Refer to figure.
In a four-stroke engine, the conversion of chemical energy into mechanical energy occurs over a four-stroke "Otto" or theoretical operating cycle, which consists of the following:

• First stroke - Intake or induction;
• Second stroke - Compression;
• Third stroke - Power; and
• Fourth stroke - Exhaust stroke.

In the image is shown the "First stroke - induction" where:

• the piston is traveling from the Top dead center (TDC) to the Bottom Dead Centre (BDC);
• the inlet valve is open;
• the fresh mixture is entering the cylinder;
• the spark plug is inactive, and
• the exhaust valve is closed.