Induction, cooling and exhaust

Carburetor ice

Inside a carburetor, the pressure drop in the venturi and the evaporating fuel can chill the air by 20 °C or more, so ice can form even on warm, humid days. It shows as a slow drop in rpm with a fixed-pitch propeller, or a drop in manifold pressure with a constant-speed one. Carburetor heat brings in hot air from a shroud around the exhaust. Hot air is less dense, so the mixture richens and power drops: applying it gives an rpm drop, and if ice was there, the rpm then rises as it melts. Carb heat air isn't filtered, so it's used sparingly on the ground. Fuel-injected engines don't get carburetor ice but can get intake ice, so they have an alternate air door.

Boosting

A supercharger is a compressor driven by the engine's gears. A turbocharger is a compressor driven by a turbine in the exhaust; a wastegate sends some exhaust past the turbine to control how much it boosts. A turbocharger lets the engine keep sea-level power up to its critical altitude. Compressing air heats it, so some systems add an intercooler.

Cooling

Air-cooled cylinders have fins; baffles and seals force the incoming air down through the fins instead of around them, so damaged or missing baffles cause hot cylinders. Cowl flaps open to let more air out on the ground and in the climb and close in cruise and descent. Cylinder head temperature (CHT) is measured by a thermocouple under a spark plug or in the head.

Exhaust

Exhaust leaks are dangerous twice over: fire in the engine compartment, and carbon monoxide in the cabin heat. Look for cracks, especially at welds and slip joints, gray or white streaks that mark a leak, and loose or missing clamps, at every inspection.