But how can we estimate the amount of heat that has entered the general cycle? It also does not seem clear what the low and high temperature heat sources are for the general cycle.
These quantities are much easier to see on a $TS$ diagram (a temperature-entropy diagram) than on a $PV$ diagram. Hopefully it should be obvious that the any closed cycle which returns to its original state will also form a closed loop in the $TS$ plane.
By Eric_DUMINIL at English Wikipedia
To interpret this, note that:
- Since $dQ = T \, dS$ for a reversible process, heat is being added to the working fluid at all points for which $dS > 0$, and removed at all points for which $dS < 0$.
- This implies that the area under the "upper" part of the curve connecting $S_A$ and $S_B$ above is the heat $Q_H$ added to the fluid from the hot reservoir, while the area under the "lower" part of the curve is the heat rejected to the cold reservoir $Q_C$.
- By conservation of energy, the work done by the gas is $Q_H - Q_C$, or the area enclosed by the loop (just like in a $PV$ diagram.)
- The hottest temperature reached in the cycle and the coldest temperature reached in the cycle are quite obvious: they're the $T$ values at uppermost and lowermost points on the loop, respectively.
Finally, this perspective allows us to see why the Carnot Cycle is the most efficient. Suppose that we have some cycle in which the operating fluid cycles between $S_A$ and $S_B$, and is in contact with some hot reservoir $T_H$ and cold reservoir $T_C$. What kind of loop gives us the most efficiency?
Well, the most efficient loop will be the one for which the least heat is wasted, i.e., the one for which $Q_C$ is smallest when expressed as a fraction of $Q_H$. This implies that we want a loop that makes $Q_H$ as big as possible while making $Q_C$ as small as possible. In other words, the top part of the loop should be as far up as possible, while the bottom part should be as low down as possible. This implies that the top part of the curve should be an isothermal process at $T_H$ from $S_A$ to $S_B$, while the bottom part should an isothermal process at $T_C$ from $S_B$ to $S_A$. These two processes are connected by isentropic processes at $S_A$ & $S_B$, forming a rectangle in the $TS$ plane. This exactly describes the Carnot Cycle.