Question

An aircraft engine takes in 9000 j of heat and discards 6400 j each cycle. (a) what is the mechanical work output of the engine during one cycle? (b) what is the thermal efficiency of the engine?

Answer 1

Work done = heat in - heat out 
= 9000 -6400 = 3600 J per cycle. 

And the efficiency = work out / heat in 
= 3600/9000 
= 0.4 or 40%

Sorry if this does not help.