How a piston engine works

This course covers the Powerplant rating: piston and turbine engines, their fuel, ignition, oil and cooling, and propellers. It assumes the General course; the formulas it uses (displacement, pressure, horsepower) are recapped where they come up.

Most light aircraft engines are four-stroke, spark-ignition piston engines working on the Otto cycle:

1. Intake: the piston moves down with the intake valve open, drawing in fuel and air.
2. Compression: both valves closed, the piston moves up and squeezes the mixture.
3. Power: a spark lights the mixture near the top; expanding gas drives the piston down.
4. Exhaust: the exhaust valve opens and the piston pushes the burned gas out.

The cycle takes two turns of the crankshaft (720°), so each cylinder fires once every two revolutions and the camshaft turns at half crankshaft speed. Valves don't open exactly at top or bottom: they open early (lead) and close late (lag) to use the gas's momentum, and near the top of the exhaust stroke both are open for a moment, called valve overlap, which helps clear the cylinder and cool it.

Compression ratio is the cylinder's volume with the piston at the bottom divided by its volume at the top: 120 cubic inches down to 15 is 8:1. Higher ratios give more power from the fuel but need higher-octane fuel to avoid detonation.

Layouts and parts

Most light aircraft use horizontally opposed engines (cylinders flat on each side, compact and smooth). Radial engines have an odd number of cylinders in each row so that firing every other cylinder works out evenly: a nine-cylinder radial fires 1-3-5-7-9-2-4-6-8. Oil can drain into a radial's lower cylinders when it sits, causing hydraulic lock, so the propeller is pulled through by hand before starting.

Inside: the crankcase holds the crankshaft, which connecting rods turn from the pistons. Pistons carry compression rings (seal) and oil control rings. Cylinders have steel barrels and aluminum heads with cooling fins. The valve train is cam, lifters (often hydraulic, which keep zero clearance), pushrods and rocker arms. Exhaust valves may be sodium filled: the sodium melts and sloshes heat from the valve head to the stem.