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- auto parts

- Concept of the electronic system
- Ignition system timing
- Professional brake repair
- Right way to handle the warped and worn brake disc

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Ignition occurs some time before top dead center (BTDC) during the compression stroke of the piston. This time is measured in degrees of crankshaft rotation BTDC. For a modern SI engine, this timing is typically 8 to 10 degrees for the basic mechanical setting with the engine running at low speed (low rpm). This basic timing is set by the design of the mechanical coupling between the crankshaft and the distributor. The basic timing may be adjusted slightly in many older cars by physically rotating the distributor housing. As the engine speed increases, the angle through which the crankshaft rotates in the time required to burn the fuel and air mixture increases. For this reason, the spark must occur at a larger angle BTDC for higher engine speeds. This change in ignition timing is called spark advance. That is, spark advance should increase with increasing engine rpm. In a conventional ignition system, the mechanism for this is called a centrifugal spark advance. It is shown in Figure 1.10. As engine speed increases, the distributor shaft rotates faster, and the weights are thrown outward by centrifugal force. The weights operate through a mechanical lever, so their movement causes a change in the relative angular position between the rubbing block on the breaker points and the distributor cam, and advances the time when the lobe opens the points. In addition to speed-dependent spark advance, the ignition timing needs to be adjusted as a function of intake manifold pressure. Whenever the throttle is nearly closed, the manifold pressure is low (i.e., nearly a vacuum). The combustion time for the air—fuel mixture is longer for low manifold pressure conditions than for high manifold pressure conditions (i.e., near atmospheric pressure). As a result, the spark timing must be advanced for low pressure conditions to maintain maximum power and fuel economy. The mechanism to do this is a vacuum-operated spark advance, also shown in Figure 1.10. The vacuum advance mechanism has a flexible diaphragm connected through a tie rod to the plate on which the breaker points are mounted. One side of the diaphragm is open to atmospheric pressure; the other side is connected through a hose to manifold vacuum. As manifold vacuum increases, the diaphragm is deflected (atmospheric pressure pushes it) and moves the breaker point plate to advance the timing. Ignition timing significantly affects engine performance and exhaust emissions; therefore, it is one of the major factors that are electronically controlled in the modern SI engine. The performance of the ignition system and the spark advance mechanism has been greatly improved by electronics parts control systems. Because ignition timing is critical to engine performance, controlling it precisely through all operating conditions has become a major application of digital electronics, as explained in Chapter 7. It will be shown in Chapter 7 that ignition timing is actually computed as a function of engine operating conditions in a special-purpose digital computer known as the electronic engine control system. This computation of spark timing has much greater flexibility for optimizing engine performance than a mechanical distributor and is one of the great benefits of electronic engine control.

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  Blog créé le 16-04-2011 à 07h53 | Mis à jour le 01-06-2011 à 07h15 | Note : Pas de note