Skip to main content
Northern IrelandMotor Vehicle & Road User StudiesSyllabus dot point

How does a four-stroke petrol engine turn fuel into movement?

The four strokes of the petrol engine cycle (induction, compression, power, exhaust), the valve positions in each, and the main engine components.

A CCEA GCSE Motor Vehicle and Road User Studies answer on the four-stroke petrol engine: the induction, compression, power and exhaust strokes, the valve positions in each, and the main engine parts.

Generated by Claude Opus 4.810 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

CCEA wants you to know the four strokes of the petrol engine cycle in the correct order, what the piston, valves and spark plug do in each, and which strokes have both valves closed. The four-stroke cycle is one of the most reliably examined engine topics, including "name the strokes" and "which strokes have both valves closed" questions.

The answer

The main engine components

Inside each cylinder:

  • A piston slides up and down.
  • The inlet valve lets the fuel-air mixture in; the exhaust valve lets the burnt gases out.
  • A spark plug provides the spark to ignite the mixture.
  • The piston is joined by a connecting rod to the crankshaft, which turns the up-and-down motion into rotation.

The four strokes

The cycle has four strokes (two up, two down); the crankshaft turns twice for one complete cycle.

Only the power stroke produces useful work; the other three are driven by the momentum of the crankshaft and flywheel.

Valve positions - a key exam point

Stroke Inlet valve Exhaust valve
Induction Open Closed
Compression Closed Closed
Power Closed Closed
Exhaust Closed Open

So both valves are closed during compression and power - the part of the cycle that must be sealed for compression and combustion.

A useful memory aid

The cycle is often remembered as "Suck, Squeeze, Bang, Blow": suck (induction), squeeze (compression), bang (power), blow (exhaust).

Worked example: engine capacity

Examples in context

Example 1. The flywheel's job. Because only one stroke in four gives power, a heavy flywheel stores energy and keeps the crankshaft turning smoothly through the other three strokes.

Example 2. More cylinders, smoother power. A four-cylinder engine has power strokes overlapping across the cylinders, so the engine runs more smoothly than a single-cylinder engine.

Try this

Q1. Give the four strokes in the correct order. [2 marks]

  • Cue. Induction, compression, power, exhaust.

Q2. During which stroke does the spark plug ignite the mixture? [1 mark]

  • Cue. At the start of the power stroke (end of compression).

Q3. Name the two strokes during which both valves are closed. [1 mark]

  • Cue. Compression and power.

Exam-style practice questions

Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

CCEA style4 marksList the four strokes of the four-stroke cycle in the correct order, and name two strokes during which both valves are closed.
Show worked answer →

The four strokes in order are: induction (intake), compression, power (combustion), exhaust.

Both valves are closed during the compression stroke and the power stroke. (During induction the inlet valve is open; during exhaust the exhaust valve is open.)

Markers reward the correct order of the four strokes and that compression and power are the two strokes with both valves closed.

CCEA style4 marksDescribe what happens during (a) the induction stroke and (b) the power stroke of a four-stroke petrol engine, mentioning the piston, the valves and the spark plug where relevant.
Show worked answer →

(a) Induction (intake) stroke - the piston moves down the cylinder, the inlet valve opens (exhaust valve closed), and the downward movement draws a mixture of petrol and air into the cylinder.

(b) Power (combustion) stroke - both valves are closed; the spark plug ignites the compressed petrol-air mixture, the burning gases expand rapidly and force the piston down. This is the only stroke that produces power, and it turns the crankshaft.

Markers reward: induction = piston down, inlet valve open, mixture drawn in; power = spark ignites mixture, both valves closed, piston forced down.

Related dot points

Sources & how we know this