Auto questions: how automobile ignition systems work

A car's ignition system is primarily responsible for starting the engine, but it also controls some electrical systems as well.

Before you can fully understand the role a car's ignition system plays, you must understand the concept behind an internal combustion engine. An explosive fuel such as gasoline is sprayed into a series of chambers as a very fine mist. A spark plug emits a tiny burst of electricity, which causes the atomized gasoline to explode. This explosion pushes down a series of valves and pistons which ultimately turn the engine's main crankshaft. The crankshaft then provides the force for the wheels to propel the car forward through a series of gears. As long as the spark plugs are igniting the gasoline vapors, the engine will continue to provide power. Once the electricity is turned off, however, the entire process quickly stops.

A car's ignition system provides the initial mechanical energy needed to start the combustion process and keep the engine running. It does this by completing several important electrical circuits, depending on the position of the ignition switch.

The ignition switch is the device which receives your car key, usually located in the steering column or on the dashboard near the steering wheel. An ignition switch generally has 4 positions which determine which electrical systems can be turned on and which will be turned off. The first position is usually marked "Acc"- an abbreviation for 'accessories'. Your car key allows the switch to be turned to the Acc position. Certain electrical systems can be operated without need for the car's engine to be running. The Acc position allows occupants to turn on the radio or adjust power windows, for example. It helps prevent unnecessary engine idling for routine accessory functions.

The next position is 'Off'. This is always the default setting for any ignition system. Certain electrical systems not wired to the ignition system will still function, such as the headlights, power locks and interior lights, but the engine, air conditioner, radio and power windows will generally not work if the ignition is turned off. This insures that the engine will not receive any spark from the plugs and the electrical systems will not drain the battery.

The third position of an ignition switch is 'On'. This will activate all the necessary electrical systems for starting. Dashboard indicator lights will become active, computer systems and navigational equipment should work and all of the optional electronics should function. The engine will not start from the 'On' position, but the driver will be able to prepare for ignition.

The final position is the one which matters most for starting an engine. It is usually spring-loaded to prevent drivers from keeping the key in that position for too long and possibly burning out the starter. Once a driver turns the ignition switch complete over to the starting position, a number of things begin to happen.

The ignition switch completes an electrical circuit which leads to a small device called a 'starter solenoid'. It derives its power from the car's battery and causes the actual starter to turn. The starter is a small but powerful electric motor which has a special gear assembly at the end of its shaft. This gear meshes perfectly with a large gear located at the front of the engine itself. This large circular gear is called a flywheel. As the starter's gear turns the flywheel, the engine's valves and pistons also begin to move in sequence. At the same moment, the ignition switch also allows electricity from the alternator and battery to reach the spark plugs.

When the engine's fuel system becomes activated or the driver pushes slightly on the gas pedal, vaporized gas enters the chambers above the valves and the spark plugs ignite the gasoline in an order controlled by the car's distributor. This entire process usually only takes a few seconds, unless something fails between the ignition switch and the engine. Most starters can only maintain their proper cranking speed for a few seconds at a time, so a driver may inadvertently burn out the starter solenoid or the starter by holding the key down too long.

Batteries are also affected by weather conditions such as excessive cold, so they may not provide enough power to the starter for proper ignition. The starter itself may be defective or the gear may be worn out, causing a spinning sensation instead of a solid connection. The flywheel may also be damaged, preventing the starter from meshing fully with the flywheel's sprockets. If the engine is not kept properly lubricated or is driven under extreme conditions, it may seize up completely and not allow the flywheel to turn at all. Sparkplugs could fail to fire, causing the individual valves and pistons to move out of order and break under the stress.

Ignition systems in general are the beginning and ending of a car's daily existence. They provide the initial power to start the engine, the continued sparks that keep an engine running and the responsibility of bringing all that power to an end safely. The design of an ignition system also encourages security, since very little can be done without a key in the ignition switch and all the electrical circuits up and running.