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Four stroke refers to the quantity of times a piston travel the bore of an ICE in order to complete the combustion process.

Combustion Process

  • Intake

The piston is pulled down by the connecting rod attached to the crankshaft as the intake valve opens. The combustion charge is pulled down into the bore of the cylinder as the piston descends. The intake valve closes at the bottom of the piston stroke

  • Compression

As the crank rotates the connecting rod pushes the piston to the top of the cylinder bore. The air fuel mixture is compressed by the rising piston. Just before TDC (top dead center) the spark plug has an electric charge jump across it's center and ground electrodes igniting the air fuel mixture in a process called combustion.

  • Power

The combustion of the compressed air fuel mixture expands as the gasses undergo rapid oxidation, pushing the piston down the bore of the cylinder thereby forcing the connecting rod down and forcing the crankshaft to spin.

  • Exhaust

At the end of the power stroke the exhaust valve begins to open allowing the exhaust gasses to escape. The piston begins to rise again in the cylinder forcing any remaining exhaust gasses to be pushed out of the cylinder and into the exhaust manifold.

Illustration of the 4 cycle combustion process.

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