The ignition switch is a part of the car with which everyone is familiar. We all know that when you put the key in the ignition switch and turn it all the way forward the car starts. The term “ignition switch” actually refers to something very specific, that in many ways doesn’t apply to modern vehicles any more.
The ignition system is used to ignite the air/fuel mixture in the combustion chamber. When you turn the ignition switch on you are turning on the flow of electricity to the ignition system enabling it to work. The secondary function of the ignition switch is connecting power to the starter motor. At least this is what used to happen. The ignition switch was actually a switch.
In the old days (anytime before the 1980’s) the fuel system on the car had no electrical controls. Turning on the ignition switch, and getting the starter cranking were the only things that needed to be switched on. The carburetor that controlled the fuel began working when air started flowing through it as the engine was cranking. On modern cars and trucks it’s not exactly that simple. What a surprise.
Once computers became common (1980’s) the ignition switch was still sending power to the ignition system anytime you would turn on the key, but it would also send power to the computers that were in charge of things such as engine and transmission function. The ignition switch was still literally connecting power to all of these systems, or at least connecting power to the relays that would power up these systems.
Many cars today don’t even have a place for a traditional key. Smart key systems today use a transponder in a fob that first gets noticed by the vehicle computers as you approach. The engine still has an ignition system because it still must have a way to ignite the air fuel mixture, but the ignition system is completely computer controlled. Even the modern cars that still use a traditional key, do not actually use the key to turn on a switch.
When you turn on the “ignition switch” you activate a computer that controls the engine. The switch is not actually connecting power; it works more like a sensor that sends a signal to the engine control computer telling it that the driver is requesting engine start, or requesting accessory mode, or whatever.
This engine control computer will activate the starter, turn on the fuel pump and the fuel injectors, and fire up the ignition system to start firing the coils on the spark plugs. Old cars had only one ignition coil and a distributor. The coil produced the spark, and the distributor did exactly what the name implies. A rotor inside the distributor spun around taking the spark from the coil and shooting it to the proper cylinder at the proper time. The spark would leave the distributor through a wire that would carry it to the spark plug mounted in the top of the cylinder.
While modern cars are computer controlled, the ignition system is much simpler in its function. Instead of the coil, distributor, and spark plug wires, modern ignition systems have a separate coil on top of each spark plug. The computer has control of each coil individually, and it knows the position of each piston as soon as the engine starts to turn. When the time is right the computer simply fires the coil directly into the spark plug. This is referred to as coil-on-plug ignition.
This is a fantastic design because we have eliminated all of the moving parts and most of the wear items from the old style ignition systems. Because of this, the old tune up that used to be a common service on all cars and trucks has been mostly eliminated. The only items left in the ignition system that require regular service are the spark plugs. Modern technology has even given us spark plugs that only require service every 100k miles or so.
With no distributor this means there is is no distributor adjustment needed. During the decades and decades that such a device was in service, the exact position of the distributor had a major affect on the way the engine ran because the timing of the spark at each coil was determined by the position of the distributor. The engine position sensor that would determine when each spark should fire was nearly always located in the distributor. This sensor is watching the rotation of the distributor shaft which must correlate correctly with piston position.
Rotating the distributor in its mounting changes the position of the sensor. In ancient times (before the 1970’s, sorry old people), we used mechanical mechanisms to trigger the coil. These systems used a set of contact points to ultimately control the coil. They were terribly unreliable, and if not kept in proper adjustment caused the vehicle to run terribly. These mechanical devices still tracked rotation of the distributor shaft and so adjustment of the distributor, or ignition timing, still worked the same way. Having this timing variation inherent in every ignition system meant that regular adjustment was necessary to keep the ignition event as precise as possible.
In modern vehicles (everything since the late 1990’s) the computer has been more fully and completely in charge of when each coil fires, and the timing of each ignition event is very precise. The point at which the air fuel mixture must be ignited changes constantly because the engine is always operating under variable conditions. Sometimes it turns quickly, sometimes it turns slowly, sometimes it operates under heavy load, and sometimes it operates under no load.
No matter how the engine operates, the time it takes to burn the air fuel mixtures doesn’t change much, and the piston must be in the same place on the power stroke once the fuel is finished burning. This ultimately means we need to begin burning the fuel sooner, or later, depending on conditions. With a computer controlling individual coils this timing can be nearly exact for every combustion event. This means the ignition system is able to get every bit of power out of the fuel. Since very little fuel is wasted, the car will use less fuel overall, and it will produce less harmful emissions.
The ignition system has changed a lot over the years but fundamentally it’s still doing the same thing, with many of the same components. Someday when engineers perfect the gasoline compression ignition, or even laser ignition systems that they currently have in research and development, we will see a more significant change to the way we ignite the fuel in our engines. That will be a topic for another column.