4-Stroke Engine

This is a breakdown of how an internal combustion engine works. The 4-stroke cycle requires the piston to make two trips up the cylinder and two trips down the cylinder to get one mass of air to combust. In gas engines, this requires a spark delivered by a spark plug. In diesel engines, the combustion takes place due to pressure alone.

Pistons are moved by the use of rotating lobes on the camshaft. In overhead valve (OHV) engines, the camshaft lobes move a pushrob which actuates the valves instead of moving the valves directly.

The piston is moved by a rotating crankshaft.

Intake Stroke
Piston is at the top and starts moving down. The intake valve opens, allowing air to enter.

Compression Stroke
Piston is at the botom and starts moving up. The intake valve closes, sealing the air in the cylinder.

Power Stroke
Piston reaches the top. In gas engines, a spark ignites the compressed air. In diesel engines, no spark is necessary.

Exhaust Stroke
Piston is at the bottom. The exhaust valve opens, allowing the combusted air to exit as the piston rises.