A heat engine converts heat into mechanical work by passing through a hot source and a cold sink in a specific thermodynamic cycle. Examples include steam engines and gas turbines. Efficiency depends on the temperature difference, and is limited by the cold sink’s temperature. The efficiency of heat engines varies, with car engines achieving around 25% and power plants being the most efficient.
A heat engine is a device used to convert thermal energy, or heat, into mechanical work. This occurs when heat from a hot source passes through the motor itself and enters a cold sink. The cold sink is the lower temperature part of a thermodynamic cycle, such as the condensing unit found in the Rankine cycle, or steam. There are many different types of heat engines, each of which has its own specific cycle. Some examples of heat engines include steam and internal combustion engines, Stirling engines, and gas turbines.
Commonly, a heat engine will be confused with the thermodynamic cycle that takes place inside the engine itself. This is mainly due to the fact that heat engines are often classified according to their specific thermodynamic cycles. The device itself that converts thermal energy into work is known as the “engine”, while the thermodynamic model applied to the engine is the “cycle”. For this reason, steam engines are not referred to as Rankine engines.
An efficient heat engine will try to imitate its respective cycle as best as possible. The greater the temperature difference between the hot source and the cold sink within the cycle, the more efficient the motor is. For example, an efficient steam engine requires both a high-temperature heat source and a low-temperature cold sink. In the Rankine cycle, a boiler uses a high-temperature burner to convert water into steam. This vapor passes through the engine and is then recondensed into water through a low temperature condenser.
The colder the condenser, the more vapor will be condensed back into water. This is because the condensers are designed to effectively reverse the saturation process carried out by the boiler. This will help achieve higher condensation rates; the higher the rate, the more water will be returned. This helps to increase the overall efficiency of the steam cycle.
Although the heat engine efficiency can be highly optimized through a large temperature difference between the hot source and the cold sink, it is still limited. This is because the temperature of the cold sink depends on the surrounding temperature, which in some situations cannot be cooled down to ideal conditions. Because of this, the efficiency of a heat engine is limited to the temperature limits of the cold sink. A common fix to this is to raise the temperature of the hot spring; however even this is limited to the material’s lack of resistance to high temperatures.
The efficiency of the heat engine varies depending on the specific engine and cycle. Thermal efficiency ranges from 3% to around 70%, with car engines achieving thermal efficiency around 25%. The most efficient heat engines are found in large power plants, where both gas turbines and steam turbines are used to generate electricity.
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