1985-1998 [Subscribe to Daily Digest] |
Gord,
Here's what I can summon from memory:
All ignition systems use a coil (or coils) to generate the voltage necessary to jump a spark across the spark plug gap. This coil is actually a high-voltage transformer that takes the 12v supply from the battery and ramps it up to about 40,000 volts or more.
Conventional ignitions (at least in a Saab 9000) use a single coil that is routed to each individual spark plug by the "distributor." The distributor is a rotating device that has two primary components. The "cap" has five wires connected to it, one coming from the coil, and four wires leading to the spark plugs. The "rotor" spins inside of the cap and directs the charge from the coil to each spark plug wire. The rotor is connected to one of the cam shafts and is therefore timed to direct a charge to each plug at precicely the correct moment (that is, when the cylinder is at the top of it's "compression stroke").
Direct Ignition also uses coils to produce high voltage. However, the ignition casette contains four separate coils, one for each cylinder, positioned directly over each spark plug (this is often referred to as a "coil-over" system). The timing of the spark is controlled entirely by the ECU, which knows the position of each cylinder by reading from a separate electronic "crankshaft position sensor".
A conventional ignition system is very limited in its ability to adjust the timing and duration of the spark. Typically, the timing can be moderately advanced in proportion to engine speed and load in order to increase power and/or efficiency. However, the advance is always applied equally to all four cylinders, and each plug can only produce one spark at most per engine rotation.
Direct Ignition is completely free from any limitations in spark timing, and can direct each plug to produce multiple (even several thousand) sparks per engine rotation. DI also utilizes a "burn-off cycle" which fires a powerful burst of sparks after the engine is turned off in order to clean deposits off of the spark plug tips.
The "Trionic" engine control system takes is a step further and uses each spark plug as a combustion sensor. By measuring the ionization across the spark plug gap after each combustion, the system can monitor each individual cylinder for knock or "detonation," which occurs when fuel is inadvertently ignited during the exhaust stroke (very bad). This is especially important in turbocharged engines, where the high pressures leave little room for error. If knocking is detected, the ECU can retard the timing in an individual cylinder in order to keep the temperature below the damage threshold.
Although I'm sure there is more to be added or something that I left out, I have to go to bed now, so I'll let our fellow saabers fill in if necessary.
Hope that this was helpful.
Take Care,
-Kyle
'91 9000T
147,000 Mi
posted by 24.60.14...
, Wed, 6 Nov 2002 22:04:51
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