Definition
A small turbine, driven by the exhaust gases of a reciprocating engine, that is mechanically coupled back to the engine crankshaft through a fluid coupling and gearing. It extracts additional energy from the exhaust stream and returns it to the crankshaft as added shaft horsepower, increasing engine output without burning more fuel.
Plain English
A spinning wheel placed in the engine's exhaust pipe. The hot exhaust gases blow through it and make it spin, and that spin is fed back into the engine to give it extra power for free.
Context Anchor
Seen in discussions of large piston engines, especially turbo-compound engine systems.
Derivation
Called 'power recovery' because it recovers energy that would otherwise be wasted out the exhaust. A turbine is any wheel with blades that is turned by a flowing fluid or gas — from the Latin 'turbo,' meaning a spinning thing or whirlwind.
Why Pilots Care
Increases power output and fuel efficiency on certain high-performance piston engines without requiring additional fuel flow.
Analogy
It is like putting a small waterwheel in a fast-moving stream and using that turning motion to help drive a machine.
Intuition Check
Do not assume a power recovery turbine is a turbocharger. A turbocharger uses exhaust energy to help the engine breathe; a power recovery turbine uses exhaust energy to help turn the engine’s main shaft.
Example Sentence 1
The R-3350 Turbo-Compound used three power recovery turbines, each driven by the exhaust from six cylinders, to add roughly 450 horsepower at takeoff.
Example Sentence 2
On takeoff the power recovery turbines added usable horsepower from exhaust that would have been lost.