A useful engine exhibits two important features: 1 an engine must work in cycles 2 a cyeclic engine must include more than one thermal reservoir The efficiency of heat machine(热机效率) The ratio of the work done to the total positive heat W 9 flow provided by the 7 =1- thermal reservoir 2A useful engine exhibits two important features: 1 an engine must work in cycles 2 a cyclic engine must include more than one thermal reservoir The efficiency of heat machine(热机效率） 1 2 1 1 Q Q Q W     The ratio of the work done to the total positive heat flow provided by the thermal reservoir