Ignition cuts for rev limits / traction control

Bernd Felsche bernie at innovative.iinet.net.au
Thu Jun 22 13:18:06 GMT 2000


Murphy, Ian tapped away at the keyboard with:

> Does anyone have any cunning ideas for implementing ignition cut strategies
> for rev limits and traction control purposes?

Cutting ignition is generally a bad idea because it results in
unburnt fuel going down the exhaust.

> I'm struggling with the following concern: You detect an over-rev (or
> wheelspin) event and want to cut spark ASAP, but the coil driver(s) are
> already charging the coil(s) for the next combustion event (or 2, or 3,
> depending on your system). Do you wait until the next available 'slot' when
> you can avoid charging a coil up (and therefore possibly risk over-reving
> further) or can you 'bleed down' the charging coils somehow without causing
> a spark to fire?

Retard the ignition until after TDC. Torque drops off rapidly if you
retard the timing to after the peak torque ignition point.

Retard probably isn't sufficient to stop an engine over-revving
under no-load, but it does give you time to reduce/cut the fuel 
supply which may have accumulated ahead of the inlet valve.
i.e. retarding ignition gives you a measure of control within a
cycle; fueling the control over several/many cycles.
(Fuelling can also be useful within the cycle, but the control path
is far longer for all but direct-injection.)

> The only other solution that springs to mind is retarding the already
> charging coils to limit torque output while waiting for the cut to be
> available.

Cut off the fuel first, not the spark. Make sure that the residual
fuel is burnt (perhaps giving it an extra cycle or two) before
stopping the spark. 

A millisecond of retard w.r.t. optimum will cause a significant drop
in torque (especially at high engine speeds). Retarding two
milliseconds makes more than twice the difference. If you retard too
much, you may get a negative torque which would be _unpleasant_.
Retarding even further could cause a significant amount of unburnt
mixture to expand down the exhaust largely defeating the purpose of
firing the spark in the first place.

Exact timing will depend on the physical characteristics of the
engine. e.g. bore/stroke, valve timing, fuel injection, spark plug
type and location.

> Another thing that bothers me is engine stalling: If you stall an engine,
> and have a charging coil, what do you do with the spark? If you just fire it
> off after the maximum safe dwell for your coil, the engine could be in any
> position and you could blow up your inlet manifold, or push the engine
> backwards...

You can fire the coil into an empty cylinder. Depending on the type
of ignition, you could fire it near the end of the exhaust cycle.
Much simpler is to cut the fuel, then run the spark until the engine
stops turning.

How do you get the engine stalling? It won't stall instantaneously.
You probably have at least two revolutions (an eternity for an ECU)
to cut fuel below a threshold speed.

Weigh up your ideas against what would happen with a Kettering
ignition in the worst case; the coil either fires, or doesn't.

-- 
Bernd Felsche - Innovative Reckoning
Perth, Western Australia
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