How the engine works
8 min readLesson 17 of 593D lab
By the end of this lesson you'll be able to:
- Describe the four strokes of a piston engine
- Explain why aircraft engines are air-cooled and horizontally opposed
- Read the basic engine instruments
Most training airplanes use a reciprocating (piston) engine: four cylinders, horizontally opposed (two on each side), and air-cooled. The opposed layout is compact and smooth, and air cooling saves the weight of a radiator.
The four strokes
- Intake. The piston moves down and the intake valve opens, drawing in the fuel-air mixture.
- Compression. Both valves close and the piston moves up, squeezing the mixture.
- Power. The spark plugs fire, the mixture burns, and the expanding gas pushes the piston down. This is the only stroke that makes power.
- Exhaust. The exhaust valve opens and the piston pushes the burned gases out.
Each cylinder completes the cycle in two revolutions of the crankshaft. The pistons turn the crankshaft, and the crankshaft turns the propeller.
Engine instruments
- Tachometer: engine RPM. In a fixed-pitch prop airplane, it's your power gauge.
- Oil pressure: check it within 30 seconds of starting (sooner in cold weather, per your POH). No oil pressure means shut down.
- Oil temperature: slower to respond, rises during the first minutes of operation.
- Cylinder head temperature (CHT) and exhaust gas temperature (EGT), if installed, help you manage cooling and leaning.
Key takeaways
- Four strokes: intake, compression, power, exhaust; only the power stroke makes power.
- Trainer engines are horizontally opposed and air-cooled.
- Check oil pressure right after start (per the POH).
What the examiner may ask
Practice answering out loud before you open each answer.
Describe your airplane's engine.
Know the POH description: for example, four-cylinder, horizontally opposed, air-cooled, normally aspirated, carbureted, and its horsepower.
What does the tachometer show?
Engine RPM, the power indication in a fixed-pitch propeller airplane.
What do you do if oil pressure doesn't rise after start?
Shut the engine down, per the POH.
Common mistakes
- Not knowing basic engine specs for your airplane.
- Ignoring a low oil pressure indication after start.
- Running the engine at high RPM before the oil has warmed.
Read it in the FAA sources
Lesson check
Get every question right to complete the lesson. Take your time; wrong answers explain why.
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