概念汽车实用英语毛矛概念汽车实用英语U11章节.pptVIP

PPT研究院 POWERPOINT ACADEMY * * IGNITION SYSTEM The points ride on the cam so that as the engine turns and reaches the top of the engines compression cycle, a high point in the cam causes the breaker points to open. This breaks the primary windings circuit and abruptly stops the current through the breaker points. * IGNITION SYSTEM Without the steady current through the points, the magnetic field generated in the coil immediately begins to quickly collapse. This rapid decay of the magnetic field induces a high voltage in the coils secondary windings. * IGNITION SYSTEM The ignition coils secondary windings are connected to the distributor cap. A turning rotor, located on top of the breaker cam within the distributor cap, sequentially connects the coils secondary windings to one of the several wires leading to each cylinders spark plug. * IGNITION SYSTEM The extremely high voltage from the coils secondary — often higher than 1000 volts — causes a spark to form across the gap of the spark plug. This, in turn, ignites the compressed air-fuel mixture within the engine. It is the creation of this spark which consumes the energy that was originally stored in the ignition coil’s magnetic field. * IGNITION SYSTEM The disadvantage of the mechanical system is the use of breaker points to interrupt the low voltage high current through the primary winding of the coil; the points are subject to mechanical wear where they ride the cam to open and shut, as well as oxidation and burning at the contact surfaces from the constant sparking. They require regular adjustment to compensate for wear, and the opening of the contact breakers, which is responsible for spark timing, is subject to mechanical variations. * IGNITION SYSTEM In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead to lower engine efficiency. A mechanical contact breaker system cannot control an average ignition current of more than about 3 A while still giving a reasonabl