How Does an Ignition System Work Anyway?%20Spark.JPG "(6) Spark.JPG") 
The squeezed air/fuel mixture in a gasoline engine cylinder has to be ignited just before the top of the compression stroke so the explosion can reach its climax when the piston is on its way back down. To make that happen, a spark is required, and the spark plug provides a dandy little air gap where that spark can start the oxidation process we call ‘fire.’ As the hydrocarbon molecules in the fuel unite with oxygen (the most efficient mix is two molecules of oxygen for each molecule of carbon), CO2 is born.
So let’s talk about creating the spark.
The ‘pop’ of an ignition spark in the open air is actually thunder on a tiny scale, since the stream of electrons in an ignition spark displaces air molecules the way lightning does when it stabs through the air in a thunderstorm. So how do we make artificial lightning and harness it for our purposes? The step-up transformer we all know as an ignition coil has been the heart of gasoline ignition systems for decades, and the principle is fairly simple:
Magnetism induces voltage in copper windings. When very thinly insulated copper wire is wound in significant volume around a metal core and is saturated with a magnetic field, the collapse of that field produces a voltage spike that can be very powerful and very useful if properly controlled and timed.
Ignition coils contain two windings:
Winding # 1 is known as a ‘primary’ winding, and one end of the primary winding is connected, either directly or indirectly to the ignition switch. For decades, the ground side of the ignition coil primary winding was switched on and off by contact breaker points (mechanics just called ‘em ‘points’) wired parallel with a capacitor and located in the base of the distributor. The distributor on these archaic old systems rotates at camshaft speed (half crankshaft speed). The breaker points on older ignition systems open once each cycle per cylinder, i.e., four times for a four cylinder, six times for a six cylinder, and so on. The primary and secondary windings in the old oil-filled coils were connected at the positive end (fed from the ignition switch through a voltage-dropping resistor with about 1.2 ohms of resistance), but newer coils are ‘potted’ with special epoxy-type resin with both windings wrapped around a metal core to amplify the magnetic saturation.
Winding #2 turns out to be the ‘secondary’ winding and it consists of a much greater number of windings than the primary. When the breaker points or ignition module circuitry interrupt the current flow through the primary circuit, the magnetic field that has saturated both the primary and secondary windings collapses, causing a several hundred volt spike in the primary winding and spike in the secondary winding that is downright intimidating to people that don’t like a jolt. The old oil-filled coils generally popped about 50,000 or so volts, but newer coils are routinely capable of murderous voltages that can actually be fatal if they happen to cross a person’s chest cavity in search for a ground path!
Distributor, or not?
Incidentally, the distributor, when a vehicle has one, sorts out the spark and sends it to the cylinder that needs it at a given moment in time. The rotor (see photo) spins under the distributor cap, and when the coil fires, the tip of the rotor is near the terminal that feeds the appropriate spark plug. The ignition spark happens earlier as the engine picks up speed, because the time interval from the moment the spark ignites the mix until the fire goes out remains constant, and when the pistons are moving faster, the spark has to happen sooner.
%20Distributor.jpg "(8) Distributor.jpg")
That timing advance adjustment happens automatically was made by a vacuum diaphragm on older distributors and was supplemented by a centrifugal device inside the distributor that would change the indexing of the reluctor based on engine speed.
The old breaker points that provided the coil primary circuit ground path were initially replaced by a magnetic distributor pole piece (also called a “pickup coil” that sent an Alternating Current (AC) to electronic ignition modules that used transistors instead of breaker points to control the coil’s primary current.
In the original electronic ignition setup as it widely appeared on American makes, the reluctor in the distributor (see photo) whirled past a wire-wound magnet to make the signal; a four cylinder had four reluctor teeth, a six cylinder had six, and so on and while Chrysler and Ford held on to the oil filled coil for years even on their electronic ignition systems, GM introduced the hotter-sparking 80,000 volt HEI coil in 1975, complete with a tiny distributor-mounted ignition module that would fit in the palm of your hand, and by 1981, the Engine Controller (now known as the Powertrain Control Module because it also controls the transmission) would be in charge of ignition timing on many, if not most cars on the American road. 
%20Coils.jpg "(5) Coils.jpg") With the new laminated core ignition coils came the need to limit the current that flowed through the primary windings so as not to damage the delicate primary windings.
The crank sensor in some foreign cars consists a light emitter and a detector mounted above and below a thin plate with a lot of tiny slots cut in it. The light emitting diode shoots through those slots to its light detecting counterpart on the other side of the slotted plate and the circuitry connected to the light detecting device creates the waveform that the PCM or ignition module uses in the work that it does firing and timing the ignition coil pulses.
 The old round oil-filled coils were a bit more robust in that regard, and didn’t need current limiting, although they did produce lower voltages, usually no more than about 50,000 volts. The newer e-core coils such as the one in the larger photo tend to produce nearly twice that much.
Nowadays, just about every vehicle has a crankshaft sensor (see Jeep 4.0L crank sensor in photo), which generates a waveform of some kind as the teeth of a crankshaft-mounted wheel spin past it.
%20Crank%20Sensor.JPG "(4) Crank Sensor.JPG")
Modern vehicles (beginning in 1985 with some GM cars) had a coil with two secondary towers for each pair of companion cylinders. A four cylinder would have two coils with two towers. One coil fires cylinders 1 and 4 and the other coil fires cylinders 2 and 3. The coil fires on both the intake and exhaust strokes of each cylinder, but most of the energy goes to the cylinder where the gasoline charge is, because the gas charge is conductive and the exhaust isn’t. Six cylinder engines have three two-towered coils. An eight cylinder has have four of those. Knowing which cylinders are companions (both companions travel up and down together) is important and on six and eight cylinder engines for the troubleshooter.
Most of the newest vehicles have a coil for each cylinder, but all engines receive crankshaft speed and position information from a crankshaft sensor. The wheel that gives the sensor its information can have just about any number and configuration of teeth that the engineers want it to have, and can be mounted either in front of the engine, in the center of the engine, or in the flywheel area.
Before very many years went by the function of the ignition module was moved into the Engine Controller (PCM or ECM), which read a signal from the distributor or crankshaft sensor and responded by firing the ignition coil. This was pretty handy, because the PCM also knows what should be done with the ignition timing and can make the necessary changes based on temperature, driver demand and engine load.
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Richard
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Basics 
