©By: John T. Blair (WA4OHZ)
1133 Chatmoss Dr., Va. Beach, Va. 23464; (757) 495-8229

Originally written: circa 1996
Last update: March 12, 2006 - Changed title, and email address

The ignition subsystem of a car is probably one of the least understood items by most hobbyists. So, in this article, we'll take a look at the parts comprise this subsystem, and how they work.

Actually 2, 2, 2 pieces in one
The ignition system can be broken down into two electrical sub circuits, the low voltage (12V - primary) side and the hi voltage (20,000V+ secondary or output) side. There are a total of 9 parts in both circuits: the battery, ignition switch, coil, points, condenser, the distributor cap and rotor, the spark plugs, and plug wire. The article will NOT deal with the newer electronic ignition modules and computer control circuits.

Why a spark?
In order for an engine to run, a combustible mixture of gasoline and air (1 part gasoline to 14 parts air) is trapped in the engines cylinders. The gas is burned causing its volume to expand and push down on the piston, which in turn rotates the crank shaft and eventually turns the wheels. Something is required to make the gas burn. A spark!. Each part of the ignition system has a part to play in generating and delivering that spark to the correct place (the cylinder) and at the correct time; (as the piston is approaching the end of its upward motion with the valves closed). When any part fails, the engine will not run or run very poorly.

This discussion will assume that the reader has a basic knowledge of electricity theory. If not, it is suggested that you read the article - Electricity primer.

System overview
The heart of the primary side is the battery. It stores low voltage (12V) Direct Current (DC) electricity that the car's systems will draw from when electricity is needed. The output from the battery is sent to the ignition switch. When the switch is open there is no complete circuit for the electrons, no current flow, no spark, no bang bang, and the engine does not run.

When the ignition switch is closed, the electrons can flow along the wire, into and out of the coil, and to the points. If the points are open, then again the circuit is broken (opened) and no current can flow. When the points close, the current can pass across the points to ground, via the metal parts of the distributor, engine block, ground straps, and back to the battery. Simple!

The secondary side is a little harder to understand. Notice there is no battery! So how does it get any electricity? This is where the coil comes into play. The term coil is actually a misnomer here, it is really a voltage step up transformer. When an alternating current (AC) flows in the primary of a coil, a voltage is induced, created, in the secondary. This secondary voltage is very high, starting at about 20,000V.

To get this high voltage to the spark plugs another switch is needed, enter the distributor cap and rotor button. The high voltage enters the distributor via the thick wire from the coil to the center of the distributor cap. Inside the cap, there is a small rod that is spring loaded and will rest on the top center of the rotor button. The rotor button is keyed so that it will stay in place on the distributor shaft. As the engine turns, so does the distributor shaft, and since the rotor button is physically locked to this shaft, the button turns also. As the tip of the rotor button goes around inside the distributor cap, it almost touches the numerous contacts that the spark plug wires are attached to. If there happens to be any hi voltage present in the wires, as the rotor passes a contact, the current will flow to the contact and the attached spark plug wire, in turn.

Now that we have an understanding of the basics lets start looking at some of the parts more closely and get a better understanding of the theory behind how this all works.

How The Coil works
The ignition coil is actually a transformer. Internally it consists of 2 very long pieces of wire wrapped around some kind of holder (called the core). (SEE coil experiment at the end of this article, for more information on coils.) It transforms the 12V from the battery on it's primary side into the kilovolt-class voltages needed for spark plugs. Most ignition coils have three terminals: input, return and high voltage.

1. The input wire is connected to the ignition switch.
2. The return wire is connected to the points which switch the DC on and off.
3. The high voltage output is connected in the center of the distributor.

It should be noted here that there are both 6V and 12V coils. The main difference between them is the amount of amplification they performs on the input voltage. For example: The output of a given coil is to be 120V. If 6V were to be applied to the coil, it would have to be multiplied 20 times. If 12V were applied to a coil, it would have to be multiplied only 10 times. This amplification factor and many other parameters go into the actual design of the coil.

Remember that a plain piece of wire has very low resistance, and the primary of the coil is just a long piece of wire to a DC voltage. If the ignition switch were in the "on" position, the engine not running, and the points just happen to be closed; there would be a complete circuit through the coil to ground allowing for very high current to flow. Consequently, the coil's primary or the wiring harness would burn up. Enter the ballast resistor.

Ballast resistor
To prevent the high current flow, a ballast resistor is installed. Some coils have an external ballasted. This requires a resistor between the ignition switch and the coil. Other coils have internal ballast. They have the resistor built into the coil itself. Again this is another of the design parameters.

A resistor is used to control electron flow and in so doing will cause the voltage to decrease across the resistor. (Remember Ohms Law, E=I*R; - from the electrical primer.) The 12V, from the battery, is applied to the input of the resistor. The resistor changes some of that energy to heat and the output voltage of the resistor becomes less than 12V - 7V for example. So the actual voltage at the input terminal of the coil will depend on whether there is an external ballast resistor or not. If the voltage is less than 12V there is an external ballast resistor somewhere. This resistor may be a resistor or special resistive wire.

On some ignitions systems the external ballast resistor is automatically switched out of the circuit when the car is being started. The rational is that the voltage applied to the coil will be higher and since the coil was designed for a specific multiplication factor, the output will be higher. For example: normally 7v applied to the coil. If there is a multiplication factor of 10 then the output of the coil is 70V. When the input voltage is allowed to be 12V, with the same x10 factor, the output is 120V or 50v higher. This can be very helpful in cold weather starting.Most of the British cars of the 50s and 60s had their ballast resistor built into the coil. I hadn't seen the external resistors, on any of my British cars, until I started playing with my 77 Spit.

The points
The coil will not work with the DC from the car battery. Some how the DC has to be changed to AC. That is the job of the points in the older cars (or opto-electronic in the more modern cars). The points are simply a switch that is turned on and off very fast buy the lobes on the distributor shaft. This lets the DC start and stop and is called "pulsating DC". While not true AC it is close enough for the coil. A pulsating DC signal is very rich in noise as it is composed of all odd harmonic of the fundamental frequency, (ie. for a 60 cycle square wavy - pulsed DC - it consists of frequencies of 60, 180, 300, 420, ....).

Once the ignition switch is closed, the primary circuit has voltage, 12V, applied all the time. The coil's primary return wire goes to the points and condenser. When the points are open there is NO ground on the coil's primary side and the condenser is allowed to charge. When the points are closed, it **shorts** out the primary's voltage to ground and the condenser is allowed to discharge.

The condenser
The condenser is another term that is archaic. The modern term is capacitor and their job is to store electricity. A condenser is simply 2 metallic plates separated by some material called a dielectric. This dielectric can be made of anything from air to paper, but it must NOT conduct electricity. Again, like anything else, there are a lot of parameters that go into designing a condenser and are beyond the scope here.

There is a "Charge time" (time constant) associated with the a condenser. The charge time must be designed to match the time between required pulses to the spark plugs.

The condenser has 3 basic functions.

1. The charging time constant. The cap. hold the voltage charge while the distributor turns to the point where the spark needs to go (on a V8, less than 1/8 turn on distributor shaft).

2. It passes the AC component of the "pulsed" DC to ground. Remember that stuff about the odd harmonics. This help reduce Electro Magnetic Interference (EMI).

3. Helps reduce the "arcing" at the points to increase points life.

1. If it shorts out, it acts like a closed switch across the points. Consequently, the voltage on the primary of the coil can not fluctuate nor induce the higher voltage on the secondary. The result: NO spark.

2. If it changes value, the charge time will change. This will result in the spark being "out of time" or the magnetic field is not allowed to fully build and collapse to induce enough voltage at the secondary to fire the plugs. Results: weak or no spark.

In the primary circuit, the lobes on the distributor shaft open and close the points, helping to turn the DC from the battery into pulsating DC. There will be 1 lobe on the distributor shaft for each spark plug. In the secondary circuit, the distributor shaft and rotor button form a large rotating mechanical switch to distribute the high voltage to each of the spark plugs at the required time. The distributor cap is the static part of the switch and routes the spark to each plug.

In addition to distributing the spark, the distributor with its mechanical and vacuum advance mechanisms can change the timing of when the spark is delivered to the plugs.

The distributor cap can collect moisture and or carbon dirt. When this happens, the high voltage is allowed to travel through the water or carbon dirt to the metal base and "short" out the distributor. The engine will miss. This can be readily fixed by removing the distributor cap and wipping it with a clean paper towel.

I have seen in a few rare cases, the rotor button short out. The high voltage line comes into the center of the distributor to a spring loaded contact. This contact presses on the rotor button, which sits on the distributor shaft. If the material the rotor button is made of breaks down, the high voltage is allowed to go through the rotor button and to ground via the distributor's drive shaft.

To test the low voltage side of the distributor and coil, remove the high voltage wire from the distributor cap. Hold it near a ground, any metal on the engine block, and have someone try to start the car. If there is a nice spark everything including the coil is functioning correctly.

To test the high voltage side of the distributor, simply remove a spark plug wire from one of the spark plugs and insert a Phillips screwdriver into the connector at the end of the plug wire. Hold the shaft of the screwdriver near a ground, any metal on the engine block, while someone tries to start the car. If there is a nice spark everything is functioning correctly.

Plug wires
The spark plug wire's job is to carry the high voltage from the distributor to the spark plug. The modern wire is called "resistive" wire as it has a resistor built into it and helps reduce engine noise in the cars radio.. This wire is not as strong as the older copper wire ones. Care must be taken when removing the wires from a spark plug. Do not pull on the wire itself as this can break the conducting wire inside.

The spark plugs
The spark plug's job is to ignite the fuel-air mixture in the cylinder. There are many different type of plugs and "heat" ranges for various applications. But on the whole, a spark plug is a spark plug.

This ends the basics of the ignition system. I hope this has helped take some of the mistery out of this system. A more indepth information on transformer theory follows for those that are interested.

Enjoy your Morgan

John

To make a transformer a core (something to wind the wire around) is needed. The core is usually made of soft iron and have any shape. However, the material and shape will determine how it works in an AC circuit (beyond the scope).

In a DC circuit. When power is applied to the transformer primary a magnetic field will be established. This building of the field will induce 1 spike across a transformer. To induce a continual voltage on the output (secondary of a transformer) the magnetic field must continually build and collapse, build an collapse. This can be achieved by using AC source or in automotive use - pulsating DC.

In an AC circuit, as the positive half of an AC signal flows in the primary it sets up a magnetic field around the secondary windings. This will produce 1 and only spike (its size, duration, and ringing are based on things like the ratio of primary to secondary windings, RC time constants). When the incoming AC signal starts to flow in reverse, the initial magnetic field collapses across the secondary and again induces a spike. Therefore, the output frequency of the coil is the same as the input frequency. Or if you put a coil (transformer) on a typical 60Hz household circuit, the output will be 60Hz also.

Since a coil is nothing more than 2 very long pieces of wire, and the primary wire is relatively large, it has only a few Ohms resistance to DC (from the battery).

Transformer Theory
Voltage and Current in a transformer are inversely related. The power contained in an electrical circuit, or consumed by an electrical device is based on the applied voltage times the current, Power= Current * Voltage (P=I*E).

To paraphrase the conservation of energy, You can't get something for nothing. The best you can hope for is for the input and output to be almost equal. Therefore, in a perfect environment, power into a device would be equal to the power out of a device, Pin=Pout. We can substitute the P=I*E into the conservation of energy equation and get: Iin*Ein = Iout*Eout. And this can be rearranged to:

    Ein        Iout
    -----  =  ------
    Eout       Iin

This equation, shows that the Voltage and Current across a transformer are inversely related. That is to say that if the output voltage is increased across a transformer, the output current is decreased by the same factor. This is why you don't get killed when you pull a spark plug wire off a running engine. There is enough voltage to cause your muscles to spasms but there isn't enough current to kill you.

A transformer is nothing more than 2 coils of wire, a primary and a secondary. For a voltage step up transformer the secondary will have more turns of wire than the primary. So if you want to multiply the input voltage by a factor of 2, then the secondary will have to have twice as many windings as the primary. The equation for this becomes:

         Vi   =  Vo
        ---     ---
         Np      Ns

where Vi is the voltage applied to the primary, Vo is the voltage from the secondary Np is the number of turns (coils of wire) in the primary Ns is the number of turns (coils of wire) in the secondary

Making a simple transformer - a mind experiment
Do not actually build this and plug it into a house hold wall socket. If you want a hands on experiment check with your local electronics hobby store (ie. Radio Shack) or library for how to books and experiments on the subject.

To make a step up transformer, we will need 3 items:

1. A core - something to hold the wire. A stick or a pencil would be fine.
2. A piece of insulated wire about 10 feet long for the primary winding.
3. A piece of insulated wire about 100 feet long for the secondary winding.

To email me with comments or questions.