So I read that in a motoring test (to test the engine power), the engine is run to the desired speed by its own power. The power is absorbed by an electric dynamometer. Then, the fuel supply is cut-off and the dynamometer is converted to work as a motor to drive the engine at the same speed. After that the power supply to the motor becomes a measure of the friction power. My question is, How is this a measure of friction power? Shouldn't the power supply to motor be the same as the output power of the engine to overcome friction and also make it work at the same speed? how is it only a measure of friction power?