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DI ignition introduction
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Posted by Dean (more from Dean) on Wed, 28 Nov 2001 17:30:54 Share Post by Email
In Reply to: What does direct ignition do that non-direct doesn't?nm, Adrian, Wed, 28 Nov 2001 16:05:26
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In general, there is one coil per plug or for two plugs as in the Quad-4 GM engine. The coil snaps onto the plug. There is no rotor, no distributor cap and no cables.

As implemented by Saab, the DI is much more complex then that. It generates and fires the primaries of the coils with 400 volts DC pulses. It monitors the ionization resistance of the plugs to detect conditions condusive to knocking. There is no descrete knock sensor. The plugs do it all. The same effect is also used to detect top dead center so that the ECU can figure out the relationship between the crank and cam shafts. Yep, there is no crankshaft sensor. Aquiring this at startup time is one of the reasons that these engines do not startup as fast as one would hope for. The DI allows for sensing of a mis fire, and the ECU can at lower RPM's fire a second spark to ignite the fuel air mixture.

Because of the above, the DI is often not happy when someone puts in some trick plug. Bosch plugs are known to foul up and get black causing severe missing and loss of power. Stick with ngk bcpr6es11's gapped to .039".

There are probably other things that don't come to mind right away.

The ECU does lots of these things. The ECU can vary the timing and mixture in each cylinder to some extent as well. But that is straying away from the DI somewhat.

Why do these things? For one, the denser fuel air charge in turbo engines is a stronger insulstor that for lower pressures. So when on boost, the breakdown voltage in the spark gap is much higher and conventional ignition systems are simply not adequate. Yes you can get stronger conventional ignition systems, but then you will get more problems with leaky and damp cables etc at the higher voltages. This is why you will often see the plug spark gaps spec'd smaller on turbo vatiants of an engine compared to the normall apserated (NA) variants of an engine.

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