The gas expansion is propelled by absorption of heat energy Q1 and of entropy Δ S H = Q H / T H {\displaystyle \Delta S_{\text{H}}=Q_{\text{H}}/T_{\text{H}}} from the high temperature reservoir. Isentropic (reversible adiabatic) expansion of the gas (isentropic work output). For this step (2 to 3 on Figure 1, B to C in Figure 2) the piston and cylinder are assumed to be thermally insulated, thus they neither gain nor lose heat.