Proper rev limiting

[cng1]

I've been thinking about how difficult it would be to add proper rev limiter support to the megajolt. It strikes me that for a simplistic implementation the approach could be:

if (rpm > limit) advance = -10;

That on its own should give the effect of a soft cut by virtue of retarting it far enough that peak cylinder pressure is late enough in the cycle that it generates very little power. AIUI -10 is the maximum retardation that you can get the EDIS unit to supply but it's conceivable that this could be a configurable number or that you could have multiple limit points ie:

limit1 6000rpm advance = 10
limit2 6250rpm advance = 0
limit3 6500rpm advance = -10

Is something like this sensible to try implementing for the next iteration of the firmware?

[cng1]

I've had a ponder and I <b>think</b> that something like the code below may do the job.

At the top of SET_ADVANCE_DEGREES add:
ldx REV_LIMITER
jsr COMPARE_RPM
bne SOFT_CUT
beq TEST_RPM_BIN

SOFT_CUT:
lda #0
sta AdvanceDegrees
rts


Brent, if the above or something similar works please feel free to incorporate it, if it's complete rubbish and won't work feel free to say, I won't take offence - I'm not a programmer and haven't done any assembly to speak of before.

[brentp]

That would basically work. I'll work on including this in the beta firmware before it gets released.

Thanks,
Brent

[cng1]

It occured to me overnight that we could do a progressive cut, ie once the soft cut limit is hit to retard a degree for every 8rpm above the soft limit. On most engines that's going to mean that once you've hit the soft limiter you would have 200-300rpm of progressive cut.

A progressive cut would be a lot nicer to the engine but I'm not convinced that the extra complexity and user confusion (that the "limit" isn't actually quite the limit) is worth it. Technically I don't think it's that costly to do as once you've done the rpm-rev_limit arithmetic comparison it's just a case of shifting the result right 3 places to get the amount of retard.

Anyone have any thoughts?