Dual spark plug question...

[Sir James]

I'm just trying to figure out exactly how this engine works. I'm familiar with Internal Combustion engines, but not motors with 2 spark plugs per cylinder. I would think that the second plug is just an extra spark to make sure the fuel vapor is all burned up but, it doesn't seem like that's the case. If the 2nd plug is bad or not firing you can definitely feel it. Why is that? What is the "job" of the secondary plug, does it fire during a different part of the stroke or something? I just can't figure out why that second plug not firing would make such a difference??
(2000 RSV Mille)

[BrummyMax]

From the what I have heard, the design of the original Rotax heads was not the most efficient. The fuel and air will only completely burn with two simultaneous sparks. When the engine was updated in 2004, the combustion chambers were redesigned. This negated the effect of the the second spark plug. Apparently.

Now that doesnt exactly ring right with me. Why couldnt rotax get it right first time, especially if it meant having to have a second spark? Anyway, a number of things about this engine are odd. The number of temp sensors for one, but dont get me started.

[DanV990]

The reason you feel the difference when one plug in the dual plug head is not working is due to the difference in spark advance. The dual plug design uses a significantly different spark advance than a single plug design does and when one plug is not working the advance is off. Because the dual spark heads start the flame front in two locations less advance is required to create a complete fuel burn compared to a single plug design. If one plug is not firing this effectively is the same as retarding the ignition and the fuel charge does not get completely burned in the time available, reducing power.

[Sir James]

The reason you feel the difference when one plug in the dual plug head is not working is due to the difference in spark advance. The dual plug design uses a significantly different spark advance than a single plug design does and when one plug is not working the advance is off. Because the dual spark heads start the flame front in two locations less advance is required to create a complete fuel burn compared to a single plug design. If one plug is not firing this effectively is the same as retarding the ignition and the fuel charge does not get completely burned in the time available, reducing power.

That's exactly what it felt like, felt like the ignition was way retarded! I had very little power under 5K rpms. I cruised on the freeway in 4th because 5th and 6th gear had no power at 79mph. When I throttled it in 6th, it would just bog, 5th went no where fast... The front spark plug boot with the straight plug wasn't all the way on.

[SoulDaddy]

James, how did it run in the lower revs at a constant throttle setting? Were the rpms constant or did it hunt? Did you get any pops/misses/backfires on trailing throttle?

And you missed the boat last Sunday!

[Matt fe2o3]

A while back someone in caponords posted an electronic overlay graph using an oscilloscope, dammed if I can find it however the spark was apparently shown to be timed exactly....so if I read correctly the engine ignition is designed for simultaneous ignition in two a read of the head and one failing then has the same effect as changing the timing?

Interesting anyway, i'll get out the tub of popcorn and learn something here.

Cheers.

[DanV990]

For a given design two simultaneously firing sparks plugs will start the charge burning in two locations and it will burn more quickly than with a single spark plug. Since the fuel charge burns more quickly with two plugs than one, less advance is required. Another way of looking at it is two plugs effectively create more advance for a given design. When one plug is not working you lose that effective advance. The actual timing of the spark does not change, but having only one plug operating in a combustion chamber designed and timed for two has the same effect as retarding the timing.

[Micah / AF1 Racing]

Temperature control I think was the real reason for the dual plugs early on inthe design life. Say this allows for 10 degrees less advance in most of the rev range, well that is an automatic 20% or so reduction in the amount of time per combustion event for the fire to heat the metal. Disconnect a side plug and you may not even notice it, a center plug is a different beast though. Chamber efficiency on the early dual spark V990's was fairly poor and the second plug helped a lot with HC reduction by making sure the least turbulent portion of the chamber gasses got lit at the same time as the most turbelent portion of the chamber gasses. Hope this helps more than hinders.

[Matt fe2o3]

Cool, thanks all. Micah really nice description.

[FORZA]

Emissions was the reason behind it, spoke with Rotax engineers at the factory when the '04 came out & they said why.
The extra squish pads in the later chambers negated the need for twin plugs.

Dave Ward
Moto Italia
Aprilia-Ducati-Moto Guzzi

www.motoitalia.com.au

0432 441 985