Two Stroke Ignition

I helped a friend build an ignition system for his 3 cyl two-stroke engine last winter. It's for a snow mobile:

We used a 6-1 trigger wheel with a Honeywell GT1 Hall sensor :


Here is the engine assembled, all machine work has been done by the owner/builder :


The only problem is starting it - our current method of triggering the ECU in combination with the pull start mechanism makes it tough!

It runs great once started, but it simply won't consistently fire the ignition while the engine has momentum from the pull start, and we suspect that this is due to the 6-1 trigger. Every pull only rotates the engine two turns which can be too little to sync the ECU quickly enough. Factory three cylinder snowmobile engines don't have this problem, of course many factory engines have electrical starters but this is not an option on most racing machines and there is very little room for it on this one since the chassis is originally made for a two cylinder engine, not three.

The stock ignition systems on these two strokes fire wasted spark, on every cylinder event, two cylinder engines do it all the time, and three cylinders at least while cranking/pulling. This is what we would like to do as well, but that will probably require a mix of changes in both the firmware and trigger system.

One plan we have come up with is this :

Fastest syncup with only 1 sectrig pulse

We don't know yet, but simply changing from a 6-1 missing tooth trigger to a 3+1 trigger could make starting significantly better. This is only speculation on my part (Mattias), my reasoning is that the ECU can fail to notice that it just passed the missing tooth, in case the crank rotation starts just before the missing tooth - resulting in a lost crank rotation before the ECU properly sees the missing tooth and starts firing the coils. With the 3+1 trigger the ECU will hopefully be more consistent and sync up properly every time.
Both 6-1 and 3+1 methods still (should) require one rotation to sync up, but 3+1 can be used to fire on each event using TableSwitch.


Other suggestions from Marcell that needs more investigation:

I will return to this page in a week or two with a triggerlog from cranking/pulling and an updated view on how easy it is to start with the most recent firmware (1.1.84+).