The four-stroke engine

A piston rises during both compression and exhaust. What tells those strokes apart?

A piston slides inside a cylinder. Combustion creates pressure that pushes it down; a connecting rod turns that motion into crankshaft rotation. In a four-stroke petrol engine, intake, compression, power and exhaust complete one cycle.

One four-stroke cycle spans two crankshaft revolutions. The intake valve admits air, both valves close for compression and power, and the exhaust valve opens to release spent gas. A spark initiates combustion in a petrol engine.

Combustion converts only part of fuel energy into useful work. Heat and friction account for losses. Petrol uses spark ignition; a diesel compresses air until it is hot enough to ignite injected fuel. This diagram intentionally simplifies valve timing.

1. 1
   Intake

   The piston descends. An open intake valve lets fresh air enter.

2. 2
   Compression

   The piston rises with the valves closed, compressing the charge.

3. 3
   Power

   Combustion pressure drives the piston down and turns the crankshaft.

4. 4
   Exhaust

   The piston rises while an open exhaust valve releases spent gas.

Think it through

In the simplified diagram the piston rises and the exhaust valve is open. Name the stroke. What valve state would distinguish the other upward stroke?

Show the reasoning

This is exhaust: the rising piston pushes gas out through the open exhaust valve. During simplified compression the valves are closed, trapping the charge as the piston rises.