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Author Topic: lpg hick ups.  (Read 738 times)

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feeutfo

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lpg hick ups.
« on: 13 June 2009, 11:01:31 »

its no massive issue tbh. It runs fine under normal driving. Problem only arises when you bury the pedal and start the revs spinning toward the red. It revs out all the way i think, although i have not really tested this, but as soon as you lift off the gas it cuts the power dead. Gives a bit of a jolt.

I think, from driving it, that its not a starvation problem but am by no means sure. Im thinking lpg ecu may not be managing well under those conditions. But wanted to see what you guys say.

Its a single hole upright donut tank. Stag 300 system.
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Lazydocker

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Re: lpg hick ups.
« Reply #1 on: 13 June 2009, 22:29:29 »

Sounds similar to my issue... The LPG pressure is probably dropping when red lining it. I just have it switch to petrol over 5200RPM at the moment as it's rare it gets that high anyway!

I'm planning to have a good look at it in the near future but have slight Laptop issues at the moment!
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feeutfo

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Re: lpg hick ups.
« Reply #2 on: 14 June 2009, 01:05:13 »

think i will have to do the same, as you say doesnt happen often but if you do need a bit of poke you dont really want it buggering about. Its fine under normal conditions and only really plays up above legal speeds.

Sassanach said he changed his vapourisor, wonder if that will sort it. Or is it worth a bit more Bar...?
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Kevin Wood

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Re: lpg hick ups.
« Reply #3 on: 14 June 2009, 18:46:13 »

Mine is the same. I believe it stumbles because of the sudden change in engine operating conditions. Can't really see enough detail in the log to see what's going on.

I have a theory that it might actually be a spike in the vapour pressure, because it is maintained constant relative to the manifold pressure. Manifold pressure instantly goes from 100kPa to probably around 20 kPa when you lift off. Until the engine burns the excess gas in the pipework that's an extra 0.8 bar.

However, this whould be compensated by the Stag ECU. Not sure how often it samples the pressure though.

Kevin
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feeutfo

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Re: lpg hick ups.
« Reply #4 on: 15 June 2009, 00:01:55 »

er.... kpa? I havent fully understood but are we talking momentary fuel air imbalance?(basically?)

It is comforting to know its not just me, and nothing i have done specific to my install.

Maybe a chat with Tailo will give some more info. Ill try and ring him this week. :-)
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cem_devecioglu

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Re: lpg hick ups.
« Reply #5 on: 15 June 2009, 08:16:05 »

Dont know its the same thing or not , but I read many threads about different lpg systems switching to petrol on high rpms on local forms..

And dont see any cure for that :-/
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Kevin Wood

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Re: lpg hick ups.
« Reply #6 on: 15 June 2009, 09:35:00 »

I know some LPG systems don't flow enough fuel for full chat. That's not the case here. We're achieving a solid "rich" indication from the Lambda sensors up to the red line without the injector duty cycle reaching 100%.

The stumble happens when you lift off, from what I can see. Now, a petrol ECU would normally cut the fuel quite aggressively during such a situation. This is because, with the intake system at atmospheric pressure there is quite a potential for fuel to "wet" the walls of the intake system. Take the pressure down again (by closing the throttle) and this fuel is liberated and burnt, requiring much less or even no fuel to be injected during this phase.

The situation with LPG is much more simple because the vapour (as opposed to liquid) passes straight into the cylinder in all instances. This means the petrol ECU might be backing off the fuel to compensate for an effect that's not happening. Difficult to see how it can be prevented in that case. :-/

Kevin
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cem_devecioglu

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Re: lpg hick ups.
« Reply #7 on: 15 June 2009, 11:52:44 »

Quote
I know some LPG systems don't flow enough fuel for full chat. That's not the case here. We're achieving a solid "rich" indication from the Lambda sensors up to the red line without the injector duty cycle reaching 100%.

The stumble happens when you lift off, from what I can see. Now, a petrol ECU would normally cut the fuel quite aggressively during such a situation. This is because, with the intake system at atmospheric pressure there is quite a potential for fuel to "wet" the walls of the intake system. Take the pressure down again (by closing the throttle) and this fuel is liberated and burnt, requiring much less or even no fuel to be injected during this phase.

The situation with LPG is much more simple because the vapour (as opposed to liquid) passes straight into the cylinder in all instances. This means the petrol ECU might be backing off the fuel to compensate for an effect that's not happening. Difficult to see how it can be prevented in that case. :-/

Kevin

Thanks for detailed explanation Kevin  :y

I'm sure you can find some way of simulation to solve this.. :y
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feeutfo

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Re: lpg hick ups.
« Reply #8 on: 15 June 2009, 17:34:07 »

hmm...we where checking a throttle dody on a polo a while back and noticed that when blipped, the throttle  would open and close suddenly,as you would expect, but then would open again, say 10%, then close, then open again, say 5%, or what ever then settle. It seemed to be managing the drop in revs so as not to be too sudden, i didnt make too much of it as with the inlet removed and hence upsetting maf readings it could be doing all sorts of strange things it wouldnt normally. I'm also under the impression smaller engines seem to run on a bit when the throttle is shut so "may" be more prominent on the polo...

But re lpg, your above description made me think of this same throttle action from a throttle body when driving, in that it seems to cut the power instantly and then come back in as if the ecu opens the throttle to manage the revs drop off, in a similar fassion as on the polo.

As i write i realise cable operated throttles would not have this "managment" and presumably still have the issue with lpg as we are experiencing? but am i getting the general idea of the problem at least...sort of ...ish? :-/

Edit to add, if carefull when closing the throttle it seems to improve things slightly...i think...not really played with it enough to be sure, but obviously depends how carefull you are...
« Last Edit: 15 June 2009, 17:44:43 by chrisgixer »
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Kevin Wood

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Re: lpg hick ups.
« Reply #9 on: 15 June 2009, 21:50:26 »

Was this Polo a DBW engine?

It's almost impossible for the ECU to keep up with the correct fuelling when the throttle is snapped open / shut and one of the side-effects is poor emissions. One of the advantages of DBW is that the action of the throttle can be damped and modulated such that the emissions are improved (marginally, but every little counts).

Some cars that aren't DBW (single point injection setups particularly) have a stepper motor driving the throttle stop so it can come up and "meet" the throttle as it's snapped closed and bring the revs down slightly more gently.

It all points to it being something transient because, as you say, it's fine when the throttle is backed off smoothly. Unfortunately transient things are the most difficult to tune out, and the LPG ECU settings are a bit of a blunt instrument in this regard. :-/

I will have to do a few datalogs sometime and see if I can glean a little more about what's happening.

Kevin
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feeutfo

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Re: lpg hick ups.
« Reply #10 on: 15 June 2009, 22:41:52 »

polo was not true drive by wire, the throttle cable still direct to the pedal but also had a stepper motor as you say, but iirc stepper motor was actuating the butterfly "within" the cable movement if you know what i mean, taking over so to speak. But certainly understanding a bit more i think. Thanks for explaining. Anytime you want to play.... :-)
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Kevin Wood

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Re: lpg hick ups.
« Reply #11 on: 15 June 2009, 22:49:29 »

Quote
polo was not true drive by wire, the throttle cable still direct to the pedal but also had a stepper motor as you say, but iirc stepper motor was actuating the butterfly "within" the cable movement if you know what i mean, taking over so to speak. But certainly understanding a bit more i think. Thanks for explaining. Anytime you want to play.... :-)


Off to Munich in the morning but will take another look when I get a minute and give you a shout.

Kevin
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