Why do they call the key (or push to start button) the ignition? It isn't setting anything on fire, is it? Turning the key or pushing the button energizes your car’s ignition system, which produces the sparks used to ignite the air/fuel mixture. (Diesel engines don't need spark plugs or an ignition system because the fuel ignites itself due to heat at the right moment.)
The ignition coil (or coils) transforms the 12 volt electricity from the battery into high-voltage electricity, which is sent along the ignition wires to the spark plugs. On more modern cars, the coil packs are mounted directly to the top of the plugs and fired via computer, not mechanical distributor. The spark plugs are screwed into the cylinder head, and produce sparks inside the combustion chambers to make the air fuel mixture explode on cue. With this overview in mind, let’s delve a little deeper into the subject…
The exact instant at which the air/fuel mixture inside each cylinder is ignited by the spark plug is critical, and has an important effect on how the engine performs. The timing of the spark is known as ignition timing, and this can be controlled very accurately by the engine management system or less accurately on older cars by mechanical, vacuum, and electronic means.
On most modern systems the ignition timing is varied constantly, and the timing for each of the engine’s cylinders is controlled individually, which helps to maximize the efficiency of the engine. Most modern cars utilize a crank position sensor, or a cam position sensor, which allow the engine control module (ECM) computer to know exactly where each piston is in its four strokes up and down its cylinder.
Distributor-based ignition systems
Until the introduction of electronic engine management systems, a distributor was the heart of the ignition system, switching the power to the coil, adjusting the ignition timing, and distributing the high voltage from the coil to the spark plugs. Nowadays, distributors are seldom used, and if they are their sole job is to distribute voltage to the spark plug wires.
A "rotor" inside the distributor is driven by the camshaft or jackshaft at half engine speed, and rotates inside the distributor cap. The center terminal (usually) on the distributor cap sends power from the coil to the rotor, which spins past a series of contacts – one for each of the engine’s spark plugs.
Precisely as the rotor passes each contact in the distributor cap, a pulse of electricity from the ignition coil jumps the tiny gap between the rotor and the contact (they don’t actually touch) and then passes down the ignition wire to the appropriate spark plug.
Direct ignition systems
A direct ignition system (sometimes known as DIS) does not have a distributor. The ignition wires run directly from the coil or coils to the spark plugs and the ignition timing is controlled by the engine management system. Some direct systems have one coil per spark plug, and some have a coil with multiple outputs on it.
Most recent engines actually have a separate coil for each cylinder, often fitted directly above the spark plugs, which means that long high voltage ignition wires aren’t needed.
Spark plugs are fitted to all gasoline engines (and propane, LPG, or ethanol), and their job is to ignite the air/fuel mixture in the cylinders at the correct instant. The spark ignites the explosive air/fuel mixture, which expands, pushing the piston down the cylinder. This is the heart of the internal internal combustion engine.
When the ignition system sends voltage down to the spark plug, it causes a spark to jump between the spark plug center electrode and the side electrode to ground. The "spark plug gap" is the space between the the center and side electrodes. The size of the gap is very important, because the larger it is the higher the voltage has to be to make the spark jump. A larger gap typically promotes better performance by exposing more air/fuel mixture to the spark, but larger gap plugs are more likely to foul due to fuel or oil contamination.
Some spark plugs have more than one ground electrode, in an attempt to give the spark better access to the explosive mixture.