Page 30 - Understanding Automotive Electronics

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AUTOMOTIVE FUNDAMENTALS 1

IGNITION TIMING

The point at which igni- Ignition occurs some time before top dead center (BTDC) during the
tion occurs, in relation to compression stroke of the piston. This time is measured in degrees of
the top dead center of crankshaft rotation BTDC. For a modern SI engine, this timing is typically 8
the piston’s compression to 10 degrees for the basic mechanical setting with the engine running at low
stroke, is known as igni- speed (low rpm). This basic timing is set by the design of the mechanical
tion timing. 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 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 is electronically controlled in the modern SI
engine.
The performance of the ignition system and the spark advance
mechanism has been greatly improved by electronic 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.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 17