Automotive Ignition Systems

Ignition Systems:

New and Old

Jeff Tuttle

Period 3

9/9/04

Ignitions systems in motor vehicles have evolved in the past thirty years. Points was a simple concept but was not reliable and needed adjustment and replacement of components seemed constant. Today a magnetic sensor relays a signal to the computer which in turn sends the voltage to the selected cylinder to ignite the fuel/air mixture. There are not mechanical parts to fail or become corroded and brittle. There are many different parts to an ignition system. These parts differ between modern and old ignition systems. There is a coil. Sometimes one coil provides the increased voltage to the distributor or there is no distributor at all and each cylinder has its own coil to provide voltage for the spark plug.

The coil is a compact, electrical transformer that boosts the battery's 12 volts to as high as 20,000 volts. The incoming 12 volts of electricity pass through a primary winding of about 200 turns of copper wire that raises the power to about 250 volts. Inside the distributor, this low-voltage circuit is continuously broken by the opening and closing of the points, each interruption causing a breakdown in the coil's electromagnetic field. Each time the field collapses, a surge of electricity passes to a secondary winding made up of more than a mile of hair-like wire twisted into 25,000 turns. At this point, the current is boosted to the high voltage needed for ignition and is then relayed to the rotor.

The distributor is separated into three sections: the upper, middle, and lower. In the middle section, the corners of the spinning breaker cam strike the breaker arm and separate the points some 160 miles an hour. High-voltage surges generated by the action of the coil travel to the rotor that whirls inside a circle of high-tension terminals in the distributor cap, at each terminal, current is transferred to wires that lead to the spark plugs. Two other devices - the vacuum advance and the centrifugal advance - precisely coordinate the functions of the points and the rotor assembly as the requirements of the engine vary.

An ignition circuit consists of two sub-circuits: the primary, which carries low voltage; and the secondary, which carries high voltage. The primary circuit, controlled by the ignition key, releases 12 volts of electricity from the battery or alternator through the coil to a set of breaker points in the lower part of the distributor, or to the relay in electronic ignition applications. When the points or relay are closed, current flows through the chassis back to the battery, completing the circuit. When the points or relay are open, the flow stops, causing a high-voltage surge to pass from the coil through a rotor in the top of the distributor to the spark plugs. Once the car has started, the voltage regulator protects the battery from being overcharged by the alternator. The condenser absorbs part of the low-voltage current when the points are open.

In an electronic ignition, a rotating reluctor and magnetic-pickup coil replace the traditional cam, breaker points and condenser in the distributors of cars equipped for electronic ignition. This system reduces the time between tune-ups. The high spots of the reluctor interrupt the magnetic field of the pickup coil and the permanent magnet. These interruptions, or pulses, are transmitted from the pickup to a nearby electronic control unit. There, the pulses signal a transistor to break the low-voltage sub-circuit and release high voltage from the coil to the spark plugs. The short-lived electronic ignition system was a transition from the points and condenser system to the computerized ignition system. It came into widespread use in the mid-1970s, but there are still a few engines that use electronic ignition.

The starter circuit is activated when the ignition switch is turned on. This opens a second switch in the solenoid, permitting a second flow of electricity from the battery to the starter motor. The engine cranking circuit is made up of a battery, starting motor, ignition switch, and electrical wiring. When the ignition switch is placed in the "start" position, the solenoid windings are energized and the resulting shift lever movement causes the drive pinion gear to engage the flywheel ring gear, and cranking takes place. When the engine starts, an overrunning clutch (part of the drive assembly) protects the armature from too much speed until the switch is opened. At this time, a return spring causes the pinion gear to disengage from the flywheel.

The spark plug wire carries 20,000 or more volts from the distributor cap to the spark plug. Spark plug wires are made of various layers of materials. The fiber core, inside the spark plug wire carries the high voltage. The older design of spark plug wires used a metallic wire to carry the high voltage. This caused electrical interference with the radio and TV reception. Some spark plug wires have a locking connection at the distributor cap. The distributor cap must first be removed and the terminals be squeezed together, and then the spark plug wire can be removed from the distributor cap. To reduce interference with radio and TV reception, ignition systems are provided with resistance in the secondary circuit. Resistor spark plugs or special resistor type ignition cable may be used. To work effectively in modern ignition systems, it is important that the resistor ignition cable is capable of producing a specifically designed resistance. The cable must also have enough insulation so that it can withstand heat, cold, moisture, oil, grease, and chafing. High tension electricity passing through a cable builds up a surrounding electrical field. The electrical field frees oxygen in the surrounding air to form ozone, which will attach to the rubber insulation if it is not properly protected. Ozone causes the rubber to deteriorate and lose its insulating qualities. Electrical losses will seriously weaken the spark at the plug gap.

As the rotor rotates inside the cap, it receives the high voltage from the ignition coil, and then passes it to the nearest connection, which is a metal projection in the cap, which is connected to a spark plug. The distributor cap should be checked to see that the sparks have not been arcing from point to point within the cap. The inside of the cap must be clean. The firing points should not be eroded, and the inside of the towers must be clean and free from corrosion. A distributor rotor is designed to rotate and distribute the high tension current to the towers of the distributor cap. The firing end