Definition
The ratio of the brake horsepower (power actually delivered at the engine output shaft) to the indicated horsepower (power developed inside the cylinders by the burning fuel-air mixture). It expresses how much of the power produced inside the engine is actually available to do useful work after friction and other internal mechanical losses are subtracted.
Plain English
How much of the power an engine makes inside its cylinders actually reaches the propeller shaft, rather than being lost to friction inside the engine itself.
Context Anchor
Seen in aircraft piston-engine theory, engine performance discussions, and maintenance training when comparing power produced inside the engine with power available to the propeller.
Derivation
From Latin 'mechanicus' (relating to machines) and 'efficere' (to accomplish, bring about). 'Efficiency' here means how much of the work put in actually comes out the other side as useful output. The aviation meaning follows directly: the engine's mechanical efficiency is the share of internal power that survives to do useful work.
Why Pilots Care
Lower mechanical efficiency reduces usable power, directly affecting climb rate, cruise speed, and fuel burn.
Analogy
It is like pedaling a bicycle: your legs make the power, but some of it is lost in the chain and gears before it reaches the rear wheel. Mechanical efficiency describes how much useful power makes it through the machine.
Intuition Check
Mechanical efficiency does not just mean the engine is “working well” in a general sense. Here it means a specific power ratio: usable output power compared with power made inside the engine.
Example Sentence 1
A typical aircraft reciprocating engine has a mechanical efficiency of about 85 to 90 percent, meaning roughly 10 to 15 percent of the power produced in the cylinders is lost to internal friction.
Example Sentence 2
A drop in mechanical efficiency can explain why an engine produces less thrust than expected even though compression readings remain normal.