Internal Combustion Engine

Internal Combustion Engine

Internal Combustion Engine

Definition of Heat Engine

A heat engine is a mechanical device which firstly, converts the chemical energy (in the form of fuel) into the thermal energy (in the form of hot gases after the heat addition) then further this thermal energy is converted into the mechanical energy (in the form of piston and crankshaft movement).

A heat engine can be classified as follows: –


  1. Internal Combustion Engine (I.C. Engine):-
    Rotary Engine
    Reciprocating/I.C. Engine
    Wankel Engine

    An Internal combustion Engine is heat engine in which the combustion/ignition of fuel (Petrol or gasoline & Diesel) takes place inside the engine or piston-cylinder.

    For example: Automobile engines, Aircraft engines etc.

  2. External Combustion Engine (E.C. Engine): –
    • External Combustion engine is a type of heat engine in which the combustion/ignition of the fuel takes place outside of the cylinder.
    • It uses of the heat exchanger to transfer heat to the working fluid
    •  Open or closed cycle
    • Example: In the steam engine or a steam turbine plant, the heat of combustion is employed to generate steam which is used in a piston engine (reciprocating type engine) or a turbine (rotary type engine) for 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.

    External combustion (E.C.) Engine

    Stirling Engine

In the following sections, we will be studying the Internal Combustion (I.C.) engine in details.


   As we have already been familiar in the previous section with the definition of the I.C. engine that an engine in which the combustion of fuel takes place inside the cylinder. Like automobile and motorbike engines etc.

   In other ways, we can also say that in I.C. engines, the gases produced from the combustion of gaseous or liquid fuels at high pressure expanded in a piston-cylinder arrangement which helps to transmit mechanical energy to the crankshaft. Therefore this device is used to convert the thermal energy to mechanical energy.

Based on the various important phenomenon I.C. engines can be classified into the following : –

It can be classified into the following types:

  1. According to the basic engine design- (a) Reciprocating engine (Use of cylinder piston arrangement), (b) Rotary engine (Use of turbine)
  2.  According to the type of fuel used- (a) Petrol engine, (b) diesel engine, (c) gas engine (CNG, LPG), (d) Alcohol engine (ethanol, methanol etc)
  3. According to the number of strokes per cycle- (a) Four stroke and (b) Two stroke engine
  4. According to the method of igniting the fuel(a) Spark ignition engine (S.I. Engine) and (b) Compression Ignition engine (C.I. Engine)
  5. According to the Working cycle
    (a) Otto cycle (constant volume cycle) engine,
    (b) Diesel cycle (constant pressure cycle) engine,
     (c) Dual combustion cycle (semi diesel cycle) engine.
  6. According to the fuel supply and mixture preparation-
    (a) Carburetted type (fuel supplied through the carburettor),
    (b) Injection type (fuel injected into inlet ports or inlet manifold, fuel injected into the cylinder just before ignition).
  7. According to the number of cylinder-
    (a) Single cylinder and
    (b) multi-cylinder engine
  8. Method of cooling- (a) Water cooled or (b) Air cooled
  9. Cylinder arrangement-Vertical, horizontal, inline, V-type, radial, opposed cylinder or piston engines.
  10. Valve or port design and location- Overhead (I head), side valve (L head); in two stroke engines: cross scavenging, loop scavenging, uniflow scavenging.

Also read: Engineering Thermodynamics

Main components of Internal Combustion (I.C.) Engine

A simple sectional elevation of a 4-stroke engine is given in below figure indicating the various components.

Figure:- Components of the four-stroke engine

  1. Cylinder: -the cylinder allows the piston to move to and fro. The cylinder is made of cast iron or steel or aluminium alloy.
  2.  Cylinder head: -It is fitted on the top of the cylinder. A gasket is provided between the cylinder and cylinder head to prevent the leakage of hot gases. The cylinder head also accommodates the inlet valve, the exhaust valve and spark plug or injector.
  3. Piston: -It transmits the force exerted by the burning gases to the connecting rod and finally the crankshaft. The piston is usually made of cast iron or steel or an aluminium alloy.
  4. Piston rings: -Two different types of piston rings are housed in the circumference grooves provided on the outer surface of the piston. The function of upper rings known as compression ring is to provide gas-tight sealing to maintain the compression pressure inside the cylinder and to prevent the leakage of burnt gases into the crankcase. The function of the lower rings is to scrap the used lubricating oil into the crankcase. These rings are called scraper rings.
  5. Connecting Rods: -It helps in converting the reciprocating motion of the piston into the rotary motion of the crankshaft.
  6. Crankshaft: -Alloys steels are used for the crankshaft to withstand the high stress & strain.
  7. Flywheel: -It stores the excess energy during the power stroke of the engine and helps the movement of the piston during the remaining idle strokes.
  8. Cams: -Properly designed cams control the opening and closing of the inlet and exhaust valves in the case of four-stroke engines. Cams are rotated by a camshaft driven by the crankshaft through gears.

Also read:Engine Performance Parameters

Working of Four-Stroke Petrol/Spark-Ignition (S.I.) Engine: –

The following figure depicts the working of a typical four stroke petrol engine. The different strokes are described in this section.


Working of the Four-stroke petrol/S.I. engine
  1. Suction Stroke: -During this stroke, the inlet valve is kept opened and the exhaust valve is closed. This piston comes down to the bottom dead centre (BDC) from the top dead centre (TDC). Pressure in the cylinder will be slightly less than the atmospheric pressure. Petrol-air mixture in the correct proportion from the carburettor is drawn inside the engine cylinder through the inlet valve.
  2. Compression Stroke: -In this stroke, both inlet and exhaust valves are kept closed. The mixture of petrol-air is compressed when the piston moves up to TDC. The compression ratio varies from 7-10 for petrol engines. The ratio between the cylinder volume before compression to the volume after compression is known as the compression ratio. At the end of the compression stroke, a spark is produced at the spark plug due to which combustion starts resulting in high pressure and temperature which are comparatively less than that of a diesel engine.
  3. Working or Power stroke: -During this stroke, both valves are kept closed. The piston is pushed down from TDC to BDC. The force above the piston is transmitted to the crankshaft through the connecting rod & crank mechanism.
  4. Exhaust Stroke: -During the stoke, the exhaust valve is kept opened and the inlet valve is kept closed. The piston moves up from BDC to TDC. The waste gases are sent out through the exhaust valve and the cycle is repeated.
  5. During the four strokes, The crankshaft will make two revolutions. The thermal efficiency of a four-stroke engine is higher compared to a two-stroke cycle engine.

The spark ignition engine works on the air standard cycle known as the Otto cycle, which is an ideal cycle.

Also read:Terms Frequently Used in Thermodynamics

Otto cycle: –

     Nicolaus Otto (1876) proposed a constant-volume heat addition which forms the basis for the working of today’s spark-ignition engines. The cycle is shown on p-V and T-s diagrams in the following figures: –

Video 1: Working Principle of 4-Stroke Petrol Engine.

Video 2: Actual Working of 4-Stroke Petrol Engine

Air Standard Efficiency of otto cycle: –

Theory notes of IC Engine

Efficiency and numerical on the efficiency of otto (petrol) and diesel cycle

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