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PostPosted: Wed Apr 11, 2007 3:34 am 
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Joined: Fri Apr 06, 2007 6:09 am
Posts: 116
Location: France
You have to be very carefull with this capability. Agree that it can be very helpfull for fuel tuning as it will guaranty that what you get won't drift in time, but as Crazymikie noticed, if for some reason the ECU needs to compensate against something, it won't be able to do so.
The ECU LTFT capability has originally been implemented to compensate for parts becoming less and less accurate in time (MAF, fuel pump, injectors, fuel rails, fuel pressure regulator...). After 2 or 3 year of use of a car, all these components are getting old, still functionnal, but need some compensation to guaranty constant AFR in time.
The only way for good fuel tuning consists in the verification that LTFT will be kept close to zero, its longer but more accurate for long term reliability :wink:
I would advise that forcing LTFT to 0 be done only to cars constantly monitored with WB and owned by "aware" guys :wink:


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PostPosted: Wed Apr 11, 2007 6:27 am 
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Joined: Mon Apr 10, 2006 11:49 am
Posts: 169
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I don't think we should be trying to turn this feature off at all. As Jon has said, it should be tuned for the car. We need to focus more on this.

Jon, you say that 'If you see values getting closer to zero as RPM rises, then adjust injector latency', can you expand on this please. Adjust up or down etc.

My AFR has varied from the low det fuel map in places by varying degrees. In the past I have adjusted the map to give the desired afr as measured on the wideband, taking into account the ltft. This has given reasonable afrs, although the fuel table doesn't always look neat and smooth.

I fitted a new maf sensor to the car when I bought it, and have logged this against the old one. There is no difference between them which would suggest the original one is fine.

One thing to point out is that my ltft will be updated really quickly after an ecu reset. I don't know if this is because it is a jdm car.


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PostPosted: Wed Apr 11, 2007 8:28 am 
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Joined: Thu Aug 03, 2006 10:40 am
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swifty wrote:
Jon, you say that 'If you see values getting closer to zero as RPM rises, then adjust injector latency', can you expand on this please. Adjust up or down etc.


well obviously that depends on the "sign" of the correction. :)

if the trim is more NEGATIVE as rpm rises then the ecu is injecting MORE fuel than expected in that range. that means that the ecu is injecting LESS fuel at lower rpms than it "thinks" it is, which means that the latency could be adjusted to a LARGER value so that the effective duty cycle is increased.

conversely, if the trim is more POSITIVE as rpm rises then the ecu is injecting LESS fuel than expected in that range. that means that the ecu is injecting MORE fuel at lower rpms than it "thinks" it is, which means that the latency could be adjusted to a SMALLER value so that the effective duty cycle is decreased.

since it is a static duty cycle offset, adjusting latency will have the greatest effect when duty cycles are lowest.

Quote:
One thing to point out is that my ltft will be updated really quickly after an ecu reset. I don't know if this is because it is a jdm car.


my usdm 16bit ecu does the same thing. i believe that the quick learning of LTFT immediately after a reset has to do with a built in averaging algorythm.


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 Post subject:
PostPosted: Sat Apr 14, 2007 8:44 pm 
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Joined: Sat Feb 03, 2007 3:58 pm
Posts: 166
Jon [in CT] wrote:
This is nuts. You people are trying to defeat one of the features the ECU does so well, responding to varying fuel chemistry. I don't know about the UK, but in the US it's possible to fill up with Shell super premium (~98 RON) and get fuel with a stoichiometric AFR of 14.8:1 and then refill one week later from the same pump and get fuel with a stoichiometric AFR of 14.0:1. If you had tuned on the 14.8:1 fuel, changing to 14.0:1 fuel should eventually result in a +5.7% long term fuel trim. Now suppose you've disabled LTFT for open loop and you've targeted 11:1 for some open loop AFR cell. What you'll end up with, as reported by a wideband, is 11.6:1.

What we really need is a way to display the LTFT table maintained in RAM. The goal is to have all entries in that table be the same value when you're tuning. If you see values getting closer to zero as RPM rises, then adjust injector latency. If all the LTFT values are the same, but target AFR is consistently higher or lower than AFR measured by a wideband, adjust injector flow rate.


i understand the need to account for this in closed loop mode, but when in open loop, will the LTFT actually keep the A/F ratio constant over multiple fillups with different gas? All it has ever done for me is mess up my closed loop fuel tables. I started off with a measured AFR of 11.3-11.4. A week later, my wideband is measuring 12.1-12.2 in WOT. What gives? It seems it doesnt matter if the measured afrs are different from what is dictated in the tables, but I would want it to still be consistent. Say I have the target set to 11.2, and its actually reading 11.4. I wouldnt care so much about that, but the fact that after a week the mixture has leaned out to over 12:1 annoys me to no extent!


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 Post subject: Re: Why can't we make LTFT go away?
PostPosted: Fri Mar 07, 2008 3:57 pm 
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Joined: Fri Mar 17, 2006 8:42 pm
Posts: 189
Location: ARIZONA
I ll try to cero it out today and post result on the weekend. I just finish breaking-in my hybrid and the fuel values are way off on the open loop fueling ( 12.9=10.8afrs ) so i ll try this and see if it help. My feul learning do not got over ( or under ) -3.15 so the scaling is good ( I haven't play with the latencies yet ).

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 Post subject: Re: Why can't we make LTFT go away?
PostPosted: Fri Mar 07, 2008 6:58 pm 
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Joined: Thu Aug 30, 2007 1:59 am
Posts: 70
this sounds exactly what I've been dealing with lately. Is this the same thing I'm seeing in this thread? topic2832.html

I thought my car was possessed when over the next 2 or 3 days after a map it would lean out.


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 Post subject: Re: Why can't we make LTFT go away?
PostPosted: Sat Mar 08, 2008 1:33 pm 
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Joined: Fri Mar 17, 2006 8:42 pm
Posts: 189
Location: ARIZONA
I have a stock air box but still gonna try to rescale the maf an see if it help. if not i will play with the scaling ( sti pink inj ). I ll post in an hour or two.

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 Post subject: Re: Why can't we make LTFT go away?
PostPosted: Sat Mar 08, 2008 5:17 pm 
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Location: ARIZONA
I got it. By rescaling my maf like is reading less air I got the afr's close or similar to the ones on my fuel map. still got the fuel learning on 0's and the close loop too. I ll be done with the scaling today, already drove 30 miles just tunning so if you got the same set up let me know and i ll post the scaling.

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