Demonstrate construction and working of different types of ignition systems

Demonstrate construction and working of different types of ignition systems

INTRODUCTION

The spark ignition engines require some device to ignite the compressed air-fuel mixture inside the cylinder at the end of the compression stroke. Ignition system serves this purpose. It is a part of electrical system which carries the electrical current to spark plug which g ives park to ignite the air-fuel mixture at the correct time. The ignition system consists of a battery, switch, ignition distributor, ignition coil, spark plugs and necessary wiring . Some systems use transistors to reduce the load on the distributor contact points. Other systems use a combination of transistors and a magnetic pick-up in the distributor.

Compression ignition engines do not have such ignition syste ms. In a compression ignition engine. Only air is compressed in the cylinder, and at the end of compression stroke the fuel is injected which catch fire due to the high temperature and pressure of the compressed air.

REQUIREMENTS OF IGNITION SYSTEM

The ignition system supplies high voltage surges of current as high as 30,000 volts to the spark plug. These surges produce the electric sparks at the spark plug gap that ignite or set fire to the compressed air-fuel mixture in the combustion chamber. The sparking must take place at the correct tune at the end of the compression stroke in every cycle of operation. At high speed or daring part throttle operation, the spark is advanced so that it occurs somewhat earlier in the cycle, the mixture thus has sufficient time to bum and deliver its power. The ignition system should function efficiently at the maximum and minimu m speeds of the engine. It should be easy to maintain, light and compact. It should not cause any interference.

TYPES OF IGNITION SYSTEM

1. Battery Ignition System

2. Magneto Ignition System

Both the ignition systems are based on the principle of mutual electromagnetic induction. The battery ignition system is mostly used in passenger cars and light trucks. In the battery ignition system, the current in the primary winding is supplied by the battery whereas in magneto ignition system, the magneto produces and supplies the current in the primary winding.

BATTERY IGNITION SYSTEM

Battery Ignition System mainly consist a battery, ammeter, ignition switch, ignition coil, condenser, contact breaker, distributor and spark plug. The primary ignition circuit starts at the battery and passes through the switch, ammeter, primary winding, and contact breaker points to the ground. A condenser is also connected in parallel to the contact breaker points. One end of the condenser is connected to the contact breaker arm and the other end is grounded.

The secondary ignition circuit is not connected electrically to the primary ignition circuit it starts from the ground and passes through the secondary winding, distributors, spark plug to the ground.

The ignition coil steps up 6 or 12 volts from the battery to the high tension voltage of about 20,000 to 30,000 volts required to jump the spark at the spark plug gap, which ignites to combustible charge in the cylinder. The rotor of the distributor revolves and distributes the current to the four segments which in turn, send it to the spark plugs. The purpose of the condenser is to reduce arcing at the breaker points and thereby prolong their life. Because the ignition system is of four-cylinder engine, the cam of the contact breaker has four lobes. It makes and breaks the contact of the primary circuit four times in eve ry revolution of the cam.

When the ignition switch is on, the current will flow from the battery through the primary winding. It produces magnetic field in the coil. When the contact points open, the magnetic field collapses and the movement of the magnetic field induces current in the secondary winding coil Because the secondary winding has many more turns about 21,000 turns of fine wire (40 SWG), the voltage increases upto 30,000 volts. The primary winding consists of 200—300 turns of thick wire (20 SWG). About 15,000 volts are necessary to make the spark jump at 1 mm gap. The distributor then directs this high voltage to the proper spark plug when it jumps the gap, producing a spark which ignites the combustible mixture in the cylinder.

MAGNETO IGNITION SYSTEM

Magneto Ignition System mainly consist a magneto, instead of a battery, which produces and supplies current in the primary winding. The remaining arrangement in this system is the same as that in the battery ignition system. The magneto consists of a fixed armature having primary and secondary windings and a rotating magnetic assembly which is driven by the engine. When the magnets rotate, current flows in the primary winding. The secondary winding gives high voltage current to the distributor, which distributes it to the respective spark plugs.

In a magneto, the magnetic field is produced by means of permanent magnets whereas to conventional generator, the magnetic field it produced by passing some of the generated current through the field winding which produces the magnetic field.

The magneto may be either rotating armature type or rotating magnet type. In rotating armature type magneto, the armature carrying the primary and secondary windings and the condenser rotate between the poles of a stationary horse shoe magnet.

COMPONENTS OF BATTERY IGNITION SYSTEM

Battery ignition system consists of the following components:

i. Battery

ii. Ignition Coil

iii. Contact Breaker iv. Condenser

v. Distributor

vi. Spark Plug

IGNITION COIL

The ignition coil is simply a transformer with certain characteristics making it suitable for its special use but impairing to some extent its efficiency as a transformer. It serves to convert the relatively low battery voltage into high voltage.

The battery winding consists of 200-300 turns of thick wire (about 20 S.W.G) of total resistance about 3 ohms while the secondary is made up oflarge number of turns (about 15,000-20,000) of fine wire (about 40 S.W.G.). As the voltage difference between adjacent turns is small, only thin insulation is required and enamelled wire is used for this purpose However, there is sufficient voltage drop between the layers and for this paper insulation is used. The whole assembly is then impregnated in wax under vacuum to remove any air pockets or submerged in thin insulating mineral oil and hermetically sealed and in an aluminum case. The oil acts as an insulator to prevent high voltage arcing within the coil. Hermetic sealing prevents the entry of moisture in the coil which if not prevented, would cause coil failure.

The core is formed of either laminations of silicon steel or annealed iron wire insulated by varnish from each other. The laminated core has lower hysteresis loss. However, it is not easily produced in circular cross-section and because of this, and square section is often used, although when square, for the same area it has a greater perimeter than that of a core formed of iron wires stacked in a circular tube. Only open cores are used; closed cores tend to cause excessive damping.

With the advancement in technology it is now possible to use powder metal as coil cores. This reduces eddy currents drastically but the magnetism is also reduced. However, overall, it gives a more efficient and higher output ignition coil.

When the contact breaker points are closed a magnetic field is built up in the ignition coil. The opening of the breaker points causes the magnetic field to collapse suddenly as a result of which high voltage surge is induced in the secondary winding and this voltage cause a spark to jump across the spark plug gap. This collapse of magnetic field must be very rapid to produce the desired voltage. This is achieved with the help of condenser. Without the use of a condenser, the energy stored in the coil in the form of magne tic flux would be dissipated in an arc across the points and no high voltage Surge would be induced in the secondary winding. The condenser momentarily absorbs the current and brings the flow to a quick stop causing the magnetic field to collapse rapidly.

There are two types of ignition coil

1. Core type (Shell type)

2. Metal clad or Can type

CORE TYPE IGNITION COIL

In this type, primary is wound first on the core and then outside it the secondary is wound, the proper insulation, of course, being provided between the two.

METAL CLAD OR CAN TYPE COIL

In this, the secondary is first wound over the core and then the primary over this. The inner end of the secondary winding is connected to the core, while the other end of the secondary winding is connected to the primary winding. The ends of the primary winding are connected to the L.T. terminals of the coil, one of which is connected further to the contact breaker and the other to the ignition switch.

Between the primary and the case, iron strips are sometimes placed. These increase the inductance and prevent losses that would occur otherwise if the flux is linked with the metal case.

Comparison of Core and Can type ignition coil

The heat losses in the primary winding in the core type have to pass through the secondary, whereas in the can type, the primary being situated immediately under the metal case, can readily part with its heat without affecting secondary. In the core type, the primary being inside, its mean turn is relatively short, therefore, it is necessary to add external resistance called “ballast resistance” in order to limit the battery current in the oil. In the can type, however, no ballast resistance is necessary.

BALLAST RESISTOR

When the engine is running at slow speed, the contact breaker points are closed for a comparatively longer period of time. This causes overheating of the induction coil. To avoid this a ballast resistance is sometimes inserted in the primary circuit of the coil. This increased resistance decreases the amount of current in the primary circuit and thus avoids overheating of the coil at low engine speeds.

At higher engine speeds, however, due to smaller closing time of the contact breaker points, the current in the primary circuit and thus its temperature decreases, due to which the resistance value of the ballast resistor decreases. Thus it guards against any drop of secondary voltage at higher speeds, which if occurred would be quite opposite to what is desirable.

Further during starting of the engine, the resistance is bypassed from the ignition circuit to provide correct voltage to the coil when the starting motor is drawing a very heavy current, thereby drawing a very heavy current, thereby draining the battery voltage. This is done by the solenoid switch which short circuits the resistance, thus putting it out of the primary circuit completely. The value of the ballast resistance varies generally from 1 to 4 ohms. The ignition coil in Maruti 800 employs such a resistance.

CONTACT BREAKER

The function of a contact breaker is to make and break the primary circuit. This is probably the weakest member of an ignition system. This is clear from the fact that a four-cylinder engine operating at 4000 r.p.m. must make and break the circuit 8,000 times a minute. The essential requirements of a good contact breaker are:

The contacts must open and close at the correct time. The contacts must close without bounce.The contacts must open without “Fling”. Flinging occurs when the spring force is too low. In this, heel is flung clear of the cam surface. Excessive oxidation of the contacts should be avoided. The oxide film that may form on the contact faces is particularly detrimental in a low - resistance circuit of the primary winding. The corrosion of the points should be minimum.

The contact breaker is placed in the distributor housing itself. The cam is fitted to the distributor spindle.A hardened steel cam attached to the end of the driving spindle actuates the lever through the heel. The speed of cam is always one half of the engine speed. However, the number of cam faces is always equal to the number of cylinders except in case of the double lever type contact breakers.

The main components of a contact breaker are the lever, heel, bush and the contacts. One contact point is mounted on a steel pressing fixed by a screw or screws to the distributor base plate, thus making it adjustable. The other contact is mounted on the pivoted lever which is made of steel. The heel and the bush are generally of plastic. For the contact tungsten or alloy of platinum and iridium (with iridium content varying from 10 to 25 per cent, depending upon the degree of hardness required) is used. The tungsten isquite hard and has high melting point, but it has the disadvantage that it is liable to oxidation. Platinum- iridium contacts, however, are costlier and therefore not used so frequently. Generally their use is restricted to airc raft magnetos. Compared with tungsten they are relatively free from oxidation, but do not resist impact deformation so well.

The double arm type has certain advantages over the single arm type. In this for the same contact force, the force and wear on the heel are smaller, and because of smaller moment of inertia, contact bounce is less. This is why the double arm contact breaker is widely used in magnetos when operating speeds are normally higher than with the battery ignition.

CONDENSER

Condenser is connected across the correct breaker. It may be considered as a kind of elastic container in which the energy due to the inertia of the current flowing during the contact period is stored. The functions of a condenser are:

To minimize arcing and pitting of contact breaker points.To intensity the spark.It absorbs the excess energy during the ‘break’ period and gives out the same at the ‘make’ thereby intensifying the spark and protecting the contact points. The condenser consists of metallic plates usually of tin foil or aluminum foil separated by thin sheets of insulating paper or mica. These insulating sheets are kept larger than the metal plates, to prevent leakage. Recently, metalized paper on which a very thin film of metal is deposited has been used instead of aluminum foil. Such a condenser is very small in size.

DISTRIBUTOR

Situated in the same housing as the contact breaker, is the ignition distributor. However, in practice, the whole housing is called the distributor which contains the contact breaker, condenser, ignition advance mechanism and the distributor proper. The function of the distributor is to distribute the high voltage impulses to each of the sparking plugs at regularly timed intervals in the sequence of the engine’s firing order.

The common firing orders are:

4-cylinder in-line engine 1-3-4-2 or 1-2-4-3

6-cylinder in-line engine 1-5-3-6-2-4

8-cylinder in-line engine 1-4-7-3-8-5-2-6

8-cylinder v-type engine 1-5-4-8-6-3-7-2

The distributor proper consists of a rotor and a cap; both of these are made of Bakelite. Apart from Bakelite, the other materials used are alkyl plastic phenol resin, fiber glass - reinforced polyester resin plastic. All these are high dielectric strength materials. For correct location on the distributor housing, every cap has a locating tab or notch on its bottom which matches with a corresponding area on the housing. The cap is held firmly in place by spring - type clips or screws; the housing is attached to the engine block or head by means of a screw and retainer plate with a gasket in between the housing and the engine. This ensures that the housing will not move.

The rotor point is made of nickel and is inserted in the Bakelite casting. Rotor is fitted on the top of the shaft carrying the breaker cam. The distrib utor cap contains the same number of contacts as the number of cylinders. These contacts are made either from brass or nickel. The outer distributor body is usually formed of either zinc alloy or cast iron. The spindle bearing hole is bored directly along the body axis. The external surface of the distributor stem is also machined. The distributor is so installed on the engine that the drive connection aligns the rotor tip with the cap terminal for cylinder number 1 when the piston is at T.D.C.

After this initial setting, the ignition timing still has to be set according to manufacturer’s specifications. Each cap terminal is connected to a spark plug on the engine by means of h.t. cables and according to the firing order of the engine. The h.t. cable cores w ere previously of copper or aluminum but recent practice is to use carbon impre gnated linen cores or graphite-saturated fiber-glass cores which reduce the radio and television interference.

SPARK PLUG

It is mounted in the combustion chamber of the engine, where working conditions are severe. During peak combustion conditions the temperature of gases in the combustion chamber in a modem car engine may be around 2500°C and the pressure about 7 MPa. Moreover, a spark plug is also exposed to thermal and load cycling fatigue due to sudden changes in temperature and pressure - from the high temperature of burnt gas to the relatively low temperature of the air/fuel mixture and from the high pressure at the time of explosion of the air/fuel mixture to low pressure during induction. In addition, the spark plug has to endure high voltage, mechanical vibration and the corrosive atmosphere of combustion gases. A modem spark plug has an economical life of about 10,000-16,000 km.The minimum voltage required to make the spark jump across the air gap depends upon the compression pressure, mixture strength, air gap, electrode temperature, vehicle speed and load.

Construction

The plug has three main parts, the center electrode, the ground electrode and the insulator separating them. Besides these, there are the body shell, the sealing ring and the gasket washer. The upper end of the center electrode is connected to the spark plug terminal, where H.T. cable from the ignition coil in case of single cylinder engines (or from distributor in case of multi-cylinder engines) is connected. The lower end of the center electrode projects beyond the insulator to form a gap with the ground electrode.

The insulator is meant to fulfill the following functions:

(i) To insulate the center electrode from body shell, thereby preventing the leakage of high voltage surge from leaking to earth within the shell.

(ii) To control the working temperature of the center electrode by suitably adopting the thermal conductivity of the insulating material, its shape and the length of the heat path, while designing the spark plug for a given engine.

The body shell serves to house the electrodes and the insulator. Gas-tight seal is necessary to prevent the hot gas from leaking between the insulator and the body shell and between the insulator and the center electrode. Such seals may be of different types, e.g. solid ring, dry powder, metal powder fused into glass, etc. Besides above the hot gas from the combustion chamber may also leak between the plug and the cylinder head. To seal this flat ring gasket washer is commonly used.

In some modem spark plugs, the center electrode is made in two pieces. By doing so the designer can use different metals to suit best the different requirements of the upper piec e which has to be connected to the H.T. cable and the lower piece which has to go into the combustion chamber.

In some spark plugs, center electrode contains a built-in resistance in series with the electrode. This resistance reduces the number of high voltage surges accompanying the full voltage surge for the spark, thereby reducing the radio and television interference due to ignition system and also providing protection to the on-board computer. The resistance also increases the plug life by cutting down peak current that would bum the electrodes. However, the resistance increases the voltage required to jump across the spark plug gap. Suc h plugs are called resistor spark plugs and their life is about double the life of ordinary plugs.

Materials

The body shell is generally made of low carbon steel. The body is formed by impact extrusion. The threads are cold rolled to provide a low-friction profile. Porcelain was used initially for insulation, but this has the disadvantages of brittleness and low resistance to thermal shock. Porcelain was therefore replaced by mica. The use of mica, too, was restricted by leaded fuels which attached it. Present day insulators are almost exclusively of sintered alumina. These possess some distinct advantages:They are much stronger than porcelain.

Their thermal conductivity is higher than in a case of porcelain. These possess high electrical resistivity at operating temperatures. These can be molded accurately and with ease, to the desired shape.These have a high resistance to abrasion, erosion and chemical attack from the products of combustion. Thus with these materials, the plug can be cleaned by sand blasting in contrast to mica and porcelain plugs which are eroded by sand blasting.

The ground electrodes are usually formed of nickel or sometime an alloy of nickel and manganese. The addition of manganese (2.5—3.5 per cent) improves the tensile strength and the resistance to sulphur attack at high temperatures. For center electrodes silicon-manganese- nickel alloys are used. These are particularly resistant to the high temperature effects of leaded fuels. Copper-cored central electrodes are also used which increase the heat range of a plug. A nickel alloy jacket around the copper core, due to its very good anti-corrosion and anti-erosion properties, protects it from the aggressive combustion environment.

Types

The spark plugs may be long reach or short reach type depending upon the length of threaded portion (Fig. 3.6) and should be used only in the corresponding hole in the combustion chamber as described earlier.From heat dissipation point of view also the spark plugs are also divided into two classes, ‘hot’ or ‘hard’ and ‘cold’ or ‘soft’. Hot plug runs hotter than the cold plug because the path of heat dissipation of the cooling water in the jackets is longer in the hot plug than in the cold plug (Fig. 3.7). Some cold spark plugs have a copper core in the center electrode to help carry heat from the tip of the electrode. Cold plugs are generally used in heavy duty, high speed engines where high temperatures are encountered. Lower speed, medium duty and colder operating conditions require a hot plug.