The engine in which combustion of fuel takes place inside the engine cylinder. It is more compact to occupy less space, more efficiency, and portable.

Two principal types of reciprocating internal combustion engines are in general use: the Otto Cycle engine & the Diesel engine. The inventor of Otto cycle engine was the German technician Nikolaus August Otto and the Diesel engine was French-born German engineer Rudolf Christian Karl Diesel.

Both Otto-cycle and Diesel-cycle engines are manufactured in two-stroke and four-stroke cycle models.

Classification of Internal Combustion Engines:

Internal Combustion engines are classified in following types:

According to the type of fuel used-

  1. Gas engine
  2. Petrol engine
  3. Light oil engine (kerosene)
  4. Heavy oil engine (diesel)

According to the working cycle-

  1. Otto cycle engine
  2. Diesel cycle engine
  3. Dual combustion engine

According to all the speed of the engine –

  1. Low speed engine-N < 300 RPM
  2. Medium speed engine- 500<N< 1000 RPM
  3. High speed engine – 3600<N< 6000 RPM

According to the method of ignition-

  1. Spark ignition engine
  2. Compression ignition engine

According to the arrangement of the cylinder-

  1. Horizontal engine
  2. Inline engine
  3. V – engine
  4. Opposed Piston engine
  5. Radial engine

According to the method of cooling-

  1. Air cooled engine
  2. Water cooled engine

Components of Internal Combustion Engines:


The cylinders of internal combustion generally made of cast iron and Nickel cast iron. In case of a large engine, all the cylinders are cast in one block. For smaller engines the cylinder with water jacket is cast in one block; for a large engine, however cylinder liners are used with the main cylinder. In the case of water, it is sufficient to replace the liner. A cylinder head containing the spark plug. And atomizer is fitted with the cylinder on the head and side of the Piston through studs and nuts. The cylinder body contains the passage for the inlet and outlet of the gases.

Motorcycle cylinder


Piston commonly used in I.C. engines moving to the end inside the cylinder are known as trunk piston. A number of piston rings are fitted in the groups towards the top of the Piston to prevent the leakage of charge passes the piston. Piston rings are known as the compression ring.


Connecting rod:

It is the link between the piston and the crank. The small end of the connecting rod is connected with the Piston through wrist pin or Piston Pin. The big end of the connecting rod is connected with the Crank through a pin known as a crank pin. Through the connecting rod, the pressure of the working gases is transmitted to the Crank. The connecting rod is made of steel or Nickel steel or duralumin for Enlarge engines.

Connecting Rod

Crank and crankshaft:

The crankshaft which is the rotating member of the engine is built with one or more concentric portion called the cranks, the big end of the connecting rod is connected with the crank pin. The crank is rectangular in cross-section. The crank and crankshaft are made of steel forging. The main body of the engine which contents the crankshaft and bearing is known as crankcase.


To control the fluctuation of speed due to the fluctuation of energy during a cycle of operation, a heavy mass in the form of a wheel is mounted on the crankshaft of the engine.

The wheel is known as flywheels. In case of a multi-cylinder engine, the size of the flywheel is smaller. It is made of cast iron.

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To control the fluctuation of speed of the engine due to fluctuation in external load, the mechanism used in known as governor. It also connects with the engine crankshaft

Valve Gear:

The mechanism for controlling the admission and exhaust of the gases to and from the cylinder is known as valve gear. It consists of valves and their driving mechanism. Generally, Poppet valves are used in internal combustion engines but in some cases, light valves are also used for the purpose. In two-stroke cycle engine the Piston itself serves the purpose of a slide valve in controlling admission and exhaust of the gases to and from the cylinder.

The motion to the valve is imparted from the camshaft through the cam, push Road, and the rocker arm, the profile of the cam depends on the type of motion to be imparted to the valve.

Intake manifold:

Intake manifold means the pipe through which admission charge is admitted into the engine cylinder. The best cross section of an intake manifold is circular because it produces list resistant to the flow of charge. The amount of charge admitted to each cylinder must be the same. This requires that equal resistance should exist between the carburetor and the intake port to each cylinder. Since the distance to different intake ports is different, the resistance in the intake manifold is made equal by charging the cross-section of the manifold.

Intake manifold

Exhaust manifold:

Exhaust manifold means the pipe through which the exhaust gas goes out of the engine cylinders. The heat exhaust system has a separate connection from each cylinder into one or several exhaust pipes.

Piston rings:

Piston rings are accommodated in the grooves provided around the outer surface of the piston. The Piston rings provide gas-tight fitted between the Piston and the cylinder so that leakage of high-pressure gas is prevented. The Piston rings are made of special quality cast iron which can retain its elastic property event at high temperature. The Piston rings near the head are called “compression rings” and those away from compression ring are called “oil rings”.


It is a part of the valve gear mechanism. The function of the camshaft is to operate the intake and exhaust valve through the cam, push road and rocker arm. The camshaft is driven by the crankshaft at a speed which is the half speed of the crankshaft.

Push Road and Rocker arm:

The motion of the cam is transmitted to the valve through the push rod and rocker arm which swings in a vertical plane about a fulcrum due to rotation of the cam.


It is a casing which accommodates the cylinder, the connecting rod, and the crankshaft. It is also used as a sump for storing the lubricating oil.


Exhaust valve:

The function of this valve is to control the exhaust of the burnt gases from the engine cylinder into the atmosphere.

Valve spring:

valve spring helps to close the valve.

Valve spring

Inner dead center:

The extreme position of the Piston at the head end of the cylinder is called the inner dead center. In case of horizontal engine or top dead center in case of the vertical engine.

Outer dead center:

The extreme position of the Piston at the Crank end side of the cylinder is called outer dead center, in case of horizontal engine or bottom dead center in case of the vertical engine at O.D.C OR B.D.C

Stroke or stroke length:

It is the distance through which Piston moves in the cylinder during one stroke. In other words, the stroke length is the distance between two dead centers. Numerically, it is equal to twice the crank length or crank radius.

Cylinder bore:

It is the internal diameter of the cylinder.

Stroke volume:

It is the volume through which the Piston moves in one stroke within the cylinder.

Clearance volume:

It is the volume left between the cylinder head and the piston when the piston is at I.D.C or T.D.C.

Cycles of Internal Combustion Engine:

  • Otto cycle:

It consists of two reversible adiabatic, two constant volume process. Heat is supplied during process 2-3 at constant volume and heat is rejected during the process 4-1 at constant volume. During adiabatic process 1- 2 and 3- 4, no heat is supplied rejected.

P-V diagram of Otto Cycle

T-S diagram of Otto Cycle

NoteAll gas and petrol engine run on this Otto-cycle.

  • Diesel cycle:

It consists of constant pressure and a constant volume, and two adiabatic processes. Heat is supplied during constant pressure process 2-3 and is rejected during constant volume process 4-1. During adiabatic process 1-2 and 3-4, no heat is supplied or rejected.

P-V Diagram Diesel Cycle T-S diagram of diesel cycle

  • Dual combustion cycle:

It consists of two constant volume process, two adiabatic processes, and one constant pressure process. Heat is supplied partly at constant volume process 2-3 and partly at constant pressure process 3-4. Heat is rejected at constant volume process 5-1. No heat is added during adiabatic process 1-2 & 4-5.

P-V diagram of dual combustion cycle T-S diagram of dual combustion cycle

What is 2-Stroke Engine?

If the basic four events of operation i.e. suction, compression, expansion and exhaust stroke are completed in two-stroke of piston i.e. one revolution of the crankshaft, the engine is called two-stroke engine.

What is 4-Stroke Engine?

If the four events of operation i.e. suction, compression, expansion, and exhaust are completed in 4 strokes of piston i.e. by two revolutions of the crankshaft, the engine is called four stroke engine.

The working principle of 4-Stroke Petrol engine:

It based on Otto cycle. Inlet valve, exhaust valve, and spark plugs are mounted over the cylinder head. It runs in the following sequence of operation

  • Suction stroke:

In this stroke, the Piston moves outward. The pressure within the cylinder falls and the mixture of oil and fuel enters into the cylinder through the inlet valve till the end of the stroke. In this stroke, the exhaust valve will remain closed and no Spark will give by the spark plug.

  • Compression stroke:

In the stroke, the Piston moves inward and inlet and exhaust valve are in closed condition. So the charge within the cylinder is compressed adiabatically. At the end of the stroke, the spark plug supplies the spark and burning of charge starts.

  • Expansion stroke:

As the charge burnt, the high pressure is built up above the piston and for which Piston moves outward and expansion of burnt gas take place adiabatically. So power is produced in this stroke. At the end of the stroke, the exhaust valve open and some of the burnt gases escape into the atmosphere and pressure Falls.

  • Exhaust stroke:

In the stroke, the inlet valve is in the closed condition and the exhaust valve is in open condition and Piston moves inward. So, burn gases are driven out from the cylinder. At the end of the stroke and exhaust valve will close and inlet valve will open to start the next cycle.


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Working principle of 4-stroke diesel engine:

It consists of three valves, inlet and exhaust and a fuel injection valve in the following sequence of operation.

  • Suction stroke:

During the stroke, the Piston moves outward within the cylinder and only air is drawn through the inlet valve. The exhaust and fuel injection valve remain closed and this process continues till the end of the stroke.

  • Compression stroke:

In the stroke, the Piston moves inward with all valve are closed condition. So, the air inside the cylinder compressed adiabatically and high pressure with high temperature of the air is attended.

At the end of the stroke, the fuel injection pump supply the fuel due to the high temperature of compressed air, fuel gets ignited automatically.

  • Expansion stroke:

In this stroke, the Piston moves outward due to the burnt gases exerted a force on the piston and then the burnt gas expands adiabatically. At the end of the stroke, the exhaust valve open

  • Exhaust stroke:

In the stroke, the Piston moves inward and remaining burnt gases are driven out from the cylinder. The exhaust valve will be closed and the inlet valve will open at the end of the stroke to start the next cycle.

The working principle of 2-Stroke Petrol engine:

It consists of three parts namely inlet, exhaust, and transport instead of the valve.

It worked in the following ways:

  • Upstroke:

The Piston moves upward and first cover the inlet port. So the charge enters into the casing through the inlet port, for the movement of the piston, It covers transfer port and then exhausts port. So the charge above the piston compressed adiabatically. Thus in this stroke suction and compression are completed in single Piston movement. After compression ignition of charge starts by means of the spark plug.

  • Down stroke:

The Piston moves downward due to a force exerted by the burnt gases. So, expansion burnt gases take place. After expansion, it fast uncovers the exhaust port and then transfer port. So, scavenging of burnt gases takes place. For further movement of the piston, it covers the inlet port. Thus, the cycle is repeated.

Difference between 2-stroke and 4- stroke engine:

Difference between 2 stroke and 4 stroke engine

Difference between Otto cycle and diesel cycle engine:

Petrol engine vs diesel engine

What is Scavenging in IC Engine?

The process of removing or cleaning out burnt gases by blowing the fresh air or charge in the engine cylinder is known as scavenging.

In this process, a mixture of air and fuel or pure air is admitted into the engine cylinder during exhaust at a pressure slightly above at pressure to remove the burnt gases from the cylinder for two-stroke engines.

For four-stroke engines at the end of the exhaust stroke by opening the inlet valve before closing the exhaust valve. The Fresh charge is admitted at a pressure slightly above the atmospheric pressure.

What is Supercharging?

The process of admitting air or mixture of air and fuel in large quantity then the cylinder volume during normal suction is known as supercharging or boosting. the device is used for supercharging is called supercharger

The objective of supercharging:

  1. Increase the output power of the engine.
  2. To overcome the effect of high altitude.
  3. To reduce the weight of the engine Per KW power developed.
  4. To reduce the volume of the engine.
  5. To enable the engine to take overloads.

What is Turbocharger?

By this device, a large amount of energy within the exhaust gas is used for other purposes. The outgoing exhaust gas is allowed to expand in a nozzle and a huge quantity K.E. is obtained which is used to drive the exhaust gas turbine.

The exhaust gas turbine is used to drive the supercharger. Combination of a supercharger and exhaust gas turbine is called turbocharger.

turbo Charger

What is the process of Combustion in SI engine?

When the electric Spark is produced in the spark plug at first there is no rise of pressure or temperature which is called delay time or lag of ignition. But a change of chemical reaction takes place Which produced a flame that moves radially from the spark plug. This flame progressive across the combustion chamber, the temperature of the burnt fuel within the flame front increases rapidly and flame moves at high velocity. The pressure rises during combustion because of the increases of temperature within the flame front.

The combustion process in CI engine:

When the fuel is sprayed into the highly heated atmosphere, a delay occurs during which the fuel particles are being evaporated. The vapor of fuel then comes in contact with the oxygen at high temperature. Thus, fuel combustion takes place and high pressure is developed.

What is Detonation?

Very rapid combustion of the last portion of the Unburnt charge will generate high-pressure waves and unusual sound in the cylinder.

  • Cause:

If the ignition temperature is reached and the delay period is shorter then the time is taken by the flame front to reach the last portion of the Unburnt charge, detonation will take place.

  • Effect of detonation:

  1. Reverberating shock wave in the combustion chamber.
  2. Break of parts due to high-pressure waves.
  3. Loss of power.
  4. Overheating of spark plug causes charge of Pre-ignition.
  5. Increase heat loss and reduce thermal efficiency.
  • Method of preventing detonation:

  1. The air-fuel mixture is to be reduced during admission.
  2. The temperature of charge to be reduced.
  3. The rich air-fuel mixture is to be used.
  4. And then of the mixture may be added with the fuel

What is Pre-ignition in Spark Ignition engine?

The automatic ignition of charge before normal ignition given by spark plug. It occurs in Spark ignition engine.

  • Cause:

The overheat carbon deposit on the combustion chamber, exhaust valve, spark plug supply to spark to ignite the fresh charge before the spark plug.

  • Effects:
  1. It causes detonation
  2. Reduce output power
  3. Rough engine operation
  4. Damage of part due to high temperature.

Working Principle of Simple Carburetor:

It consists of two main following parts:

  1. Float chamber containing a float and needle valve.
  2. Mixing chamber containing the main nozzle, throttle and chock valve.
  • Float Chamber:

It is a small tank where the level of the fuel is maintained within it. And the flow of fuel from it is regulated by the needle valve.

If metering jet and the metering rod is installed, it measures the amount of fuel going out from the float chamber to the main nozzle.

  • Mixing Chamber:

It is a long barrel attached with float chamber and contained venturi. It contains throttle valve which regulated by the flow of air-fuel mixture going out from it. A chock valve which controls the flow of air into the mixing chamber.

  • Working Principle:

During suction stroke of the engine, the atmosphere air rushes into the carburetor through the air filter and passes through the fuel nozzle and made the air-fuel mixture. The amount of air-fuel mixture entering to the engine cylinders is controlled by the throttle valve which is operated by acceleration. So, more fuel and the small amount of air is mixed to get a rich mixture.

I wrote an article on different types of carburetor with diagrams. You can also take a look on that.

What is Firing Oder in I.C. Engine?

The sequence in which the firing takes place in the different cylinder in a multi-cylinder engine. Proper firing order reduce the engine vibration, ensure the balancing of the engine and provide even flow of power from different cylinder to the common crankshaft

The firing order of multi-cylinder engine:

  • 3-cylinder engine: 1-3-2
  • 4-cylinder engine:1-3-4-2 or 1-2-4-3
  • 6-cylinder engine:1-5-3-6-2-4 or 1-2-4-6-5-3
  • 8-cylinder engine:1-6-2-5-8-3-7-4 or 1-4-7-3-8-5-2-6

Lubrication of Internal Combustion Engine:

The function of Lubrication:

  1. To minimize wear and friction.
  2. To prevent metal to metal contact of moving part.
  3. To remove the heat of the engine parts by acting as a cooling agent.
  4. To absorve shock between the bearing and other engine parts thus reducing noise and improves life.
  5. To form an effective seal between the piston ring and cylinders walls.
  6. To keep the engine parts, clean by carrying way parts.
  7. To prevent oxidation of engine parts.

Parts to be lubricated:

The following parts of I.C. Engine are to be lubricated:

  1. Main bearing of the crankshaft.
  2. Big end bearing of the connecting rod,
  3. Cylinder wall.
  4. Small end bearing.
  5. Valves guides.
  6. Camshaft bearing.
  7. Camshaft driving gears.
  • Properties of lubricating oil:

  1. Viscosity
  2. Physical stability.
  3. Chemical stability.
  4. Corrosion resistance.
  5. Pour point.
  6. Flashpoint.

Method of lubrication:

  • Splash Method:

In this system, a dipper on the lower part of the connecting rod bearing cap enters into the oil through containing lubrication oil with each revolution the crankshaft. The dipper flashes the oil to the upper part of the engine as a fine oil spray which lubricates the cylinder wall, piston ring, big end bearing and small end bearing of the connecting rod.

  • Forced feed system method:

In this system, the oil is pumped to the all lubricating point. The lubricating oil is supplied under pressure by means of a pump operated by the camshaft of the engine.

The cooling system of the Internal Combustion engine:

The necessity of cooling:

Cooling of I.C. Engine is necessary because of following ways-

  • To prevent damage of engine parts due to high temperature.
  • To prevent the breakdown of the lubricating system.
  • To prevent overheating valves, its guide/ seats to prevent wear of valves.
  • To prevent loss of strength of piston and other parts.

The purpose of the cooling system is to keep the engine at is efficient temperature (700– 900) at all engine speed and operating condition.

Method of cooling:

It is classified as follows-

  • Air cooling:

In this system, the engine cylinder is cast with a no. of extended surface known as fines provided around the surface of the cylinder. Atmospheric air flows through this fine and carried away the heat generated from the cylinder into the atmospheric. This type of cooling is used in the scooter/motorcycle etc.

  • Liquid Cooling:

Water is used for cooling media. In this system cylinder and cylinder head are surrounded by water jackets and which cold water is circulated. It is subdivided as-

  • Thermosyphon/natural or gravity circulated:

It is simplest but not suitable for large engines. The cooling tank is placed slightly above the engine level. The hot water comes from the engine jackets fitted into the tank through the hot water outlet pipe where it cooled by radiation. The cooled water from the bottom of the tank flows back to the engine through a pipe. The H2O level in the tank should be above the level of hot water inlet to the tank.

  • Forced or pump circulation:

In this system, a pump is used in near the cylinder head and is driven by the engine through belt and pulleys. The pump forced the water flow through the system high rate. The hot water comes from the cylinder jackets enter the top of the radiation and cools. The cold water again circulated through this pump.

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About the Author

Saswata Baksi

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Saswata Baksi is an Excutive Editor and Lead Content Writer of Learn Mechanical, with a background degree in Mechanical Engineering from KIIT- An Institute of Eminence. Apart from that, he is also a certified SEO expert.

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