Kettering Ignition System
An internal combustion engine requires a source of energy to ignite the fuel-air mixture in the cylinder at the right time. In a diesel engine, the rapid compression of the inlet charge raises its temperature to the point of self-ignition, hence the alternative name, compression-ignition engine. This method needs high compression ratios, typically 22:1 and a heavily built engine to withstand the forces involved.
The more lightly built petrol engine has a typical compression ratio of 8 or 9:1 and the compressed charge does not get hot enough. Some outside help is needed to initiate combustion. Various ideas were tried in the very early days, such as the hot tube system which had a piece of metal protruding into the combustion chamber and brought to red heat by an external blow-lamp arrangement.
An electric spark plug works much better. The problem is (a) turning six or twelve volts from a battery into the 20,000+ volts needed to jump across the 1mm spark plug gap and (b) persuading the spark to occur at just the right moment of the pistons travel.
The standard Kettering circuit fulfils these requirements. As illustrated for a four-cylinder engine, it consists of a battery, an ignition coil, a capacitor, a pair of points arranged as a contact breaker, an ignition distributor and at least one spark plug per cylinder.
The ignition coil commonly consists of a secondary winding of many thousands of turns of fine wire wound around a laminated iron core. Wrapped over the outside of the secondary is a primary winding of a few hundred turns of thicker wire. The whole lot is housed in a metal can, usually filled with dielectric oil to aid heat dissipation.The secondary has a resistance of some thousands of ohms. The primary has a resistance of between one half and several ohms depending on the design. The lower ends of the two coils are joined inside the can and are brought out to the negative terminal on the coil top. The upper end of the primary comes out at the positive terminal while the upper end of the secondary is connected to a well insulated socket for the high voltage lead to the distributor.
The ignition points are housed within the distributor base and are opened by a cam on the distributor shaft which turns at half engine speed. Closure is effected by a spring.The distributor cam has one lobe for each cylinder and is positioned on the shaft so that the points open near the start of each power stroke. The moving arm of the points set is connected to the negative terminal of the ignition coil ( in a negative earthed electrical system) by an insulated lead passing through the distributor base.
When the points are closed, a current of up to ten amps flows from the battery, through the primary winding and away to ground via the points. The current flow produces a magnetic field around the coil windings which is enhanced and concentrated by the iron core. When the ignition points open, the primary current suddenly stops and the magnetic field collapses. The rapidly changing magnetic field induces a sharp voltage spike in the primary winding. This voltage spike is several hundred volts negative with respect to the +12volt applied to the battery side of the primary and because the coil is acting as a transformer, a pulse of up to 20,000 volts appears across the secondary winding. The voltage required to fire the sparkplug determines the maximum voltage which actually appears on the High Tension terminal and this can range from 5000 to over 15000 volts, depending on the plug gaps and the conditions inside the combustion chamber. If a sparkplug lead happens to be disconnected, the secondary voltage can rise to the maximum possible figure and cause unnecessary strain on the insulation of the coil and the high voltage leads.
The voltage spike in the primary will strike an energy wasting arc across the just-opened points unless something is done about it. This is one function of the capacitor connected across the points, an electrical device which has the ability to accept an inflow of current until it is fully charged. Coil current continues to flow briefly into the uncharged capacitor while the points move apart to the point where an arc can't be struck by the few hundred volts of the primary spike. Once the capacitor is fully charged, the current flow stops and the magnetic field can collapse. The voltage spike in the primary rapidly rises to its peak value, inducing the multi-thousands of volt pulse in the secondary. Without the capacitor, the points will be rapidly eroded and the collapsing magnetic field will expend a good part of its energy as a few dozen volts maintaining the unwanted arc across the points.
The High Tension voltage pulses induced in the secondary are conducted from the top of the coil back to the distributor cap centre post, from where the rotor arm feeds it to the correct spark plug lead at the appropriate time.
At ordinary engine operating speeds, the points open and close a couple of hundred times per second, the exact number depending on the number of cylinders and the engine RPM. The points need to be closed for a appreciable time in order to build up the maximum magnetic flux in the ignition coil core. The period of points closure is specified by the ignition system designer and is typically expressed as degrees of distributor rotation. In a four cylinder engine, the angle between each ignition cam lobe is 90 degrees and the period of points closure or "dwell" is usually a bit over 45 degrees of distributor rotation. In a six cylinder engine, the lobes are 60 degrees apart and the dwell time is 30 to 35 degrees. The dwell is adjusted by setting the points gap to a specified distance at maximum opening. A narrower gap gives more dwell and a wider gap gives less. Taking it to extremes, excessive dwell means that the points close too soon after opening, cutting off the magnetic field collapse before it delivers all its energy. Too little dwell gives the magnetic flux insufficient time to build up to the maximum. Both conditions give a a weak spark which gets even weaker as the engine RPM rises and produces misfiring at normal operating speeds.
Advantages of the points/coil system.
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