Definition
A measure of how much fuel a piston engine burns to produce one unit of usable shaft power over one hour, usually expressed in pounds of fuel per brake horsepower per hour (lb/bhp/hr). It is a standard way to compare the fuel efficiency of reciprocating engines independent of their size.
Plain English
How much fuel an engine has to burn to deliver one horsepower of useful work for one hour. The lower the number, the more efficient the engine.
Context Anchor
Seen in aircraft engine performance, maintenance, and fuel efficiency discussions.
Derivation
‘Brake’ comes from the prony brake, an old device clamped onto an engine’s output shaft to measure its actual usable power. ‘Specific’ means ‘per unit’ — in this case, per horsepower per hour. So the term literally describes fuel use measured against real, delivered shaft power.
Why Pilots Care
Helps evaluate engine efficiency and compare fuel economy across different engines during maintenance or performance tuning.
Intuition Check
“Brake” does not mean the aircraft’s wheel brakes here. It means the engine’s usable power measured at its output shaft during testing.
Example Sentence 1
At a properly leaned cruise setting, the engine showed a BSFC close to the manufacturer’s published figure of 0.45 lb/bhp/hr.
Example Sentence 2
A lower BSFC value indicates better engine performance in converting fuel to usable power.