combustion chamber design
2.28 IGNITION SYSTEMS
2.28.1 The ignition system
Early designs of engine used a ‘hot tube’ type of ignition system. This was an externally heated tube attached to the combustion chamber, which was designed to glow red and ignite the fuel-air mixture about TDC, at the end of the compression stroke.
As engines developed and the speed of the engine increased, the time, relative to crank position, at which the charge was ignited had to be set more accurately, so designers turned to an electric spark system invented by Lenoir.
2.28.2 Requirements of an ignition system
Towards the end of the compression stroke, an electric spark is required to ignite the petrol/air mixture inside the cylinder. The spark must have sufficient energy to start to ignite the mixture, and must also occur at the correct time.
A system that produces a ‘fixed’ spark at TDC does not suit modern engines that are designed to operate over a wide speed range, and at various load settings.
To suit these conditions, the spark must occur before TDC and, in addition, the spark timing must be precise and be made to vary when conditions change. In early systems the driver of the vehicle adjusted the ignition timing, but today these duties are automated by mechanical or electronic means.
Altering the timing to make the spark occur earlier in the operating cycle is described as ‘advancing’ the ignition. Conversely, the term ‘retard’ is used when the timing is changed to make it occur later.
To obtain a high power output from an engine, the maximum cylinder pressure should occur at 10° after TDC, irrespective of engine speed and mixture/load conditions. Since it takes a comparatively long time for the burning mixture to build up to its maximum pressure, the spark must be set to allow for this time period.
The angle of advance is enlarged when the load is decreased or if either the air/fuel mixture ratio or engine speed is increased. The change in timing is necessary because the crankshaft moves through a larger angle during the extra time taken by the gases to build up to the maximum pressure.
2.28.3 Production of high voltage
Although early designs used a comparatively low voltage ‘trembler’ arrangement, the introduction of the
‘high-voltage, timed spark system’ considerably improved engine performance.
The voltage needed on a high-voltage system to produce a spark depends on the size of the sparking plug gap and on the gas pressure within the cylinder. A normal gap is about 0.6 mm (0.024 inch), and although only a small charge of about 600 volts is required to produce a spark across this gap in the open air, it will take 10–50 times this voltage to fire a sparking plug that is under combustion pressure in a cylinder.
The combustion pressure should be taken into account when an ignition system is being tested – a spark produced at a plug outside the cylinder does not guarantee that it will spark when it is subjected to cylinder pressure.
A high-voltage spark can be generated by a
‘magneto’ or ‘coil-ignition’ system.
Magneto
This is a small, self-contained ignition unit which generates pulses of high-tension current and distributes them to the appropriate cylinders at the correct time.
Some motorcycles and small single-cylinder engines still use this system, but it is not used on modern vehicles.
Coil ignition
This system uses electrical energy produced by a battery or alternator to supply the low-tension ignition current.
The ignition coil transforms the low voltage (battery voltage) to that required to produce a high-voltage spark (several thousands of volts). Compared with a magneto system, the coil-ignition system makes engine starting much easier. It is also simpler to control the maximum voltage to suit conditions.
2.28.4 Coil-ignition systems
The conventional battery-inductive ignition system was introduced by Kettering in 1908, but it was not until the mid-1920s that the system was accepted as a successor to the magneto for use on cars.
Until recently, the main layout of a coil-ignition system, as it is commonly known, did not change, but the current need to design cleaner, more efficient engines has demanded ignition systems that produce a higher energy spark that is timed far more accurately.
Electronically controlled coil-ignition systems of the
‘breakerless’ type meet these requirements. To avoid confusion, the conventional breakerless system will be called the ‘Kettering-type’.
The Kettering-type coil-ignition system
Figure 2.250 shows the layout of a basic system that has been in use for many years. Although more modern electronic ignition systems appear more complicated (with the introduction of electronic control modules) the principles remain the same. The main components are:
Ignition coil
The ignition coil produces the high voltage necessary to cause a spark at the sparking plug. It transforms the battery voltage of 12 V to a low-current, high-voltage charge that is required to jump the plug gap. Inside the case of the ignition coil there are two windings connected to three external electrical terminals:
1 Low tension (LT) to the battery via the ignition switch.
2 Low tension (CB) to the contact breaker.
3 High tension (HT) to the sparking plug via the distributor. This lead, often referred to as a ‘king lead’ must be highly insulated to prevent loss of the HT current.
Contact breaker
A high-tension current is produced by the ignition coil, when the low-tension circuit from the battery through the coil to the earth connection (vehicle frame) is interrupted.
The contact breaker is a mechanical switch consisting of two contact points, a fixed contact, screwed down to a base plate, and a movable contact that is insulated from the metal parts that surround it. A cam, normally driven by the engine camshaft, operates the movable contact against the reaction of a strip-type spring. The number of lobes on the cam matches the number of cylinders (e.g. a four-cylinder engine has four lobes) so one revolution of the cam will give a spark for each cylinder. On a two-stroke engine, one spark per engine revolution per cylinder is required, so the cam is driven at the same speed as the crankshaft instead of half crankshaft speed as required by a four-stroke unit. A spark at the plug occurs at the instant the contacts open, so the assembly is set (timed), in relation to the crankshaft, to give a spark at the correct time.
Capacitor (often referred to as a condenser) The capacitor is connected in the circuit across the contacts, (i.e. in parallel). The condenser reduces arcing of the contacts and in consequence gives a rapid interruption of the LT circuit.
Distributor
On a single-cylinder engine the HT lead from the coil is connected directly to the sparking plug. A multi-cylinder engine has a number of sparking plugs and each one of these has to be connected to the ignition coil when the high-voltage charge is supplied to the plug. The HT switch used to select the appropriate cylinder’s spark plug is called a ‘distributor’.
It consists of a hard plastic distributor cap, inside which is a rotor arm, which is rotated at half crankshaft speed by the shaft that drives the contact breaker cam.
The HT lead (king lead) from the coil fits in the centre of the cap and a carbon brush rubs on the rotor to transmit the electrical charge through to the rotor arm.
At the instant the points open and the spark occurs, the rotor arm is set to point to the ‘segment’ in the cap that is connected to the correct cylinder sparking plug that requires the high voltage charge.
The distributor cap, rotor, contact breaker assembly and automatic advance systems are all incorporated in one unit, which is referred to as the ‘distributor unit’.
Figure 2.250 A coil-ignition system of the Kettering type
High-tension lead
Highly insulated with PVC or thick rubber, the HT leads connect the ignition coil to the distributor and to the sparking plugs.
Electrical interference is reduced by using
‘suppression’ leads. These leads have a high electrical resistance. The conductor used for the HT lead is either carbon impregnated cotton or glass fibre. These leads are generally made up in a set to suit the suppression requirements of the particular vehicle.
Sparking plug
The plug consists of a highly insulated centre electrode, and an earth electrode, which is welded to the metal body of the sparking plug. A typical gap of 0.6 mm (0.024 inch) between the electrodes enables a spark to be produced when a high voltage is delivered to the plug.
2.28.5 Magnetism and induction
To help you understand the operation of the ignition, and other systems, consider the following.
Figure 2.251 shows a conductor passing through a piece of paper, on to which are scattered some iron filings. When current flows through the conductor, the iron filings arrange themselves in a series of concentric circles. The iron filings arranged in this manner indicate the presence of a magnetic field. If the process could be slowed down, you would see that on making the circuit, the field moves outwards from the conductor. When the flow of current is interrupted, the reverse occurs – the field collapses from the outer edge.
This core becomes a magnet when current is flowing through the coil (2.253). The strength of the magnet is governed by the amount of current flowing, and the number of turns on the coil.
Figure 2.251 Demonstrating a magnetic field using iron filings
Figure 2.252 Production of a magnetic field using a conductor
Figure 2.253 Generation of a field by an electromagnet
Figure 2.254 Inducing current in a secondary winding
Figure 2.254 shows another coil, termed a ‘secondary winding’, which is wound around, or placed near, the
‘primary winding’. A galvanometer (an instrument used for detecting small electrical currents) is connected in the secondary circuit. The switch is then operated.
On closing and opening the switch, the galvanometer needle momentarily flicks one way on switching the circuit on, and the opposite way on switching the circuit off. This was first discovered by Faraday, and from his experiments it was concluded that when a magnetic line of force cuts a conductor (or vice versa), an electro magnetic force (EMF) is induced in that conductor. The magnitude of the EMF, depends on:
1 the rate of change of the magnetic field 2 the number of turns on the coil.
In the apparatus shown in Figure 2.254, the build-up of the field around the primary winding causes the lines of force to cut the secondary circuit, therefore an EMF is induced in the secondary circuit. Since the build-up is slower than the collapse, a much higher EMF is induced when the circuit is broken.
If the secondary circuit contains more turns than the primary winding (2.255), it is possible to obtain a higher voltage in the secondary circuit. For example if the secondary contains 100 times the number of turns wound on the primary winding, the EMF will be 100 times greater than the EMF in the primary, assuming the efficiency is 100%. This increase in voltage in the secondary circuit is balanced by a proportional decrease in the current of the secondary circuit.
Figure 2.252 shows a length of wire wound in the form of a coil. On closing the switch, the magnetic field surrounding each turn of the coil combines with other fields to produce a larger field. A soft iron core, mounted in the centre of the coil concentrates and intensifies the field.
2.28.6 Principle of operation of a coil-ignition system
The layout of an earth-return ignition system is shown in Figure 2.257. The system consists of two circuits:
primary and secondary.
Figure 2.255 Stepping up EMF with more turns on the secondary coil
Primary circuit
Wound around a soft iron core are several hundred turns of comparatively heavy enamelled wire. This is arranged in series with a battery, ignition switch and contact breaker. A capacitor is connected in parallel with the contact breaker.
Secondary circuit
A secondary coil winding, consisting of several thousand turns of fine enamelled wire, is wound under the primary coil. One end of this winding is joined to the contact breaker terminal, the other end is connected in series with the distributor and sparking plugs. A
‘return path’ from the plug, via the earth electrode, passes through the battery and primary winding, and so EMF induced in the primary winding is added to the large EMF produced in the secondary winding. This gives the coil a higher efficiency.
When the ignition is switched on and the contacts are closed, the current flowing in the primary winding sets up a magnetic field around the iron core of the coil.
Opening the contacts interrupts the current flowing in the primary circuit, and causes the magnetic field to collapse. During this collapse, the lines of force cut the secondary winding, and induce an EMF in the secondary circuit. A higher EMF than that acting in the primary is obtained, since the secondary coil contains more turns. The HT current is conveyed from the ignition coil to the rotor arm. The rotor arm should be pointing to the correct distributor segment, which is connected to the sparking plug of the cylinder that requires the ignition HT charge.
Figure 2.256 Internals of a coil indicate the primary/secondary and core
Figure 2.257 An earth-return coil – ignition system
Capacitor
A capacitor is fitted for two reasons:
1 It reduces arcing of the contacts.
2 It ensures a quicker collapse of the magnetic field.
It consists of two sheets of foil or metallized paper, which are separated from each other by at least two sheets of insulating material such as waxed paper.
These are wound into a cylinder shape, and inserted into a metal container. One sheet is joined to the earthed container, and the other sheet is connected by a wire or metal strip to the insulated side of the contact breaker.
When a voltage is applied to the terminals, a current can be made to flow into, but not through, the capacitor. The voltage charges the capacitor to a value equal to the supply voltage, and when this point is reached, the flow of current ceases.
If the supply is now disconnected, the charge will be retained for a time, but will gradually ‘leak away’.
When a charged capacitor is connected to a circuit, the charge produces a current flow in the opposite direction to the original supply current.
In order to understand the function of the capacitor, consider the operation of an ignition system which has the capacitor disconnected. When the contacts open, the lines of force cut the primary winding as well as the secondary, and therefore an EMF is also induced in the primary winding.
The build up of EMF is sufficient to cause a spark to jump the small contact points gap. Arcing maintains current flow in the primary circuit, and thereby prevents the rapid collapse of the magnetic field, as well as causing serious burning of the contacts.
The action is in many ways similar to the hydraulic analogy shown in Figure 2.258. Water flowing along a pipe at great speed will produce a sudden pressure rise if the tap is shut off quickly. The surge of pressure may lead to the discharge of an amount of water through the
‘closed’ tap, by lifting the tap washer off its seat.
Fitting an air dome (as illustrated) to the system allows the water to flow into the dome as the tap is suddenly shut off, but after a short time the air forces the water back into the pipe.
The capacitor must perform a duty similar to the buffer action of the air dome. When the contacts open, the capacitor absorbs the self-induced current, and by the time the capacitor is fully charged the contacts have opened sufficiently to prevent a spark occurring at the contact breaker points.
Ignition coil output
The voltage needed to produce a spark sufficient to ignite the air/fuel mixture is increased when the engine compression pressure is raised or if the mixture strength is weakened.
To meet modern engine requirements various refinements are made to the ignition system.
Oil-filled coil
Immersing the windings in oil gives the following improvements:
● Provides better insulation and greater resistance to moisture.
● The primary winding operates at a lower temperature, so the lower resistance allows more current to flow.
● Reduces corona (glow of light) effect.
2.28.7 Distributor
Figure 2.259 shows the construction of the complete distributor assembly, which comprises the distributor, contact breaker assembly and automatic advance mechanism.
A rotor arm, mounted above the cam and driven at half engine speed, is contacted by a spring-loaded carbon brush. The brush allows the rotor arm to rotate but still maintain an electrical contact with the HT terminal at the centre of a moulded ‘Bakelite’ distributor cap. Cables from the cylinder sparking plugs connect, in the engine firing order, with segments held in the cap.
These are positioned so that there is a small gap between the rotor arm and the segment.
Contact breaker
The contact breaker consists of two tungsten contacts, a fixed earth contact and a movable insulated contact.
The insulated terminal is linked by its return spring to a terminal on the side of the main body (Figure 2.260).
A cam with four lobes (four-cylinder) or six lobes (six-cylinder) opens and closes the contacts as the cam rotates. Elongated holes through which the contact breaker clamping screw passes, allows for adjustment of the contact gap. The gap is set to provide the correct ignition coil charge time, (if in doubt refer to manufacturer information).
A capacitor, earthed by a fixing screw to the contact base plate, connects with the insulated screw on the side of the main body.
Figure 2.258 Action of a capacitor
Figure 2.259 A distributor unit
Figure 2.260 The contact breaker assembly
2.28.8 Ignition timing advance
Maximum cylinder pressure should be developed just after TDC, which generally means that the spark must be produced before TDC, since a period of time elapses while the gas builds up to its maximum combustion pressure. The time factor is fairly constant, but the angle moved by the crankshaft during this time varies in proportion to the engine speed. This means that, as the engine speed is increased, the timing of the spark must be advanced.
Alteration of the ignition timing to suit speed and load conditions is performed by two automatic advance and retard mechanisms. These are:
● Centrifugal – advances the spark as the speed increases (i.e. engine speed sensitive)
● Vacuum – uses induction manifold depression to advance the spark during light load (cruising) conditions (i.e. engine load sensitive).
To advance the timing of the spark, either the cam is turned in the direction of rotation (DOR) or the base-plate is moved against the DOR.
Centrifugal advance
The advancement of the distributor cam in the arrangement shown in Figure 2.261 is produced by two fly-weights (bob-weights). These are pivoted on a baseplate, which is rotated by the distributor shaft. A contoured face on the driving side of each flyweight acts against a camplate. The camplate is integral with
Figure 2.261 Contact breaker assembly