# 11.4 Other expressions for efficiency - and SFC

Sometimes the overall efficiency of aircraft engines is expressed in alternative parameters: specific impulse, , and thrust specific fuel consumption, TSFC or just SFC. Both of these parameters measure how effectively fuel is used, and have dimensions.

Specific Impulse ( or ):

where is the fuel mass flow rate. To find the fuel-air ratio, , we employ a control volume around the combustor and carry out an energy balance.

Specific Fuel Consumption (SFC or TSFC):

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