Specific fuel consumption is the ratio that compares the fuel used by the engine to the amount of power the engine produces. Specific fuel consumption allows manufacturers to see which engine use the least fuel while still producing high amount of power. It allows engines of all different sizes to be compared to see which is the most fuel efficient.
There are different types of specific fuel consumption of internal combustion engines: indicated specific fuel consumption (ISFC) and brake specific fuel consumption (BSFC). The formula for calculating the specific fuel consumption is written as:
SFC = mass flow rate (kg/s) / power (kw) ..........(kg/kw.s)
ISFC = mass flow rate (kg/s) / IHP (kw) ............(kg/kw.s)
BSFC = mass flow rate (kg/s) / BHP (kw) ...........(kg/kw.s)
Brake specific fuel consumption is a type of comparison ratio which looks at engine's fuel efficiency in terms of how much fuel the car uses versus how much power it produces. The formula for calculating the brake specific fuel consumption is fuel consumption divided by brake power, and often the results are expressed in kilogram per kilowatt hour. Equipment known as dynamo-meter is used to keep necessary information to calculate brake specific fuel consumption. The dynamo-meter measures different forces while the engine is running. The mechanic takes the results, and finds out what is BSFC measurement for this specific running engine under those specific conditions. Changing the conditions will change the result of the BSFC test.
A lower number equals a higher efficiency because the engine is creating a high level of power while using a low amount of fuel. Diesel engines typically perform better than gasoline engines in term of BSFC.