The sole purpose of the ignition system is to provide the spark necessary to ignite the fuel-air mixture during the cumbustion stroke of each cylinder. This task can be acheived in a number of different ways, some of which will be discussed here.
Regardless of the method used, one or more ignition coils are used to provide sufficient voltage to provide a spark across the electrodes of the spark plug to ignite the fuel-air mixture (see engine). The ignition coil can be triggered in a number of different ways. In older cars, a mechanical system consisting of electrical contacts that were opened and closed by a rotating cam inside the distributor were used. These contacts were commonly referred to as points. As the points closed, the ignition coil built up a high voltage charge on its secondary winding, similar to the operation of a transformer. Upon opening, however, this charge became a source of high voltage energy sufficient to cause electrical energy to flow across a gap, such as could be found on a spark plug. This pulse of energy was distributed to each cylinder by means of an electrical contact called the rotor which rotated inside the distributor cap. As the rotor passed each electrode in the cap, it would send the high voltage discharge through the electrode and spark plug wire to the spark plug itself, where the electrical energy would be dissipated through the plug's gap. The timing of the points and the rotor was set so that each time the points opened, the rotor was passing the appropriate cylinder's electrode in the cap.
The problem with systems that used points was that the contact surface on the points would quickly become burned and pitted, resulting in frequent tune-ups. Additionally, the energy available for spark was limited and would frequently result in incomplete combustion of the air-fuel mixture. Finally, the point contacts were prone to bouncing, and were susceptible to "floating," a situation in which the cam surface that opened and closed them would be turning so rapidly that the points would not close consistently. When floating, there would be no spark, and the engine would immediately lose power. This would occur at higher engine speeds.
The next phase in ignition systems was the capacitor discharge or electronic ignition system. Like its predecessor, it used a distributor, but instead of points, it used a rotating magnet called a reluctor to initiate spark timing. This reluctor had several surfaces protuding outward from the center in a star-like fashion. There was one protrusion for each cylinder of the vehicle's engine. As this star-shaped reluctor rotated (along with the distributor shaft), these protrusions would pass by a small electrical coil, causing an induced voltage to occur on the coil. The coil was wired to an amplifying device which made the tiny signal stronger. This stronger signal, in turn, would trigger an electronic switch of sorts called a transistor. The transistor, when turned on, would conduct the charge which had been simultaneously built up on a capacitor. This discharge of the capacitor would then charge the ignition coil, whose energy was delivered again through the rotor and cap to the spark plugs.
By using electronics, this system was shown to be much more reliable than older systems with points. A stronger spark could be produced, by using a larger capacitor to energize the coil. There were no points to wear out. A larger gap in the spark plug could be used, since there was more energy for the spark.
Modern engines use the engine's computer to operate the ignition system. The newest have done away with the distributor completely, instead using a seperate ignition coil for each spark plug. The computer takes care of the timing of the spark and sends the appropriate pulse of energy to the coil precisely when it is needed. There are no mechanical parts to wear in these systems. By using a seperate coil for each plug, these newer systems avoid the problem of incomplete discharge of the ignition coil. This problem was prevalent in older systems with only one coil. At high engine RPM, the coil would not have time to completely discharge before it was again called upon to charge up for the next spark. This resulted in a weaker spark. This is not a problem with engines that have seperate coils for each spark plug. There is ample time for complete discharging and charging of each ignition coil no matter how rapidly the engine is spinning.
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