1)INTAKE  STROKE: Air fuel mixture is taken to the combustion room in this stroke. Initially both valves are closed and piston is in the top dead center position. Later in this stroke, piston moves down to the bottom dead center and intake valve opens. As intake valve opens mixture comes in due to the pressure difference between environment and the combustion room. (This is like a syringe). Finally piston is in the bottom dead center and intake valve is open.

2)COMPRESSION STROKE: After intake of mixture we need to exert some pressure on them to make the mixture heated which is a need for the reaction. To do this piston which is initially at the bottom dead center goes up and exert a pressure on the mixture. Before this movement of the piston of course both valves are closed not to have leakage.

3)POWER STROKE: This is the stroke where energy is gained by explosion. Since here we get energy this stroke is called “active”. As you can guess the other strokes are said to be “passive”. There is no change about valves here but piston moves down due to the explosion. Explosion is ignited by using spark plugs. (Spark plug is not used for  diesel engines.) Spark plugs give  the spark which  will make the mixture react each other and make  an explosion.

4)EXHAUST STROKE: This stroke is needed to throw the exhaust gases away from the engine to the outside. Simply exhaust valve opens and piston moves up to the top dead center. After exhaust gases are thrown away engine is ready for new cycle. New cycle begins with the intake. ..

There we say sometimes piston moves down up etc. Without giving any energy. This can be asked  where the energy for piston moving down up in the other strokes (other than power strokebecause in power stroke energy to move piston is gained by explosion) comes from? Here a new part which is called flywheel is the answer. It is used to store energy produced in the power stroke to use for the other strokes.

As you can see also for 2 up and down movement of the piston is equal to one cycle.

Energy conversions in the Otto cycle can be written like this in order:

Chemical energy (in the air-fuel mixture) converted to heat and straight movement of the piston due to explosion (or reaction) in the combusiton room and this straight movement is converted to the rotational movement by use of the slider-crank mechanism.  Also at the end of the combustion some amount of energy is stored at the flywheel for other (passive) cycles.