Heat engines are most commonly used in different worldwide applications, such as cars, power plants, trains, and buses. This article describes the heat engine working, efficiency, types, and applications.
What is a Heat Engine?
A heat engine is a mechanical machine that changes the thermal or heat energy of the fuel into mechanical power, which is further used to perform useful work. This is achieved by moving the work material from a higher temperature to a lower temperature.
When someone rubs his hand one on the other hand, then friction produces due to rubbing. This friction converts mechanical energy (hand movements) into heat (the sensation of warmth).
The heat engine works in opposite phenomena because this type of engine takes energy from a warm object (compared to the environment) and converts it into movement. Usually, this movement converts into electrical energy by a generator.
The majority of energy utilized for vehicles and electricity generation is generated by heat engines. Warm objects, including gases, possess thermal energy that may convert into a useful form.
A heat engine usually transfers energy from a hot source to a cold sink and converts a portion of that energy into mechanical energy. A temperature difference is essential for heat engines to operate.
Various processes are used in different types of engines, such as combustion for gasoline and coal engines or harnessing energy from nuclear reactions for uranium-based reactors. Regardless of the process, the ultimate objective remains the same: converting heat into useful work.
The car engine is a well-known example of a heat engine, but most power plants, including nuclear, natural gas, and coal plants, also operate as heat engines.
Working of Heat Engine
A heat engine essentially consists of a cold sink, an engine, and a heat reservoir. The heat produced through external or internal combustion is supplied to the engine, which causes the piston to move.
The output power is then transferred to a connected machine, enabling it to perform work. Additional heat is released to a cold sink, maintaining a consistent temperature between the sink and the reservoir.
Heat engines, such as the car engine, operate in a cyclical manner. Such types of engines absorb energy in the form of heat during one phase of the cycle and utilize this energy to perform useful work during the other phase of the cycle.
Types of Heat Engines
The heat engine has the following types:
- External Combustion Engine (ECE)
- Internal Combustion Engine (ICE)
1) External Combustion Engine
An external combustion engine is one of the most famous types of heat engines in which an external heat source is used to heat the (internal) working fluid through a heat exchanger or engine walls. As the fluid gets heat, it expands, generates power, and moves the vehicle.
EC engines don’t fit in the internal part of the vehicle like the IC engine. The external combustion engine attaches to the external portion of the vehicle. In this engine, combustion does not take place in the cylinder, but the piston is driven through steam. It’s out to that place.
The external combustion chamber has a mixture of air and fuel that ignites to generate heat. This generated heat is utilized to heat the inner fluid through the wall of the heat exchanger or the engine. When heated, the liquid expands and acts on the engine machinery to create movement and useful work. This fluid is then poured (open cycle) or cooled, compressed, and use again (closed circuit).
External combustion engine uses combustion primarily as a medium of heat, and this engine operates equally well with other heat source types.
The steam engine and Stirling engine are examples of external combustion engines. A maximum part of the world’s electricity produces through steam turbines. The train having a steam engine is also a well-known example of an external combustion engine.
Read More: External Combustion Engine Working and Types
2) Internal Combustion Engine
Internal combustion engine or IC engine is one of the most prevalent types of heat engines. These engines are most commonly found in trains, airplanes, ships, boats, and vehicles.
In these engines, the air-fuel mixture is ignited within the engine to perform work. This same fuel-air mixture is later discharged as exhaust. While pistons are commonly used for this process, turbines can also be employed.
Read More: IC Engine Parts and Types
Heat Engine PV Diagram
A heat engine generally takes in energy in the form of heat, performs work, and then releases the remaining heat that can’t be utilized for work. Thermodynamics investigates the relationships between work and heat, and the operation of a heat engine is constrained by the first and second laws of thermodynamics.
The first law involves the conservation of energy within the system, while the second law establishes the maximum efficiency of the equipment and measures the energy flow’s direction.
To understand the efficiency of an engine cycle, one can compare the pressure-volume (PV) diagram with the Carnot cycle, which represents the most efficient form of a heat engine.
The explanation of the PV diagram is given below:
- During the process where the fluid transforms from a liquid state to a vapor state, it occurs isothermally when the source has a high temperature. This vaporization takes place at a constant pressure while the volume increases.
- When the steam enters the turbine, it expands in a reversible and adiabatic manner, adhering to the equation of state for such a process.
- When the steam transforms from a steam to a water liquid and the source has a low temperature, it occurs isothermally. This condensation process takes place at constant pressure while the volume decreases.
- When the liquid enters the compressor, the compressor compresses the liquid reversibly and adiabatically, with its pressure increasing back to the initial point.
Energy Reservoir Model
A common method to represent a heat engine is through the energy reservoir model. The engine extracts energy from a hot reservoir and utilizes a portion of this heat to perform work. However, due to the limitations imposed by the second law of thermodynamics, the engine must release some of the energy to a cold reservoir.
In the case of the vehicle engine, the combusting fuel acts as the hot reservoir, while the cold reservoir is the surrounding environment to which the exhaust gases are discharged.
The above-given efficiency expression is a general one, but the maximum efficiency is restricted to that of the Carnot cycle. This limitation is usually referred to as the thermal bottleneck.
Heat Engine Efficiency
Efficiency refers to the proportion of heat input at a high temperature that is converted into work. According to the second law of thermodynamics, no engine can achieve 100% efficiency.
Efficiency, η = Work done / Heat input
In the above equation,
Efficiency, η = W / Q1
Work done, W = Q1 – Q2
Heat input = Q1
The maximum efficiency of the reciprocating engine is usually about 50%, while an average coal-fired power plant has an efficiency of around 33%. The latest constructed power plants can achieve efficiencies of over 40%.
However, the output power of a small heat engine, such as a car engine, is expressed in horsepower. The power of the larger heat engines is expressed in megawatts (MW). However, output may also be calculated in any unit of power, including watts.
The power plants also have an additional electric output power. To differentiate between the two powers, electric power output is denoted in megawatts electric (MWe), while thermal power (input power) is denoted in MWt. For the heat engine that produces motion instead of electricity, the output power is in the form of mechanical work.
Applications of Heat Engines
- Heat engines are most commonly used in different vehicles such as cars, buses, trains, tractors, and small aircraft.
- They are used in ships.
- They are used in power plants.
- They are used to produce electricity.
- Heat pumps are employed in refrigeration and heat pumps.
These types of engines may use waste heat produced in power plants or industrial processes, transforming it into electricity or useful work, thus increasing overall efficiency.
FAQ Section
What is a heat engine?
A heat engine is a mechanical device that transforms heat energy into useful work. Car engines, motorcycle engines, bus engines, and tractor engines are the most common examples of heat engines.
Heat energy is converted into what form of energy in a heat engine?
In the heat engine, the heat energy is transformed into mechanical energy.
What is the efficiency of the heat engine?
The ratio of the mechanical work done to the heat absorbed is known as the efficiency of the heat engine.
What is the difference between the heat engine and the Carnot engine?
A heat engine is a mechanical device that transforms heat energy into useful work, and its efficiency varies according to its material and design. Heat engines are real devices used in many applications, such as steam engines and IC engines. However, a Carnot engine is a theoretical concept of an ideal heat engine that works at the maximum possible efficiency. Carnot engines are not practical for real-world applications because of their idealized molds.
What are the examples of heat engines?
What are the two types of heat engines?
- Internal combustion (IC) engine
- External combustion (EC) engine