Transients.

[John Dammeyer]

>
>Date: Thu, 2 Dec 1999 09:43:31 +1300
>From: Tony Bryant <Tony.Bryant at psc.fp.co.nz>
>Subject: RE: Transients
>
>
>> | 6) A richer mixture (i.e. more powerful) is needed to accelerate the
>> engine.
>> | Bollocks. Running a seriously rich mixture at say
>> 1300RPM/0" vac, does not
>> | help
>> | the problem. Neither does serious amounts of dMap/dt.
>> dTPS/dt helps a
>> | little,
>> | but not much.
>>
>> Then try going very lean, ahh, see, problem gets worse
>
>That because on your first 100% VE cycle after the transition, the
>fuelling will still generally be set-up for the pre-transition
>conditions. Leaner there, gives much leaner post transition.
>

I have experienced much the same sort of problem and also have that post
Throttle Open Stumble.  I use a dTPS/dt to increase the amount of fuel injected
based on how large the dTPS/dt is.  I also have a timer that runs for 1 second
maintaining the longer pulse width but I still have a stumble of sorts.

>> | ----------------------
>> | Having stated what I think its not, here's what I think it is:
>> | 1) Fueling delays. The ECU not responding to changes in MAP as
>> | quickly as the engine. The engine will respond on a cycle by cycle
>> | basis, whereas an ECU and MAP sensor most likely will not.
>>


I don't think it's totally engine response on a cycle by cycle basis but maybe a
lack of knowledge of how much extra fuel to pour in.  Here's my quandary.  My
maximum pulse width is limited to 12.8 milliseconds at 7000RPM.  So the dTPS/dt
calculation can, at most inject no more than 12.8ms per cylinder for 'n' seconds
after the throttle is snapped open.  I suspect that the engine requires far more
than 12.8ms of fuel at this point even though at 6600RPM I'm using that much to
generate maximum horsepower.

i.e.: theory says that at WOT and 95% VE that if the 12.8ms is enough at 6600RPM
then it should also be enough at high dTPS/dt;  yet it's not.  So why?

At 1200RPm each engine rev takes 50ms. Each stroke therefore takes 25ms.  This
means if I'm updating my TPS and therefore my Injector pulse width more often
than every 25ms I should see the 12.8ms pulse width within one stroke at low
RPM.  That's not equivalent to a .25 to .5 second stumble that real life
provides so why the stumble?  This stumble is quite real, the engine can
actually cough and almost stall if the mixture or the duration of the enrichment
is too slow.

My Jeep Cherokee, when the throttle is matted to the floor,  has a brief brief
pause before it winds up but it never really stumbles.  Same with my SAAB Turbo
(MAF sensor and turbo) so things are different.  But in either case the FI
handles the dTPS/dt without a real stumble before accelerating.  What are they
doing?

Regards,

John