Mechanical Technology

A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a heat pump rather than a heat engine.

The Carnot cycle is the most efficient cycle possible for converting a given amount of thermal energy into work or, conversely, for using a given amount of work for refrigeration purposes.

The Carnot cycle

The Carnot cycle when acting as a heat engine consists of the following steps:

1. Reversible isothermal expansion of the gas at the "hot" temperature, T_H (isothermal heat addition). During this step (A to B on Figure 1, 1 to 2 in Figure 2) the expanding gas causes the piston to do work on the surroundings. The gas expansion is propelled by absorption of quantity Q₁ of heat from the high temperature reservoir.
2. Isentropic (Reversible adiabatic) expansion of the gas. For this step (B to C on Figure 1, 2 to 3 in Figure 2) we assume the piston and cylinder are thermally insulated, so that no heat is gained or lost. The gas continues to expand, doing work on the surroundings. The gas expansion causes it to cool to the "cold" temperature, T_C.
3. Reversible isothermal compression of the gas at the "cold" temperature, T_C. (isothermal heat rejection) (C to D on Figure 1, 3 to 4 on Figure 2) Now the surroundings do work on the gas, causing quantity Q₂ of heat to flow out of the gas to the low temperature reservoir.
4. Isentropic compression of the gas. (D to A on Figure 1, 4 to 1 in Figure 2) Once again we assume the piston and cylinder are thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to T_H. At this point the gas is in the same state as at the start of step 1.

Figure 1: A Carnot cycle acting as a heat engine, illustrated on a temperature-entropy diagram. The cycle takes place between a hot reservoir at temperature T_H and a cold reservoir at temperature T_C. The vertical axis is temperature, the horizontal axis is entropy.

Figure 2: A Carnot cycle acting as a heat engine, illustrated on a pressure-volume diagram to illustrate the work done.