IC Engine - Basics

INTERNAL COMBUSTION INTERNAL COMBUSTION ENGINESENGINES

IntroductionIntroduction

A heat engineheat engine is a device which transforms the chemical energy of a fuel into thermal energy and uses this thermal energy to produce mechanical work.

Heat engines are classified as

(i) External combustion engines (EC Engines). (ii) Internal combustion engines (IC Engines).

In an External combustion engine the combustion takes place outside the engine.

In steam engine, the working fluid (high pressure steam) is generated outside the cylinder by the combustion of fuel.

The steam is then passed to a reciprocating engine or turbine for useful work.

In an Internal combustion engine the combustion takes place inside the cylinder

Chemical energy is released when the fuel-air mixture is burnt in the combustion chamber.

I.C engines directly convert chemical energy of the fuel into useful mechanical work.

The gas produced in this reaction rapidly expands forcing the piston down the cylinder on the power stroke.

THE BASIC COMPONENTS OF AN IC ENGINE

Cylinder block cylinder Piston piston rings Inlet and exhaust valves Connecting rod Flywheel Crankshaft gudgeon pin Camshaft cams

Cylinder

Cylinder is made up of cast iron or an aluminum alloy.

Inside the cylinder, piston is fitted which makes the reciprocating motion.

This reciprocating motion is converted to rotary motion through a mechanism consisting of a connecting rod and crank shaft (slider – crank mechanism).

Piston

It is cylindrical in shape fitted tightly inside the cylinder.

It moves up and down inside the cylinder bore and transmits the force exerted by the combustion of fuel to the crankshaft.

Piston rings

Piston rings provide a sliding seal between the external cylindrical surface of the piston and the inner surface of the cylinder.

The rings serve two purposes:

They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.

They keep oil in the sump from leaking into the combustion area, where it would be burnt and lost.

Intake and exhaust valves

The intake and exhaust valves open at the proper time to let in air and fuel and to let out the products of burnt fuel (exhaust)

During compression and combustion both the valves are closed

Spark plug

Used to generate the spark which ignites the compressed fuel and air mixture in the spark ignition engine.

To generate the spark, a high voltage of around 20,000 Volts is applied.

Connecting rodConnecting rod

The connecting rod connects the piston to the crankshaft.

It can rotate at both ends so that its inclination angle can change as the piston moves up and down and the crankshaft rotates.

Crankshaft

The crankshaft converts up and down motion of the piston into circular motion.

Flywheel

It is mounted on the crankshaft which stores excess energy during the power stroke and returns that energy during the other strokes and hence reduces the fluctuation in speed.

Gudgeon pin (Piston pin)

It is the pin which links the connecting rod with the piston

CamshaftCamshaft

It is used to operate the inlet and exhaust valves through cams, cam followers, push rods and rocker arms.

Cams

Used to operate the valves.

Designed in a way to open the valves in correct time and to keep open for necessary duration

Sump (Oil pan)

Sump surrounds the crankshaft

Contains oil, which collects in the bottom of the sump (the oil pan)

Terminology in I.C engines

BoreThe inside diameter of the cylinder is known as bore.

StrokeMaximum distance traveled by the piston inside the cylinder in one direction is known as stroke.Equals twice the radius of the crank.

Top Dead Center (TDC)Extreme position of piston at the top of the cylinder.

Bottom Dead Center (BDC)Extreme position of piston at the bottom of the cylinder

Terminology in I.C engines

Clearance Volume

The volume contained in the cylinder above the top of the piston when it is at TDC and is denoted Vc.

Piston Displacement or Swept Volume (Vs)

Volume swept by the piston when traveling from one dead center to the other is called piston displacement.

Expressed as cubic centimeter (cc)

Vs = A x L = (Π/4) d2 L

Compression Ratio (r)

Ratio of the total volume of the cylinder above the piston when it is at BDC to the volume when the piston is at TDC, denoted as ‘r’.

r = (Vc+Vs)/Vc

4 - Stroke SI engine working principle

Movement of the piston up or down the cylinder makes up one stroke. Movement of the piston up or down the cylinder makes up one stroke.

It is the distance between TDC and BDC and is equal to twice the radius It is the distance between TDC and BDC and is equal to twice the radius of the crank.of the crank.

In the four stroke engine cycle there are four strokes namely, In the four stroke engine cycle there are four strokes namely,

Suction or Intake stroke, Suction or Intake stroke,

Compression stroke, Compression stroke,

Expansion stroke or power stroke, Expansion stroke or power stroke,

exhaust stroke. exhaust stroke.

One cycle completes when piston moves for four strokes or two One cycle completes when piston moves for four strokes or two revolutions of the crankshaft.revolutions of the crankshaft.

(i) Suction or Intake stroke.

In the suction stoke piston moves from top dead center to bottom dead center.

Inlet valve opens at this time and the exhaust valve is closed.

The fuel air mixture is sucked into the cylinder during the intake stroke.

The suction is shown in the Ideal PV diagram (path 0-1)

(ii) Compression stroke

Piston starts moving from bottom to top (path 1-2) compressing the air-fuel mixture.

Both inlet and exhaust valves are closed during this stroke. At the end of the compression stroke the fuel-air mixture is

ignited with the spark plug leads to increase in pressure and temperature which is around 2000ºC (path 2-3).

The heat addition takes place at constant volume process.

(iii) Expansion or Power stroke.

Piston is pushed to BDC (path 3-4) with high force due to high pressure released by burnt gases.

During expansion the pressure and temperature decreases (path 4-5).

This is known as power or expansion stoke.

Both inlet and outlet valves are in closed position during the power stroke.

(iv) Exhaust stroke.

The piston which is in BDC starts moving to TDC (path 5-0).

During this stroke the exhaust valve opens and the inlet valve remains closed.

The burnt gases are pushed out to the atmosphere by the piston.

The exhaust valve closes when the piston reaches TDC.

Thus one cycle completes.

During a cycle the crank shaft turns by two revolutions.

Ideal PV diagram of a four stroke SI engine (Otto cycle)

Working of 4 stroke 4 cylinder diesel engine

2 – Stroke engine working principle

In the 2 stroke engine, the working cycle completes in two strokes of the piston movement or one revolution of the crank shaft.

The valves are replaced by ports.

There are three ports in two stroke engine namely inlet port, transfer port and the exhaust port.

The ports are opened and closed by the movement of piston, here cams are not used for operating to open and close the ports.

Exhaust port is located slightly above the inlet port and the transfer port is located in between the exhaust port and inlet port.

When the piston is at TDC and about to move downwards, only the inlet port is kept opened and other two ports are closed.

The air and fuel (petrol) mixture is drawn into the crankcase due to vacuum produced by the upward movement of the piston.

It is to be noted that top surface of the piston controls the opening and closing of the exhaust port and transfer port whereas the bottom surface of the piston controls the opening and closing of the inlet port.

Inlet port closes

CONTD...

During the movement of piston from BDC to TDC the mixture inside the cylinder gets compressed.

As the piston reaches TDC the spark is produced by the spark plug.

As the pressure and temperature of the burnt gases increase the gases push the piston towards downward direction.

When the exhaust port open during the movement of piston from TDC to BDC the burnt gases leave the cylinder through exhaust port.

Transfer and exhaust ports open

CONTD..

When the piston moves down, the inlet port also closes due to which the air fuel mixture trapped in the crankcase will get compressed.

As the piston moves still towards BDC the transfer port is opened due to which the mixture from the crank case enters the cylinder.

CONTD...

The piston crown is specially made in a shape such that the fresh mixture hits the piston crown and gets deflected up and is prevented from going out directly to the atmosphere through the exhaust port.

The deflected fresh mixture pushes the burnt gases out.

This action of sweeping out the exhaust gases with the help of fresh charge is known as “Scavenging”.

During the downward movement of piston the power, exhaust and suction process takes place.

CONTD...

When the piston moves from BDC to TDC first transfer port closes, then exhaust port closes finally the inlet port will open.

After closing the exhaust port the air-fuel mixture gets compressed and the cycle is repeated.

The inlet port is opened and closed by the bottom portion of the piston.

Two stroke SI engine with Inlet valve.

Nowadays in the two stroke SI engine instead of inlet port, spring loaded inlet valves are used for suction of fuel air mixture.

2 Stroke Diesel engine

Working principle of two stroke diesel engine is similar to two stroke petrol engine.

During suction stroke, only air is sucked instead air-fuel mixture.

The fuel is injected using fuel injector during the power stroke;

here there is no spark plug.

Comparison between Four stroke and Two stroke engine

Four stroke Two stroke

The cycle completes in four strokes or two revolution of the crank shaft

The cycle completes in two strokes or one revolution of the crank shaft

For the same power the engine is bulkier and heavier For the same power the engine is small and light weight

Efficiency is high. Efficiency is low.

It has valves and valve actuating mechanisms It has ports (some 2 stroke engines are fitted with conventional exhaust valve or reed valve)

Used where efficiency is important viz., cars, buses, trucks. Used where low cost, compactness and for light weight application viz., motor cycles and mopeds.

Comparison between SI and CI engine

Cooling System

To maintain the temperature of I.C engine in an optimum value, cooling of engine is essential.

If cooling is not provided the temperature of I.C engine rises and the expansion of piston and burning out of lubricating oil occurs which leads to seizure of piston.

Also the strength of the materials used for various engine parts usually decreases with the increase in temperature.

Two types of cooling are adopted

(i) Air cooling

(ii) Water cooling

Air cooling

Air cooling is used for low power engines like the ones used for motor cycles and also for aero engines to reduce the weight.

Cooling fins are provided in the cylinder head and the outer surface of the cylinder as shown in the figure.

The amount of heat dissipated depends upon the surface area of the fins, amount of air circulated, the velocity of air and the temperature difference between the engine block and the surrounding.

In large size engine the circulation of air is enhanced by a fan.

Water coolingWater cooling is commonly used in cars, buses, heavy trucks and in stationary engines.

Water passage is provided between the walls of cylinder and the cylinder heads for the flow of water, these passages are known as water jackets.

Water is circulated along the cylinder by a pump driven by the crankshaft.

The water from the over flow tank flows to the engine via radiator.

The heat will be carried away by the water passing over the engine.

The hot water will be cooled when it passes over the radiator.

The water in the radiator is cooled by the air circulated by the cooling fan as shown in the figure.

Water cooling

Lubrication

Friction, Wear and Lubrication are three important terms in mechanical engineering. In fact the study of these three items is called ‘Tribology’.

Lubrication is the activity of providing a lubricant (a substance) between solid surfaces in contact having relative motion to reduce friction.

Functions of lubrication

Reduce the friction between the rubbing parts Reduce the wear and tear Reduce the power lossForm good seal between piston rings and

cylinder walls Keep the rubbing parts clean

SAE (Society of Automotive Engineers) has allocated numbers for specifying the viscosity of Engine Oils.

The SAE grades 0W through 25W, where W stands for Winter, have a maximum viscosity specified at low temperatures (-5°C to -35°C), to ensure easy starting under low temperature conditions.

The SAE grades 20 through 60 only have limits set at 100°C as these grades are not intended for use under low temperature conditions.

For marine applications, monograde oils of SAE 30 or SAE 40 are used because of the steady operating conditions in a ship's engine room.

CONTD...

Automotive oils are normally formulated with Viscosity Index Improvers (VI Improvers) to provide multigrade performance.

VI Improvers are very large molecules, which are chemically made by linking together smaller molecules in a process called polymerization.

The resulting products, called polymers, may have molecular weights 1000 times or more greater than the base stock molecules.

The use of these special polymers makes it possible to meet both the low temperature viscosity requirements of the W grades as well as the high temperature requirements of the non-W grades.

SAE 20W40 has the low temperature viscosity value of a SAE 20W oil at low temperatures and the high temperature viscosity of a SAE 40 oil at the higher temperature.

The parts required lubrication in I.C engines are

Piston

cylinder

crankshaft

cam shaft

connecting rod, etc.

Normally the lubrication is applied to these parts by splash lubrication system where oil is placed on the oil sump.

In two stroke SI engines the lubricating oil will be directly mixed with petrol for lubrication (Petroil lubrication).

Types of Lubricants

Solid lubricants – Graphite

Liquid lubricants – Mineral oils, Vegetable oils

Semi solid lubricants - Grease

Requirements of good lubricating oil

A good lubricant should possess the following requirementsA good lubricant should possess the following requirements

Sufficient viscosity to work under high and low temperature.

Normally high viscosity index is preferred for engine lubrication.

Should not react with the surfaces being lubricated

Should be noncorrosive and should provide protection against corrosion.

Must have good detergent quality to keep the rubbing parts clean

Must be Non toxic and non-flammable

Applications of I.C engines

For transport on land, sea and air

Industrial power and prime movers for electric generators. plants

For Irrigation purpose

Earth moving equipment

Classification of Ic engines

Questions

Give four applications of I.C engines

Give the difference between (a) Petrol and diesel engines (b) Two stroke and four stroke engine

Explain the construction of a four stroke single cylinder I.C engine.

Explain the working principle of (a) Four stroke petrol engine (b) Four stroke diesel engine (c) Two stroke petrol engine with intake port.

Contd..

With a PV diagram explain the process in Otto cycle.

With a neat sketch explain the water cooling system used in cars

Give a comparison between water cooling and air cooling.

Explain the need for lubrication in I.C engine