Multispark / Wasted Spark - ignition timing questions

[Prof]

Couple of questions on the stock Ignition setup.
 
Question relates to FC\'s, but I guess is applicable to any 2 rotor.

The leading coil drives both leading plugs, so it\'s wasted spark. If you have a Multispark ignition, how many degrees of rotation from the initial firing point are you allowed to fire mulitsparks before the wasted spark plug is in the \'wrong\' chamber.

It might help if I explain this a little - when the coil fires, one rotor face is up against the plugs and this is the main spark event, but there is also a wasted spark event in the exhaust cycle on the other rotor. Looking at the way these things rotate around, there doesn\'t seem to be much rotation before the plug carrying the wasted spark is actually in the chamber doing the \'compression\' cycle, and so you wouldn\'t want to fire it.

So if I have a unit that\'s doing multiple sparks, how many sparks or how much time (would be a better quesion) would I have before that other plug is in the wrong cycle and shouldn\'t be fired ?

The 2nd question is kind of related to the first:

Is multispark a bit of a waste of time on the leading ignition, and better kept for the trailing\'s, as these will follow the leading in any case, and the same effect could I guess be achieved by firing the trailing plug multiple times, and that would also avoid the \'wrong firing\' scenario I described above ?

The other option would appear to be the replacement of the one leading coil pack with two coils, and with some additional logic, use the ignition select signal and fire them alternately like the trailings.

[bnaellis]

Couple of questions on the stock Ignition setup.
 
Question relates to FC\'s, but I guess is applicable to any 2 rotor.

The leading coil drives both leading plugs, so it\'s wasted spark. If you have a Multispark ignition, how many degrees of rotation from the initial firing point are you allowed to fire mulitsparks before the wasted spark plug is in the \'wrong\' chamber.

It might help if I explain this a little - when the coil fires, one rotor face is up against the plugs and this is the main spark event, but there is also a wasted spark event in the exhaust cycle on the other rotor. Looking at the way these things rotate around, there doesn\'t seem to be much rotation before the plug carrying the wasted spark is actually in the chamber doing the \'compression\' cycle, and so you wouldn\'t want to fire it.

So if I have a unit that\'s doing multiple sparks, how many sparks or how much time (would be a better quesion) would I have before that other plug is in the wrong cycle and shouldn\'t be fired ?

The 2nd question is kind of related to the first:

Is multispark a bit of a waste of time on the leading ignition, and better kept for the trailing\'s, as these will follow the leading in any case, and the same effect could I guess be achieved by firing the trailing plug multiple times, and that would also avoid the \'wrong firing\' scenario I described above ?

The other option would appear to be the replacement of the one leading coil pack with two coils, and with some additional logic, use the ignition select signal and fire them alternately like the trailings.

Hi Mate,

I think you\'ll find that the multispark function is only up to a certain rpm typically to around 3,000 rpm or so to aid in a better idle fule economy for cruising. Once you go above a certain rpm there is no more multispark.
Check the manual of any igintion amp and they will specify what range the multispark work up to.


rdgs
Brian

[Prof]

Thanks Brian,
 
I know most if not all of the commercial units cut the multispark feature at a certain RPM - some of them vary the number of sparks depending on revs as well.
 
I\'m looking at building my own unit - based on some research and a recent article in one of the electronics mags.
 
My concern was that because the leading plugs use wasted spark, the point at which the wasted spark fires relative to the cycle is quite late in that cycle, and although the ignition is advanced with RPM up to about 4000 RPM off boost, this only buys a bit of time.
 
Looking at the diagrams of the rotor\'s as they rotate, it doesn\'t look like there is much of time gap for firing multiple sparks in the wasted spark chamber before you end up firing it early in the compression stroke - which would be a no no.
 
As I see it I could:

• Limit the number of multiple sparks based on RPM/Load to avoid firing the wasted spark in the compression stroke
• Not fire multiple sparks on the leading coils, but do it on the trailing side only, where each coil fires only in it\'s ignition cycle
• Run 2 independant leading coils and do away with wasted spark - potentially using that ignition select signal to fire the correct coil

Dave.
 
PS I was looking to see if there was any known timing data on this or whether the companies that offer these devices keep it close to their chest.

[kbareham]

There was a time when I would be in the pub by now on a Friday, but now I\'m looking at rotor housing geometry!

So as far as I can see, my map has the latest firing position on the leading plugs at 0 degrees. This puts the rotor chamber centrally about the plug centreline on the rotor I want to fire. That is half way between the two plugs. Therefore the apex seal on the other rotor must be exactly half way between the two plugs. Therefore the maximum delay on multi sparc cannot be any more than half the number of degrees (at the rotor, not e shaft) between the plugs. I\'m not sure what that is, but its not much.

So if you are going to do this, you really need to disable multi sparc in areas of the map where there is very late timing, because I don\'t think that you have time to get more that one spark in.

I am thinking of a more conventional approach. Why not run the leading plugs on a two channel capacitor discharge system? It would be best to add the logic two split the two plugs, and only fire on the appropriate cycle. That way you double the capacitor charge time available, and I know from experience that this will help prevent misfiring the plug at high RPM where you don\'t get enough charge time.

BTW, how were you thinking of identifying which leading plug to fire?


Keith

[Prof]

Hi Keith,
 
From this site:
 
http://www.teamfc3s.org/info/articles/demystifying.html
 
It looks like the front rotor trailing coil fires on the leading edge of the select signal, and the rear rotor trailing coil fires on the falling edge.
 
This means that the select signal is low when the front rotor leading should fire, and high when the rear rotor leading should fire.  By using that signal to gate the leading ignition output from the ECU it should be possible to set it up to fire only one of two coils - like the trailing.
 
It seems spark events occur every 180 crank degress (every 60 degrees rotor) Am I right into thinking that if the timing fires dead centre (no advance) then you\'ve half the time so 30 degrees to fire multiple sparks - that would be 90 degrees crank angle. (a 1/4 turn).