Engine Tech

Next comes the ignition distributor, the ignition points it houses, a capacitor and some sort of timing advance mechanism (one mechanical and one, usually, governed by intake manifold vacuum). We’ve already mentioned that a distributor’s ignition points serve as an electrical “switch” for the interruption of current (voltage) flow through the ignition points. Such points are opened and closed Dy the rotation of an eccentric cam fixed to the distributor shaft which is driven by the engine (and in speed proportion to engine rpm, normally at one-half engine speed). Also attached to the distributor shaft is a rotor which is in electrical contact with a terminal in the distributor cap and secondary voltage wires leading directly from the ignition coil. As the rotor turns (sounds like some sort of soap opera), its tip comes into alignment with terminals located around the inner circumference of the distributor cap, each of which leads to one of the engine’s spark plugs by way of a plug wire. This provides plug voltage “timed” with the engine’s piston position, since the distributor shaft (and, consequently, its rotor) is related to crankshaft position, which is related to piston position—which lives in the house that Jack built!
Also inside the distributor, or at least an electrical part of its operation, is a condenser (or capacitor).

This device, sometimes called an “electrical shock absorber,” is responsible for the prevention of excessive sparking at the ignition points (when they open) and the speeding up of the magnetic field collapse (thus affecting the rate of induced voltage in the secondary windings of the ignition coil). Since the capacitor provides an alternate path for current (voltage) passage through the points at the time of point opening, collapse of the magnetic field is rapid and ignition point “pitting” is reduced.
In addition to these distributor components, there is also the capability of providing variable amounts of timing based on engine conditions. One of these methods relies on distributor shaft rotation speed and is called centrifugal advance, since a set of weights is caused to move radially outward as a function of distributor shaft speed. Working against spring tension, these weights cause the ignition points to “move ahead” on the distributor point cam, resulting in earlier point opening and ignition timing as engine rpm increase.
The other basic type of timing advance mechanism uses intake manifold vacuum. Especially during part-throttle engine operation, manifold vacuum is high. As a result, smaller amounts of air and fuel enter the engine’s cylinders.
Such mixtures are less compressed, causing slower combustion rates (burning times) and requiring earlier ignition timing for increased fuel economy. So don’t disconnect your engine’s vacuum advance if mileage is the object.
The reasons underlying an ignition’s ability to vary timing according to engine conditions can be discussed as follows: Part-throttle operation does not require the production of maximum power. Mixture compression pressures are reduced, resulting in the need for earlier ignition timing to compensate for slower combustion rates. If it burns slower, we need to get the fire started sooner to make certain a high percentage of mix is consumed.
Under conditions of wide-open throttle and low engine speed, large quantities of air/fuel mixtures are passing into the engine, resulting in greater mixture compression, faster burning rates and the need for less ignition timing (in order to prevent detonation). At such times, intake manifold vacuum drops, vacuum advance is reduced, and the engine relies more on centrifugal advance and initial timing for best combustion efficiency.

G. In a transistor ignition system, beneficial because of much reduced current passage across the ignition points (and subsequent increased point life), a conventional positive-negative-positive (P-N-P) transistor is used, consisting of a collector (C), base (B) and emitter (E). You students of electronics know this is pretty basic, so be patient with those of us who still think of current as how fast the creek flows. Part (a) shows current flow from the ground side of the transistor through the primary side of the ignition coil and back through the battery to ground. Simultaneously, a small amount of current passes from ground through the ignition points to the transistor base (B). When the points open (by action of the distributor cam), the coil’s primary field collapses, causing induced secondary voltage to pass along the spark plug(s). If it were any more simple, we’d understand it, too.

This entry was posted on Thursday, April 15th, 2010 at 2:11 AM and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.