Comparison of two-stroke and four-stroke cycles

The main difference between the two cycles is the power developed. The

two-stroke cycle engine, with one working or power stroke every

revolution, will, theoretically, develop twice the power of a four-stroke

engine of the same swept volume. Inefficient scavenging however and

other losses, reduce the power advantage to about 1.8. For a particular

engine power the two-stroke engine will be considerably lighter—an

important consideration for ships. Nor does the two-stroke engine

require the complicated valve operating mechanism of the four-stroke.

The four-stroke engine however can operate efficiently at high speeds

which offsets its power disadvantage; it also consumes less lubricating

oil.

Each type of engine has its applications which on board ship have

resulted in the slow speed (i.e. 80— 100 rev/min) main propulsion diesel

operating on the two-stroke cycle. At this low speed the engine requires

no reduction gearbox between it and the propeller. The four-stroke

engine (usually rotating at medium speed, between 250 and 750 rev/

min) is used for auxiliaries such as alternators and sometimes for main

propulsion with a gearbox to provide a propeller speed of between 80

and 100 rev/min.

The four-stroke cycle is completed in four strokes of the piston, or two

revolutions of the crankshaft. In order to operate this cycle the engine

requires a mechanism to open and close the inlet and exhaust valves.

Consider the piston at the top of its stroke, a position known as top

dead centre (TDC). The inlet valve opens and fresh air is drawn in as the

piston moves down (Figure 2.1 (a)). At the bottom of the stroke, i.e.

bottom dead centre (BDC), the inlet valve closes and the air in the

cylinder is compressed (and consequently raised in temperature) as the

piston rises (Figure 2.1(b)). Fuel is injected as the piston reaches top

dead centre and combustion takes place, producing very high pressure

in the gases (Figure 2. l(c)). The piston is now forced down by these gases

and at bottom dead centre the exhaust valve opens. The final stroke is

the exhausting of the burnt gases as the piston rises to top dead centre to

complete the cycle (Figure 2.1(d)). The four distinct strokes are known

as 'inlet' (or suction), 'compression', 'power' (or working stroke) and

'exhaust'.