Brake Specific Fuel Consumption (BSFC) Calculator

Background.

An engine on a dynamometer burns 100 lb of fuel an hour while holding 250 horsepower at the crankshaft. Brake specific fuel consumption folds those two readings into one figure, 0.4 lb of fuel per horsepower-hour, which can be set against another engine, another calibration or another point on the same engine's map whatever their sizes. Lower is better: less fuel mass for the same shaft work.

The US EPA describes BSFC as fuel consumption divided by engine power, quoted in grams per kilowatt-hour or pounds per brake-horsepower-hour, and you get both units here. 'Brake' means power measured at the output shaft by an absorbing dynamometer, a descendant of the friction brakes once used for the job, so transmission and axle losses are not part of it.

Fuel goes in as mass. Gallons or litres per hour would need the measured density of the actual fuel at its actual temperature, and no density is assumed. Each result also describes a single operating point: BSFC shifts with speed, load, temperature, calibration and transient running, so one value is neither a full fuel map nor a prediction of a vehicle's MPG.

What is brake specific fuel consumption calculator?

Brake specific fuel consumption is the mass of fuel an engine burns for each unit of work it delivers at the output shaft. Dividing by power removes engine size from the comparison, so a 100 hp engine and a 1,000 hp engine with the same BSFC use the same fuel per unit of work at those points. It behaves as an inverse efficiency: once the fuel's heating value is known, BSFC converts directly into brake thermal efficiency.

How to use this calculator.

1. 01Record fuel mass flow in pounds per hour over a steady interval, from a mass-flow meter or by weighing the fuel used and dividing by the elapsed time.
2. 02Enter the brake horsepower measured at the crankshaft over that same interval. Wheel horsepower from a chassis dyno includes drivetrain losses and would inflate the result.
3. 03Read BSFC in lb/(hp·h) and g/kWh; the kg/h figure lets you check the fuel flow in metric units.
4. 04Compare results only between tests on the same fuel under comparable conditions.

The formula.

BSFC = ṁ ÷ P, fuel mass flow divided by brake power. With pounds per hour and horsepower that gives lb/(hp·h) directly. The metric figure converts each side using exact definitions, 1 lb = 453.59237 g and 1 mechanical horsepower = 0.74569987158227022 kW, so 1 lb/(hp·h) equals 608.2774 g/kWh. No fuel property enters either calculation.

To turn BSFC into brake thermal efficiency you need the fuel's lower heating value in MJ/kg: efficiency = 3,600 ÷ (BSFC in g/kWh × heating value), where 3,600 comes from 1 kWh = 3.6 MJ. For a fuel with a heating value of 43 MJ/kg, 243.31 g/kWh works out to about 34.4%. Because fuels differ in heating value, equal BSFC on two different fuels does not mean equal efficiency.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. EPA. CleanBoost test and quality-assurance plan; defines BSFC as fuel consumption divided by engine power with units g/kWh or lb/bhp-h.