The four-stroke piston engine

Intake, compression, power, exhaust: how a light-airplane engine turns fuel into thrust.

13 min read · The airplane: engine and systems

After this lesson you can

  • Name the four strokes of the Otto cycle in order and identify the only one that produces power.
  • Explain why the spark fires before the piston reaches top dead center.
  • Distinguish detonation from pre-ignition and state their most common operational causes.
  • Explain why the mixture must be leaned as altitude increases, and why it must be enriched again on the way down.

Almost every trainer you will fly is built around the same machine: a horizontally opposed, air-cooled, four-stroke piston engine. The layout has barely changed since the 1950s, and that is good news -- what you learn here will still apply to nearly the whole general aviation fleet, from a Cessna 150 to a Cirrus SR22.

The goal of this lesson is not to make you a mechanic. It is to give you enough of the internal picture that engine indications in the cockpit stop being mysterious numbers: why you lean the mixture, why there are two spark plugs per cylinder, why a cylinder head temperature gauge matters as much as the tachometer.

The rest of this lesson

  1. 1.How reciprocating engines are built
  2. 2.The four strokes
  3. 3.Why the spark fires early
  4. 4.The fuel-air mixture
  5. 5.Detonation and pre-ignition
  6. 6.Fuel grades and 100LL

These 6 sections, their figures, the checks along the way and the end-of-lesson quiz come with the PPL track. Paid once, yours for good.

See what the PPL track costsEvery module 1 is free to read, no account, no card.

Key terms

Try to recall each definition before turning the card.