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 are the two primary types of heat engines as per the CBSE syllabus for 2025-26?
A heat engine is a device that converts thermal energy into mechanical work. The two primary types of heat engines are:
- Internal Combustion (IC) Engine: In this type, the combustion of fuel (like petrol or diesel) occurs inside the engine's combustion chamber. The high-pressure gas produced directly pushes a piston to do work.
- External Combustion (EC) Engine: Here, the fuel is burned outside the engine. The heat generated is used to heat a separate working fluid (like water turning into steam), which then expands and does work.
2. What are the essential components required for any heat engine to function?
Every heat engine, regardless of its type, requires three essential components to operate in a cycle:
- Source (Hot Reservoir): A body at a higher temperature (T₁) that supplies heat energy to the engine.
- Working Substance: A substance (like a gas or liquid) inside the engine that absorbs heat, expands, does work, and then rejects heat.
- Sink (Cold Reservoir): A body at a lower temperature (T₂) that absorbs the waste heat rejected by the working substance.
3. How does an Internal Combustion (IC) engine work? Provide some common examples.
An Internal Combustion (IC) engine generates mechanical power by burning a fuel-air mixture directly within a combustion chamber. This rapid combustion creates a high-pressure, high-temperature gas that expands and pushes a piston. This linear motion is then converted into rotational motion to power a vehicle or machine. Common examples include the petrol engines in cars and motorcycles, and diesel engines in trucks and buses.
4. What is the working principle of an External Combustion (EC) engine? Give some examples.
In an External Combustion (EC) engine, the fuel is burned in a separate furnace outside the engine's main body. The heat from this combustion is transferred to a working fluid, such as water. This fluid turns into a high-pressure gas (steam), which is then directed into the engine to push a piston or spin a turbine. Examples include the steam engines used in historic trains and ships, and the steam turbines used in most thermal power plants.
5. 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 heat energy into useful work. It is calculated as the ratio of the net work done (W) by the engine to the amount of heat absorbed (Q₁) from the hot reservoir. The formula is:
η = Work Output / Heat Input = W / Q₁
Since W = Q₁ - Q₂, where Q₂ is the heat rejected to the sink, efficiency can also be expressed as η = 1 - (Q₂ / Q₁).
6. What is the fundamental difference between a petrol engine and a diesel engine?
While both are types of Internal Combustion engines, the fundamental difference lies in their ignition method. A petrol engine uses a spark plug to ignite the compressed fuel-air mixture. In contrast, a diesel engine has no spark plug; it compresses only air to a very high pressure and temperature, and fuel is then injected, which ignites instantly upon contact with the hot, compressed air. This is known as compression ignition.
7. Why can a real-world heat engine never be 100% efficient?
A heat engine can never be 100% efficient due to the Second Law of Thermodynamics. This law states that to operate in a continuous cycle, an engine must reject some of its heat to a colder reservoir (the sink). It is impossible to convert all the heat taken from the source entirely into work. Some energy is always lost as waste heat to the surroundings, which is a necessary part of the thermodynamic cycle.
8. How is a refrigerator considered a type of heat engine, even though its purpose is to cool?
A refrigerator is essentially a heat engine running in reverse, more accurately called a heat pump. Instead of using a temperature difference to produce work, it uses external work (from electricity) to move heat from a cold space (inside the fridge) to a warmer space (the room). It actively pumps heat against its natural flow, which is why it requires energy to operate.
9. What is the importance of the Carnot Cycle in understanding heat engines?
The Carnot Cycle is a theoretical and idealised thermodynamic cycle. Its importance is not in its practical application, but as a benchmark for performance. It represents the maximum possible efficiency that any heat engine can achieve when operating between two specific temperatures. Real engines are always compared to the Carnot efficiency to gauge how well they perform, as no real engine can exceed this theoretical limit.
10. In what real-world scenarios is an External Combustion (EC) engine more suitable than an Internal Combustion (IC) engine?
External Combustion engines are preferred in scenarios where fuel flexibility is crucial and engine size is not a major constraint. Because the combustion happens separately, EC engines can use a wide variety of fuels, including lower-grade or solid fuels like coal, biomass, or wood. This makes them ideal for large-scale, stationary applications like thermal power plants, where using cheaper, abundant fuel is a primary economic driver.
