1-Is the efficiency of heat engines working in cycles other than Carnot independent of the nature of substance used? Can we still claim that maximum efficiency in converting heat to work is attained during reversible processes for such cycles? For which engines/cycles can we do this? 2- Why is the Carnot cycle used to prove ∫dqrev/T = 0 in a cycle? Why not use some other cycle? We use the ideal gas as our working substance because it's easier to calculate dq/T for ideal gas along the path of the cycle, but why does the cycle have to be Carnot? Or does it? Of course the integral will be zero for any substance in ANY reversible cycle, but can you actually PROVE this general result to be true using some cycle other than Carnot? In other words, in his quest to prove ∫dqrev/T =0 solely from the second law, how was man motivated to pick the Carnot cycle (among other cycles) as his cycle of choice for the proof?