The Ignition System

05/21/12

We have talked about how the air and fuel get into the cylinders. Now that we have the correct fuel charge, we have to ignite that charge and burn that fuel mixture to get power to run the engine. The basic operation of the ignition system has changed little over the years but innovations in the last few years have made it more precise. The basic components of the ignition system are the ignition coil, high voltage ignition wires, distributor and of course, the spark plugs. Let's see how the system has evolved over the years.

The battery is the source of the voltage for the spark we need to ignite the mixture at the proper microsecond in the cylinder. This spark requires thousands of volts to occur, as much as 120,000 volts. Where do we get these thousands of volts? The ignition coil is the source of the high voltage we need.

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The coil is a simple transformer the steps up battery voltage to the thousands of volts the spark plugs need. The coils has two sides, there is the 12 volt or primary side. This side has a few hundred turns of a large diameter wire and builds up the magnetic field in the coils. The other side is the high voltage or secondary side. This side has thousands of turns of smaller diameter wire. The coil uses "electromagnetic induction" to create the high voltage. When we turn off the voltage on the primary side, the collapsing magnetic field induces a voltage in the secondary side producing the thousands of volts the spark plugs need.

In a conventional ignition system, the switching on and off of the primary voltage was done with a set of Breaker Points. The points were set inside the distributor and rode on a cam on the distributor shaft. This cam would have 4, 6 or 8 lobes, depending on how many cylinders the engine had. When the points were closed, current flowed into the primary side of the coil creating the magnetic field. When the cam lobe opens the points, the current is turned off and the magnetic field collapses.

In an electronic ignition system, the points were replaced with a control module and the lobes on the cam were replaced with a trigger device. The trigger device uses a magnetic force field to induce a small "trigger" voltage in the control module to turn off the current to the coil. As it passes, the module turns the current back on. It is extremely accurate in when it does this. Another advantage of an electronic ignition system can produce higher voltages, up to twice the voltage a conventional system can produce.

In a computer controlled ignition system, the control module and the triggering device are replaced by a Crank Angle Sensor (CAS) and an ignition control unit. In newer cars, it has even replaced the distributor. The CAS has a plate that has 360 one-degree marks, four 90-degree marks and two 180-degree marks. There is an infrared sensor that "sees" these marks and tells the control unit exactly where the crankshaft is and the control unit turns off the current to the coil at the precise instant the spark is needed.

Since the control unit can do so many calculations per second and the CAS doesn't have to be in the distributor, manufacturers simply did away with the distributor entirely. Now a separate coil is provided for each spark plug. The control unit signals each coil independently. Since the CAS tells the control unit where the crankshaft is at any given moment; it's easy to do. This has made the system more reliable also. In conventional and electronic systems if the coil went bad, you were stuck. Now with 4, 6 or 8 coils, losing one will affect engine performance, but you can still drive. Speaking of reliability, most engines will continue to run even if the CAS fails. The ignition control unit stores basic timing data and when the signal from the CAS stops, it will go into a "fail safe" mode and use the basic data to continue engine operation. In most cases the fail-safe mode will limit speed and/or engine rpm to protect the engine and, in addition to the Malfunction Indicator Light (MIL) alert the driver that there is a problem.

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