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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:

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.

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