In 1824, Sadi Carnot designed a theoretical Heat engine whose working efficiency was 100%. Heat engines are devices that can convert heat into mechanical energy. Like a steam engine or Motor vehicle. The working principle of all heat engines are the same. In which any working substance gas or vapour is brought back to the initial stage by going through different thermodynamic stages of a cyclic process. Carnot suggested that, if a heat engine goes through fixed thermodynamic stages of a specified process, then the efficiency of the heat engine may be 100% possible. This is called the Carnot engine or ideal heat engine. Ideal, because this engine gives maximum efficiency. No other engine can be more efficient than this. This specified cyclic process suggested by Carnot is called Carnot cycle. The Carnot engine has three main parts. First, an insulated, non-conducting stand, so that the transfer of energy is not possible. Second, a hot reservoir with infinite heat capacity. Heat capacity is Infinite so that the temperature of the reservoir remains constant after taking any energy from it. It is also called Heat source. Third, a cold reservoir. Its heat capacity is also infinite. So that any amount of heat is given, its temperature does not change. This is called heat sink. Fourth, a cylinder in which the working substance can be kept. working substance can be anything gas, or liquid. cylinder is fitted with a movable piston. The piston and cylinder walls are made of non-conducting material. Due to which transfer of heat is not possible. While the base of the cylinder is conducting. A Carnot engine goes through four different stages of the Carnot cycle. which is graphically displayed on a PV graph. For this, first place the cylinder on the hot reservoir. Since the temperature of the reservoir is higher than the temperature of the gas, the heat flows from the heat reservoir to the gas. Due to which the gas starts to expand. Since due to heat transfer gas is expanding, therefore the temperature of the gas remains constant. This process is called Isothermal expansion. As the gas expands, the volume of the gas increases and the pressure decreases. In the next step, take off the cylinder from the hot reservoir and place it on the insulating stand. When doing so, the gas is confined to the non-conducting wall. Now Let Piston rise slowly upward. Due to which the gas continues to expand. But this time the gas is not getting any heat to expand. Such expansion is called adiabatic expansion. Since this expansion is taking place without heat, the temperature of the gas starts to decrease. Due to this expansion, the volume of the gas also increases and the pressure decreases. In the next step, lift the cylinder from the stand and place it on the cold reservoir. Now press the Piston downward which causes compression of the gas. The reservoir absorbs whatever extra heat is produced due to compression of the gas. In this way the gas compresses at the same constant temperature. This is called isothermal compression. Due to this compression the volume of the gas decreases and the pressure is increased. In the final step, keeping the cylinder on stand again, press Piston downward. But now there is no cold reservoir to absorb the heat getting produced extra. Therefore, the temperature of the gas starts increasing. This compression is called adiabatic compression. Press the piston until the temperature of the gas rises back to equal that of the hot reservoir. This decreases the volume of the gas and increases the pressure. At the end of this step, the gas returns to its initial stage. This complete cycle is called the Carnot cycle and the engine working on it is called the Carnot engine. The efficiency of a Carnot engine depends on the temperature of both reservoirs. Either rise the temperature of the hot reservoir to infinite kelvin or low down the temperature of the cold reservoir to zero kelvin. In both cases, the engine efficiency will be 100%. But it is not possible to do so. Hence the Carnot engine is just a theoretical ideal engine. To increase the efficiency of the practical engine, we keep as much difference as possible in the temperature of both reservoirs.
