FIRST STEP As the piston starts going down, the inlet valve is opened (mechanically opened by the turning of the crank-shaft). A mixture of air and gas is sucked in. As it reaches the bottom, the valve closes.
SECOND STEP Then the piston starts going up, caused by the force of the turning crankshaft. When it reaches the top, the air/gas mixture reaches a certain compression which is highly explosive.
THIRD STEP At this point, the electric circuit connected to the spark plug is turned on (driven mechanically by the position of the crank shaft. The spark plug causes a spark in the dense mixture of air and gas. The spark starts the explosion in the cylinder on top of the piston. This expansion caused by the explosion pushes the piston down. This force turns the crank-shaft around.
FOURTH STEP As the piston goes up again, the outlet valve opens (being driven mechanically. The used air (smoke) from the explosion escapes through the outlet valve. As the piston reaches the top, the outlet valve closes. The cycle is then repeated.

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