Why Select Two Strokes?

By: Thomas Yoon

In our previous publication, we touched on the subject
of 2-stroke diesel engines. http://www.free-engineering.com/fwezine26.htm .
Today, we ask why we choose to use 2-strokes?

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. HoweverFind Article, there are some
disadvantages too. That could be discussed later...

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