What starts the fire
Learn about the ignition system
Do you ever wonder how the ignition system works? Let me explain it to you…
The components of the ignition system consist of:
- Ignition coil
- Spark plugs
- Advance mechanism
The ignition system ignites the air/fuel mixture inside the cylinders, or combustion chambers. All internal combustion engines require an ignition system to work. The entire system consists of two electrical systems. One is a low voltage circuit induced by the voltage supplied by the BATTERY. The other electrical system is the high voltage circuit. This circuit provides the necessary voltage to create a spark at the SPARK PLUG.
The IGNITION COIL generates a high voltage electrical pulse of about 45,000 volts to the spark plugs. The coil is made up of two sets of wire windings, one inside the other. When the ignition switch is turned on, battery current flows from the battery to the outer, or primary winding of the ignition coil, creating a magnetic field. When the electricity is turned off, the field collapses, producing an extremely high voltage in the inner, or secondary winding. This high voltage is used to create the ignition spark at the spark plug.
The spark plug, which is common to all ignition systems, is basically a pair of electrodes, one down the middle, another attached to the sides. The electrodes’ points, which protrude into the combustion chamber, are only about 1/8 inches apart. When the coil generates a high voltage pulse, a spark jumps across the gap to the side electrode, creating the electric spark that ignites the air/fuel mixture.
The DISTRIBUTOR is the heart of the ignition system. It triggers the coil at the appropriate moments to distribute the high voltage electrical pulses to the spark plugs that the coil creates. The distributor has a shaft that runs through it. This shaft is driven by a gear on the camshaft or the camshaft itself at exactly one half engine RPM. The distributor shaft operates a pair of breaker points. These contacts open and close by a cam on the shaft. Each time the points open, the primary circuit in the coil is interrupted, a high voltage pulse is created and travels to the spark plugs through the coil wire to the center terminal of the distributor cap. In addition to the central terminal, the distributor cap has terminals for each of the engine’s cylinders. The distributor rotor on top of the shaft is always in contact with the caps central terminal. As the shaft turns, the arm on the rotor comes in contact with each of the terminals for each cylinder in turn. The spark plugs are connected to the distributor cap by heavy wires. This circuit from the coil to the spark plugs is known as the secondary ignition circuit.
The spark from the coil is generated at precise moments during the piston travel. For optimum performance and mileage, ignition should occur just before the piston reaches the top of its stroke, so that the air/fuel mixture in the combustion chamber is completely burned at the top of the piston’s compression stroke. The heat and pressure of the expanding gases in the combustion chamber force the piston downward. At higher engine speeds, the piston travels faster and ignition must occur earlier, so that combustion is complete when the piston reaches the top of its compression stroke. The ADVANCE MECHANISM alters the position of the rotor and cam on the distributor shaft causing earlier, or advanced, ignition. Most engines use both vacuum and centrifugal advance mechanisms to optimize ignition timing. Coolant temperature, air temperature, air pressure, altitude are all variables that change the timing to optimize the combustion process.
On some models, electronic ignition systems have replaced the breaker point systems. In an electronic ignition system, a toothed wheel takes the place of the distributor cam, breaker points and condenser. A stationary sensor inside the distributor senses the passage of each tooth. Electronic ignition systems require less maintenance because of fewer moving parts and are more reliable. Sophisticated electronics automatically advance the timing as necessary for varying engine temperatures and load eliminating the need for mechanical advance mechanisms.
Some vehicles are equipped with ignition and fuel injection systems controlled by a single computer, providing better fuel economy and engine performance. Also, some vehicles have no distributors at all. They have individual coils on each spark plug controlled by a computer using the various engine sensors to time the spark precisely.
…and that’s the way the ignition system works!!