Brake specific fuel consumption

Brake specific fuel consumption (BSFC) is a measure of fuel efficiency within a shaft reciprocating engine. It is the rate of fuel consumption divided by the power produced. BSFC allows the fuel efficiency of different reciprocating engines to be directly compared.

The BSFC Calculation (in SI)

To calculate BSFC, use the formula BSFC = Fuel rate / Power
Where:

Fuel rate is the fuel consumption in grams per second (g·s^-1)

Power is the power produced in Watts where P = Tω

ω is the engine speed in radians per second (rad·s^-1)

T is the engine torque in newton meters (N·m)

The resulting units of BSFC are grams per joule (g·J^-1)

Commonly BSFC is expressed in units of grams per Kilowatt-hour (g/kWh). The conversion factor is as follows:

BSFC(g/kWh) = BSFC(g/J) * 3.6*10⁶

The conversion between metric and U.S. units is:

BSFC_METRIC(g/kWh) = BSFC_US(Lbs/(HP*Hr)) * 608.277

BSFC_US(Lbs/(HP*Hr)) = BSFC_METRIC(g/kWh) * 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 fuels heating value. The lower heating value LHV is used for internal combustion engines efficiency calculations because the heat at temperatures below 150 °C cannot be put to use.

Some examples of lower heating values for vehicle fuels are:

Certification gasoline = 18640 BTU/lb = 0.01204 kW·h/g

Regular gasoline = 18917 BTU/lb = 0.0122225 kW·h/g

Diesel fuel = 18500 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/kWh] 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%. 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 37%.

The Significance of BSFC Numbers for Engine Design and Class

The beauty of BSFC numbers is that they remain similar over a wide range of engine sizes. These numbers only change for different engine designs and compression ratios. For example, a small one cylinder 50cc four-stroke and a large V8 engine can both have the same BSFC number. However, engines of different classes like diesels and gasoline engines will have very different BSFC numbers.

Typical values of BSFC for shaft engines

The following table gives the minimum specific fuel consumption of several types of engine. For comparison, the theoretical work that can be derived from burning octane (based on change in Gibbs free energy going to gaseous H₂O and CO₂) is 45.7 MJ/kg, corresponding to 79 g/(kW·h).

See also

• Fuel economy in automobiles
• Specific fuel consumption (thrust) - jet engine fuel efficiency
• Fuel efficiency
• Fuel management systems
• Marine fuel management
• Fuel
• FuelTrax

References

• Reciprocating engine types
• HowStuffWorks: How Car Engines Work
• Reciprocating Engines at infoplease
• Piston Engines US Centennial of Flight Commission
• Effect of EGR on the exhaust gas temperature and exhaust opacity in compression ignition engines
• Heywood J B 1988 Pollutant formation and control. Internal combustion engine fundamentals Int. edn (New York: Mc-Graw Hill) pp 572–577
• Well-to-Wheel Studies, Heating Values, and the Energy Conservation Principle

External links