1. The problem statement, all variables and given/known data Consider the following reversible cycle: Where S2=S3=(S1+S4)/2 . Prove that the efficiency η is: η= 1 - 2*T6 / (T1+T3) 2. Relevant equations 3. The attempt at a solution I know that in a T-S diagram the heat received by the system is equal to the area delimited by the upper boundaries of the diagram and the line T=0. I also know that the heat dispensed by the system is equal to the area under the diagram (I got the same results calculating both heats analitically). Therefore I get: η = 1 - (S4-S1)T6 / [ (S2-S1)T1+(S4-S2)T3 ] = = 1 - 2*S2T6 / [ (S2-S1)T1+(S4-S2)T3 ] I don't know how to get rid of the entropies tho... If anyone could point me in the right direction I'd appreciate. Thanks!