Mechanical Basics MECHANICAL THEORY Thermal Engineering

Classification of IC Engines-Types of Heat Engines,Basic Engine Components & IC Engine Nomenclature

Classification of IC Engines-Types of Heat Engines, Basic Engine Components & IC Engine Nomenclature:Any type of engine or machine which derives heat energy from the combustion of fuel or any other sources and converts that energy into mechanical work is termed as a heat engine.

Heat engines may be classified into two types.

1.External Combustion Engine and 

2.Internal Combustion Engine

The detailed explanation of the above engines is as follows.

1.External Combustion Engine:

In this case, combustion of fuel takes place outside of the cylinder as in the case of steam engines, where the heat of combustion is employed to generate steam which is used to move a piston in a cylinder.

Example: The steam turbine and closed-cycle gas turbine.

These engines are generally used for driving locomotives, ships, generation of electric power etc.

2.Internal Combustion Engine:

In this case, combustion of the fuel occurs within the cylinder of the engine. The IC Engines group includes engines employing mixture of combustible gases and air known as gas engines.

Those using lighter liquid fuel are known as Petrol Engines and those using heavier liquid fuels are known as oil compression ignition or diesel engines.

Combustion: Burning of the fuel (Air Fuel Mixture) is known as Combustion.

Advantages of IC Engines over EC Engines:

  • Requires less space
  • Simple in Design
  • Low initial cost
  • High Thermal Efficiency
  • Easy to start even in cold conditions

Components of IC Engines:

Classification of IC Engines-components of IC Engines
components of IC Engines

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The components of IC Engines are as follows.


It is a cylindrical vessel or space in which the piston makes a reciprocating motion.

The varying volume created in the cylinder during the operation of the engine is filled with working fluid and is subjected to different thermodynamic processes.

2.Piston: It is a cylindrical component fitted into the cylinder for converting one form of energy into another form. It fits perfectly into the cylinder providing a gas-tight space with the Piston rings and the lubricant.

3.Cylinder head:

The main function of the cylinder head is to close the cylinder to make a confined space. The valves(Inlet and Exhaust Valve), Spark plug, cam and camshaft are present in the head section only.

4.Connecting rod:

It interconnects Piston and the crankshaft and transmits the gas forces from the Piston to the crankshaft.


It is enclosed in a crankcase. It converts the reciprocating motion of the piston into useful rotary motion of the output shaft.

6.Piston rings:

They are fitted into the slots around the piston, providing a tight seal between the piston and the cylindrical wall thus preventing the leakage of combustion gases.

7.Gudgeon Pin:

 It forms the link between the small end of the connecting rod and the Piston. 


 It is used to connect the connecting rod and piston.

8.Inlet and Exhaust valves:

 They are provided on the cylinder head or on the side of the cylinder for regulating the charge coming into the cylinder(inlet valve) and for discharging the products of combustion(exhaust valve) from the cylinder.

9.Spark Plug:

 It is a component to initiate the combustion process in SI engines and is usually located on the cylinder head.


 It is a stationary body of the engine which serves as an enclosure for moving parts.


It is used to control the opening and closing of the two valves.

It also provides the drive to the ignition systems and the camshaft is driven by the crankshaft.


 They are made as integral parts of the camshaft and are designed in such a way to open the valves at the correct timing and to keep them open for the necessary duration.

IC Engines Nomenclature:

1.Cylinder Bore (d):

The nominal inside diameter of the engine cylinder is called a Cylinder bore. It is designated by the letter “d” and is expressed in “mm”.

2.Piston Area:

Area of a circle of diameter equal to the cylinder bore is called the Piston Area. It is designated by “A” and expressed in cm2.


The distance travelled by the piston from one Dead Centre to the other Dead Centre is called Piston Stroke.

During one stroke, the crankshaft rotates half a turn.

It is denoted by letter “L” and expressed it in “millimetre”

4.Dead Centre:

 There are two dead centres in the engine.

1.Top Dead Centre(TDC) and 

2.Bottom Dead Centre(BDC)

The piston reciprocates in the cylinder between two fixed positions. The uppermost position i.e. position nearest to the cylinder cover is termed as Top Dead Centre(TDC) while the lowermost position(nearer to the crankshaft) is Bottom Dead Centre(BDC).

For a horizontal engine, these terms are referred as Inner Dead Centre(IDC) and Outer Dead Centre(ODC).

5.Swept Volume:

The volume displaced by the piston, when travelling from one dead Centre to the other Dead center is called as swept volume.

It is expressed in terms of cubic-centimetre(cm3) and is given by

Vs = (π/4)d2L

6.Clearance Volume:

 The space between the cylinder head and the Piston face at the Top Dead Centre(TDC) is known as the clearance volume. It is designated as Vc and is expressed in cubic-centimetre  (cm3)

7.Compression ratio:

It is the ratio of the total cylinder volume when the piston is at the bottom dead centre(VT) to the clearance volume. It is designated by the letter “r”.

Compression ratio,r = (VT/VC)

                                    = (Vc + Vs)/Vc

                                    = 1+(Vs/Vc)

11 Classification of IC Engines-Every Mechanical Engineer must know:

IC Engines stands for Internal Combustion Engines which can have Higher Overall Efficiency, Low Weight to Power Ratio, Requires less space and has Greater Mechanical Simplicity.IC Engines are classified in various ways depending on their functional characteristics. In this Article,11 Classification of IC Engines were discussed and are as follows.

11 Classification of IC Engines:

The classification of IC Engines are as follows.

  1. Basic Engine Design
  2. A cycle of Operation(Working Cycle)
  3. Number of Strokes
  4. Type of Ignition
  5. Method of Cooling
  6. Method of Charging
  7. No. of Cylinders
  8. Cylinder Arrangement or Type of cylinder
  9. Fuel used
  10. Method of Fuel Supply
  11. Engine Speed
  12. Applications

Detailed classification of IC Engines:

The Detailed classification of IC Engines was presented below.

1.Basic Engine Design:

  1. Reciprocating Engines
  2. Rotary Engines (Wankel Engine)

2.Cycle of Operation(Working Cycle):

  1. Engines working on OTTO Cycle (SI Engines)
  2. Engines working on DIESEL Cycle (CI Engines)

3.Number of Strokes:

  1. Four Stroke Engines
  2. Two Stroke Engines

4.Based On Type of Ignition

  1. Spark Ignition Engines

             Spark Is Generated through an External Source

  b.Compression Ignition Engines

              Air Is Heated to a Sufficiently High Temperature Because of High Compression Ratio

5.Method of Cooling

  • Cooling Is Essential for the Satisfactory and Healthy Working Of the Engine (Otherwise Results In Engine Seizing)

Two Types of Engine Cooling In Practice

  1. Air-cooled Engine
  2. Water Cooled Engine

6.Classification Based On Method of Charging

a.Naturally Aspirated Engines

Admission of Fuel-air Mixture at Near Atmospheric Pressure

b.Super-charged Engines

Admission of Fuel- Air Mixture Under Pressure (Above Atmospheric)

7.No. of Cylinders:

  • Single Cylinder
  • Multi Cylinder

8.Type Of Cylinder Arrangements

  • V- Engine
  • Opposed Cylinder Engine
  • Opposed Piston Engine
  • Radial Engine
  • X-type Engine
  • H-type Engine

9.Fuel used :

  1. Gasoline or Petrol Engines
  2. Compressed Natural Gas (CNG) Engines
  3. Diesel Engines

10.Fuel Supply:

  1. Carbureted type- Fuel supplied through Carburetor
  2. Injection type- Fuel supplied through injector

    Fuel injected into the cylinder just before Ignition.

11.Classification Based On Engine Speed

  • Low Speed – up to 500 rpm
  • Medium Speed – 500 to 1000 rpm
  • High Speed – above 1000 rpm

12.Classification Based On Applications:

  • Motor cycle engine
  • Automobiles
  • Earth Movers
  • Locomotive engine
  • Marine engine
  • Aero engine
  • Prime movers for Electric Generators

This is the complete Classification of IC Engines in detailed.If you have any doubt feel free to ask from the comments section.


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