An Introduction
Heat engines are found everywhere you go. There is a heat engine present in a refrigerator that helps to cool your food items. You can also find heat engines in your car. They are also present in all kinds of air-conditioning units. The primary importance of a heat engine is to produce mechanical energy with the help of heat energy. There are two primary classifications of heat engines based on the combustion process. The two types of heat engines are internal combustion engines and external combustion engines. Before we look into the classification of heat engines, let us understand what exactly a heat engine is.
What is a Heat Engine?
We know that a heat engine is used to produce mechanical energy by using heat energy as a source. To be precise, it uses fuel such as petrol or diesel and converts its chemical energy into thermal energy by burning it. This thermal energy is then used to move mechanical parts, thus producing mechanical energy. By definition, it is considered a thermodynamic system.
Let's consider a heat engine system, as depicted in the image. For a heat engine to work, we need three essential components. Those components are the engine that produces work, a source that provides heat energy at a higher temperature reading, and a sink used to cool down the engine to protect the engine from overheating. Now that you know how a heat engine works, we will look at the different types of heat engines.
1. Internal Combustion Engine
As the name suggests, the burning of fuel, commonly known as combustion, occurs inside the system. It is commonly known as IC engines. An IC engine is a type of heat engine that uses working fuel such as petrol and diesel as a heat energy source. The working principle is that it produces work by burning fuel and creating a high-pressure environment. This high pressure is then used to run a turbine or a piston, which converts the heat energy to mechanical energy.
The internal combustion engine is classified into three major types, and they are as follows.
Petrol engine or Spark-ignition engine: The basic principle is that a piston is moved up and down by burning the fuel using a spark.
Diesel engine or Compression ignition engine: It has the same principle as the spark-ignition engine. The only difference is that the fuel (diesel) is made to combust by producing high pressure. It is commonly used in heavy-duty vehicles.
Gas turbines: It uses steam as a medium to produce mechanical energy instead of fuel. Gas turbines have high power output, but it also requires a lot of space as the engines are massive.
The Advantages of IC Engines
It requires a low initial cost.
It has better mechanical simplicity.
It can produce higher power output per unit weight of the fuel.
These units are very compact and require less space.
It has a better brake thermal efficiency, as only a small amount of thermal energy is lost.
It is easy starting from cold conditions.
The Disadvantage of IC Engines
It cannot use solid fuels such as coal which are far cheaper than petrol or diesel.
The IC engines have a lot of moving parts; therefore, it requires a lot of maintenance.
2. External Combustion Engine
As the name suggests, the burning of fuel, or commonly known as combustion, takes place outside the system. It is commonly known as EC engines. In this engine, heat from the burnt fuel is transferred to a secondary liquid, which acts as fuel for the engine.
Examples: In the steam engine, the heat of combustion is employed to generate steam, which is used in a reciprocating engine to produce useful work.
In a closed cycle gas turbine, the heat of combustion in an external furnace is transferred to gas, usually air, which is the working fluid of the cycle.
The Advantage of EC Engines
Almost any fuel can be used as the primary fuel.
It is very economical for considerable power generation.
It can also burn wood and use solar energy.
Its output and efficiency are not affected by altitude, and it is very suitable for use in high-altitude areas.
Engine emissions are also very low.
The Disadvantage of EC Engines
A high level of maintenance is required.
The size of the engine is enormous and requires a lot of space.
The working temperature of such engines is very high.
Fun Facts
The first heat engine was a Stirling engine that was developed by Robert Stirling in 1816.
Heat engines play a vital role in the overall process of industrialisation.
Did you know that there is a toy that tends to utilise a heat engine? It is called “the drinking bird” and it works according to a thermodynamic cycle, just like all the other types of heat engines.
The Carnot Cycle is known to be the most efficient heat engine.
FAQs on Heat Engine
1. What is a heat engine? List a few examples of the same.
A heat engine is basically a thermodynamic system. It is a type of engine that converts the chemical energy of the fuel it uses like diesel or petrol, into thermal energy through burning the former. This thermal energy is then used to produce mechanical energy which is further used to do various things that are related to mechanical work. Some examples of a heat engine are as follows:
A steam engine is one of the most commonly known examples of a heat engine. Its working is quite similar to that of other types of heat engines.
A steam turbine is yet another well-known example of a heat engine.
Heat pumps.
Refrigerators.
Air conditioners.
Power plants.
Internal combustion engine.
Power stations.
2. Discuss the advantages and drawbacks of a heat engine.
Several advantages and benefits of a heat engine are as follows:
Almost any type of energy can be converted into heat energy without much of a hassle through the processes of combustion, friction, and the absorption of light.
The mechanism of a heat engine is utilised in various everyday life applications from refrigerators and air conditioners to certain modes of transport as well.
Internal combustion engines, a type of heat engine, tend to require a low initial cost and are also able to produce the higher power output.
And external combustion engines, yet another type of heat engine, have very low engine emissions and can utilise almost any fuel as their primary fuel.
Several disadvantages of a heat engine are as follows:
The limitation of efficiency is the main drawback of a heat engine. Its operating efficiency doesn’t allow it to be a hundred percent efficient. The number is limited to a mere 30-50 percent. This is mainly because a heat engine isn’t capable of converting all the heat energy into mechanical energy to utilise it.
3. What are the characteristics and elements of a heat engine?
There are three main parts of any heat engine. These include:
The source, or the hot body: this is the source of all the heat which is supposed to be at infinite thermal capacity. This should be at a high temperature at all points of time to stop any amount of withdrawn heat from affecting its overall temperature.
A working substance: this substance is the kind that is used for absorbing the heat or rejecting it into the other body, i.e., the sink.
A body known as the sink: the sink is supposed to be at a finite thermal capacity, however, like the source, it is also supposed to be at a high temperature constantly to restrict the withdrawn heat from affecting its temperature.
4. Differentiate between the EC and IC engines.
There are two main types of heat engines, namely the internal combustion engine and the external combustion engine:
Internal Combustion Engine | External Combustion Engine |
These are quite clamorously operating engines. | These are relatively more silent and tend to run more smoothly because of the outside combustion. |
An IC tends to work on the Otto cycle. | An Ec tends to work on the Dual Combustion cycle. |
These have a high fuel cost. | These have a low fuel cost. |
Require a less amount of water for the cooling process. | It requires quite a lot of water for the cooling process. |
The combustion of the air-fuel takes place inside the cylinder. | The combustion of the air-fuel tends to take place outside the cylinder. |
These tend to be more efficient. | These are comparatively less efficient. |
6. What is Earth’s heat engine?
Earth’s heat engine comprises the planet’s atmosphere and hydrosphere. They perform a coupled process that involves constantly evening out the imbalances associated with the solar heating through the process of evaporation. They use the process of evaporation of rainfall, surface water, ocean circulation, etc., while distributing the heat all over the globe. Earth’s heat engine more or less functions in the same way as the engine of one’s car, only, the fuel in question is the energy acquired from the sun.
