Ignition System Basics
The ignition system on your car has to work in perfect concert with the rest of the engine. The goal is to ignite the fuel at exactly the right time so that the expanding gases can do the maximum amount of work. If the ignition system fires at the wrong time, power will fall and gas consumption and emissions can increase. When the fuel/air mixture in the cylinder burns, the temperature rises and the fuel is converted to exhaust gas. This transformation causes the pressure in the cylinder to increase dramatically and forces the piston down.
In order to get the most torque and power from the engine, the goal is to maximize the pressure in the cylinder during the power stroke . Maximizing pressure will also produce the best engine efficiency, which translates directly into better mileage. The timing of the spark is critical to success.
There is a small delay from the time of the spark to the time when the fuel/air mixture is all burning and the pressure in the cylinder reaches its maximum. If the spark occurs right when the piston reaches the top of the compression stroke, the piston will have already moved down part of the way into its power stroke before the gases in the cylinder have reached their highest pressures.
To make the best use of the fuel, the spark should occur before the piston reaches the top of the compression stroke , so by the time the piston starts down into its power stroke the pressures are high enough to start producing useful work.
Work = Force X Distance
In a cylinder:
TIMING
The timing of the spark is important, and the timing can either be advanced or retarded depending on conditions.
The time that the fuel takes to burn is roughly constant. But the speed of the pistons increases as the engine speed increases. This means that the faster the engine goes, the earlier the spark has to occur. This is called spark advance : The faster the engine speed, the more advance is required.
SPARK PLUG
The spark plug is quite simple in theory: It forces electricity to arc across a gap, just like a bolt of lightning. The electricity must be at a very high voltage in order to travel across the gap and create a good spark. Voltage at the spark plug can be anywhere from 40,000 to 100,000 volts.
The spark plug must have an insulated passageway for this high voltage to travel down to the electrode, where it can jump the gap and, from there, be conducted into the engine block and grounded. The plug also has to withstand the extreme heat and pressure inside the cylinder, and must be designed so that deposits from fuel additives do not build up on the plug.
Spark plugs use a ceramic insert to isolate the high voltage at the electrode, ensuring that the spark happens at the tip of the electrode and not anywhere else on the plug; this insert does double-duty by helping to burn off deposits. Ceramic is a fairly poor heat conductor, so the material gets quite hot during operation. This heat helps to burn off deposits from the electrode.
COIL
The coil is the device that generates the high voltages required to create a spark. It is a simple device -- essentially a high-voltage transformer made up of two coils of wire. One coil of wire is called the primary coil . Wrapped around it is the secondary coil . The secondary coil normally has hundreds of times more turns of wire than the primary coil.
Current flows from the battery through the primary winding of the coil. The primary coil's current can be suddenly disrupted by the breaker points , or by a solid-state device in an electronic ignition. The coil looks like an electromagnetic, but it is also an inductor. The key to the coil's operation is what happens when the circuit is suddenly broken by the points. The magnetic field of the primary coil collapses rapidly. The secondary coil is engulfed by a powerful and changing magnetic field. This field induces a current in the coil, which is a very high voltage current (up to 100,000 volts) because of the number of coils in the secondary winding. The secondary coil feeds this voltage to the distributor via a very well insulated, high-voltage wire.
DISTRIBUTOR
The distributor handles several jobs. Its first job is to distribute the high voltage from the coil to the correct cylinder. This is done by the cap and rotor . The coil is connected to the rotor, which spins inside the cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder. Older distributors with breaker points have another section in the bottom half of the distributor -- this section does the job of breaking the current to the coil. The ground side of the coil is connected to the breaker points.
