Types of Heat Engines

The invention of the heat engine was one of the revolutionary discoveries of human beings. A heat engine follows the first law of thermodynamics, which states that energy cannot be created or destroyed, but it can be transformed from one form into another. Heat energy is converted into mechanical energy in a heat engine. Today, different types of heat engines are found in our daily lives, such as automobiles, marine engines, thermal power stations, etc.

Types of Heat Engines

Heat engines are mainly classified into two types based on their working principle.

• Internal Combustion engine (IC Engine)
• External Combustion Engine (EC Engine)

Internal Combustion Engines (IC Engines)

An internal combustion engine is defined as an engine where the combustion of fuel occurs in the engine.

Principle

Chemical energy is supplied in the form of petrol, diesel, or gas. These fuels combust in the cylinder, producing heat energy within the system. This heat later transforms into mechanical energy.

Advantages of IC Engines

• This type of heat engine has a low starting time.
• It takes up little space. 
• The ratio of power to weight is very high.
• It is highly efficient.

Disadvantages of IC Engines

• They are not ideal for a massive amount of power generation.
• Only a limited number of fuels can be used, like petrol or diesel.
• Exhaust pollution is high.

Applications:

• Chainsaws
• Portable generators
• Aircraft

Classifications of Internal Combustion Engines

IC engines are classified according to the following factors: 

1. Based on the number of strokes:

• Four-stroke cycle engines: Here, one complete power cycle is occurred by the four-stroke of the piston. The four strokes are suction, compression, ignition, and exhaust.

• Two-stroke cycle engines: In a two-stroke engine, one complete Power cycle occurs by the two-stroke piston where the strokes are upstrokes and downstrokes.

2. Based on thermodynamic working cycles:

• Otto cycle engines: Engines, heat addition and rejections occur at constant volume in an Otto cycle. Example: petrol engine vehicles.
• Diesel cycle engines: In diesel cycle engines, heat addition is at constant pressure and heat rejections occur at constant volume. Therefore, the combustion occurs under continuous pressure. Example: diesel engine vehicles, diesel generators.
• Dual cycle engines: In dual cycle engines, heat addition will partially occur at constant volume and constant pressure. Heat rejections take place at a regular volume process.

3. Based on fuel used:

• Petrol engines: Petrol is used as fuel to work the engine, whereas a spark plug is used for the ignition process to produce a power stroke.
• Diesel engines: Diesel is used as fuel in these types of heat engines. Air compressed during compression stroke with atomised diesel is used for the ignition process to produce a power stroke.
• Gas engines: Gases such as compressed natural gas, biogas and LPG are used to fuel these engines.

4. Based on the number of cylinders used:

• Single-cylinder engines: This type of heat engine converts heat energy to mechanical energy in a single-cylinder machine.
• Multi-cylinder engines: In a multi-cylinder engine, multiple cylinders convert heat energy to mechanical energy. As a result, more power is generated compared to a single-cylinder engine.

5. Based on cylinder arrangement:

• Horizontal cylinder engines: Here, the cylinder is arranged in a horizontal position. Generally, most single-cylinder engines are arranged in horizontal positions.
• Vertical cylinder engines: The cylinder is arranged in a vertical position.

6. Based on engine speed:

• Low-speed engines: In a low-speed engine, the speed of the crankshaft is lower than 100 rpm.
• Medium-speed engines: In this type of engine, the range of speed lies between 100 to 250 rpm.
• High-speed engines: If the speed of the engine is more than 250 rpm, it is known as a high-speed engine.

7. Based on the cooling system used:

• Air-cooled engines: In air-cooled engines, the cylinder is cooled by fins or propellers that ease cooling operations.
• Water-cooled engines: Here, a water pump is used to protect engines from overheating. A device pumps water from the radiator.

External Combustion Engines (EC Engines)

In an external combustion engine, the combustion of fuel occurs outside of the engine cylinder.

Principle

In this type of heat engine, an external source is used to heat an internal working fluid. Due to the heat, the fluid will expand. This expansion will generate power by moving the mechanism of engines.

Advantages of EC Engines

• They produce low exhaust pollution.
• They offer a high starting torque.
• EC engines create less operational noise.
• They are ideal for high power generation.

Disadvantages of EC Engines

• EC engines need large space.
• They consume a significant amount of lubricants. 
• The high overall temperature of this heat engine requires more care and maintenance.

Applications

• Nuclear and coal-fired power plants
• Marine engines

Classifications of External Combustion Engines

EC engines are classified as follows:

• Steam engine: A steam engine is a type of heat engine where the thermal energy of steam is converted into mechanical energy to work.
• Stirling engine: In a stirling engine, air or gas is compressed and expanded in a closed cycle at different temperatures. This mechanism creates mechanical energy.

Conclusion

In different types of heat engines, thermal energy produced by fuel or gas is converted into mechanical energy. These types of engines follow the first law of thermodynamics, which states that heat cannot be created or destroyed, only transformed. 

Heat engines can be categorised as internal combustion (IC) engines and external combustion (EC) engines. More questions on the types of heat engines have been answered below.