Calculation of work done by/on engine

[aks2708]

Homework Statement

During the working cycle of an engine, the heat transferred out of the system was 200kj/kg of working fluid. The internal energy also decreased by 300kj/kg of working fluid. find the work done and state whether it is work done on the engine or by the engine.

Homework Equations

W = Q- E

The Attempt at a Solution

W = Q-E

Since heat transferred out so Q = -200
Decrease in internal energy E = -300

Work done = -200-(-300) = 100.

Please correct me if i am wrong. I am not sure if it is work done by the engine or on the engine

 

[diazona]

Think of it in terms of energy conservation. Considering 1kg of fluid as a "system", the system went through a cycle and in the process lost 300kJ of energy. All those 300kJ had to be transferred as either heat or work. Now, if you measured 200kJ of heat coming out, what can you say about the energy that was transferred in the form of work (i.e. the rest of it)? Did the work take extra energy out of the system, or did it add energy back into the system?