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Figure 1 above shows the effects of ignition timing that is too far advanced, too far retarded, and spot-on in terms of developing optimum cylinder pressure.  If the ignition sequence works as planned, the voltage at the electrodes climbs from 0 to the ignition voltage (that voltage which is required to produce a spark arc under the instantaneous conditions) causing the arc to form at which point the voltage drops back to the ignition voltage level for the duration of the spark, usually between 1.5 - 2.0 milliseconds.  During this time the fuel mixture ignites and you achieve combustion within the cylinder.  When ignition energy dwindles and the ignition maintenance voltage can no longer be sustained, the spark arc extinguishes and the fuel mixture continues to burn as best it can.

To accomplish all of the above, one needs a transformer to step up the battery voltage.  We'll call it an ignition coil.  You also need something to control the ignition coil primary circuit ground path since we connect our battery positive power to one side of the ignition coil promary directly.  To control the coil primary ground (the other side of the coil primary circuit, opposite the power) manufacturers commonly use a transistorized ignition module.  Note that some vehicles have a separate stand-alone ignition module that can be replaced independently of the ignition system while others may have these transistors fitted within the Engine Control Module (ECM) or within the ignition coil pack itself.  The ignition coil secondary circuit may be connected directly to the spark plug and an electrical ground source (commonly called a "Coil-on-Plug" configuration) or it could be connected to the spark plug via a set of spark plug wires (a.k.a. "ignition wires" or "high-tension leads").  Most modern engines have multiple coils each serving one or two spark plugs while some older engine designs utilize a distributor that sends the high-voltage generated from one ignition coil to multiple spark plugs via the cap and rotor.

Figure 1.

manufacturers up until the early 1970's.  At that time the adoption of the electronic transistor improved the operation of the ignition system by eliminating the antiquated "breaker-points" component thus making the ignition system virtually maintenance-free as well as making it more robust and reliable.  With the adoption of stringent emissions regulations in the 1980's, the transistorized inductive igntion system soon became the electronically-controlled ignition system.  With computer-controlled inductive ignition a significant increase in performance and efficiency was realized over what Kettering orginially created, but the basic principles are still exactly the same, as is the purpose.

The purpose of the ignition system is to initiate combustion in the compressed fuel mixture at the precise moment required under the instantaneous conditions by igniting the fuel mixture so that pressure builds in the combustion chamber.  This pressure then acts on the piston.  The pressure rise exerts a force on the piston which acts on the crankshaft by way of the connecting rod and thus produces usable power at the flywheel.  In modern gasoline powered engines this is accomplished by an electric spark arcing across the spark plug electrodes that is created by the inductive effect of the ignition system.  Computer-controlled igntion system timing advance is properly mapped to allow the engine to produce the desired compromise between lowest exhaust emissions, avoidance of engine knock, best fuel economy and maximum power output (generally in that order as well, so far as the priorities of the OE manufacturers go).  Consistent ignition performance is required to obtain optimum power and efficiency as well as to allow the catalytic converter to function properly.  Ignition misfires result in a dramatic loss in power and efficiency, increased exhaust emissions and catalytic converter failure.  In severe cases, ignition misfires can cause the converter to overheat and fail due to unburned combustion gasses igniting within the converter.  So, how does the electric spark arc do all of this?  It's complicated, but it's also easy to understand.

First let's examine what goes on in the cylinder when the spark arc is created.  As the fuel and air have been compressed within the cylinder, the inductive system creates a spark arc across the spark plug gap.  An ignitable mixture cloud surrounding the spark arc (also known as the spark "kernel") is enough to initiate combustion within the cylinder.  Stoichiometric ignition requires spark arc energy of approximately 0.2 mJ, but this energy value requirement changes depending on spark plug gap and thus spark arc size (think "kernal size"), spark arc location within the cylinder (cylinder head design, spark plug location and design) as well as spark arc duration (this depends on igntion energy available).  All of the preceding factors have an impact on ignition characteristics.  Leaner or richer mixtures may require up to 15 times that ignition energy, and forced induction applications, including nitrous, may require even more.  The spark arc energy is only a fraction of the total ignition energy as some energy is lost in transit and as heat.  Total ignition energy will affect spark duration and intensity.  The location and length of the spark arc are determined by the type and design of the ignition system as well as instantaneous cylinder conditions.  You can't do much about the mechanical characteristics of the design but you can change the plug gap and improve your ignition system components to ensure adequate performance.  If ignition energy is inadequate, a misfire will occur.  When an engine misfires, power output is greatly reduced and exhaust emissions go through the roof.

To generate a spark arc, the ignition system must supply a voltage level high enough to cause the spark to arc from one electrode to the other.  When the precise moment for ignition within the cylinder is reached, the ignition system must provide an adequate high-voltage level and trigger it at precisely the right moment.

Charles Kettering designed what is today commonly known as the inductive ignition system.  At the time, Kettering needed to create a system that could reliably and efficiently introduce a heat source into the combustion chamber to allow efficient and properly-timed combustion of the air-fuel mixture in the reciprocating-piston internal combustion engine.  Kettering's system was simple and effective and it was used for decades by almost all auto