How The Internal Combustion Ignition System Works
Heat initiates the internal combustion process. Diesel engines utilize the temperature buildup from extremely high compression (pressure) to ignite the air/fuel mixture, with a little help from glow plugs on a cold startup. But more volatile fuels like gasoline require a spark to light the fire. This electrically produced form of heat is the basis behind an ignition system.
Ignition spark is the flow of high-voltage electric current across a sparkplug’s air gap. Producing this current is based on the theory of electromagnetic induction. Simply put, moving a magnetic field across a conductor (coil of wire) induces electricity.
The ignition coil is where it all comes together with the use of two coils of wire wrapped around an iron core. The “Primary” coil consists of fewer windings of heavier gauge wire, while the “Secondary” uses many more windings of a thinner diameter wire. Both coils are electrically insulated from each other.
The primary coil is where 12 V from the battery is applied with the ignition “On,” and grounding the other end completes the circuit. The ground is opened and closed by a transistor (or ignition points) in time with piston position. We’ll get into that later.
Resistors are used to limit the amount of current in the primary coil, while a condenser (capacitor) is used to temporarily store energy. Consider voltage as electrical pressure, while current (amps) is the volume of electrical flow. Ignition spark can reach the 50,000 V range when jumping a sparkplug gap, while the amperage is relatively low and less harsh on ignition components.
Let’s fire it up. The primary coil is energized with 12 V from the ignition and grounded by a transistor (solid-state points). The voltage builds up and the primary side becomes an electromagnet. When the circuit is quickly broken (opened) by the transistor, the magnetic field collapses and engulfs the secondary windings. The powerful application of a magnetic field induces a high-voltage spike into the secondary windings. Through a well-insulated conductor (plug wires, etc.) the high-voltage current travels from the secondary coil and jumps the sparkplug gap. This ignites the air/fuel mixture, leading to combustion, power-stroke, and so on.
A very similar ignition system was developed with the use of a magneto before roadworthy batteries and charging systems were available. The principles of inducing the voltage from the primary to the secondary coil are the same. However, the voltage source to the primary coil comes from a permanent magnet. The magnet is manually rotated across and induces electricity into the primary windings, and with a breaker, the circuit is opened. The collapsing electromagnetic field induces voltage to the secondary coil and again delivers a spark at the plug.
Magnetos were, and still are, a reliable source of ignition because they don’t require an external source of electricity—no battery required. Every time you kick-start a dirt bike or pull-start a lawn mower, you’re taking advantage of an ignition magneto.
Ignition system distributors have evolved and eventually been replaced over the years. With a distributor cap on top and insulated sparkplug wires attached to each terminal, a distributor is geared to, or driven by, the four-stroke engine’s camshaft. This allows it to distribute spark at the plugs in time with each piston approaching top dead center of its compression stroke.
Ignition points were originally used. Points are just a simple pair of electric contacts, which ride on a cam affixed to the distributor shaft. The cam has high and low points (4 on a 4 cylinder, 6 on 6, etc.). At the cam’s low points, the contacts are closed energizing the primary side of the ignition coil. Coming up on the high points the contacts open, collapse the primary magnetic field, induce voltage to the secondary windings, and the spark is created.
Most use an external ignition coil (some inside the cap). The spark travels through an insulated coil wire to the center terminal of the distributor cap. This is where the spark contacts the center of the distributor rotor. The rotor is rotating on top of the distributor shaft, and extends from the center outward, to each cylinder’s sparkplug wire terminal. This is how it delivers a spark at the correct time to each cylinder.
The distributor shaft and rotor rotate clockwise or counterclockwise, depending on the application. However, all sparkplug wires must be connected to the correct terminals on the cap in the correct firing order. On a small-block Chevy engine, the firing order is 1-8-4-3-6-5-7-2.
Electronic ignition refers to contact points being replaced with a transistor to open and close the primary coil circuit. A transistor is simply an electrically powered On/Off switch with no moving parts, located inside an ignition control module. The distributor’s cam, which opened and closed the ignition points, is replaced by a toothed (or notched) reluctor wheel, which triggers (most commonly) a Hall Effect switch or pickup coil. The reluctor’s teeth interrupt the magnetic field at the sensor and produce an electronic pulse. The big advantage was eliminating the unreliability and needed maintenance of points, along with more precisely controlling primary coil voltage and spark timing.
Distributorless Ignition System (DIS)
Today’s mass-produced cars and trucks have moved away from distributors altogether. Instead, similar to the electronic ignition’s Hall Effect switch, crankshaft and camshaft position sensors are used to determine engine speed and camshaft position. This also gives the PCM the ability to set a diagnostic trouble code reflecting a worn timing chain or jumping a tooth on a timing belt. The crankshaft position sensor can also detect engine misfires by variations in crankshaft speed.
The PCM receives all the necessary data to calculate ignition timing and controls independent ignition coils sequentially. The coil-on-plug configuration eliminates the use of high-maintenance sparkplug wires. Some coil-on-plug systems control the primary coil ground circuit with a transistor (driver) built into the PCM, while others incorporate an ignition module with each coil assembly: the PCM sends a command signal and the transistor within the ignition module opens/closes the primary circuit.
Before coil-on-plug systems were introduced a few manufacturers used coil packs. The principles of operation were basically the same, coils controlled by the PCM without a distributor, but grouped together as an assembly – and sparkplug wires were still required. These coil packs often used one coil for two cylinders that fired a waste spark during the exhaust stroke on the non-firing cylinder. Coil packs are still used with engine configurations where sparkplug location isn’t practical for coil-on-plug.
Timing is the point the sparkplug fires in relation to each piston’s position on its compression stroke. This is defined in degrees BTDC (before top dead center) or ATDC (after top dead center). Ignition timing is crucial to engine performance. Typically timing is advanced (BTDC). When the air/fuel mixture ignites, it takes a period of time for the combustion process to complete the burn and fully expand in the combustion chamber. As such, it’s important to time the ignition in advance of TDC where the combustion chamber is at its minimal size and highest pressure. This helps provide maximum force driving the piston downward on the power stroke.
Ignition timing excessively retarded (less advance) can produce a lack of power, decreased fuel economy, and high combustion temperatures. Too far advanced may cause engine knock and damage. A knock occurs when the spark is so far advanced BTDC that combustion is attempting to drive the piston backwards (down) on the compression stroke, as opposed to ATDC on the power stroke.