What is the difference between a two stroke and a four stroke engine?

2 stroke cycle engine: The engine revolves once (two strokes of the piston, one down, one up) for a complete cycle of the engine. It is very complex to explain the action of intake, compression, ignition/power, and exhaust because they vary between engines quite significantly, but all these functions occur sometime during each revolution of the two-stroke cycle engine.

4-stroke cycle engine: Each stroke is involved with a separate function which is occurring during a particular stroke. Each complete cycle of the engine involves four strokes of the piston, a down, an up, a down, and an up stroke for each complete cycle of the engine (which is two revolutions of the engine). The four stroke cycle engine is more clearly understood: the first down stroke is called the intake stroke which brings an air charge into the engine. In direct injection engines, air is all that is admitted during the intake stroke; the fuel is injected directly into the cylinder when the engine is ready. The next stroke, an up stroke of the piston, compresses either the air or the air/fuel mixture; the rising pressure heats the contents of the cylinder. Once ignition takes place, the resulting "explosion" forces the piston down through the power stroke. Momentum (or other cylinders) carries the piston back up through the exhaust stroke. Obviously there are many other things going on during all of this, but this is the basic "MO" of each type of engine.