Thermal efficiency is the relationship between the potential for power contained in a specific heat source, and how much usable power is actually created when that heat source is used. The formula for calculating thermal efficiency is:
Thermal Efficiency =
Horsepower Produced ÷ Potential Horsepower in Fuel
For example, consider the piston engine used in a small general aviation airplane, which typically consumes 0.5 lb of fuel per hour for each horsepower it creates. Imagine that the engine is creating 200 hp. If we multiply 0.5 by the horsepower of 200, we find the engine is consuming 100 lb of fuel per hour, or 1.67 lb per minute. Earlier in this chapter, one horsepower was found to be 33,000 ft-lb of work per minute. The potential horsepower in the fuel burned for this example engine would be:
The example engine is burning enough fuel that it has the potential to create 744 horsepower, but it is only creating 200. The thermal efficiency of the engine would be:
More than 70 percent of the energy in the fuel is not being used to create usable horsepower. The wasted energy is in the form of friction and heat. A tremendous amount of heat is given up to the atmosphere and not used inside the engine to create power.