How a two-cycle diesel works

You have learned how a four-cycle diesel works. You remember that it needs four full strokes of the piston to go through one cycle intake, compression, power, and exhaust. Since there are two strokes for each complete rotation or revolution of the crankshaft, it takes two revolutions to complete a cycle. Thus there is only one power stroke for every two revolutions.

The idea of a two-cycle engine is to complete the cycle in only two strokes of the piston, that is, in only one revolution of the crankshaft. Thus a two-cycle engine running at the same speed («revolutions per minute») as a four-cycle engine will have twice as many power strokes. So here's another way of getting more power out of the same size cylinders, or of getting an engine weighing less for the same power.

Function of Air Pump. The two strokes we must have in any diesel engine are the compression and power strokes. The other two strokes are really wasted strokes, which we would like to get rid of. They are the intake and exhaust strokes. During these two strokes, the piston of the four-cycle engine is really acting as an air pump. The way we get rid of the pumping strokes in the two-cycle engine is by using a separate air pump, which takes in atmospheric air, compresses it slightly and feeds it to the engine cylinder. This separate air pump on a two-cycle engine is sometimes a rotary blower, and might be mistaken for a supercharger on a four-cycle engine, which you learned about in the previous article. However, there's an important difference: a supercharger for a four-cycle engine is just an accessory added to give the engine more power, whereas an air pump must be used on a two-cycle engine - otherwise it won't get any air and therefore won't run at all. Air pumps for two-cycle engines are made in many forms, but we won't go into details now. The important point to remember is that a blower on a two-cycle engine is simply an air pump.