When the piston is at its highest point in the cylinder, it is in a position called top dead center. When the piston is at its lowest point in the cylinder, it is in a position called bottom dead center. As the piston moves from top dead center to bottom dead center, or vice versa, the crankshaft rotates exactly one half of a revolution.

(a) Intake Stroke.
The intake stroke begins at top dead center. As the piston moves down, the intake valve opens. The downward movement of the piston with the exhaust valve closed creates a vacuum in the cylinder.

The vacuum causes a fuel and air mixture to be drawn through the intake port into the combustion chamber. As the piston reaches bottom dead center, the intake valve closes.

(b) Compression Stroke. 
The compression stroke begins with the piston at bottom dead center. Both the intake and the exhaust valves remain closed.

As the piston moves toward top dead center, the amount of space in the upper cylinder gets smaller. The fuel and air mixture is compressed and the potential energy in the fuel is concentrated. The compression stroke ends when the piston reaches top dead center.

(c) Power Stroke. 
As the piston reaches top dead center, ending the power stroke, the spark plug ignites the compressed fuel and air mixture. Because both valves are closed, the force of the resulting explosion pushes the piston down, giving a powerful driving thrust to the crankshaft. The power stroke ends as the piston reaches bottom dead center.

(d) Exhaust Stroke. 
As the piston reaches bottom dead center, ending the power stroke, the exhaust valve opens, beginning the exhaust stroke. As the piston moves upward toward top dead center, it pushes the burnt gases from the fuel and air mixture out of the combustion chamber through the exhaust port.

As the piston reaches top dead center, ending the exhaust stroke, the exhaust valve closes. As the exhaust valve closes, the intake valve opens to begin the intake stroke in the next cycle.

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