How this calculator works
Brake specific fuel consumption divides the mass of fuel burned per hour by the brake power produced. A lower number means a more efficient engine: less fuel for the same shaft work. Because fuel flow is often measured by volume, the calculator converts litres per hour to mass using the fuel density before dividing. Multiplying BSFC by the fuel lower heating value and comparing with the 3.6 MJ in one kilowatt hour gives brake thermal efficiency. Engine friction, pumping losses and accessory drives are included in the brake figure, so BSFC is an overall engine efficiency, not a combustion-only efficiency.
Formula
BSFC(g/kWh) = fuel mass flow (g/h) / brake power (kW). Thermal efficiency = 3600 / (BSFC x LHV), with LHV in MJ/kg. To convert, 1 g/kWh = 0.001644 lb/(hp h).
Worked example
A gasoline engine burns 30 L/h at 0.745 kg/L, which is 22.35 kg/h, while producing 100 kW. BSFC is 22,350 / 100 = 223.5 g/kWh. With a lower heating value of 44.4 MJ/kg the thermal efficiency is 3600 / (223.5 x 44.4) = 36.3 percent.
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Frequently asked questions
What is a good BSFC?
Spark-ignition gasoline engines usually sit between about 220 and 350 g/kWh, with the best efficiency near peak torque and moderate load. Modern turbocharged diesel engines can reach roughly 190 to 220 g/kWh in their best operating region. Small engines and part-load operation are often much worse.
Why does BSFC change with engine speed and load?
Friction and pumping losses are a larger share of the fuel energy at low load, and combustion phasing changes with speed. BSFC is lowest in a compact island on the engine map, usually near peak torque at moderate speed.
Should I use diesel or gasoline heating values?
Use the lower heating value of the fuel actually burned, because exhaust water is assumed to leave as vapour. The page supplies typical values, but a supplier certificate is more accurate for a blend such as E10 or B20.
Why is my imperial BSFC different from a catalog value?
Some catalogues quote lb/(hp h) for mechanical horsepower while others use metric horsepower or a different fuel flow basis. This page uses mechanical horsepower, 745.6999 W, and the same fuel mass flow for both figures.