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Re: Direct Ignition Cassette: Layman's Function Pls. 97 Aer
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 22 Feb 2005 06:03:46 Share Post by Email
In Reply to: Direct Ignition Cassette: Layman's Function Pls. 97 Aer, Brian Lafazanos, Mon, 21 Feb 2005 16:46:20
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The DI replaces the distributor and single ignition coil of the old days. Inside the cassette there is circuitry and four small coils, one sitting over each spark plug. There is also a control box right next to the fuel control computer, behind the false firewall/at the base of the windshield.

The DI system controls the spark for each cylinder individually. This allows very precise timing, which reduces emissions and improves performance. It can even retard the timing in a single cylinder if that cylinder is prone to knock. It also allows very high spark plug voltages (no wires) for better combustion.

The DI also has some other features. It has a feature where it can supply multiple sparks when starting, and also to clean fouled plugs. A neat one is a knock sensor. A spark is normally 40Kv or so. The DI applies 80 volts, not enough to spark, to the electrodes when not sparking. It uses this voltage to measure the conditions inside the cylinder. It can detect the conditions of pre-ignition (knock) and change the timing/fuel/boost to avoid knock, on a cylinder-by-cylinder basis. Pretty neat. It's also used to determine engine timing along with the crank position sensor.

It's critical to use the exact right NGK plugs, because the plugs are part of the sensing system inside the cylinder. Gap is also critial - too large a gap can damage the coils.

Most other cars have gone to some form of Direct Ignition, because it's just about impossible to meet emissions without it. It also provides lots of advantages. But sitting on top of an engine, with all that heat and vibration, is hard on electronics. This is a common failure item on lots of cars. About 18 months ago VW had a massive recall on a couple of years worth of DI systems.

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