The main advantages of the four-stroke cycle over its two stroke

(a) the longer period available for the gas exhange process and the separation of the exhaust and inlet periods— apart from the comparatively short overlap—resulting in a purer trapped charge.

(b) the lower thermal loading associated with engines in which pistons, cylinder heads and liners are exposed to the most severe pressures and temperatures associated with combustion only every other revolution.

(c) Easier lubrication conditions for pistons, rings and liners due to the absence of ports, and the idle stroke renewing liner lubrication and giving inertia lift off to rings and small and large end bearings.

These factors make it possible for the four-stroke engine to achieve output levels of the order of 75% of equivalent two stroke engines. In recent years attention has focused particularly on three-cylinder high speed passenger car two-stroke engines as a possible replacement for conventional four-cylinder, four stroke engines with considerable potential savings in space and weight.