Heat Engines 

Introduction

Heat engines are machines or systems that convert thermal energy into mechanical energy. A basic heat engine may do this by absorbing heat at a higher temperature, such as from solar energy or a furnace, and converting a portion of that heat to work, often by rotating a shaft.

The engine then ejects waste heat at a lower temperature, maybe into its surroundings or a water supply, before preparing to start over again. It’s a cyclical process.

Working of Heat Engine 

The working principle of all heat engines is the same. Any working substance gas or vapor is brought back to the initial stage by going through different thermodynamic stages of a cyclic process.

Each cycle is a series of events or stages that repeat in the same order in which net heat is transferred, and motion is produced after some heat is rejected.

Thermal efficiency of Heat Engine:

The efficiency of a heat engine is defined as the ratio of work produced to heat supply. Only a fraction of the heat input is converted to work, with the rest being rejected. Let us derive an equation for a heat engine’s efficiency.

η =W/Q1

Where Q1 = heat supplied to the system, kJ

W = net work done by a system, kJ

After each cycle, the engine returns to its previous state, preserving its internal energy.

ΔU = 0

W = Q1−Q2, where Q2 = heat rejected from the system, kJ

Hence, the engine efficiency is:

η =(Q1−Q2)/Q1

η = 1−Q2/Q1

So, in this case, efficiency will be 100%, but this is not achievable since there will be some energy loss in the system. As a result, the efficiency of any engine has a limitation.

Characteristics of a heat engine are:

1. It receives heat from a high-temperature source (furnace, nuclear reactor, etc.). Denoted by Q1
2. It converts the part of that heat into work (mainly in a rotating shaft). 
3. It rejects the leftover waste heat to a low-temperature sink (the atmosphere, rivers). Denoted by Q2
4. It operates completely on the thermodynamic cycle. 

Types of Heat Engine  

The principle on which heat engines operate has been used to classify them. Even though all the principles are drawn from thermodynamics, each type of heat engine employs a unique principle to transform heat energy into mechanical work. 

The following are the several types of heat engines known in thermodynamics:

• Internal Combustion Engine: In these heat engines, the fuel is burned within the cylinder. A vehicle engine is an example of an internal combustion engine. Internal combustion engines are much more economical than external combustion engines because no energy is lost during heat transfer between the boiler and the engine.
• External Combustion Engine or Stirling Engine: An external combustion engine is a type of reciprocating heat engine in which a working fluid is heated by combustion in an external source, such as a heat exchanger. The fluid then creates motion and usable work by expanding and working on the engine’s mechanism. It can run without valves or an ignition system, allowing for longer service intervals and cheap operating costs.

Everyday Examples of Heat Engine:

Thermal power plants, internal combustion engines, weapons, freezers, and heat pumps are all modern examples of heat engines. 

Power plants are an example of heat engines that operate forward, with heat flowing from a hot reservoir into a cool reservoir to create work. 

Refrigerators, air conditioners, and heat pumps are all examples of heat engines that work in reverse. They utilize work to convert heat energy at a low temperature to a higher temperature more efficiently than just converting work into heat.

Refrigerators extract heat from a thermally closed container at a low temperature and expel waste heat to the environment at a high temperature. On the other hand, heat pumps collect heat from a low-temperature environment and ‘vent’ it into a thermally closed container (a house) at a higher temperature.

Carnot Engine

In 1824, Sadi Carnot suggested 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.

But the efficiency of a heat engine can never be 100%. So it’s a theoretical engine. To increase the efficiency of the practical engine, we keep as much difference as possible in the temperature of both reservoirs.

A Carnot cycle is a closed thermodynamic cycle that is reversible. This includes processes such as isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. During these procedures, the substance can be expanded and compressed to the desired degree before returning to its original form.

The four processes of the Carnot cycle are as follows:

1. Isothermal gas expansion: The amount of heat absorbed by the ideal gas in this process is Qin from the heat source at a temperature of Th. The gas expands and has an effect on the surroundings.
2. Adiabatic gas expansion: The system is thermally insulated here, and the gas expands while the surroundings are being worked on. Here the temperature is lower Ti. 
3. Isothermal gas compression: The heat loss Qout happens when the surroundings do the job at temperature Ti.
4. Adiabatic gas compression: The system is once again thermally insulated. As the surrounding environment works on the gas, the temperature increases back to Th.

Conclusion: 

A thermodynamic system that turns heat energy into mechanical energy is known as a heat engine. Any heat engine requires a heat sink or a high-temperature heat source, which can come in various shapes and sizes (for example, a nuclear reactor is the heat source in a nuclear power plant, but in many cases burning fuel is used as a heat source).

The engine receives heat from the hot reservoir and expands, and this expansion process exerts work on the environment, normally harnessed with a piston. The system then returns to its initial condition by releasing heat energy into the cold reservoir. The process is then repeated cyclically to create helpful work constantly called a heat engine.