In a gasoline-powered internal combustion engine, the method of triggering the timed detonations in the engine's cylinders. The ignition system typically detonates the volatile fuel/air mixture in the engine's cylinders by way of generating an electrical arc at the tip of a spark plug.
Diesel engines compress their fuel/air mixture to extremely high pressure when compared with gasoline engines. This very high pressure, combined with the heat of the operating cylinder causes the mixture to ignite spontaneously, without the aid of an ignition spark. Most diesel engines utilize a glow plug - a filament heated electrically to the glowing point - to begin combustion in a cold cylinder. Once the four stroke cycle has begun, the glow plug is shut off. Because the combustion is created spontaneously by at high pressure and temperature, a diesel engine does not require the timing control of the previously described ignition systems, and requires no electric current at all after the glow plugs have been shut off. The ability of diesel engines to run with no ongoing ignition gives rise to the term 'dieseling' which describes a gasoline powered engine running (usually sputtering) after the ignition has been switched off.
Gasoline engines use two generic types of ignition system, differentiated by their method of timing:
- Mechanical Ignition Systems
Usually found in older automobiles, a mechanical ignition system times its sparks by means of a cam, ignition points, and a rotor within the distributor. As the distributor shaft spins, driven by the mechanical action of the drive shaft, the cam alternately makes then breaks contact with the points. This action generates high-voltage spikes from the ignition coil. This current flows to the distributor rotor. Each spark plug in the engine is connected to a contact in the distributor, and as the rotor spins, it makes contact with each of these in sequence, passing the current to the appropriate spark plug. Ideally, this current creates an arc at the tip of the spark plug just after the piston reaches its highest point, and has begun its downstroke, to make most efficient use of the force generated by the exploding mixture.
- Electronic Ignition Systems
Nearly all modern cars utilize an electronic ignition, first introduced in the 1960s. The electronic ignition is very similar in principle to the mechanical ignition, governing the flow of current to the spark plugs. Where the mechanical ignition relies on high-precision moving parts to time and direct the flow of current, however, the electronic ignition makes use of solid state components to achieve the same effect, usually with greater reliability and precision.
Manual and electronic ignition systems require a source of electricity to create the arc. Ignitions of both types can be further differentiated by the way in which they derive their current:
- Battery Ignition Systems
Battery ignition systems rely on the nominal battery (usually 12-volt in an automobile) to supply power to the coil. The battery's power is constantly recharged by a generator (usually called an 'alternator') which is driven by the mechanical action of the engine. Both electronic and mechanical ignition systems can incorporate a battery.
- Magneto Ignition Systems
Magneto ignition systems differ from battery systems by powering the coil directly from a permanent magnet generator or magneto, rather than a stored battery source. The generator requires some external, mechanical action to start, often a crank or pedal of some kind. Some aircraft and motorcycles utilize magneto ignition systems. Both electronic and mechanical ignition systems can incorporate a magneto.