Four-stroke engines
The vast majority of current diesel engines operate on the four-stroke principle in which combustion occurs only every other
revolution, again in the region of top deadcentre (TDC), and with the intermediate revolution and its associated piston strokes given over to the gas exchange process. In practice the exhaust valve(s) open well before bottom dead centre (BDC) following the expansion stroke and only close well after the following top dead centre (TDC) position is reached. The inlet valve(s) open
before this latter TDC, giving a period of overlap between inlet valve opening (IVO) and exhaust valve closing (EVC) during
which the comparatively small clearance volume is scavenged of most of the remaining products of combustion. Following
completion of the inlet stroke, the inlet valve(s) close well after the following bottom dead centre (BDC), after which the 'closed' portion of the cycle, i.e. the sequence compression, combustion, expansion, leads to the next cycle, commencing again with exhaust valve opening (EVO).
The main advantages of the four-stroke cycle over its two- stroke counterpart are:
(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.
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