Ignition Systems

Basically Convectional Ignition systems are of 2 types :
                                       (a) Battery or Coil Ignition System, and
                                       (b) Magneto Ignition System.
Both these conventional, ignition systems work on mutual electromagnetic inductionprinciple.
Battery ignition system was generally used in 4-wheelers, but now-a-days it is morecommonly used in wheelers also (i.e. Button start, 2-wheelers like Pulsar, Kinetic Honda; Honda-Activa, Scooty, Fiero, etc.). In this case 6 V or 12 V batteries will supplynecessary current in the primary winding.Magneto ignition system is mainly used in 2-wheelers, kick start engines.(Example, Bajaj Scooters, Boxer, Victor, Splendor, Passion, etc.).In this case magneto will produce and supply current to the primary winding. So inmagneto ignition system magneto replaces the battery.

Battery or Coil Ignition System:
           It mainly consists of a 6 or 12 volt battery, ammeter, ignition switch,auto-transformer (step up transformer), contact breaker, capacitor, distributor rotor, distributor contact points, spark plugs, etc.Note that the Figure 4.1 shows the ignition system for 4-cylinder petrol engine,here there are 4-spark plugs and contact breaker cam has 4-corners. (If it is for 6-cylinder engine it will have 6-spark plugs and contact breaker cam will be a perfect hexagon).

The ignition system is divided into 2-circuits :
(i) Primary Circuit : It consists of 6 or 12 V battery, ammeter, ignition switch, primary winding it has 200-300 turns of 20 SWG (Sharps Wire Gauge) gauge wire, contact breaker, capacitor.

(ii) Secondary Circuit : It consists of secondary winding. Secondary Ignition Systems winding consists of about 21000 turns of 40 (S WG) gauge wire.Bottom end of which is connected to bottom end of primary and top end of secondary winding is connected to centre of distributor rotor.
Distributor rotors rotate and make contacts with contact points and are connected to spark plugs which are fitted in cylinder heads (engine earth).

(iii) Working : When the ignition switch is closed and engine in cranked, as soon as the contact breaker closes, a low voltage current will flow through the primary winding. It is also to be noted that the contact
beaker cam opens and closes the circuit 4-times (for 4 cylinders) in one revolution. When the contact breaker opens the contact, the magnetic field begins to collapse. Because of this collapsing magnetic
field, current will be induced in the secondary winding. And because of more turns (@ 21000 turns) of secondary, voltage goes unto 28000-30000 volts.

This high voltage current is brought to centre of the distributor rotor. Distributor rotor rotates and supplies this high voltage current to proper stark plug depending upon the engine firing order. When the high voltage current jumps the spark plug gap, it produces the spark and the charge is ignited-combustion starts-products of combustion expand and produce power.

Note :
(a) The Function of the capacitor is to reduce arcing at the contact breaker (CB) points. Also when the CB        opens the magnetic field in the primary winding begins to collapse. When the magnetic field is
     collapsing capacitor gets fully charged and then it starts discharging and helps in building up of voltage in        secondary winding.
(b) Contact breaker cam and distributor rotor are mounted on the same shaft.
     In 2-stroke cycle engines these are motored at the same engine speed. And in 4-stroke cycle engines            they are motored at half the engine speed.


Magneto Ignition System:
            An ignition magneto is a magneto that provides current for the ignition system of a spark-ignition engine, such as a petrol engine. It produces pulses of high voltage for the spark plugs.The use of ignition magnetos is now confined mainly to engines where there is no other available electrical supply, for example in lawnmowers and chainsaws. It is also widely used in aviation piston engines even though an electrical supply is usually available. This is because a magneto ignition system is more reliable than a battery-coil system. People discussing magnetos and coils used in early internal-combustion engines generally used the term "tension" instead of the more modern term "voltage."

Operation:
                In the type known as a shuttle magneto, the engine rotates a coil of wire between the poles of a magnet. In the inductor magneto, the magnet is rotated and the coil remains stationary.

               On each revolution, a cam opens the contact breaker one or more times, interrupting the current, which causes the electromagnetic field in the primary coil to collapse. As the field collapses there is a voltage induced (as described by Faraday's Law) across the primary coil. As the points open, point spacing is such that the voltage across the primary coil would arc across the points. A capacitor is placed across the points which absorbs the energy stored in the primary coil. The capacitor and the coil together form a resonant circuit which allows the energy to oscillate from the capacitor to the coil and back again. Due to the inevitable losses in the system, this oscillation decays fairly rapidly.

                A second coil, with many more turns than the primary, is wound on the same iron core to form an electrical transformer. The ratio of turns in the secondary winding to the number of turns in the primary winding, is called the turns ratio. Voltage across the primary coil results in a proportional voltage being induced across the secondary winding of the coil. The turns ratio between the primary and secondary coil is selected so that the voltage across the secondary reaches a very high value, enough to arc across the gap of the spark plug.

                 In a modern installation, the magneto only has a single low tension winding which is connected to an external ignition coil which not only has a low tension winding, but also a secondary winding of many thousands of turns to deliver the high voltage required for the spark plug(s). Such a system is known as an "energy transfer" ignition system. Initially this was done because it was easier to provide good insulation for the secondary winding of an external coil than it was in a coil buried in the construction of the magneto (early magnetos had the coil assembly externally to the rotating parts to make them easier to insulate—at the expense of efficiency). In more modern times, insulation materials have improved to the point where constructing self-contained magnetos is relatively easy, but energy transfer systems are still used where the ultimate in reliability is required such as in aviation engines.

COMPARISION BETWEEN COIL & MAGNETO IGNITION SYSTEM:

Coil Ignition	Magneto Ignition
Battery is a must.	No battery needed
Battery supplies current in primary circuit.	Magneto produces the required current for primary circuit.
A good spark is available at low speed also.	During starting the quality of spark is poor due to slow speed.
Occupies more space.	Very much compact.
Recharging is a must in case battery gets discharged.	No such arrangement required.
Mostly employed in car and bus for which it is required to crank the engine.	Used on motorcycles, scooters, etc.
Battery maintenance is required.	No battery maintenance problems.
Bad for Economical	Good for Economical

DRAWBACKS OF CONVENTIONAL IGNITION SYSTEM:
 Following are the drawbacks of conventional ignition systems :

(a) Because of arcing, pitting of contact breaker point and which will lead toregular maintenance problems.
(b) Poor starting : After few thousands of kilometers of running, the timingbecomes inaccurate, which results         into poor starting (Starting trouble).
(c) At very high engine speed, performance is poor because of inertia effects ofthe moving parts in the                system.
(d) Some times it is not possible to produce spark properly in fouled spark plugs.
In order to overcome these drawbacks Electronic Ignition system is used.

ADVANTAGES OF ELECTRONIC IGNITION SYSTEM:

Following are the advantages of electronic ignition system :
(a) Moving parts are absent-so no maintenance.
(b) Contact breaker points are absent-so no arcing.
(c) Spark plug life increases by 50% and they can be used for about 60000 km without any problem.
(d) Better combustion in combustion chamber, about 90-95% of air fuel mixture is burnt compared with            70-75% with conventional ignition system.
(e) More power output.
(f) More fuel efficiency.

TYPES OF ELECTRONIC IGNITION SYSTEM:

Electronic Ignition System is as follow :
                                      (a) Capacitance Discharge Ignition system
                                      (b) Transistorized system
                                      (c) Piezo-electric Ignition system
                                      (d) The Texaco Ignition system


Capacitance Discharge Ignition System:
                     It mainly consists of 6-12 V battery, ignition switch, DC to DC convertor, charging resistance, tank capacitor, Silicon Controlled Rectifier (SCR), SCR-triggering device, step up transformer, spark plugs. A 6-12 volt battery is connected to DC to DC converter i.e. power circuit through the ignition switch, which is designed to give or increase the voltage to 250-350 volts. This high voltage is used to charge the tank capacitor (or condenser) to this voltage through the charging resistance. The charging resistance is also so designed that it controls the required current in the SCR.


Depending upon the engine firing order, whenever the SCR triggering device, sends a pulse, then the current flowing through the primary winding is stopped. And the magnetic field begins to collapse. This collapsing magnetic field will induce or step up high voltage current in the secondary, which while jumping the spark plug gap produces the spark, and the charge of air fuel mixture is ignited. Transistorized Assisted Contact (TAC) Ignition System

Advantages:
(a) The low breaker-current ensures longer life.
(b) The smaller gap and lighter point assembly increase dwell time minimize contact bouncing and improve           repeatability of secondary voltage.
(c) The low primary inductance reduces primary inductance reduces primary current drop-off at high speeds.


Disadvantages:
(a) As in the conventional system, mechanical breaker points are necessary for timing the spark.
(b) The cost of the ignition system is increased.
(c) The voltage rise-time at the spark plug is about the same as before.

Piezo-electric Ignition System:
                The development of synthetic piezo-electric materials producing about 22 kV by mechanical loading of a small crystal resulted in some ignition systems for single cylinder engines. But due to difficulties of high mechanical loading need of the order of 500 kg timely control and ability to produce sufficient voltage, these systems have not been able to come up.

The Texaco Ignition System:
              Due to the increased emphasis on exhaust emission control, there has been a sudden interest in exhaust gas recirculation systems and lean fuel-air mixtures. To avoid the problems of burning of lean mixtures, the Texaco Ignition system has been developed. It provides a spark of controlled duration which means that the spark duration in crank angle degrees can be made constant at all engine speeds. It is a AC system. This system consists of three basic units, a power unit, a control unit and a distributor sensor.This system can give stable ignition up to A/F ratios as high as 24 : 1.

Published by Ravindra.K(Mechanical Engineering)