Calculators · Fuel

BSFC, fuel flow and engine efficiency calculator

Brake-specific fuel consumption (BSFC) measures how much fuel an engine burns for each unit of work it produces, and it is the most direct measure of engine efficiency. These calculators work out BSFC from a power and fuel-flow measurement, estimate fuel flow from power, convert BSFC into thermal efficiency, and show how much energy a volume of fuel contains.

BSFC from power and fuel flow

Divides measured fuel flow by the power produced at the same moment. Engine test benches measure exactly this; the lowest BSFC an engine reaches is its best-efficiency point.

Results
BSFC—g/kWhGrams of fuel per kilowatt-hour — lower is more efficient.
BSFC—lb/hp·hThe same figure in US units.

Formula
BSFC [g/kWh] = fuel [kg/h] × 1,000 / P [kW]
1 lb/hp·h = 608.3 g/kWh

Fuel flow from power and BSFC

Estimates how much fuel an engine uses at a given output. It is how generator and machinery fuel consumption is estimated from an engine’s rated power and its published BSFC.

Results
Fuel flow—kg/hMass of fuel burned per hour.
Fuel flow—L/hVolume of fuel burned per hour.

Formula
fuel [kg/h] = P [kW] × BSFC [g/kWh] / 1,000
L/h = kg/h / density

Thermal efficiency from BSFC

Converts BSFC into brake thermal efficiency — the share of the fuel’s chemical energy that reaches the crankshaft as work. It uses the fuel’s lower heating value, the standard basis for engine efficiency.

Results
Brake thermal efficiency—%Share of fuel energy turned into useful work.

Formula
η = 3,600 / (BSFC [g/kWh] × LHV [MJ/kg])

Energy content of fuel

Shows how much chemical energy a volume of fuel holds, using typical density and lower heating value. It explains why a litre of diesel takes a car further than a litre of petrol, and a litre of ethanol less far.

Results
Energy—MJLower heating value of that volume.
Energy—kWhThe same energy in kilowatt-hours, comparable with battery capacity.

Formula
energy = volume × density × LHV

Worked example: a diesel generator

  1. An engine produces 100 kW while burning 25 kg/h of diesel: BSFC = 25,000 / 100 = 250 g/kWh.
  2. At 0.835 kg/L, 25 kg/h is 29.9 litres per hour.
  3. A modern diesel at 200 g/kWh converts 3,600 / (200 × 42.8) = 42% of its fuel energy into work.
  4. One litre of diesel holds about 35.7 MJ, or 9.9 kWh of chemical energy.

Typical fuel properties

FuelDensityLower heating valueEnergy per litre
Petrol (gasoline)0.745 kg/L43.4 MJ/kgabout 32.3 MJ (9.0 kWh)
Diesel0.835 kg/L42.8 MJ/kgabout 35.7 MJ (9.9 kWh)
Ethanol0.789 kg/L26.8 MJ/kgabout 21.1 MJ (5.9 kWh)
Methanol0.792 kg/L19.9 MJ/kgabout 15.8 MJ (4.4 kWh)
LPG (liquid)0.51 kg/L46.0 MJ/kgabout 23.5 MJ (6.5 kWh)

Related

Fuel injector size calculatorFuel savings and break-even calculatorTorque, power and BMEP calculatorMachinery specifications

About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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