IGNITION SYSTEM IN PETROL ENGINE

In a petrol-engine , electric spark produced between the points of a spark plug due to this spark the combustible mixture is ignited. 

There are two systems of electric ignition:-

1. Battery ignition ( or coil ignition )

2. Magneto ignition 

1.Battery or coil ignition system -:

It relates to four cylinder petrol engine it has two circuits, the primary circuit and the secondary circuit. The function of ignition coil is to set-up 6 or 2 volts from the battery to the high tension voltage of about 24,000 volts . Closing the switch,  the current from the battery passes through the ammeter , the primary of the induction coil and contact breaker point , the contact pint being closed. The secondary of the induction coil is connected to the distributor arm. The primary winding has a small number of turns of coarse-wire ( between 100 to 200 )whereas a secondary winding has a large number of turns of very fine wire ( of the order of 10,000).

                       When spark is required in a cylinder,  the contact breaker points are opened by the cam and this induces high voltage( the order of 2400 volts ) in secondary. At the same time the distributor arm comes opposite to the point from from where the wire leads to the saprk plug of these cylinder. The high voltage induced in secondary give spark at the spark-plug .

        

         Fig. Battery ignition system 

2. Magnet Ignition System-: 

 In magnet type ignition system , is not required a battery for voltage . It generates its own voltage for the primary. Here an armature having wound upon it in the primary coil of a few hundred turn of enamelled copper wire and secondary coil of several thousand turns of fine insulated wire, rotates in a permanent magnetic field. 

                  In the mordern trend , the armature is to keep stationary and magnets is to keep rotating. By this action,  the primary voltage is generated in accordance with the law of electric generator , which is then transformed into a very high secondary voltage exactly on the lines described in the battery ignition system. Generally magneto system is used in high speed engines like scooter,  racing cars aeroplanes etc .

Valve timing Diagrams

Valve timing Diagram -: The opening and closing of intake and exhaust valves of the engine is graphically represented by a diagram , is known as " valve timing Diagram ". The opening and closing of the valves of the engine depend upon the movement of the piston from TDC to BDC , this relation between piston and valves is controlled by setting a graphical representation between these two , which is known as valve timing Diagram .

                  Theoretically the valve should open and close at the end or start of the stroke. But in actual practice it is not desirable to do so in order to obtain better results. The valves are generally opened earlier and closed later . 

                     Valve timing Diagram comprises of a 360 degrees figure which represent the movement of the piston from Top dead centre to bottom dead centre . In all the stroke of the engine cycle , which is measured in degrees and the opening and closing of the valves is controlled according to these degrees. 

Starting from the suction stroke , the intake valve is opened about 10 ° to 30 ° begore the TDC . Exhaust valve is also open at this movement . By opening the intake valve earlier,  more chances to fresh charge coming in are made . As fresh chagre move in , it helps in expelling the exhaust gases also from the chamber. 

                                                                                   Fig. (a) Inlet valve open, inlet valve close       

  

The inlet valve does not close at B.D.C. , but is closed at after it . It generally closes 30° to 40 ° after B.D.C. by doing so , a large amount of fresh chagre is ensured,  as the charge during suction stroke was get going in and compression by piston has not yet been able to push out the charge. 

During compression , the spark is ignited before the T.D.C. in case of otto . This is about 10 ° before T.D.C. or tge fuel is injected about  5 to 10 ° before T.D.C. . This is done so , because the whole of the fuel can't burn instantaneously.  Fuel also has some delay  period , ( i.e. period between spark and actual ignition ). Also time is required for the spark to travel and spread throughout the cylinder. 

        

        In expansion stroke , the exhaust valve opens about 30 to 40 ° before B.D.C. and closes about 10 ° after T.D.C. . Thus the period of exhaust is increased and complete  and effective scavenging is ensured. If this is not done , then residual  combustible mixture results in the dilution of fresh chagre. 

                                                                                               Fig . ( c ) expansion stroke 

FOUR STROKE ENGINE

The engine is classified as four stroke depending upon the number of stroke it requires to complete the basic four events of opration. The otto ,diesel and dual cycle are completed in four strokes , therefore engines based on these are called "four stroke engine " .

             

  It is the most common type of the internal combustion engine and used in various automobiles like cars , trucks ,some motorcycles etc . It deliver one power stroke for every two cycles of the piston or four piston stroke. Four stroke engine has valves    (inlet or exhaust) which is operated by the push rod ,cam , roker arms etc.

Four stroke engine complete two cycles in four strokes -: 

1. Suction or intake stroke 

2. Compression stroke 

3. Power or expansion stroke 

4. Exhaust stroke 

1.Suction/ Intake stroke -: In this stroke piston move downward to the bottom,  this increase the volume , inlet valve open and  allow a air -fue mixture to enter into the chamber. In this process inlet valve open and exhaust valve close. Now half cycle is complete. 

2.Compression stroke -: In compression stroke inlet valve close and the piston move upward or top dead centre. Due to movement of piston upward intake fuel mixture compress . And at the the end of this strokea spark plug is present to ignition. At this stage one cycle is complete .

3.Power stroke -: In the power stroke the compressed air-fuel mixture is ignited by the spark plug and combustion is start. Due to burning of fuel very high pressure gases produced which oppose the piston at the top face and now piston move downward and create the power output. Now one and half cycle is completed. In this stroke both valves are closed .

4. Exhaust stroke -: At the end of compression stroke the brunt gases is remain into the chember and which pushes the piston top side . At this time of syroke exhaust valve open and due to upward motion of piston waste gases is expelled outsude from the chamber through the exhaust valve. At the end of this stroke two complete cycles of four stroke engine is complete. 

               Again piston move downward and allow to enter fuel into the chember process will again start and cycle begins again. 

TWO STROKE ENGINE

Two stroke engine-: As the name implies, the two stroke engine only requires two piston movement or one cycle. In two stroke engines , the two ideal cycle i.e. suction and exhaust stroke are eliminated ( in other words both are not have separate stroke ). So we have one power stroke per revolution of crank in two stroke engine.

                      In two stroke engine ports are used instead of the inlet and exhaust valves oprated by complicated mechanism. The engine is able to produce power after one cycle because intake and exhaust of the gas occur simultaneously. 

Two - stroke engine 

Two stroke engine contain two processes:-

1. Compression stroke -: In this stroke first the inlet port open, air-fuel mixture enters in the chamber and piston move upward. Movement of the piston from downward to upward compress the fuel mixture at high pressure ratio. due to compression temperature and pressure of fuel increase and now spark plug ignite and burn the mixture at this point compression stroke end.

2. Power stroke-: In the power stroke the brunt fuel gases exerted very high pressure on the piston ,due to this pressure piston move downward ( expansion) and waste or exhaust gases is out from the exhaust port due to piston movement. Now one cycle is complete. 

As compare to four-stroke engine , two stroke engines are lighter , more efficient and also have tha ability to use lower- grade fuel. Therefore the lighter engines results in a higher power to weight ratio.

Two stroke engine can proceed with intaking the fuel mixture as seen in processes --:

Process 1-2-: During process 1-2 inlet port open and piston at the bottom dead centre to come upward top dead centre . Now fuel-air mixture intake and piston move upward, compresse the fuel. Due to compression cause the mixture to increase temperature and pressure and no heat exchange in this process. This is a adiabatic process. At the end of the process fuel is met by the spark plug to be ignited. 

Process 2-3-: In this process, fuel is burn by the sapkr plug ignition and combustion of fuel occurs. Due to combustion high pressure heat is generated at this point this process is end. 

 P -V diagram of two stroke engine 

Process 3-4-: The thermal energy in the chamber as a result of combustion is used to do work on the piston which pushes the piston downward and increasing the volume of the chamber. This process also known as power stroke.

Process 4-1-: In this process 4-1 , all the waste gases is expelled from the engine chamber.  As the heat leave the gas , molecules lose kinetic energy causing the decreasing in pressure. There is no exhaust phase in a two stroke engine, so the cycle begins again by allowing the air-fuel mixture in the combustion chamber. 

Two stroke engines ideal for ships , motorcycles and lawnmower etc.

Components of engine

There are various components are used in the structure of the engine.Every component have specific quality by which an engine work with desired efficiency .

1. Cylinder -: Internal combustion engine cylinder is designed to withstand the high pressure and temperature conditions. It should be able to transfer unused heat efficiently so that metal temperature does not approach at danger.In this type of engine cylinder liners or sleeves are use because these are more economical because of ease of replacement after wear and tear.also instead of making of the whole of the cylinder of best grade material ,only liner can be made of better grade. 

             In the cylinder of the I.C. engine all the process take place ( fuel intake , compression, expansion etc.) .

2. Cylinder head -: cylinder head close one end of the cylinder.  Air and fuel valves are also accommodate in it. A copper gasket is placed between the cylinder and cylinder head to make gas tight joint .

3.piston-:Piston is an important part of an I.C. engine which receive impulse from the expanding gases in side the cylinder and transmits the energy to the crankshaft through the connecting rod.It also disperses large amount of heat from the combustion chamber to the cylinder walls .

               Its construction should be rigid enough to withstand mechanical and thermal distortion. I.C. engine have trunk type piston which consists of head or crown to carry the cylinder pressure. To maintain the piston temperature within limits , the heat from the crown of piston must be dissipated quickly. It should have least friction and noiseless opration. Material of the piston must possess good wearing qualities ,so that the piston is able to maintain surface hardness upto the operating temperature and there should be little or not tendency towards corrosion. The most common used material for the piston of I.C. engine are: cast iron , cast aluminium, cast steel ,forged aluminium and forged steel.

4. Piston rings-: Piston rings are designed on the piston which impart the necessary radial pressure to maintain the seal between the piston and the cyliderbore .The piston rings inserted at the top function as compression rings or pressure rings and may be in number of 3,4,5,6 and 7.It absorb part of piston fluctuation due to side thrust and also transfer heat from piston to cylinder liner.

             In the oil ring , either bottom outer edge is stepped or bevelled,or slot cut at the centre of the ring all round the periphery and lower edge of the grove of the piston is bevelled and small holes drilled toward the instead of the piston so that excess oil scrapped by the ring flows through these holes and fall in the sump.

         Compression rings are made usually of rectangular cross- section and diameter is larger than cylinder bore .

Piston rings are usually made of cast iron and alloy cast iron .

5.connecting rod-: It is used to transmit the motion from the reciprocating piston to the rotating crank. It also convey the lubricating oil from the crank pin to the piston pin and provide splash or jet cooling of the piston crown. It consists an eye at the small end or piston pin bearing , a long shank usually I-section, and a big end opening which is usually split to take the crankpin bearing shells. The connecting rod of I.C. engine are mostly manufactured by drop forging with outer surfaces left unfinished. 

6.Crank shaft-: It is used to convert reciprocating motion of piston to rotary motion. Crank shaft consist of the shaft parts which revolve in the main bearing ,the crankpin to which the big end of the connecting rod are connected,  the crank webs connect the crank pin and the shaft parts.

7. Fuel nozzle-: The fuel in diesel engines is fed by an injection system consisting of a pump , fuel line and the injector/fuel nozzle. Fuel is fed by this nozzle ina fine spray under pressure. 

8. Intake and Exhaust valves-: these both valves are spring loaded and oprated by the cam. The valves are mounted either in the cylinder block or the cylinder head . Usually conical valves having angle of 30° or 45 °are used. As the head of the valves are subjected to high temperature of the burning gases ,these should not wrap under the influence of heat and there seats should not scale or corrode,otherwise they would start leaking. 

9.Cam shaft -: It is driven from the crank-shaft by a timing gear on a chain . It operates the intake and exhaust valves through the cam, cam followers ,push rods and rocker arms.

INTERNAL COMBUSTION ( I. C. )ENGINE

In internal combustion ( I.C. ) engine, the combustion of fuel takes place inside the engine. In this type of engine energy loss in the form of geat is less than steam or outside combustion engine. Because in steam engine combustion of fuel is takes place outside of the engine . 

* Theefficiency of I.C. engine is of the order of 35 % and the steam engine is about 15%. this is becauseof the fact that very high tem- perature can be achieved in I.C. engines.

* I.C. engine are mostly single acting, thereby avoiding the necessity of leak proof stuffing box, piston rod,D-slide valve etc.

* I.C. engine is more compact unit Because it does not involve several cumbersome auxi-liaries .

* Combustion of fuel takes place inside the cylinder, therefore overall size is reduced and these become portable. 

Flurial hydraulics and basic fluid mechanics

Flurial hydraulics -: The study of flow of the water in mobile boundary channels is dealt in flurial hyraulics. The channel may be classified as mobile boundary or rigid boundary channels . Mobile boundary channels include rivers and unlined alluvial chanls , the boundaries of which are made of loose soil which can be esaly eroded and transported by flowing water .

Pressure gradient -: The pressure gradient in a fluid is equal to the body force per unit volume and acts in the same direction.      The term pressure gradient ( grad p ) = ρ K = ρ ( ix + jy + kz ) , where i,j and k are the unit vectors in the direction of the axes of the coordinate system .

The one - seventh power law-: In this law of turbulent, velocity variation corresponds to a much favoured velocity profile and the mean velocity increases as the one- seventh power of the distance from the boundary wall.

Geometric similarity -:Geometric similarity imples similarity of form. The model is a geometric reduction of the prototype and is accomplished by maintaining a fixed ratio for all homologous lengths between the model and the prototype. 

Kinematic similarity -: Kinematic similarity involves the scale of time as well as length.It implies similarity in motion. Kinematic simi-larity between a model and the prototype is attained if the homologous moving particles have the same velocity ratio along geo-metrically similar path.