Types of Heat Engine

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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 classification 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 is a heat engine.

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 as a thermodynamic system. 

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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.

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. 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.

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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.

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 a 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.

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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.

FAQ (Frequently Asked Questions)

1. What is the Difference Between Heat Engines and Heat Pumps?

Answer. A heat pump is a machine or device that transfers thermal energy from the source, which is at a lower temperature, to the sink or heat sink, at a higher temperature. A heat engine is used to produce mechanical work by using heat energy as a source. The heat engine develops the work, whereas the heat pump consumes the work.

2. Compare the EC and the IC Engines?


Internal Combustion Engine

External Combustion Engine

The combustion of air-fuel is inside the engine cylinder.

The combustion of air-fuel is outside the engine cylinder.

Higher efficiency of about 35-40%.

The lower efficiency of about 15-20%.

IC engines are not self-starting.

High starting torque.

Less water for the cooling process.

It has a higher requirement of water for the dissipation of energy through a cooling system.

Very noisy, operated engine.

The engines are running smoothly and silently due to outside combustion.

It works on the Otto cycle.

It works on the Dual-combustion cycle.

The primary pollutants are oxides of nitrogen (NOx), carbon monoxide (CO), and hydrocarbons (HC).

In addition to CO, HC, and NOx, we also get soot particles.

The cost of fuel is high.

The cost of fuel is low.