In Figure 3, the top wave-form is the ignition secondary wave-form while the bottom pattern is of the ignition primary signal. Point A represents the beginning of the coil charge time. The duration between Point A and Point B is called "ignition dwell". Point B is the ignition point as commanded by the ECM. Point C is where the coil secondary voltage level reaches ignition voltage and creates the spark arc. Point D is where the spark arc extinguishes. The voltage level between Points C and D is the ignition maintenance voltage. Point D, where the spark arc extinguishes, is where the ignition voltage level is no longer sufficient to sustain a spark arc and this is where the ignition secondary circuit enters a decaying hysteresis loop until the secondary voltage returns to 0 Volts. Clearly this energy must go somewhere. If you stayed awake during high-school physics class, you'll remember that the final form of energy is always heat, and that's what happens to the remaining energy in the ignition secondary. The hysteresis loop decays due to the internal friction of the coil windings and is absorbed as heat by the surrounding elements. These hysteresis loops, due to their particular signature pattern as viewed on an oscilloscope, are commonly called "coil oscillations". As the energy is dissipated as heat, the primary and secondary voltage levels return to 0 Volts.
When you consider the above you may ask yourself "Why should I make an effort to understand the ignition system?" The answer is quite simple: To become a better Tuner/Calibrator and to become more proficient in diagnosing problematic vehicles. The latter is obvious. In the former case, if you find yourself trying to tune an engine and you're experiencing ignition misfires, understanding the ignition system and its operation will allow you to quickly diagnose the system and get back to the task of tuning. As for being a better Tuner/Calibrator, consider the following:
At idle speed, say 600 RPM, the crankshaft rotates at a rate of 3.6 degrees per millisecond. At 7,000 RPM the crankshaft is rotating at a rate of 42 degrees per millisecond. That's pretty quick. Now imagine that you've tuned an engine with a few bolt-on goodies, such as a cold-air intake and a cat-back exhaust and that you've tuned this engine to perfection; ideal fuel and ignition mapping that accomplishes exactly the goals you had set out for yourself. And then along comes your buddy with a bunch of parts because he's thinking ahead and he wants to install an ignition amplifier and a set of ignition coils so that the chances of a misfire are all but eliminated, even with the turbo kit he's planning on installing soon. If the factory OE ignition system was direct to the coils and the coils contained the power transistor (such as on GM's LS engines) you now have an issue to consider and that is of transistor latency. Nothing moves like light except light, so it stands to reason that transistors take time to switch on and off. If the transistor latency of the new ignition amplifier is 140 microseconds and the latency of the ignition module you added between the ECM and the amplifier is 120 microseconds, you have a total ignition latency of 260 microseconds for the parts you added, or 0.26 milliseconds. In other words, from the time the ECM commands spark to occur to the time that it actually does in the cylinder, by way of the new ignition parts with the additional latency of 0.26 milliseconds, the crankshaft has rotated another 10 degrees at 7,000 RPM unless you remembered your ignition theory and made these changes in advance (no pun intended). The above example is an extreme case, but it's somethig to consider when modifying or tuning your engine. Keep this in mind when you tune vehicles with improved or different ignition components and test the engine you're tuning at higher speeds to see if additional advance must be added to correct for any additional latency, or make the appropriate changes in the calibration if the ECM has a scalar that allows you to enter total ignition latency. If it does, the ECM will then adjust the ignition strategy automatically to compensate for the new components if you simply enter the new scalar value; no need to re-map the ignition tables in this case. If you don't make these changes, your customers and/or friends may wonder why the engine lost power with a new ignition system when in fact all that was needed was a simple tweak to the calibration. Shocking stuff! MET
ignition timing. To control ignition coil operation, the ECM sends a command to (or through) the ignition module to initiate ignition coil charging. Again, in some cases, the ignition transistor may be contained within the ECM or ignition coil. In the case of the Corvette ignition pictured above in Figure 2, the power transistors are fitted within the coil assembly. A magnetic field begins buidling around the coil with this commanded electric current flow. When this commanded signal is stopped, the total duration of which is called "dwell", the magnetic field collapses around the ignition coil and the high voltage necessary to create a spark arc is generated within the ignition coild secondary circuit and is passed through the ignition wires (if so equipped) to the spark plug where the arc is then created at the gap. To better understand this sequence of events, take a look at the oscilloscope wave-patterns in Figure 3.
Highlighted in Figue 2 are the typical components in a modern ignition system, in this case those of an LS engine in a Corvette. The light blue highlights the ECM, the green highlights the ignition wiring harness, the yellow highlights the ignition coils while the red highlights the spark plugs wires and the dark blue highlights the spark plugs. Not shown are the battery and the sensors the ECM uses to determine proper