How to use this calculator
Use brake specific fuel consumption based on fuel actually consumed and crankshaft power at the same operating condition. Obtain lower heating value (LHV) for that fuel from a suitable fuel specification or measurement. Record engine speed, load, fuel composition and measurement conditions with the result. If you only have fuel mass flow and brake power, calculate BSFC with the related tool first. A single operating point does not describe the complete engine map or a vehicle drive cycle.
This is a fuel-to-crankshaft energy ratio on the LHV basis, not a combustion-efficiency measurement, ideal-cycle prediction, wheel efficiency or road MPG estimate. The residual does not separate exhaust, cooling, friction or other energy paths and is not all recoverable energy. Positive-output operation only; idle, motoring, fuel cut, added external energy and mixed-fuel accounting are outside this model. Input consistency is not proof of measurement accuracy.
The formula
A worked example
For hypothetical BSFC of 250 g/kWh and LHV of 40 MJ/kg, each kWh of brake output consumes 0.25 kg of fuel. The input is 0.25 × 40 = 10 MJ; output is 3.6 MJ. Brake thermal efficiency is 3.6 ÷ 10 × 100 = 36%. The remaining 6.4 MJ is not delivered as brake output. Entering 40,000 kJ/kg gives the same result.
BSFC sensitivity at a fixed heating value
Original hypothetical examples at 40 MJ/kg LHV. These are not typical engine values or tuning targets.
| BSFC (g/kWh) | Input per kWh (MJ) | Efficiency |
|---|---|---|
| 200 | 8 | 45% |
| 250 | 10 | 36% |
| 300 | 12 | 30% |
Common questions
How is this different from BSFC?
BSFC measures fuel mass consumed per brake energy delivered. Thermal efficiency also uses the energy content of that fuel. Two fuels with the same mass-based BSFC can give different efficiency percentages because their heating values differ.
Should I use lower or higher heating value?
This page uses lower heating value. Higher heating value uses a different energy-accounting basis; inserting it changes the percentage. Do not compare LHV and HHV results as if they were on the same basis. The calculator cannot identify the basis from the number alone.
Where can I get the heating value?
Use a documented value for the actual fuel and its composition. Check both units and whether it is LHV. No universal gasoline, diesel or blend value is assumed here; the 40 MJ/kg default is only an arithmetic example.
Can I use wheel power to obtain BSFC?
Not for an engine brake thermal efficiency result. Wheel power includes additional drivetrain effects. Use crankshaft brake power with the corresponding fuel consumption and consistent accessory/test boundaries.
Does lower BSFC always mean higher efficiency?
For a fixed LHV and the same measurement boundary, yes: efficiency is inversely proportional to BSFC. Across fuels, account for both BSFC and LHV. This does not imply improved road fuel economy without vehicle and operating-cycle information.
Why reject an efficiency above 100%?
It means the entered fuel-energy input is smaller than the calculated brake output for this fuel-only model. Recheck units, heating-value basis, fuel consumption and power measurements. A result below 100% still does not establish that the measurements are accurate or the operating point is realistic.
Can I average several efficiency percentages?
Do not use a simple average unless the fuel-energy inputs are equal. For a combined interval, divide total brake energy by total fuel energy. This page accepts a single matched BSFC and fuel LHV; it does not combine changing fuels or operating periods.
Sources & calculation method
National Research Council: SI Engine Definitions and Efficiency Fundamentals describes brake thermal efficiency using fuel heating value and brake work. MathWorks: Generic Engine defines BSFC as fuel consumption rate divided by output power. The unit conversion and worked examples above are derived from these definitions; no measured engine performance is supplied by these examples.
Use the BSFC Calculator for mass flow and brake power, or Horsepower & Torque for power at a measured torque and speed. For the cylinder-to-shaft power ratio, use Engine Mechanical Efficiency. For road consumption, use the fuel log guide.
References accessed October 9, 2026. No independent professional review or endorsement is claimed.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.