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Question about EFI cold start AFR

General engine tech -- Drag Racing to Circle Track

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ptuomov
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Re: Question about EFI cold start AFR

Post by ptuomov »

adam728 wrote: Tue Nov 03, 2020 12:49 pm I'm trying to picture how a cold engine, with the same airflow and ignition timing as a hot engine, runs 200 rpm faster cold vs hot just by richening things up.
Two notes.

First, it doesn’t have the same air flow, it has the same size hole flowing more air mass when cold and less air mass when warm.

Second, the fully warmed idle is tuned to 0.95-1.00 lambda even with the oxygen sensor closed loop disabled.

The combination of more air from the same size hole and more fuel per gram of air because of warmup enrichment makes it want to run faster when cold.
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Re: Question about EFI cold start AFR

Post by Belgian1979 »

As has been mentioned you need to compensate for the loss of fuel due to the cold intake and poor evaporation of the fuel at startup. Current ecu's will allow you to automatically compensate the fuel loss at cold start. It does so to reach the set AFR goal. That is basically still what you always run.

You also need additional air to overcome internal friction and maybe some additional ignition timing.
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Re: Question about EFI cold start AFR

Post by ptuomov »

Belgian1979 wrote: Tue Nov 03, 2020 1:38 pm As has been mentioned you need to compensate for the loss of fuel due to the cold intake and poor evaporation of the fuel at startup. Current ecu's will allow you to automatically compensate the fuel loss at cold start. It does so to reach the set AFR goal. That is basically still what you always run.

You also need additional air to overcome internal friction and maybe some additional ignition timing.
The modern ECUs run massively retarded ignition timing at cold engine start up.

But I’ll bite anyway: What is the right sensor measured AFR goal for the cold engine? What is the right actual air fuel ratio for a cold engine?
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Re: Question about EFI cold start AFR

Post by adam728 »

ptuomov wrote: Tue Nov 03, 2020 1:21 pm
adam728 wrote: Tue Nov 03, 2020 12:49 pm I'm trying to picture how a cold engine, with the same airflow and ignition timing as a hot engine, runs 200 rpm faster cold vs hot just by richening things up.
Two notes.

First, it doesn’t have the same air flow, it has the same size hole flowing more air mass when cold and less air mass when warm.

Second, the fully warmed idle is tuned to 0.95-1.00 lambda even with the oxygen sensor closed loop disabled.

The combination of more air from the same size hole and more fuel per gram of air because of warmup enrichment makes it want to run faster when cold.
I guess I did word that poorly. Yes, more mass air flow, same size air passage.

I've yet to calibrate an engine that didn't want more throttle opening (ETC) or more idle passage opening (IAC) when cold. Plus the corresponding pulsewidth increase for both the increased airflow plus the unburnt fuel passing through (cold piston/valves/cylinder/head will keep a lot of hydrocarbons from being burnt).

For us the right answer has always been to watch the wideband. Yes in the case of very late idle timing burn is happening into the exhaust, and your targeted lambda shifts richer as you are guestimating how much burn is happening in the cylinder vs in the exhaust. But in most normal scenarios the wideband is giving numbers plenty close enough to use. Even at -40C.
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Re: Question about EFI cold start AFR

Post by ptuomov »

adam728 wrote: Tue Nov 03, 2020 3:16 pm I've yet to calibrate an engine that didn't want more throttle opening (ETC) or more idle passage opening (IAC) when cold. Plus the corresponding pulsewidth increase for both the increased airflow plus the unburnt fuel passing through (cold piston/valves/cylinder/head will keep a lot of hydrocarbons from being burnt).

For us the right answer has always been to watch the wideband. Yes in the case of very late idle timing burn is happening into the exhaust, and your targeted lambda shifts richer as you are guestimating how much burn is happening in the cylinder vs in the exhaust. But in most normal scenarios the wideband is giving numbers plenty close enough to use. Even at -40C.
This was unexpected to me also, but it’s a very low friction engine (now) with air run thru massive air to air intercoolers etc. so while surprising to me it’s not implausible that the engine runs better rich and cold than stoichiometric and hot.

I have a calibration algorithm that maps the IAC opening percent to rpm and it clearly shows that same size hole gives a higher rpm with cold engine than hot engine, at least with current warmup enrichment settings.
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Re: Question about EFI cold start AFR

Post by Belgian1979 »

ptuomov wrote: Tue Nov 03, 2020 1:46 pm
Belgian1979 wrote: Tue Nov 03, 2020 1:38 pm As has been mentioned you need to compensate for the loss of fuel due to the cold intake and poor evaporation of the fuel at startup. Current ecu's will allow you to automatically compensate the fuel loss at cold start. It does so to reach the set AFR goal. That is basically still what you always run.

You also need additional air to overcome internal friction and maybe some additional ignition timing.
The modern ECUs run massively retarded ignition timing at cold engine start up.

But I’ll bite anyway: What is the right sensor measured AFR goal for the cold engine? What is the right actual air fuel ratio for a cold engine?
Why so aggressive? Trying to do a piss contest?
I wasn't looking at anyone to bite. I was trying to offer some advice to someone.

Less advance during the cold start? When mixtures tend to be lean and when burn rate is slow due to reduced vaporization, calls for additional timing in some cases. It also helps the engine to overcome the higher internal friction.
The only reason I could see why any OEM (if that is what you mean with idiomen 'modern ecu') would want to run retarded timing during startup would be to reduce emissions (mainly HC) and to get the catalyc converter up to temp.
I don't use a converter and the only thing my car has to adhere to is CO emissions. Easy enough, you keep the engine close to stoich and it has best CO-emissions. Then you have the ECU compensate for any leaner than normal AFR's when cold.
And I use a wideband (in every cyl) if that was what you were asking.
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Re: Question about EFI cold start AFR

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Belgian1979 wrote: Tue Nov 03, 2020 5:19 pm Why so aggressive? Trying to do a piss contest?
I wasn't looking at anyone to bite. I was trying to offer some advice to someone.

Less advance during the cold start? When mixtures tend to be lean and when burn rate is slow due to reduced vaporization, calls for additional timing in some cases. It also helps the engine to overcome the higher internal friction.
The only reason I could see why any OEM (if that is what you mean with idiomen 'modern ecu') would want to run retarded timing during startup would be to reduce emissions (mainly HC) and to get the catalyc converter up to temp.
I don't use a converter and the only thing my car has to adhere to is CO emissions. Easy enough, you keep the engine close to stoich and it has best CO-emissions. Then you have the ECU compensate for any leaner than normal AFR's when cold.
And I use a wideband (in every cyl) if that was what you were asking.
Sorry everyone is very pissed off here in MA with the new curfew announced and shop windows being boarded to prevent election night looting...

So what's a good target for measured WBO2 sensor AFR reading at cold start up? Say 0C coolant temperature, when the normal is 87-91C?

I have the option of running cats and NBO2 closed loop, but since it's an '87 car that option would only be exercised when the car is fully in operating temperature. So I can set the cold start AFR basically wherever I want within reason whether or not I run cats.
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Re: Question about EFI cold start AFR

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ptuomov wrote: Tue Nov 03, 2020 1:21 pm
adam728 wrote: Tue Nov 03, 2020 12:49 pm I'm trying to picture how a cold engine, with the same airflow and ignition timing as a hot engine, runs 200 rpm faster cold vs hot just by richening things up.
Two notes.

First, it doesn’t have the same air flow, it has the same size hole flowing more air mass when cold and less air mass when warm.

Second, the fully warmed idle is tuned to 0.95-1.00 lambda even with the oxygen sensor closed loop disabled.

The combination of more air from the same size hole and more fuel per gram of air because of warmup enrichment makes it want to run faster when cold.
So, that would effectively make the IAC orifice a masshole? :) Sorry, you teed it up perfectly.

Belgian 1979 has summed up things quite nicely.
I worked on cold driveability for around 10 years using a chassis dyno, 2 gas IR, volt meters, scopes, and obd scanners, oh, and a sensitive built-in butt dyno.
It's pretty much all the same: give the engine the fuel it wants when it needs it, keep the idle up to keep it stable, and if it'll take more advance, use it.
You run yer hot rod at stoich at warm idle, which is around 0.6-0.8% CO which is not enough fuel for a performance cammed engine. The reason the idle speeds up when you add fuel when the engine is warm is that the idle CO is now around 1.5-2.5% CO or maybe a little more. Back in the days where one turned idle mixture screws, 2% CO was kinda universal for making an engine happy at idle and could be achieved with feel and one's ears.

Unless you have an unmeasured intake leak when cold (post MAF), I'm not sure how you will achieve increased idle speed without an increased IAC command.

If this thread hasn't done much, it sure has brought out a lot of practical, hands on info. Hopefully it'll be put to good use.
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Re: Question about EFI cold start AFR

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adam728 wrote: Tue Nov 03, 2020 12:49 pm I'm trying to picture how a cold engine, with the same airflow and ignition timing as a hot engine, runs 200 rpm faster cold vs hot just by richening things up.
Simple... if the engine is more powerful with a rich mixture, it will idle higher.

I used to use this as a cold start idle-up, by having the hot idle lean of best vacuum. For whatever reason, thoug, my current engine will not idle any leaner than about .85 lambda without surging, and if you go much more rich than that it will start to blubber from being too rich, so I can't use that tactic. (I am using a Holley 3310 as a throttle body, so having any kind of idle air control is going to be tricky at best, and to be honest I'm happier without it) This tells me that my ignition is crappy, I think.
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Re: Question about EFI cold start AFR

Post by peejay »

ptuomov wrote: Tue Nov 03, 2020 1:46 pm
Belgian1979 wrote: Tue Nov 03, 2020 1:38 pm As has been mentioned you need to compensate for the loss of fuel due to the cold intake and poor evaporation of the fuel at startup. Current ecu's will allow you to automatically compensate the fuel loss at cold start. It does so to reach the set AFR goal. That is basically still what you always run.

You also need additional air to overcome internal friction and maybe some additional ignition timing.
The modern ECUs run massively retarded ignition timing at cold engine start up.

But I’ll bite anyway: What is the right sensor measured AFR goal for the cold engine? What is the right actual air fuel ratio for a cold engine?
Yes, the priority with modern engine management is to get the engine and catalyst up to temp ASAP. Ford is proud of their Ecoboost fours coming from dead cold to operating temp in 90 seconds or less.

My Volvo not only does ignition retard but also heavy camshaft phase tuning on a cold start, because it has the converter mounted far behind the engine instead of a close coupled cat. Compromise made to be able to live at high boost for long periods of time without melting anything down. It idles at near atmospheric when cold and the exhaust's primary sound is that of screaming turbo :D Pull 10 pounds of boost just accelerating gently in the neighborhood. BUT Volvo's priority is to warm up the converter as fast as possible, to meet 2006 emissions standards. This is not YOUR priority, unless you are planning on marketing an emissions-certified product.
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Re: Question about EFI cold start AFR

Post by Belgian1979 »

ptuomov wrote: Tue Nov 03, 2020 6:08 pm
Belgian1979 wrote: Tue Nov 03, 2020 5:19 pm Why so aggressive? Trying to do a piss contest?
I wasn't looking at anyone to bite. I was trying to offer some advice to someone.

Less advance during the cold start? When mixtures tend to be lean and when burn rate is slow due to reduced vaporization, calls for additional timing in some cases. It also helps the engine to overcome the higher internal friction.
The only reason I could see why any OEM (if that is what you mean with idiomen 'modern ecu') would want to run retarded timing during startup would be to reduce emissions (mainly HC) and to get the catalyc converter up to temp.
I don't use a converter and the only thing my car has to adhere to is CO emissions. Easy enough, you keep the engine close to stoich and it has best CO-emissions. Then you have the ECU compensate for any leaner than normal AFR's when cold.
And I use a wideband (in every cyl) if that was what you were asking.
Sorry everyone is very pissed off here in MA with the new curfew announced and shop windows being boarded to prevent election night looting...

So what's a good target for measured WBO2 sensor AFR reading at cold start up? Say 0C coolant temperature, when the normal is 87-91C?

I have the option of running cats and NBO2 closed loop, but since it's an '87 car that option would only be exercised when the car is fully in operating temperature. So I can set the cold start AFR basically wherever I want within reason whether or not I run cats.
No problem. What I see on my car is that very rich AFR's, like 13.5 and below make the engine idle more difficult which I cannot really explain. Also when using the same igition timing numbers makes for a lot more IAC steps meaning that the engine needs more air to retain the same rpm.
So, I use the same AFR-target when cold and warm. Bear in mind that AFR correction only starts at around 160°F. Below that the automatic AFR-correction doesn't engage. However, when tuning cold start, you have to set the AFR-target and the ECU will compensate for the loss of fuel due to wall adherence and other factors when cold to still get to the same AFR-target. Hope that makes sense.
Roughly it will have 25% more fuel when at 80°F. Don't take this as a ballpark number as the position of the injector can have great influence and mine are not positioned well in my intake.

I assume that you can set the AFR higher for cold start within reason, however I think that your warmup enrichtment should automatically compensate if you understand what I mean. Bear in mind that the cats will heat up more with more HC going through the engine. So you'll have to return to normal numbers fairly soon.

I have 4 degrees more ignition timing when cold. You could decide not to employ cold ignition advance as the burning charge in the exhaust will heat up the cats faster. But you'll have to use the IAC to compensate. I couldn't do that on mine as I run ITB's with individual IAC lines to each barrel and due to the number of them I had to reduce their size, which means that they can only flow a certain amount of air. So in order to get the engine cold rpm up, I use more ignition timing.
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Re: Question about EFI cold start AFR

Post by digger »

1 point richer than when upto temp. Ultimately give the engine what it wants.
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Re: Question about EFI cold start AFR

Post by Belgian1979 »

digger wrote: Wed Nov 04, 2020 3:00 am 1 point richer than when upto temp. Ultimately give the engine what it wants.
Indeed, that might be what it needs, but it depends on the engine. If the goal is to get the engine to be more powerful you could just as well increase the ignition timing.
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Re: Question about EFI cold start AFR

Post by digger »

Belgian1979 wrote: Wed Nov 04, 2020 4:21 am
digger wrote: Wed Nov 04, 2020 3:00 am 1 point richer than when upto temp. Ultimately give the engine what it wants.
Indeed, that might be what it needs, but it depends on the engine. If the goal is to get the engine to be more powerful you could just as well increase the ignition timing.
Doesn’t matter if you get power through timing or bypass air the engine will tell you if it likes it richer or leaner and around 1 point richer should work ok. Even richer works to but I’d run as close to stoich as possible to avoid dilution
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Re: Question about EFI cold start AFR

Post by Orr89rocz »

David Redszus wrote: Mon Nov 02, 2020 5:51 pm The fuel enrichment should be just enough to compensate for the poor fuel evaporation due to cold temperature.
That would depend on the fuel distillation curve and ambient temperature. Fuel evaporation will lower inlet air
making fuel even harder to evaporate.

I richen mine up so that its just lean enough that engine runs smoothly. In cold 30-40 deg starts let say it def doesnt run with fuel that produces 14.7 afr once warm. So i richen up til it runs smooth which usually is low mid 13’s afr then progressively lean out as coolant temp warms enough to handle stoich idle target and even low speed driving areas target

Also note it is very common to add 2-4 deg timing during this time and idle speed demand being set few hundred rpm higher. Warm idle 25 deg and 800 rpm, cold starts may be better at 27-28 deg and 1000 rpm until it warms up
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