BSFC is the fuel mass flow rate divided by the brake (shaft) power, measured on a test stand with a load applied. Its SI form is grams per joule, but datasheets almost always quote grams per kilowatt-hour, or pounds per horsepower-hour in imperial documents (1 lb/(hp·h) is about 608 g/(kW·h)).
Dividing by the fuel’s lower heating value turns BSFC into thermal efficiency, so the figure lets engines of different size and type be compared directly. Across engine classes it varies widely: reference material gives values below 200 g/(kW·h) for large diesels at low speed and high torque, and above 1,000 g/(kW·h) for a turboprop at low power. Small spark-ignition and rotary engines used in unmanned aircraft typically sit well above the best diesel figures, which is why range-extender and long-endurance designs weigh BSFC against engine mass.
BSFC is not constant. It changes with speed and load, and is best shown as a map of iso-consumption contours. On a datasheet it appears as a single best-point value, a cruise-point value, or a curve against power, in g/(kW·h) with the fuel type and test conditions.
A frequent mistake is comparing a best-point BSFC of one engine with a cruise-point or cycle-average figure of another. A second is ignoring fuel energy content: a gasoline and a diesel engine with the same BSFC are not equally efficient, and the mass of fuel carried matters more to endurance than the volume. Check also whether the figure includes generator losses or is true shaft output.
