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From page 18-2-47 of my 1999-2002 workshop manual:
"When the ECM triggers an ignition coil to spark, current from the coil travels to one spark plug, then jumps the gap at the spark plug electrodes, igniting the mixture in the cylinder in the process".... but then...
"Current continues to travel along the earth path (via the cylinder head) to the spark plug negative electrode at the cylinder that is on the exhaust stroke. The current jumps across the spark plug electrodes and back to the coil completing the circuit. Since it has simultaneously sparked in a cylinder that is on the exhaust stroke, it has not provided an ignition source there and is consequently termed 'wasted'."
Q. How does it know this?
1-8-4-3-6-5-7-2. Cylinders 5 and 7 are next to each other. 7 should be on the compression stroke when 5 fires. It should go to 3, but what if it goes to 7 instead?
Many earlier engines, mostly magneto or pre-electronic ignition, ran on 'wasted spark' principles. A lot of petrol engines in multi-cylinder cars suffer from arcing in the distributor caps, coil packs, ignition cables etc and even via the earth paths on the cylinder heads and spark plugs. HT spark generation created for spark plugs does some strange antics during ignition cycles albeit it rarely causes noticeable problems during normal running unless under severe pressure, i.e. going up steep hills etc.
Q. How does it know this?
1-8-4-3-6-5-7-2. Cylinders 5 and 7 are next to each other. 7 should be on the compression stroke when 5 fires. It should go to 3, but what if it goes to 7 instead?
Interesting question. The RAVE does not say this, but ...
Never mind. RacerX nailed it two posts after this one.
The waste spark principle carries over to fuel injected vehicles, actually.
The gist is that two spark plugs will share a coil, and when the coil is energized both plugs will fire. The spark plugs firing into an exhaust stroke requires about 3kv to fire, while the plug firing into the compression stroke requires about 12kv to fire.
Just about every distributorless ignition system leading up to the implementation of Coil on Plug has an waste spark ignition system, including the D II.
I looked up the coil in my workshop manual. That description was poorly written. It does fire 2 cyls at once, when one is on power cycle the other is on exhaust cycle and vice versa. Economical I guess.
The waste spark principle carries over to fuel injected vehicles, actually.
The gist is that two spark plugs will share a coil, and when the coil is energized both plugs will fire. The spark plugs firing into an exhaust stroke requires about 3kv to fire, while the plug firing into the compression stroke requires about 12kv to fire.
Just about every distributorless ignition system leading up to the implementation of Coil on Plug has an waste spark ignition system, including the D II.
Thanks Hamal T, a good diagrammatic depiction of the wasted spark principle.
Wasted spark principle
