Unconventional cycles, pseudoadiabatics and multiple adiabatic points

Abstract: Unconventional cycles provide a useful didactic resource to discuss the second law of thermodynamics applied to thermal motors and their efficiency. In most cases they involve a negative slope, linear process that presents an adiabatic point where the process is tangent to an adiabatic curve and $\delta Q=0$, signalling that the flow of heat is reversed. We introduce a parabolic process, still simple enough to be fully explored analitically in order to deal with the usual follow up question on the possibility of having more than one adiabatic point. Having one (linear), two (parabolic) or more such points allow the construction of reversible, non-isoentropic processes, that we call pseudoadiabatics, whose total heat exchanged is zero.