Ignition cuts for rev limits / traction control

[Murphy, Ian]

It's common practice in motorsport systems to cut spark but leave fuel on.
This reduces the delay in restoring torque once you remove the cut, as you
don't need to restore the fuel puddles in the inlets. Obviously you throw
unburnt fuel down the exhaust. Road car systems use a complicated
combination of retard / injection cut along with catalyst temperature models
and the like to acheive torque reduction without emission concerns.

I should have mentioned flat-shift systems in my original post as another
application of this - cut the ignition to allow a clutchless gearchange then
bang 100% torque back on ASAP. Common practice in motorcycle-engined
hillclimbers and the like.

As far as engine stalling goes, if I'm using a VRS type crank sensor, I can
only get syncronisation pulses above a certain rpm. If the engine fails to
start, I may have a charged coil and a full cylinder, but no idea where the
engine is. Has anyone any idea how to handle this?

Ian

> -----Original Message-----
> From:	Bernd Felsche [SMTP:bernie at innovative.iinet.net.au]
> Sent:	Thursday, June 22, 2000 2:18 PM
> To:	diy_efi at diy-efi.org
> Subject:	Re: Ignition cuts for rev limits / traction control
> 
> 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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