Environmental Engineering Reference
Figure 8.2 (a) A diagram of the mechanical action of a reciprocating internal combustion engine and the
four piston strokes of the four-stroke cycle engine: (b) outward power stroke, (c) inward exhaust stroke,
(d) outward intake stroke, and (e) inward compression stroke.
compression, whereupon it ignites and burns quickly, without the necessity of a spark, to produce
a pressure rise.
To make a repetitive cycle out of this process, it is necessary to replace the burned gases in the
cylinder with a fresh charge (fuel-air mixture for the SI engine, pure air for the CI engine). This
is accomplished by opening ports in the cylinder connected to ducts (called manifolds) that either
conduct the fresh charge into the cylinder or collect the combustion gases leaving the cylinder at
the end of the expansion stroke. There are two mechanical schemes for effecting this replacement
of burned gas by a fresh charge. In one, called the two-stroke cycle, the fresh charge displaces the
combustion products during a short time interval when the piston is near its outermost position
(called bottom center, BC) and both an inlet port and an exhaust port are open. Only two strokes of
the piston, one inward to compress the fresh charge and the second outward to expand the burned
gases, are needed to complete this two-stroke cycle, during which the crankshaft turns through one
revolution. In the second scheme, called the four-stroke cycle [see Figure 8.2(b)-(e)], the outward
power stroke is followed by a full inward stroke in which the piston displaces the combustion
products, pushing them out through an open exhaust port located in the closed end of the cylinder
(called the cylinder head ). At the end of this inward stroke, called the exhaust stroke, the exhaust
port is closed and an inlet port is opened, allowing the piston to suck in a fresh charge as it moves
outward during the subsequent intake stroke. By adding these two extra strokes (an extra revolution
of the crankshaft), a fresh charge is prepared in the cylinder, permitting the two subsequent strokes
to compress, burn, and expand the charge, thereby producing work. These four strokes (two crank
revolutions) comprise the four-stroke cycle.
Almost all highway vehicles are powered by four-stroke cycle engines, predominantly gasoline-
fueled SI engines for passenger vehicles and diesel-fueled CI engines for trucks. Two-stroke-cycle
SI engines are mostly used to power two-wheeled vehicles, such as motorcycles and mopeds.
Engines in the latter vehicles have lower fuel efficiency and higher exhaust pollutant emissions,
but are lighter and less expensive to manufacture (for a given power). The trend of increasingly
stringent government regulation of pollutant emissions and vehicle fuel efficiency has spurred
manufacturers to improve both two- and four-stroke-cycle engines, but the predominant use of