately half of total fiction. The primary function of the
rings is to minimize leakage of the air-fuel mixture from the combustion chamber to the
crankcase, and oil leakage from the crankcase to the combustion chamber. The ring pack for most
current engines is composed of two compression rings and an oil ring. The rings have been shown
to operate hydrodynamically over the cycle, but metal-to-metal contact occurs often at the top
and bottom of the stroke. The outward radial force of the rings is a result of installed ring tension,
and contributes to effective sealing as well as fiction. Various low-tension ring designs were
introduced during the 1980s, especially since the need to conform to axial diameter variations or
bore distortions has been reduced by improved cylinder manufacturing techniques. Elimination of
one of the two compression rings has also been tried on some engines, and two-ring pistons may
be the low friction concept for the future. Here again, we found considerable disagreement, with
some manufacturers stating that two-ring pistons provided no friction benefits, while others
suggested fiction reduction of 5 to 10 percent.