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Most recent gasoline cars have a direct ignition, also called coil on plug, system where each spark plug has its own ignition coil. Before that, cars used a common ignition coil for all cylinders.

What are the benefits of such a direct ignition system? Is it there for increased reliability or is it actually cheaper to make the high voltage path as short as possible (and thus eliminate distributor and high-voltage cable costs), offsetting the costs of the additional ignition coils needed for such a system?

I recall one incident I had on a 1989 Opel Vectra where the common high-voltage wire from the ignition coil to the distributor was slightly loose and thus was disconnected due to vibration when driving, and the car stopped. Fortunately, I immediately noticed the problem when looking at the engine. Such a failure obviously can't happen on a direct ignition system.

Is it common that such systems require special purpose tools for changing the spark plugs? I at least know that some Volkswagen engines require a special tool for removing the ignition coil so that the spark plug can be accessed below the ignition coil.

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Coil on plug has several advantages over conventional distributor type systems:

1) Less voltage loss from the coil to the plug. With fewer connections and the elimination of the distributor rotor to cap air gap.

2) Can be used in conjunction with injector control to have the ECM perform misfire diagnosis.

3) The ability to control spark timing to individual cylinders, usually based on fuel mixture variation. Leaner cylinders need earlier spark timing.

4) The ability to turn off spark to a failed cylinder to limit damage to the catalyst in certain misfire conditions. It can be used in conjunction with individual control of fuel injectors to save a the catalyst and decrease tailpipe emissions.

5) Fewer maintenance related misfire issues. No crossed wires or poor connections resulting in lower fleetwide tailpipe emissions.

  • When thinking about this carefully, isn't (3) and (4) possible with traditional system as well assuming the system is computer controlled? All you need really is a camshaft position sensor. And when thinking about this more carefully, isn't the best possibility for (3) to adjust injection amount so that there are no lean cylinders (assuming multipoint injection here), and for (4) to turn off injection (again, assuming MPI)? The wasted spark in (4) should not be a problem. – juhist Feb 5 '17 at 16:13

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