In what unit is specific fuel consumption typically expressed?

Specific fuel consumption (SFC) is a measure of the efficiency of an aircraft engine design in terms of fuel consumption relative to power produced. It is typically expressed as the amount of fuel consumed per unit of thrust or power produced, commonly in pounds per hour.

SFC is an important metric in aviation because it allows for the comparison of fuel efficiency among different engines and aircraft. The units used—pounds per hour in this case—are suited for measuring the fuel consumption of jet engines, which operate over substantial periods of time and produce thrust.

The other options provided, such as gallons per hour and liters per kilometer, do not accurately represent the specific fuel consumption of engines in the context of aviation. For example, gallons per hour might indicate how much fuel is burned, but it doesn't relate that consumption to the thrust produced. Similarly, liters per kilometer is more applicable to ground vehicles where distance is a relevant factor, and kilograms per second does not align with the standard practices in the aviation industry for SFC measurements.