A water-cooled engine produces 1235 W of power. Water enters the engine block at 11.21 °C and exits at 28.23 °C. The rate of water flow is 102.3 L/h. What is the engine’s efficiency?

Question

jamesj · Answer

Efficiency = (Productive Energy out)/(Energy in) = E_out / E_in

The water is clearly taking away energy that is not used productively by the engine.  For example, if this were boat engine driving a propeller in the water, all of the energy that has gone into the water is not energy used to drive the propeller.

Hence to solve the problem, choose a time period, let's say an hour.  Calculate E_in and then calculate E_out; then take the ratio of the two.