I vaguely understand what it means - in that two spark plugs are fired at once, one of which is on the exhaust stroke so it's "wasted". My question is what's the purpose? And does effectively halve the life of the plugs?
One of the reasons is you only need 2 coil drivers, the coils have longer to recover (because they`re only firing 1/2 as much as a single coil) so RPM can be a lot higher and you`ll also get a better spark. It`s better than a single coil, but not as complicated/ expensive as seqential ignition (4 coils and drivers or 4 cop`s). The plugs will wear out twice as fast I would have tough.
I don't think the plugs do wear out considerably faster. There's virtually no spark energy in the 'wasted' spark, so it shouldn't do anywhere near as much damage to the plug as the 'proper' spark.
Are you sure? from how I`ve seen it working on motorbikes the plugs both get hit with the same voltage at the same time, or is it because there`s no combustible material in the cylinder the wasted plug doesn`t get as much hammer? Most bikes get plug changes at 16k miles.
It's all way over my head, but apparently the exhaust gases in the chamber during the wasted spark are ionised (due to the very high temperature) and so present a virtual dead short across the plug. This means that the vast majority of the coils energy goes into sparking the other plug. Apparently
Depending on the style of coils, for example 2CV, one plug is the anode (+ve) and one the cathode (-ve) and they are both linked into the same HT circuit - one wears out the tip, and one the core. The spark only has to jump 1 gap as the exhaust is ionised in the other gap, as already mentioned, so energy isn't much higher than a single coil. The trick to make plugs last in 2CV's is to keep sw*pping them over so they wear evenly, apparently. I guess this could be used in bike engines too, but considering the price of bike spark plugs I reckon the dealers are onto a good thing for themselves changing plugs every 12-16000miles! Bikes tend to run hard plugs that foul pretty easily anyway and track down the core rather than wear out. Wasted spark is usually used to eliminate the cam sensor - extra cost and usually an expensive hall effect type sensor. If you don't need to run sequential injection for emissions then it's a much simpler (cheaper) ECU!
Bike plugs normally wear evenly between cylinders, modern wasted spark systems actually have 2 coils inside one coil module which gets fired buy the ecu (well on bikes anyway). It looks like a single coil but there`s 2 cores inside.
