Definition
A turbine driven by exhaust gases leaving a reciprocating engine, used to extract additional energy from the exhaust stream. The recovered energy is typically fed back to the engine crankshaft through a fluid coupling or gear train to increase total power output, a configuration historically used on high-performance piston aircraft engines (sometimes called a turbo-compound arrangement).
Plain English
A small turbine wheel placed in the engine's exhaust pipe. The hot, fast-moving exhaust gases spin it, and that spinning is geared back into the engine to give it extra power without burning extra fuel.
Context Anchor
Seen in aircraft systems discussions involving air-driven starters, stored-pressure systems, or emergency power devices.
Derivation
"Blow-down" refers to the rapid expansion of high-pressure exhaust gas as it leaves the cylinder and "blows down" to lower pressure. The turbine captures energy from that expansion. Knowing this helps separate it from a turbocharger, which uses exhaust energy to compress intake air rather than to add power directly to the crankshaft.
Why Pilots Care
Increases engine power output and fuel efficiency without burning extra fuel.
Analogy
It is like a toy pinwheel turned by air from a compressed-air can. The pinwheel spins while the air is coming out, but it slows and stops as the pressure runs out.
Grounding Statement
Picture stored air rushing out of a tank and spinning a small wheel on its way out.
Intuition Check
“Blow-down” does not mean the turbine has failed or broken apart. Here it means the turbine is powered while stored pressure is being released.
Example Sentence 1
The Wright R-3350 used blow-down turbines to recover exhaust energy and feed it back to the crankshaft, boosting power for long-range airline operations.
Example Sentence 2
On the turbo-compound engine the blow-down turbines added nearly 150 horsepower by recovering exhaust energy.