BSFC Calculator - Brake Specific Fuel Consumption
Enter your engine's fuel flow rate and power output to calculate brake specific fuel consumption (BSFC), a key measure of how efficiently an engine converts fuel energy into useful shaft power. The calculator also derives thermal efficiency based on your fuel type and converts between the main BSFC unit systems. Swap between metric and imperial at any time.
Formula
Worked example
A turbocharged gasoline engine consumes 30,000 g/h of fuel at 120 kW output. BSFC = 30,000 / 120 = 250 g/kWh. In imperial: 30,000 / 453.59 = 66.1 lb/h; power = 120 / 0.7457 = 160.9 hp; BSFC = 66.1 / 160.9 = 0.411 lb/hp-hr. Thermal efficiency = 1 / (250 x 0.012222) = 1 / 3.056 = 32.7%. This is typical for a modern turbocharged petrol engine near its best-efficiency island.
What is brake specific fuel consumption (BSFC)?
Brake specific fuel consumption (BSFC) is the standard metric for comparing the fuel efficiency of internal combustion engines independent of their size. It is defined as the mass of fuel consumed per unit of energy output: specifically, grams of fuel per kilowatt-hour (g/kWh) in metric units, or pounds of fuel per brake horsepower-hour (lb/hp-hr) in imperial units. A lower BSFC means the engine needs less fuel to do the same amount of work, so it is more efficient. BSFC is called "brake" specific because the power is measured at the output shaft (the brake) rather than the thermodynamic power released by combustion, which is always higher.
How to use this calculator
Enter the engine's fuel flow rate and its shaft power output. Choose metric (kW and g/h) or imperial (hp and lb/h) units with the toggle at the top. If you prefer to enter torque and RPM rather than a direct power figure, switch on the "Calculate power from torque and RPM" toggle and fill in those two fields instead; power is computed as torque times angular velocity. Select your fuel type to also see thermal efficiency. The results panel shows BSFC in both unit systems and the thermal efficiency derived from the fuel's lower heating value. The "Show your work" panel below the results breaks down every arithmetic step with your actual numbers.
The BSFC formula and units
The formula is simple: BSFC = fuel mass flow rate / shaft power. In metric units that is BSFC (g/kWh) = fuel flow (g/h) / power (kW). In imperial units it is BSFC (lb/hp-hr) = fuel flow (lb/h) / power (hp). The two scales are related by 1 lb/hp-hr = 608.277 g/kWh, so you can convert by multiplying or dividing by 608.277. When power is not measured directly, it can be calculated from torque and rotational speed: P (kW) = torque (N-m) x angular velocity (rad/s) / 1000, where angular velocity = 2 x pi x RPM / 60.
Thermal efficiency and the link to fuel type
BSFC and thermal efficiency are two sides of the same coin. Thermal efficiency (eta) = 1 / (BSFC x LHV), where LHV is the fuel's lower heating value in kWh/g. Gasoline has an LHV of about 0.01222 kWh/g, so an engine with a BSFC of 250 g/kWh achieves about 32.8% efficiency. Diesel has a slightly lower LHV per gram (about 0.01195 kWh/g), so diesel engines must achieve a lower BSFC to reach the same efficiency. The most efficient production engines in the world are large low-speed two-stroke marine diesels, which reach BSFC values around 155 g/kWh, corresponding to thermal efficiencies above 54%.
BSFC maps and operating strategy
A single BSFC number describes one operating point. Real engines have a BSFC map: a contour plot of fuel consumption across the full range of speeds (RPM) and loads (typically expressed as brake mean effective pressure, BMEP, in bar). The contours form closed islands, with the lowest BSFC island sitting near peak torque and moderate speed. Hybrid vehicle calibrators and motorsport engineers use these maps to identify the most efficient operating region and control when and at what speed/load the engine runs. Turbocharged engines generally have lower minimum-BSFC values than naturally aspirated engines because forced induction allows more air and fuel per displacement unit at moderate speeds.
Typical BSFC values by engine type
| Engine type | BSFC (g/kWh) | lb/hp-hr | Thermal efficiency |
|---|---|---|---|
| Large 2-stroke marine diesel | 155-165 | 0.255-0.271 | ~54% |
| Large 4-stroke marine diesel | 170-180 | 0.280-0.296 | ~49% |
| Modern turbodiesel (automotive) | 190-210 | 0.313-0.345 | ~43% |
| Diesel passenger car (avg cycle) | 198-230 | 0.326-0.378 | ~39-43% |
| Turbocharged gasoline (DI) | 240-260 | 0.395-0.428 | ~33-36% |
| Naturally aspirated gasoline | 270-320 | 0.444-0.526 | ~27-32% |
| Aviation piston (Lycoming/Continental) | 270-300 | 0.444-0.493 | ~28-32% |
| Turboprop at cruise power | 310-340 | 0.510-0.559 | ~26-28% |
| Small 4-stroke gasoline | 330-380 | 0.543-0.625 | ~23-26% |
| Small 2-stroke gasoline | 420-450 | 0.691-0.740 | ~19-20% |
Approximate best-point BSFC values at peak efficiency. Real-world values vary with load and speed.
Frequently asked questions
What is a good BSFC value?
For automotive gasoline engines, a BSFC below 270 g/kWh (0.44 lb/hp-hr) at the peak-efficiency operating point is considered good. Modern turbocharged direct-injection gasoline engines achieve 240-260 g/kWh. Diesel automotive engines typically achieve 190-220 g/kWh. Large marine two-stroke diesels hold the record at around 155-165 g/kWh. Values above 350 g/kWh are typical of small two-stroke engines or gasoline engines operating at very light loads.
How do I convert between g/kWh and lb/hp-hr?
Divide g/kWh by 608.277 to get lb/hp-hr. Multiply lb/hp-hr by 608.277 to get g/kWh. For example, 250 g/kWh equals 250 / 608.277 = 0.411 lb/hp-hr. The factor 608.277 comes from the unit conversion: 1 kilowatt = 1.34102 horsepower, and 1 kilogram = 2.20462 pounds, combined as 2204.62 / 3.6 / 1.34102.
Why does BSFC vary with engine speed and load?
At light loads, throttling losses (for gasoline engines) and incomplete combustion increase fuel consumption relative to output power. At very high speeds, friction and pumping losses rise steeply. Peak efficiency typically occurs near maximum torque at moderate speed, where the engine is well-loaded with minimal throttling and friction losses are still manageable. This is why BSFC maps show a "best efficiency island" rather than a single value.
Can I use BSFC to compare petrol and diesel engines fairly?
BSFC in g/kWh compares engines by how many grams of fuel they use per unit of work, regardless of engine displacement or cylinder count. However, because diesel has a higher energy density per unit volume than gasoline, an engine with the same BSFC in g/kWh will consume less volume of diesel than gasoline. If you want a fair volumetric efficiency comparison, convert BSFC to fuel-specific energy consumption using the fuel's heating value, which is what the thermal efficiency figure does.
What does thermal efficiency mean in the context of BSFC?
Thermal efficiency is the fraction of the fuel's chemical energy that comes out as useful shaft work. An engine with 30% thermal efficiency converts 30% of the fuel's energy to power and loses the remaining 70% as heat through the exhaust, cooling system, and friction. It is calculated as 1 divided by (BSFC in g/kWh times the fuel's LHV in kWh/g). Because BSFC and LHV are both mass-based, the result is dimensionless and independent of engine size.
How is power calculated from torque and RPM?
Power equals torque multiplied by angular velocity. Angular velocity in radians per second equals 2 times pi times RPM divided by 60. So power (kW) = torque (N-m) x 2 x pi x RPM / 60 / 1000. In imperial units, power (hp) = torque (ft-lb) x RPM / 5,252. This calculator accepts either direct power input or torque and RPM and performs the conversion internally.
Why is marine diesel BSFC so much better than automotive?
Large marine two-stroke diesels run at very low RPM (80-120 rpm), which minimises friction losses, and they operate at near-constant high load for long periods, keeping them on their best-efficiency island. They also use long-stroke designs and high compression ratios that favour thermodynamic efficiency. Automotive engines must cover a wide range of speeds and loads, be compact and light, and meet stringent emissions targets, all of which require compromises that raise BSFC.