# Brake specific fuel consumption

Brake specific fuel consumption (BSFC) is a measure of the fuel efficiency of any prime mover that burns fuel and produces rotational, or shaft, power. It is typically used for comparing the efficiency of internal combustion engines with a shaft output.

It is the rate of fuel consumption divided by the power produced. It may also be thought of as power-specific fuel consumption, for this reason. BSFC allows the fuel efficiency of different engines to be directly compared.

## The BSFC calculation (in metric units)

To calculate BSFC, use the formula

$BSFC = \frac{r}{P}$

where:

r is the fuel consumption rate in grams per second (g/s)
P is the power produced in watts where $P = \tau \omega$
$\omega$ is the engine speed in radians per second (rad/s)
$\tau$ is the engine torque in newton meters (N·m)

The above values of r, $\omega$, and $\tau$ may be readily measured by instrumentation with an engine mounted in a test stand and a load applied to the running engine. The resulting units of BSFC are grams per joule (g/J)

Commonly BSFC is expressed in units of grams per kilowatt-hour (g/(kW·h)). The conversion factor is as follows:

BSFC [g/(kW·h)] = BSFC [g/J]×(3.6×106)

The conversion between metric and imperial units is:

BSFC [g/(kW·h)] = BSFC [lb/(hp·h)]×608.277
BSFC [lb/(hp·h)] = BSFC [g/(kW·h)]×0.001644

## The relationship between BSFC numbers and efficiency

To calculate the actual efficiency of an engine requires the energy density of the fuel being used.

Different fuels have different energy densities defined by the fuel's heating value. The lower heating value (LHV) is used for internal combustion engine efficiency calculations because the heat at temperatures below 150 °C (300 °F) cannot be put to use.

Some examples of lower heating values for vehicle fuels are:

Certification gasoline = 18,640 BTU/lb (0.01204 kW·h/g)
Regular gasoline = 18,917 BTU/lb (0.0122225 kW·h/g)
Diesel fuel = 18,500 BTU/lb (0.0119531 kW·h/g)

Thus a diesel engine's efficiency = 1/(BSFC × 0.0119531) and a gasoline engine's efficiency = 1/(BSFC × 0.0122225)

## The use of BSFC numbers as operating values and as a cycle average statistic

BSFC [g/kW·h] map

Any engine will have different BSFC values at different speeds and loads. For example, a reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak. However, the numbers often reported for a particular engine are a fuel economy cycle average statistic. For example, the cycle average value of BSFC for a gasoline engine is 322 g/kW·h, translating to an efficiency of 25% (math calculation: 1/(322 × 0.0122225) = 0.2540). However, efficiency for that engine can be lower or higher than this average statistic depending on the operating condition. In the case of a production gasoline engine, the most efficient BSFC is approximately 225 g/kW·h, which is equivalent to a thermodynamic efficiency of 36%.

An iso-BSFC map (AKA fuel island plot) of a diesel engine is shown. The sweet spot at 206 BSFC has 40.6% efficiency. x-axis is rpm, y-axis is BMEP in bar (bmep is proportional to torque)

## The significance of BSFC numbers for engine design and class

BSFC numbers change a lot for different engine design and compression ratio and power rating. Engines of different classes like diesels and gasoline engines will have very different BSFC numbers, ranging from less than 200 g/kW·h (diesel at low speed and high torque) to more than 1,000 g/kW·h (turboprop at low power level).

## Examples of values of BSFC for shaft engines

The following table takes selected values as an example for the minimum specific fuel consumption of several types of engine.

• Turboprop (aircraft engine) values are given for the complete range of power during a flight.
• Ground roll SFC value (at 0.07 Pmax) and especially idle (at rest) SFC values show dramatic increases when compared with cruise value (0.70 Pmax).[1]

For specific engines values can and often do differ from the table values shown below. For comparison, the theoretical work that can be derived from burning octane (C8H18) (based on change in Gibbs free energy going to gaseous H2O and CO2) is 45.7 MJ/kg, corresponding to 79 g/kW·h.

Power Year Engine type Application SFC (lb/hp·h) SFC (g/kW·h) Energy efficiency (%)
2,020 kW 1996 Pratt & Whitney turboprop PW127 Aircraft engine at idle 2,390
Aircraft engine at ground roll 1,270
Aircraft engine @ 0.30 Pmax .83 508 22.82
Aircraft engine @ 0.70 Pmax .538 328 24.165
Aircraft engine at Pmax .48 294 27
Turbo-prop 0.8 360–490 17–23
Otto cycle gasoline engines 0.45–0.37 273–227 30–36
80 kW/l 2011 Ford Ecoboost Downsized turbocharged otto engine[2] Automobile engine 245
2,000 kW 1945 Wright R-3350 Duplex-Cyclone gasoline turbo-compound Aircraft engine 0.4 243 33.7
57 kW Toyota Prius THS II engine only [3] automobile .362 225 37%
68 kW 2008 Revetec Controlled Combustion Engine gasoline engine[4] Automobile 212 38.6
550 kW 1931 Junkers Jumo 204 turbocharged two-stroke diesel Aircraft engine 0.345 210 39.8
36 MW 2002 Rolls-Royce Marine Trent turboshaft Marine engine 0.345 210 39.8
2,013 kW 1940 Klöckner-Humboldt-Deutz DZ 710 Diesel two stroke Aircraft engine 0.33 201 41.58
2,340 kW 1949 Napier Nomad Diesel-compound Aircraft engine 0.345 210 39.8
Diesel engine turbocharged diesels 0.34–0.30 209–178 40–47
165 kW 2000 Volkswagen 3.3 V8 TDI Automobile engine 0.33 205 41.1
43 MW General Electric LM6000[5] turboshaft Marine engine, power generation 0.32 199 42
88 kW 1990 Audi 2.5 litre TDI[6] Automobile engine 198 42.5
80 MW 1998 Wärtsilä-Sulzer RTA96-C two-stroke Marine engine 0.268 163 51.7
23 MW MAN Diesel S80ME-C Mk7 two-stroke Marine engine [7] 0.254 155 54.4