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 "Second or Compression" stroke, where:

• the piston is traveling from the Bottom Dead Centre (BDC) to the Top dead center (TDC);
• the inlet valve is closed;
• the combustion is accomplished;
• the spark plug is active, and
• the exhaust valve is closed.