What is a Heat Engine and its Types?
A heat engine is a device that turns heat into mechanical energy that can then be utilized to do work. It accomplishes this by lowering the temperature of a working substance from a higher state temperature. A heat source produces thermal energy, which raises the temperature of the working substance. The working material produces work in the engine's working body while transferring heat to a colder sink until it reaches a low-temperature state.
Using the qualities of the working substance, some of the thermal energy is turned into work throughout this process. Any system with a non-zero heat capacity can be used as the working substance, however, it is most commonly a gas or liquid. Some heat is generally lost to the environment during this process and is not converted to work. Friction and drag also render some energy ineffective.
Types of Heat Engine
Heat engines have been classified according to the concept that governs their operation. Despite the fact that all of the ideas come from Thermodynamics, each type of heat engine converts heat energy into mechanical work using a different principle. The following are examples of different types of heat engines found in thermodynamics:
Internal Combustion Engine- An internal combustion engine (ICE or IC engine) is a heat engine in which fuel is burned in a combustion chamber that is part of the working fluid flow circuit with an oxidizer (typically air). The expansion of the high-temperature and high-pressure gases produced by combustion acts directly on a component of an internal combustion engine. Pistons, turbine blades, a rotor, or a nozzle are usually the targets of the force. This force propels, moves, or powers whatever the engine is attached to by converting chemical energy into usable kinetic energy. For applications where engine weight or size are crucial, this has replaced the external combustion engine.
Stirling Engine- A Stirling engine is a heat engine that works by compressing and expanding air or another gas (the working fluid) at different temperatures in a cyclic pattern, converting heat energy to mechanical work. The Stirling engine, in particular, is a closed-cycle regenerative heat engine with a constant gaseous working fluid.
Parts of a Heat Engine-
There are 3 important parts of a heat engine, and they are:
Source- There must be an infinite thermal capacity source of heat that is kept at a constant high temperature so that whatever amount of heat is taken or added to it has no effect on its temperature.
Working Substance- It has to be some kind of substance that absorbs or rejects heat into the sink. This is the active ingredient.
Sink- There must be a finite thermal capacity sink, and it must be kept at a constant high temperature so that no quantity of heat is extracted or provided to it, and the temperature does not change.
What is an Ideal Heat Engine?
It's impossible to build a heat engine that's sole purpose is to absorb heat from a high-temperature environment and convert it all to work.
That is, there is no way to construct a heat engine that does not emit heat into the atmosphere.
Alternatively, it is impossible to create a heat engine with a 1.00 or 100% efficiency.
Conclusion
This is all about a heat engine, its different types, and how an ideal engine can be defined. Learn how an engine works and grab hold of the concept well.
FAQs on Heat Engine
1. What is a heat engine and what are its main types?
A heat engine is a system that converts thermal energy (heat) into mechanical energy, which can then be used to perform work. It operates in a cycle by absorbing heat from a high-temperature source, converting part of this heat into work, and rejecting the remaining heat to a lower-temperature sink. The two primary types are:
- Internal Combustion (IC) Engines: Where the fuel combustion process occurs inside the engine's working chamber (e.g., petrol and diesel engines in cars).
- External Combustion (EC) Engines: Where the heat is generated by burning fuel outside the engine and then transferred to a working fluid (e.g., a steam engine or a Stirling engine).
2. What is the fundamental working principle of any heat engine?
The working principle of a heat engine is based on the Second Law of Thermodynamics. It involves a cyclic process where a working substance (like a gas or liquid) undergoes a series of changes:
- It absorbs a quantity of heat (Q_H) from a high-temperature reservoir (the source).
- It converts a portion of this absorbed heat into useful mechanical work (W).
- It rejects the remaining waste heat (Q_C) to a low-temperature reservoir (the sink).
- The substance then returns to its initial state to repeat the cycle.
3. What are the three essential components that constitute a heat engine?
Every heat engine, regardless of its specific design, consists of three fundamental parts:
- Source (Hot Reservoir): A body at a high temperature (T_H) that supplies heat to the engine. It is assumed to have an infinite thermal capacity, so its temperature remains constant.
- Working Substance: The substance (e.g., air, steam, petrol-air mixture) that absorbs heat, expands to perform work, and rejects heat during the cycle.
- Sink (Cold Reservoir): A body at a lower temperature (T_C) to which the engine rejects waste heat. It also has an infinite thermal capacity, ensuring its temperature stays constant.
4. How is the thermal efficiency of a heat engine calculated?
The thermal efficiency (η) of a heat engine is a measure of how effectively it converts absorbed heat into useful work. It is defined as the ratio of the net work done (W) by the engine in one cycle to the amount of heat absorbed (Q_H) from the high-temperature source. The formula is:
η = Work Done / Heat Input = W / Q_H
Since W = Q_H - Q_C (from the first law of thermodynamics), the formula can also be expressed as:
η = (Q_H - Q_C) / Q_H = 1 - (Q_C / Q_H)
5. What are some common real-world examples of heat engines?
Heat engines are integral to modern technology and are found in various applications. Some common examples include:
- Automobile Engines: Both petrol and diesel engines are types of internal combustion engines.
- Steam Engines: Used historically in trains and factories, they are a classic example of an external combustion engine.
- Thermal Power Plants: Use steam turbines (a type of heat engine) to generate electricity from heat produced by burning coal, natural gas, or from nuclear reactions.
- Jet Engines: Operate on thermodynamic principles to produce thrust for aircraft.
6. Why can a heat engine never achieve 100% efficiency in practice?
A heat engine can never be 100% efficient due to the constraints of the Second Law of Thermodynamics, specifically the Kelvin-Planck statement. This law dictates that it is impossible to construct a device operating in a cycle that can convert all the heat it absorbs from a hot reservoir entirely into work. To complete a thermodynamic cycle and return to its initial state, the working substance must reject some amount of waste heat to a colder reservoir (the sink). Without this heat rejection, the cycle cannot be completed, and continuous operation is not possible.
7. How does a refrigerator work as a 'reversed' heat engine?
A refrigerator is a heat pump, which is essentially a heat engine operating in reverse. Instead of using heat to produce work, it uses external work to transfer heat from a cold space to a warmer space. An electric motor does work on the refrigerant, causing it to extract heat from the inside of the refrigerator (the cold reservoir) and expel it into the surrounding room (the hot reservoir). This is why the back of a running refrigerator feels warm and why leaving its door open will heat the room, not cool it.
8. What is the significance of the Carnot cycle for a real heat engine?
The Carnot cycle is a theoretical, ideal thermodynamic cycle that provides an upper limit on the efficiency that any classical heat engine can achieve. While no real engine can be perfectly reversible and operate on a Carnot cycle, it serves as a crucial benchmark. The efficiency of a Carnot engine depends only on the temperatures of the hot and cold reservoirs. By comparing a real engine's actual efficiency to the theoretical Carnot efficiency for the same temperatures, engineers can assess the performance and identify areas for improvement.
