Spark Timing
Now, in a single-spark ignition engine (one per cylinder), the combustion flame begun at the spark plug travels comparatively slowly away from the plug. As the flame moves throughout the combustion space, heat released by the burning fuel causes burned gas expansion. This tends to increase the velocity of fame travel. Spark timing, therefore, has much to do with this increase in cylinder pressure and rate of combustion flame. For example, starting it too early (too much initial timing) can lead to problems of detonation, while starting it too late can cause cylinder pressure loss and reduced power. And believe it or not, probably the most influential factor affecting flame rate (aside from actual air/ fuel ratios) is turbulence within the combustion chamber . But if you’re into building race engines, you probably already know about this. If you don’t…. But back to our cylinder.
Actual spark voltage requirements vary with the amount of spark plug electrode gap and pressure conditions
within the cylinder. Even variations in air/fuel mixture ratios can affect the amount of required spark. For example, if mixture conditions near the plug are such that a lean air/fuel ratio exists, higher spark voltages may be required to initiate combustion. This is especially true if the engine is a so-called “lean-burn” version or if the mixing of air and fuel is not uniform at the time of combustion. Cold engines also require more spark voltage, since the atomization of cold fuel is more difficult than when it’s warm. The point of all this is fairly simple: Spark is required to start the combustion process. Timing of the spark is also important, as is the amount of available spark energy. So the methods of providing ignition spark and the controls used to properly time it now become the focus of our attention.
Most cars have a wet-cell battery. Some of the voltage such a battery supplies is fed into an ignition coil consisting of
two sets of windings.
As voltage is passed through one set of “coiled wires” (windings), a magnetic field much like you’ve probably observed from science class study of electromagnets is established. That is, assuming your lab partner wasn’t one of the school’s cheer leaders. Lucky for you, ours wasn’t. In fact, we didn’t have any cheer leaders. Just labs.
Now let’s assume that this flow of primary winding voltage (see illustration of basic point-type ignition system) is interrupted. Collapse of the magnetic field “induces” a voltage in the other (secondary) windings of the ignition coil, resulting in the delivery of a much higher voltage to the engine’s spark plugs. Such a boost in secondary voltage is the result of many more turns (or coils) of wire on the secondary side of the coil. This increased voltage is then delivered to the engine’s ignition distributor, a mechanical means of providing spark to each cylinder.
C. This is a schematic of a so-called induction voltage coil. But it really isn’t all that complicated in terms of what actually happens. Around a soft iron core, two sets of wires (windings) are coiled. A magnetic field (much like the sheet of paper, iron filings and permanent magnet experiment you saw back in the seventh grade) is established. Several turns of wire (primary windings) create this field when primary voltage is passed through them. A second set of windings (many more turns than in the primary set) also experience the magnetic field, such that when the primary voltage is interrupted, an “induced” secondary voltage results from collapse of the magnetic field. Because of the greater number of secondary windings, a much higher voltage results on the secondary side of the coil. And this becomes spark plug voltage. Nothing to it, right? D. A set of ignition points is not the only method of interrupting primary voltage. Various forms of magnetic “pickup” devices are now being used in certain types of ignition distributors. Spark dwell and “point open” time are governed by the width of the distributor rotors and resulting air-gap between them. Of particular interest, of late, is the so-called Wiegand trigger distributors in which a rotating “vane” (such as the one shown here) passes between a “saturating magnet” and a Wiegand module. The result (at the risk of oversimplicity) creates a pulse which causes secondary voltage output to the spark plugs. You’ll see more of this concept in the near future.