Comparison of Two and Four Stroke Cycle Diesel Engine
A brief but critical comparison of two and four stroke diesel engine is made below:
a) Theoretically, almost twice the power can be developed with a two-stroke engine compared with a four-stroke engine.
b) A comparison between a typical 12 liter four-stroke engine and a 7 liter two-stroke engine having the same speed range would show that they would develop similar torque and power ratings. The ratio of engine capacities for equivalent performance for these four-stroke and two-stroke engines would be 1.7:1.
c) In a four-stroke engine, the same parts generate power and empty and fill the cylinders. With the two-stroke engine, the emptying and filling can be carried out by light rotary components.
d) With a two-stroke engine, 40–50% more air consumption is necessary for the same power output; therefore the air-pumping work done will be proportionally greater.
e) About 10–20% of the upward stroke of a two-stroke engine must be sacrificed to emptying and filling the cylinder.
f) The time available for emptying and filling a cylinder is considerably less in a two-stroke-cycle engine – something like 33% of the completed cycle as compared to 50% in a four-stroke engine. Therefore more power will be needed to force a greater mass of air into the cylinder in a shorter time.
g) Compared with a two-stroke engine, more power is needed by the piston for emptying and filling the cylinder in a four-stroke engine, due to pumping and friction losses at low speeds. At higher engine speeds the situation is reversed, and the two-stroke’s Rootes blower will consume proportionally more engine power – this could be up to 15% of the developed power at maximum speed.
h) With reduced engine load for a given speed, a two-stroke engine blower will consume proportionally more of the power developed by the engine.
i) A two-stroke engine runs smoother and relatively quietly, due to the absence of reversals of loading on bearings as compared with a four-stroke engine.