Specific fuel consumption, abbreviated SFC, compares the ratio of the fuel used by an engine to a certain force such as the amount of power the engine produces. It allows engines of all different sizes to be compared to see which is the most fuel efficient. It allows manufacturers to see which engine will use the least fuel while still producing a high amount of power.
There are different types of SFC: TSFC, thrust specific fuel consumption, and BSFC, brake specific fuel consumption, are two of the most common. TSFC looks at the fuel consumption of an engine with respect to the thrust output, or power, of the engine. Airplane engines, for example, can be compared to see which will produce the most thrust while using the least amount of fuel.
TSFC is expressed in the amount of fuel needed to provide a certain thrust over a period of time. This formula is written as pounds of fuel per hour of thrust. There are disadvantages to this formula, however. The most fuel efficient engine may not always be the best choice. A more lightweight engine may cut down on the need for more fuel to power it, and thus be a better choice even if a heavier engine has a lower TSFC.