BSFC Calculator

This BSFC calculator works out brake specific fuel consumption, the measure of how much fuel an engine burns for each unit of power it produces at the crankshaft. Enter the volumetric fuel flow, the fuel density and the brake power output, and the calculator converts the flow to a mass rate, divides it by the power, and returns the BSFC in grams per kilowatt hour, the fuel mass flow in kilograms per hour, the engine thermal efficiency, and the same BSFC in the imperial pounds per horsepower hour that many engine builders still quote. BSFC is the fairest single number for comparing engines of different sizes, because it strips out capacity and rates the engine purely on how efficiently it turns fuel into work. A lower figure is better. The defaults describe a petrol engine making 100 kW while drinking 30 litres an hour of fuel at 745 grams per litre, which lands around 223 grams per kilowatt hour and a thermal efficiency near 37 percent, typical of a modern petrol engine at a good operating point. Set the density to about 832 grams per litre for diesel, and adjust the lower heating value if you want the efficiency figure to match a specific fuel. Use it to compare engines, track a tune on a dyno, or turn a fuel flow reading into an efficiency you can reason about.

Brake power output

Fuel flow

L/h

Fuel density

g/L

Lower heating value

MJ/kg

223.5 g/kWh

brake specific fuel consumption

Fuel mass flow22.35 kg/h

Thermal efficiency37.1%

Imperial BSFC0.367 lb/hp·h

BSFC is fuel mass flow divided by brake power. Thermal efficiency uses the lower heating value you enter (43.4 MJ/kg suits petrol, about 42.7 for diesel). Figures apply to the single operating point you enter, not the whole engine map. Estimate only.

How it works

The fuel mass flow is the volumetric flow times the density, giving grams per hour, which is 22350 g/h at the defaults. Divide that by the power in kilowatts to get the BSFC in grams per kilowatt hour. Thermal efficiency is the useful energy out divided by the fuel energy in: 3600 divided by the product of the BSFC and the fuel's lower heating value in MJ/kg, expressed as a percentage. The imperial figure converts g/kWh to pounds per horsepower hour by dividing by 608.3, the conversion factor between the two units.

Worked example

An engine makes 100 kW and burns 30 litres an hour of petrol at 745 grams per litre. The mass flow is 30 times 745, which is 22350 grams per hour, or 22.35 kg/h. The BSFC is 22350 divided by 100, which is 223.5 grams per kilowatt hour. Thermal efficiency is 3600 divided by (223.5 times 43.4), about 37.1 percent. Converting the BSFC to imperial, 223.5 divided by 608.3 gives 0.367 pounds per horsepower hour.

Related calculators

Fuel Consumption Calculator: litres per 100 km.
Horsepower Calculator: power from torque and revs.
Carnot Efficiency Calculator: the thermodynamic ceiling.
Torque to Horsepower Calculator: convert torque to power.
Engine Displacement Calculator: swept capacity.

Frequently asked questions

What is BSFC?

Brake specific fuel consumption is the rate an engine burns fuel per unit of power it produces at the crankshaft. It is the fuel mass flow divided by the brake power, usually in grams per kilowatt hour or pounds per horsepower hour. A lower BSFC means the engine makes more power from the same fuel, so it is more efficient at that operating point.

What is a good BSFC figure?

A modern naturally aspirated petrol engine reaches its best BSFC around 230 to 260 grams per kilowatt hour, turbo diesels can dip below 200, and the very best large marine diesels approach 160. Away from the sweet spot on the map the figure rises, so quoted numbers usually refer to the most efficient operating point.

How does BSFC relate to thermal efficiency?

Thermal efficiency is the useful power out divided by the chemical energy in the fuel burned. It equals 3600 divided by the BSFC in g/kWh times the fuel's lower heating value in MJ/kg. With a BSFC of 223.5 g/kWh and a petrol heating value of 43.4 MJ/kg, the thermal efficiency works out at about 37 percent.

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