Eratic dwell

[modman]

Microsquirt 3 on a 4cyl waste spark setup, single crank wheel missing tooth, no cam sensor.
Injector pulses seem fine, but the dwell fluctuates rapidly between .5 and 8 ms while cranking. As soon as the rpm goes over 300, the dwell drops to 0.5ms and spark is lost. I have adjusted the voltage compensation to recommended points (megamanual) Accel compensation = 0, Max duration = 2.
When changing the WUE threshold to 1000RPM, the problem follows.
Any ideas?

[Bernard Fife]

modman,

Could you post a datalog and MSQ?

In addition to the battery voltage compensation, the dwell is also increased during cranking because the controller has a harder time predicting the crank position between tach pulses. This is because while cranking:
- there haven't been many crank signal to establish a trend,
- there is not much angular momentum in the rotating assembly at the low speeds of cranking, so the compression strokes affect the crank speed quite a lot,
- we want to be sure to start, so it is important to make sure all the ignition events actually produce a healthy spark.

Remember we have to start the dwell quite a bit before the spark, and guessing when the spark will need to be is most difficult while cranking. So the code tends to start the dwell earlier rather than later.

The increased dwell will not over heat the coil at cranking speeds. The duty cycle is still very low, and it is is the duty cycle (the overall percentage of time the coil is charging) that is important, not individual dwell event duration.

For example, if the dwell reaches it's upper limit at 5000 rpm (and at higher speeds there's not enough time for a full dwell), then at 250 cranking rpm the dwell may be doubled, but the time between sparks rises by 20 times, so the dwell is actual just 10% of the duty cycle it is when running at the max.

For the running dwell, perhaps the voltage compensations for dwell are not right. If this was mine, I would try setting them all to zero to see if that improves things ( http://www.megamanual.com/mt29.htm#iw ).

Lance.

[modman]

Hi Lance, Thanks for the reply. In an effort to sort this issue we have made a crank trigger simulator using the original crank wheel and pick up , but driven by an electric motor. In this way we can "crank and run" the system up to about 3000r/min without continuously operating the starter motor. This will reduce the fluctuations in speed and battery voltage. We have set all voltage compensations to zero, but to no avail. The unit produces a healthy spark under cranking r/min, but when our preset value of 300 r/min is reached, injection continues (back to single pulsing) but ignition ceases, as the dwell reduces to virtually zero. It is this unstable reducing dwell that is causing our issues. The ignition system is firing the spark plugs external to the engine at present, also so ignition events can be observed. We will look at the data logging as soon as we can return to this project. Any other ideas will be appreciated,

Regards Don (aka Modman)

[grippo]

Is your setting for cranking rpm = 300 ? If so, what happens when you set this to say 600. Does spark continue to 600 or do you still lose it at 300 ?

Usually what happens when spark, but not fuel, is lost above a certain rpm is that the timing is so far off that it tries to spark beyond 2 cylinder cycles (360 crank deg in your case), because the logic, by design, won't allow this since it only has 2 ignition outputs, so only allows 2 cylinder cycles to set up, dwell and spark. The problem shows up above a certain rpm because the window to dwell and spark shrinks at higher rpm. This is not a limitation - if timing is off that far you don't want it to spark. So you can try adjusting the cylinder offset and see if this allows you to rev higher. Try say 30 deg each way. The other thing you can do is put a timing light on the engine and see if you can read the timing during cranking.

If none of this tells us anything, we would need to see your msq.