In order to introduce those who haven't had the advantage of growing up with the older systems, I am starting this thread with a basic of the old, 60's ignition system. Hopefully, others will jump in with the theory on the new, computer controlled systems.
The basic theory behind the older ignition systems was a spark that was controlled by the physical rotation of the engine. This was accomplished by having the distributer shaft extending down into the engine and being driven by a gear within the engine. This locked the rotation of the ignition to the rotation of the engine.
The engine is a 4 cycle engine (2 cycles for power and 2 for exhaust). On the first upstroke of the piston the exhaust valve is open and the exhaust is forced out of the cylinder. When the piston is at the top the camshaft closes the exhaust valve and opens the intake valve. When the piston goes down it creates a vacuum, which pulls fuel/air into the combustion chamber.
Hopefully that fuel is well vaporized and is a highly charged mixture of vaporized gasoline and oxygen. When the piston reaches its bottom of stroke, the intake valve closes, sealing the combustion chamber. As the piston if moved upwards, the fuel/air mixture is pressurized.
Just before the piston reaches top dead center (TDC) the spark plug fires igniting the fuel. The amount of rotation that the pistion is short of TDC when the spark plug fires is known as the advance timing. This is measured in degrees of "advance". The idea is that as the timing is advanced the power that is generated is advanced. However, too high an advance can damage the engine.
When the explosion occurs, it forces the piston down, generating the power. Upon reaching bottom dead center (BDC) the exhaust valve opens so the exhaust can be forced out, completing the cycle. That is 4 distinct actions (exhaust, intake, compression, power) resulting in the 4 cycle termination.
The distributor has the number of terminals that is equal to the number of cylinders. This results in a situation where the distributor must turn at 1/2 the speed of the rotation of the engine. Typically, the shaft and gear that would drive the distributor would also drive the oil pump for the engine.
Due to the variable demand on the engine, it was found that by varying the advance of the timing, power could be developed when needed, with accelerated wear and could be retarded when that power wasn't needed. This was accomplished by a spring loaded and weighted mechanism within the distributor that controlled the advance. Higher speed increased timing. Different weights and springs could be installed to modify the advance curve for different applications.
In order to fire the spark plug, it was necessary to increase the 12VDC of the battery to a level that would fire across a gap while under extreme pressure. This was accomplished by the use of an ignition coil. This was an inductive coil was connected with its primary across the battery and controlled by a contact, call the "points". These were controlled by a cam inside the distributor that opened the points when it was desired to fire the plug.
The electrical theory was that when a voltage is applied across a coil it will develop a magnetic field. The energy of that field is transferred across the coil from the primary windings to the secondary windings at the ratio of the primary to secondary windings. This would change the voltage from 12V on the primary to 30,000 volts on the secondary.
When the points were opened by the cam in the distributor, that would attempt to stop the current. However, the field of the coil does not want to evaporate instantly. If nothing is done, an intense arc will occur across the points as a way of evaporating the energy. This would erode the points and pit them, often causing them to catch and lock together. To avoid this, a capacitor was installed across the points to eliminate that arc on the points.
With no place else to go, the energy of the coil would attempt to dissipate across the circuit of the secondary. That would generate a spark across the spark plug and cause the spark that would ignite the fuel.
If the sparkplug was gapped too large, there might not be enough energy to cause the arc. If it was gapped too short, not enough voltage would be developed on the secondary and again, no spark. An increase in the gap, or corrosion, would increase the voltage on the spark plugs and could cause the spark plug wires to arc over. This would typically occur under acceleration when more fuel was injected, the compression was greater and it was harder to arc the plug, hence higher voltage.
A simple check of the condition of the spark plug wires was to open the hood in total darkness and see if they were arcing. Often the under hood would look like a Christmas tree.
There was a vacuum advance on the mechanism that controlled the timing also. When there was high vacuum the timing would be held to a minimum. As the vacuum was reduced, as under acceleration, the timing would be advanced to create more power. This was typically a round mechanism on the side of the distributor.
The rotor inside the distributor cap would transmit the energy of the secondary of the coil from the center of the distributor to the proper terminal for the cylinder to be fired. The terminal of the rotor and the internal terminal of the distributor cap are susceptible to wear and are the main reason for them to be changed. There is a powdered metal ball in the center of the distributor cap that allows the rotor to rotate and receive energy.
Typically there is an air vent on the top of the distributor to allow it to breathe and to help dry the inside of the distributor.
Because of the characteristics of the coil and this type of energizing the coil, the resulting arc was quite long and contained a lot of energy. Much of that energy was not needed, but in order to get the arc hot enough, it was necessary to make its duration quite long. This introduced accelerated wear on the spark plugs.
In a standard ignition system, it was necessary to "tune it up" with new points, plugs, rotor, cap, capacitor in approximately 12,000 miles and plug wires in 40,000-50,000 miles.
Next, I will go into the development of the Capacitive Discharge Ignition (CDI) system and then the transistorized ignition system. These were enhancements of the standard ignition system and a precursor to the modern, computer controlled ignition system.