Brake specific fuel consumption

Brake specific fuel consumption (BSFC) is a measure of an engine's efficiency. It is the rate of fuel consumption divided by the power produced. BSFC is specific for shaft engines such as reciprocating engine.

The BSFC Calculation (in metric units)

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

Fuel rate is the fuel consumption in grams per hour (g/hr)

Power is the power produced in Kilowatts where kW = w * Tq / 9549.27

w is the engine speed in rpm

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

Note: The Power in the BSFC calculation is not weather corrected.

The resulting units of BSFC are g/(kW·h)
The conversion between metric and U.S. units is:

BSFC_US(Lbs/(HP*Hr)) * 608.277 = BSFC_METRIC(g/(kW·h))

BSFC_METRIC(g/(kW·h)) * 0.001644 = BSFC_US(Lbs/(HP*Hr))

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 lower heating value.

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)

A typical cycle average value of BSFC for a gasoline engine is 322 g/(kW·h). This means the average efficiency of a gasoline engine is only 25%. A reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak. Efficiency is lower at other operating conditions. For a gasoline engine, the most efficient BSFC is approximately 236 g/(kW·h) or an efficiency of 37%. As seen above, lower values of BSFC mean higher engine efficiency. Diesel engines are generally more efficient than gasoline engines and can have a BSFC as low as 155 g/(kW·h) (partly because of the higher calorific value for diesel fuel) and around 55% efficiency.

Significance of BSFC

SFC is dependent on engine design, but differences in the BSFC between different engines using the same underlying technology tend to be quite small. For instance, typical gasoline engines will have an SFC of about 0.5 lb/(hp·h) or (0.3 kg/(kW·h) = 83g/MJ), regardless of the design of any particular engine. Generally, SFC within a particular class of engine will decrease when the compression ratio is increased. Diesel engines have better SFCs than gasoline, largely because they have much higher compression ratios and therefore they can convert more of the heat produced into power.

In practical applications, other factors are usually highly significant in determining the fuel efficiency of a particular engine design in that particular application.

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).

Power	date	Engine type	SFC in lb/(hp·h)	SFC in g/(kW·h)	Energy efficiency
		Turbo-prop	0.8	360 to 490
		Otto cycle gasoline engine	0.5	300
		Diesel engine automotive	0.4	230 to 260
2000 kW	1945	Wright R-3350 gasoline-compound airplane engine	0.4	243
57 kW		Toyota Prius THS II engine only		236
68 kW	2008	REVETEC X4 Gasoline aircraft/auto engine		212	38.6%
550 kW	1931	Junkers Jumo 204 Turbocharged Diesel		210
2340 kW	1949	Napier Nomad Diesel-compound engine	0.345	210
165 kW	2000	Volkswagen 3.3 V8 TDI car engine	0.33	205	41.1%
43 MW		General Electric LM6000 turboshaft			42%
88 kW	1990	Audi 2.5 litre TDI		198	42.5%
213 kW		Volvo D7E 290 hp diesel truck engine		188	44.8%
80 MW	1998	Wärtsilä-Sulzer RTA96-C two-stroke marine engine		163	51.7%
23 MW		MAN B&W Diesel S80ME-C Mk7 two-stroke marine engine		155	54.4%

• Turbo-prop. The specific fuel consumption varies with speed, altitude and power SFC. For example, PT6 turbine on Cessna 208: at low power SFC may exceed 800 g/(kW·h). Cruise sfc is about 400 g/(kW·h). A turbine is SFC efficient at high power only.
• Jet engines. The General Electric CF6-80C2B2F medium sized civil turbofan (at cruise) uses 0.605 lb/h per lbf of thrust (17.1 (g/s) /kN). At a normal cruising speed of 913 km/h, this comes out at an effective BSFC of 440 g/(kW·h). Note that the figures are not directly comparable to propeller engines above as the jet engine figures allow for propulsive efficiency, whereas the above numbers are 'at the shaft' (see propulsive efficiency).

See also

• Fuel economy in automobiles
• Specific fuel consumption (thrust) - jet engine fuel efficiency

References

External links

Typical brake-specific fuel consumption map.