1. The problem statement, all variables and given/known data In 1816, Robert Stirling, a Scottish clergyman, patented the Stirling engine, which has found a wide variety of applications ever since. Fuel is burned externally to warm one of the engine’s two cylinders. A fixed quantity of inert gas moves cyclically between the cylinders, expanding in the hot one and contracting in the cold one. Consider n mol of an ideal monatomic gas being taken once through the cycle, consisting of two isothermal processes at temperatures 3Ti and Ti and two constant-volume processes. Determine in terms of n, R, and Ti (a) the net energy transferred by heat to the gas and (b) the efficiency of the engine. A Stirling engine is easier to manufacture than an internal combustion engine or a turbine. It can run on burning garbage. It can run on the energy of sunlight and produce no material exhaust. 2. Relevant equations w=-∫PdV e= w/Qh 3. The attempt at a solution So for part b I'm a little bit confused as to how to calculate work. Since two of the processes are isovolumetric the work done by them = 0. Now this is where I get confused. Doesn't the isothermal process that represents a decrease in the volume translate to work done ON the gas - therefore it should be positive. The solutions manual lists them both with the same sign. If I add my work together I do get the same result (-nRTi ln(4)). So my question I suppose is more conceptual. How does a decrease in volume represent work done by the engine?