Brake specific fuel consumption, abbreviated BSFC and also known by the term power-specific fuel consumption or simply specific fuel consumption, is a type of comparison ratio which looks at an engine’s fuel efficiency in terms of how much fuel the car uses versus how much power it produces. The formula for calculating brake specific fuel consumption is fuel consumption divided by power, and often the results are expressed in kilowatts per hour. These calculations are popular because the mechanic can compare any number of engines, regardless of size, to see which is the most efficient. While it can apply to any shaft engine, typically this test is applied to car engines to evaluate their performance.
Equipment known as a dynamometer is used to come up with the necessary information to calculate brake specific fuel consumption. The dynamometer measures different forces while the engine is running. The mechanic then takes the results, plugs them into the formula for calculating BSFC, and finds out what the BSFC measurement is for that specific engine running under those specific conditions. Changing the conditions will change the results of the BSFC test.
Though it is a useful tool for comparing two or more engines' efficiency and for monitoring what changes to the fuel and engine itself will increase a specific engine’s efficiency, the brake specific fuel efficiency formula is not without flaws. The results will vary depending on how much weight the vehicle is currently carrying and the speed at which the vehicle is traveling. Often, brake specific fuel consumption is shown on a chart showing all the different values as power and fuel consumption increase.
A lower number equals a higher efficiency because the engine is creating a high level of power while using a low amount of fuel. For example, an engine performing around 300 grams per kilowatt-hour is only getting around 20% efficiency. Diesel engines typically perform better than gasoline engines in terms of BSFC.
It’s also important for those testing BSFC to keep in mind that the results only show the efficiency between two values: fuel consumption and power. This doesn’t take into account any other factors about the engine or vehicle, and only represents one type of comparison out of many that a mechanic could take when comparing vehicles and their engines to determine the best one. Examining all the different, controllable factors on each individual vehicle is the best way a mechanic can determine which cars perform best under which situations.