How does External Combustion Engine work? | What is an EC Engine?

An engine is a mechanical equipment that converts fuel energy into mechanical work. The engines are used for different applications all over the world. There are multiple types of engines according to the application nature. However, the two most famous types of engines are external combustion (EC) engine and internal combustion (IC) engine. In the previous article, we deeply discussed internal combustion engines. Therefore, this article mainly explains the EC engine.

What is an External Combustion Engine?

An external combustion engine (EC engine) is a reciprocating engine 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.  

In simple words, an engine in which the combustion of the working fuel occurs outside the working cylinder is called an external combustion engine.

These types of engines use gas, steam, or liquid as a working fluid. In this engine, a fuel-air mixture is filled into the external combustion chamber. As the fuel-air mixture burns, a large amount of heat generates.

A heat exchanger or engine wall is used to transfer the produced heat from the external combustion chamber to the internal working fluid. When the internal fluid heats up, it expands and acts on the engine mechanisms, resulting in movement and available work.

A steam engine is a most common example of an external combustion engine. In this engine, as the steam enters the engine, it expends, rotates the piston, and converts thermal energy into mechanical power. In the case of a steam engine, the steam is generated outside the engine (i.e., boiler). Therefore, it is called an EC engine. 

In the present time, the EC engines don’t use in transportation applications due to the inefficiency of mobile design but are still used in power plants. These types of engines are most commonly used in hydroelectric power plants.   

Coal-fired power plants work in a similar way: Coal is shipped from mines to power plants and burned in boilers. The pipes use to supply water to the boiler. As the water enters the boiler, the burning coal heats up the water and converts it into steam, which drives the turbine to produce electricity.

Working of External Combustion Engine

An external combustion engine is a type of engine in which an external heat source uses to burn the internal working fluid. The Stirling engine and steam engine are the most common types of EC engine. An EC engine works in the following way:

external combustion engine working

  • First of all, the burning fluid (such as coal) is supplied to the external heat source (such as a boiler).
  • The water supply from the reservoir to the boiler.
  • As the water enters the boiler, the coal ignites and supplies heat to the water.
  • As the water temperature increases, it converts into steam.
  • After the water conversion into steam, steam passes through a compressor which compresses the steam and increases steam pressure.
  • When highly compressed steam reaches the engine cylinder, it expands and forces the piston to move forward and backward.
  • A crankshaft is connected to the piston. The piston delivers its motion to the crankshaft.
  • The crankshaft transforms the piston motion into rotatory motion and rotates the flywheel.
  • As the flywheel rotates, it further delivers power to the vehicle tires and helps to move the vehicle.

Types of External Combustion Engines

The external combustion engine has the following major two types:

  1. Steam Engine
  2. Stirling Engine

1) Steam Engine

The term “steam engine” represents the reciprocating engine, not steam turbines. This type of EC engine uses steam as a working media to produce mechanical work. A steam engine converts the thermal energy of the steam into rotational energy.

It is an engine in which the working media is separated from the burning product. An ideal thermodynamic cycle known as the “Rankine cycle” is used to analyze this process.

Steam Engine

This engine uses the thermal energy of the steam to push the piston back and forth in the cylinder. This reciprocating motion of the piston is transferred to the crankshaft and connecting rod. The crankshaft converts the piston motion into rotational motion to power the steering system of the vehicle.

2) Stirling Engine

The Stirling engine is one of the most famous types of external combustion engine. It starts quickly in cold weather. It has high flexibility.

working of Stirling engine

These types of engines have quiet operation. They have the capability to use different heat sources such as nuclear, geothermal, and biological.

Parts and Function of External Combustion Engine

The external combustion engine has the below given major parts:

  1. Cylinder
  2. Piston 
  3. Flywheel
  4. Connecting rod
  5. Crankshaft
  6. External heat source
  7. Camshaft

parts of external combustion engine, EC engine

1) Cylinder

The working cylinder is a part of the EC engine in which the working fuel(i.e., gas) is trapped. As it gets heat from the external heat source, it heats the working fluid, burns the fluid, and generates useful work.

2) Piston

This reciprocating component of the external combustion engine reciprocates inside the working cylinder.

When the working fuel ignites in the cylinder, the generated power reciprocates the piston forward and backward. This motion of the piston further rotates the crankshaft.

3) Crankshaft

As the crankshaft gets reciprocating motion by the piston, it converts this motion into rotary motion and rotates the flywheel.

4) Flywheel

When the flywheel gets power by the crankshaft, it converts received rotary motion into mechanical power and moves the vehicle wheels.

5) Camshaft

The main function of the camshaft is to control the opening and closing of the inlet and outlet valves. This part of the engine ensures a proper opening and closing of the inlet and outlet valves and ensures a proper supply of fuel.

Advantages and Disadvantages of External Combustion Engines

Advantages of External Combustion Engine

  1. These engines have the capability to use all types of fuels.
  2. The external combustion engines generate very low noise.
  3. They have a low emission rate.
  4. They are best suitable for high power generation applications.

Disadvantages of External Combustion Engine

  1. They are not ideal for low power generation applications.
  2. They have working fluid leakage issues.
  3. The EC engine has more size than the IC engine.
  4. They consume more lubricate than the IC engines.
  5. These engines have a high operational temperature. Therefore, they need more care and high construction material.

Applications of External Combustion Engine

  1. The external combustion engines are used in Marine and Locomotive.
  2. Use in the experimental space vehicles
  3. Use in Power plants and Large Marine

External Combustion Engine VS Internal Combustion Engine

The main difference between the external combustion engine and internal combustion engine is given below:

External Combustion EngineInternal Combustion Engine
In an EC engine, the burning process of the working fuel occurs outside the working cylinder.In an IC engine, the burning process of the working fuel occurs inside the cylinder.
They have a large size.They have small sizes.
The external combustion engine needs large installation space.The internal combustion engine requires low installation space than the EC engine.
These engines have high weight.They are light. 
They are difficult to start.They are very easy to start.
This engine requires more time for the initial start.It starts very quickly.
The efficiency of the external combustion engine is between 15% to 25%.The efficiency of the IC engine is between 35% to 45%.
It has a high capital cost.It has a low capital cost.
The EC engine has low thermal efficiency.The IC engine has high thermal efficiency.
The fuel used for EC engines has a low cost.The fuel used for IC engines has high costs.
It is less efficient than an IC engine.It is the most efficient engine.

FAQ Section

What are the types of the external combustion engine?

The EC engine has the following major types:

  1. Steam Engine
  2. Stirling Engine

What are External combustion engines used for?

The external combustion engines are used for Locomotive, vehicles, and Marine applications.

Who invented the EC engine?

In 1816, Robert Stirling invented the first external combustion engine known as the Stirling engine.

What are the examples of External Combustion Engine?

The EC engines are most commonly used in hydroelectric power plants and marine. Stirling engine and steam engine are the most common examples of external combustion engines.

What are the components of External Combustion Engine?

  1. External heat source
  2. Cylinder
  3. Camshaft
  4. Crankshaft
  5. Piston 
  6. Connecting rod
  7. Flywheel

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