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The Second law of thermodynamics states that total entropy of an isolated composite system cannot decrease. For a cyclic heat engine exchanging heat only with reservoirs at $T_h>T_c>0$, it gives $-Q_h/T_h+Q_c/T_c\ge 0$. Since $W=Q_h-Q_c$,The bound is attained by a reversible heat engine: no friction, no finite-temperature-difference heat exchange and no other entropy production. A Carnot cycle provides an implementation with two reversible isotherms and two reversible adiabats. All temperatures in these ratios are absolute.

Let $n_{\mathrm{mol}}$ be the fixed gas amount, $r=V_2/V_1>1$, $\Delta T=T_h-T_c$, and let $C_V$ denote the heat capacity of the whole sample. First take the unregenerated Stirling cycle, so that heat rejected during isochoric cooling is not stored and returned during isochoric heating. The reversible quasistatic gas work iswhile the heat inputs are $Q_{12}=n_{\mathrm{mol}}R{T}_h\ln r$ and $Q_{41}=C_V\Delta T$. HenceThe strict inequality follows because $C_V\Delta T>0$ adds heat input without additional work. Quasistatic motion alone does not make heat exchange with a single hot or cold reservoir reversible while the gas temperature changes on an isochore. Indeed, using those two reservoirs gives total external entropy production $C_V\Delta T(1/T_c-1/T_h)>0$ per cycle.

The strict comparison requires the no-regeneration assumption. A perfect reversible regenerator stores $C_V\Delta T$ during $2\to 3$ and returns it during $4\to 1$; only the isothermal heat transfers then enter the external heat budget, giving $\eta =1-T_c/T_h$ exactly. For example, perfect regeneration with $T_h=600\mathrm{K}$, $T_c=300\mathrm{K}$ and $r=2$ gives $\eta =1/2={\eta }_{\max }$. Thus the four gas paths by themselves do not enforce an efficiency strictly below the Carnot bound.

For the gas itself, the four entropy changes areExpansion increases the available positional states; compression reverses that change. Isochoric cooling reduces thermal populations and isochoric heating restores them. The sum is zero because entropy is a state function. This does not imply zero total entropy production in the gas plus external reservoirs.

Taking $S_1$ as reference, the isotherms are horizontal on a $T$–$S$ diagram. The isochore through state 1 obeys $S-S_1=C_V\ln (T/T_h)$, and the isochore through state 2 is shifted right by $n_{\mathrm{mol}}R\ln r$. For the internally reversible quasistatic gas paths, $\delta Q=TdS$, soThe enclosed area is the net work per engine cycle, with a clockwise orientation. It is not the total heat input.

For the refrigerator, the stated rates give the cooling coefficient of performance $\mathcal{C}={\dot{Q}}_c/P=1.9/0.1=19$. But the absolute reservoir temperatures are $T_c=277.15\mathrm{K}$ and $T_h=293.15\mathrm{K}$, so Carnot's theorem requiresAt $0.1\mathrm{kW}$ input the largest allowed cooling rate is only $1.7322\mathrm{kW}$. Equivalently the stated $2.0\mathrm{kW}$ heat rejection gives the reservoir entropy ratea direct violation of the Second law of thermodynamics for this cyclic two-reservoir device.

The unregenerated neon model makes the algebraic obstruction explicit. With $C_V/(n_{\mathrm{mol}}R)=3/2$, reversing the cycle givesSetting this to 19 produces $\ln r=-0.89385$, or $r=0.4091$, contradicting the required expansion ratio $r>1$. Perfect regeneration can raise performance only to the Carnot bound and does not fix the problem. There is no physically admissible $V_2/V_1$ for the stated data.

A vapour-power cycle using a liquid pump, high-pressure heat addition, turbine expansion and low-pressure condensation. Ideal components are reversible apart from heat transfer through finite temperature differences. With states numbered pump inlet, pump outlet, turbine inlet and turbine outlet, the net thermal efficiency is $[(h_3-h_4)-(h_2-h_1)]/(h_3-h_2)$. Liquid pumping avoids the impractical compression of a wet two-phase mixture in a steam realization of the Carnot cycle. Named after William John Macquorn Rankine.

For an ideal Diesel cycle with compression ratio $r=V_1/V_2>1$, cutoff ratio $\rho =V_3/V_2>1$ and constant heat-capacity ratio $\gamma$, its thermal efficiency is $\eta =1-({\rho }^{\gamma }-1)/[\gamma r^{\gamma -1}(\rho -1)]$. As $\rho \to 1^{+}$ the quotient tends to $\gamma$, giving $\eta \to 1-r^{1-\gamma }$. The maximum-to-minimum temperature ratio tends to $r^{\gamma -1}$, so the limiting efficiency equals that of a Carnot cycle between those temperatures. Both heat input and net work vanish in this limit: the result is a limit of their ratio, not a finite-work engine at exactly zero cutoff interval.

Vapor-compression refrigeration circulates a working fluid through cold and hot coils. In the cold coil, liquid becomes gas while absorbing heat from the refrigerated region. Compression supplies work and raises the pressure and temperature; the hot coil releases heat as the gas returns to liquid. A restriction reduces the pressure before the next cold-side passage. Changing the pressure changes the temperature of the phase transition, letting one circulating fluid receive and release heat at different temperatures. This physical implementation serves both refrigerators and heat pumps; its ordinary dissipative operation differs from the reversible Carnot cycle used to bound performance. ^{[ref] [ref]}