By: Thomas Yoon

As you might remember, for a 2-stroke engine, the piston has to move up, and then down to complete one cycle of the engine. In those 2 strokes of the piston, the crankshaft will have turned 1 revolution. In those 2 strokes of the piston the engine has also completed 4 stages of the combustion cycle – air intake, compression, combustion and exhaust.

Now, compare that with a 4-stroke engine. The piston moving down will complete the air intake stroke. When it moves up again, the piston completes the compression stroke. Next comes the injection of fuel. Combustion takes place. The piston is forced to move down by the pressure of the gases. When the piston moves up again, it drives out all the exhaust gases in the exhaust stroke.

The piston in a 4-stroke engine will move 4 strokes in order to complete the 4 stages of the combustion cycle – air intake, compression, combustion and exhaust.

The power developed in a 4-stroke engine is based on the formula, Power = PLAN/2, where,

P is the mean effective pressure inside the cylinder,
L is the stroke length of the piston travel,
A is the cylinder bore or the area of the piston top,
N is the number of revolutions per unit time.

In a 2-stroke engine, because the power is developed in the same revolution, the formula for Power becomes, P = PLAN

This means that with the same mean effective pressure, stroke length and bore (area), the 2-stroke engine can develop twice the power of a 4-stroke engine!

If size, weight, and material cost are causes for concern, the 2-stroke engine has the clear advantage.

Well folks, 2-strokes have the advantage of higher power with small size. However, there are some disadvantages too. That could be discussed later…