Step 1: The Intake Stroke

The intake stroke refers to how air and fuel enter an engine’s combustion chamber. The process behind this first step, however, varies by engine type.

In port-fuel-injected (PFI) engines, the piston moves down the cylinder while the intake valve draws an air and fuel mixture into the combustion chamber.

In gasoline-direct-injected engines (GDI), depicted in this article’s accompanying animation, fuel can be injected while the piston moves down to create a homogenous mixture or injected in a smaller amount nearer the end of the compression stroke. Most GDIs employ both depending on the duty cycle. Also, some original equipment manufacturers are even using both port and direct.

Step 2: The Compression Stroke

The intake valve closes, sealing the combustion chamber. The crankshaft rotates to complete its first full revolution, and drives the piston upwards, compressing the fuel and air mixture.

Step 3: The Power Stroke

A spark plug ignites the air-fuel mixture, and the resulting combustion quickly expands the gases, forcing the piston back down the cylinder.

Step 4: The Exhaust Stroke

Finally, the exhaust valve opens, and the piston travels back up one last time, forcing the exhaust gas to leave the cylinder, while the piston applies a fresh coating of oil.

These four strokes of the piston create one combustion cycle and require the valves, piston, crankshaft, cylinder, piston rings and oil to work together for maximum efficiency, performance, and durability.

How a Four-Stroke Gasoline Engine Works