So far in this series we have covered basic DC electricity theory and batteries, generators and alternators, voltage regulators, and then starters. Adding all those together, we have developed an electrical circuit to get the car started and charge the battery, but now we need a circuit to actually run the car. That, of course, is the ignition system and it is comprised of several parts, comprising two distinct systems, the primary and secondary ignition systems. Let's take each one in order...
The primary system consists of the ignition switch, coil primary windings, distributor contact points, condensor, ignition resistor, and starter relay.
Ignition Switch. Your ignition switch does at least three things:
First, it turns on the car's electrical system so that all accessories can be operated. It does so by providing power to the fuse panel (for those components that are controlled by the switch. Some items are independent of the ignition switch, such as headlights, horn, clock, etc.) When you insert the key and turn the switch to the "accessories" position, you are turning on the other devices in the car, such as the radio, heater, power windows, seats, defroster, etc.
Second, in the run position, everything is turned on, plus the engine's electrical components that enable it to run. Most important, it turns on the entire primary ignition system.
Third, when you move the switch to the "start" position it energizes the starter.
Wait a minute! We just learned that the starter takes enormous current from the battery through its thick cable. How can the ignition switch carry so much current if there isn't a battery cable connected to it?
Good question! The ignition switch doesn't carry the necessary current to the starter. It sends a small current to a special device called a Relay that, in turn, allows the starter to crank. We'll discuss that later in this article. Back to the primary ignition system...
The next component is the coil's primary winding. Inside the coil are two sets of wound wire, comprising of the primary and secondary windings. The primary windings carry battery voltage through and create a large magnetic field inside the coil (this is discussed thoroughly in the section on secondary windings). Although the coil's primary windings receive voltage from the ignition switch, they are actually turned on and off by the distributor's contact points.
The contact points are opened and closed by a cam on the distributor's main shaft. As it spins the cam's lobes move the actuator outward, disengaging the contacts. When the lobe passes, the contacts close, turning on the coil primary windings. The amount of time the points remain closed is referred to as dwell, and is an important factor in engine tuning.
Attached to the points is a condensor, an electrical device (capacitor) that limits current flow through the points to increase their life. The condensor is necessary because the points are opening and closing rapidly, and as they do so the voltage/current is interrupted. This causes an arc, or spark, between the contact points. Over time, this arcing will erode the material on the points and deposit carbon, and eventually the points will not pass current. The condensor acts as a current-absorber to limit the amount of arcing as the points open and close.
The next component is the ignition resistor. It is necessary because ignition coils are designed to step up battery voltage high enough — and fast enough — to keep the engine running at high rpm. That means that, as designed, the coil would produce too much high voltage at low rpm and heat up. Automakers long ago realized that there were two solutions to the problem: using two coils (one for low rpm and one for high) or an ignition resistor. Obviously, the resistor approach is the least expensive and most reliable, so that's what they did. The resistor used varies is resistance as a function of temperature, and limits the voltage to the coil accordingly. As the engine revs up the resistance lowers, allowing more voltage to the coil for fast running, and the reverse happens when the engine slows down. At idle, for instance, only about 7 volts is going through the coil primary windings.
The only time the resistor is out of the circuit is during startup, when the engine needs all the spark it can get. It's bypassed in the ignition switch's start position so that, during starting, the coil gets full battery voltage. Ignition resistors can take many forms, depending upon the manufacturer of the vehicle. Some builders mounted a big resistor on the firewall and some others utilized a special type of wire (resistance wire) running from the ignition switch to the coil. Still others used coils that were built with an internal resistor. None of these is any better an approach than the others, but it's important to know which type you have, and that you have one!
The secondary ignition system consists of the coil secondary windings, distributor cap, rotor, plug wires and spark plugs.
So just how does a coil work? Well, the principle of Inductance is the answer. Physics tells us that if you put a certain voltage through a wire (the primary) that has another wire wrapped around it, the second (hence, secondary) wire will receive an "induced" voltage from the first. Furthermore, the "induced" voltage is a function of the number of turns of wire wrapped around, so if you have two coils wrapped around the wire you'll get twice the voltage, and so on. Voltage can be stepped-up and stepped-down using inductance. Transformers are inductance devices, so a coil is a transformer.
Automotive coils generally have secondary-to-primary ratios of 200 to 1. Therefore, a 12-volt input to a coil's primary windings will result in a 24,000-volt output from the secondary winding. That's where the spark plugs get their electricity.
Inductance isn't perpetual motion, nor is it "free energy." There are many "howevers" and other considerations to worry about. The biggest one is the coil's inability to hold the induced voltage once it's been built up. In a very short time the voltage will "bleed of," leading to weak spark. Also, the coil takes a finite amount of time to build the charge up. That's the dwell time, normally defined as the degrees of rotation of the camshaft during which the points are closed. Too little dwell and the coil doesn't have time to charge up fully. Too much dwell and the coil has bled off some charge, causing a weak spark. Hesitation, low power, misfiring, pinging and a number of other conditions are symptoms of incorrect dwell.
Important Note: Since dwell is measured by camshaft rotation and the camshaft runs at one-half the speed of the crankshaft, for every two degrees of dwell an ignition is off, the engine's timing will be off one degree! If an engine needs to be re-timed when it's periodically checked, the points have worn down (thereby increasing dwell). The timing chain hasn't slipped, as so many believe.
Dwell angle has always been set by properly adjusting the ignition point gap. Your car's gap was derived by engineers to approximate the dwell angle, but individual point sets can vary considerably in their mechanical and electrical characteristics. The only way to properly set up ignition points is with a dwell meter.
The distributor cap is one of the most appropriately-named devices on the car. It's job is to distribute the high voltage pulses [generated by the coil] to the correct spark plug at the correct time. It does this through the rotor. The rotor is keyed to the distributor shaft. On the rotor is a spring-loaded "wiper" arm, whose purpose is to pick up high voltage pulses from the coil. The wiper arm is electrically connected to the rotor's tip.
Inside the distributor cap are metal nipples that are attached to the sockets holding the plug wires. As the rotor moves around its tip comes within about one millimeter of the cap's nipples, whereupon the high voltage charge jumps over. From there it travels through the plug wire to the plug, which is grounded to the engine block. The charge has nowhere else to go but to the plug's electrode, creating a spark.
Spark plug wires are greatly underappreciated and often overlooked when it comes to maintenance. They are designed to carry 20,000 to 40,000 volts (much more in modern cars) to the sparkplugs without losing the charge, breaking down electrically or "leaking" to ground. They operate in extreme heat and vibration environments.
Plug wires originally were constructed with a central copper conductor, wrapped in various layers of insulation. This was very effective, but when AM radios appeared they caused interference (high voltage creates large electromagnetic fields that in turn cause spurious radio signals. These are picked up by radio sets as static). By the 1950s many manufacturers turned to resistance wires to cure the interference problem.
Resistance wires utilize a central core made up of some flexible material impregnated with a conducting medium, usually a form of carbon, wrapped in insulation. These wires have specific internal resistance that is designed to provide proper spark with minimal electromagnetic interference. Such wires are easily damaged, especially at the ends, where the inner cores are connected to metal "boots."
Resistance wires have a finite lifetime and must be replaced after a specific number of miles or operating hours. Solid-core wires also must be replaced when the insulation becomes cracked or stiff.
It never fails. What seems the simplest of things often turns out to be the most complex, and that's true in the case of spark plugs. Looking at a typical spark plug, let's define its various parts:
The terminal is the top of the plug where the wire connects. Under that is a ceramic (insulator) section with ribs molded onto it to reduce flashover. Under that is the crimp, where the metal body begins. Below the crimp are the wrench flats, a hexagonal area that is sized for a specific wrench. Under that is the shell, which is threaded at the bottom to the size (diameter) and reach (depth) of the threaded hole in the engine's cylinder head. The plug ends at the bottom, where there is a ground strap or other device protruding over the central metal core, the electrode. Surrounding the electrode is ceramic insulation to keep it from sparking into the inside of the metal shell rather than at the end ground.
It doesn't end there. Spark plugs are designed with specific heat ranges. That is, the amount of the central insulator/electrode exposed to the heat of combustion. The deeper the electrode/insulator (and the ground piece, of course, extend into the combustion chamber, the "hotter" the plug and the less it extends, the "colder" the plug. Manufacturers specify certain heat ranges for certain conditions, even within the same engine designs.
Plugs also come in Types. "Type" indicates whether the plug's core is a resistance-type (similar in design to resistance wires) or solid-metal core, projected core nose, and single or multiple ground electrode. Depending upon the engine design, plugs may be specified to require a metal gasket between the shell and the threads.
Spark plugs vary tremendously, so it pays to study what your engine requires. Plug manufacturers publish extensive applications manuals that clearly spell out design differences and, most importantly, which plugs will work efficiently in your engine application.
How well your engine runs is totally dependent upon how well the primary and secondary ignition system components work, and whether they are matched to your engine. Of course, everything depends upon how closely timed the distributor is to the mechanical components of the engine, but that's the subject of another article, How To Tune Your Car.
We conclude this series with Part 5, which will discuss automotive relays and fuses and their importance in the overall electrical system.
Classic Car Automotive Electrical Systems - Part 1: Basic Automotive Electrical Theory
Classic Car Automotive Electrical Systems - Part 2: How Generators and Alternators Work
Classic Car Automotive Electrical Systems - Part 3: How Voltage Regulators Work
Classic Car Automotive Electrical Systems - Part 4: How Automotive Starters Work
Classic Car Automotive Electrical Systems - Part 6: How Automotive Relays and Fuses Work