VEs AFRs and Widebands
Turbo Mini Forums

I'm puzzled by the data from the dual wideband trials on my engine.

Too cold to work in the workshop today so trying to sort this out.

Questions:

1. Say you have a cylinder operating at 90% VE, fed with a mix of air and fuel at 14.7:1, what would the wideband read?

2. Take another cylinder with poor VE say 70% also fed with air/fuel at 14.7, what would the wideband read?

3. Take the cylinder in Q1 but assume that although 90% VE, 20% of that is residual exhaust gas. What would the wideband read?

Round two coming up later.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

remember to fully read the questions before answering, and do the easy ones first to build up confidence.

your exam starts.....now!

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

Give me a few more hours and I may have an initial reading....

I too am finding it rather cold outside and I'm having to search a freezing shed for one of the bits I need....

But my initial thoughts would be the wideband reading would be directly proportional to the supplied AFR,
EDIT -ie, supplied at the actual cylinder. (end EDIT)

Surely the problem with the (presumed) different VEs is that they cause the actual AFRs to be different because of the different densities of fuel and air in a dynamic/transient situation ???

A carb will only supply a single AFR at the source, but this will then change going down the siamese ports.

Edited by Rod S on 7th Dec, 2008.

Schrödinger's cat - so which one am I ???

gotta mind that fuel is heavier than air, so if it isnt atomised amasingly well, it will be harder for it to change direction than the air, resulting in different afr's / cylinder, which your question assumes doesnt change :)

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Hi,
1&2 ... 14.7 as there's the same mix of fuel and air just less of it in the cylinder, right?
3 ... 14.7 again assuming the previous charge was also 14.7..

Has there been a conclusion on whether inner and outer do have different VEs or is it due to wall wetting or some other witchcraft?
Cheers,
Gavin :)

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Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Q1 lambda will be in or around 14:1 depending on load,air density ect
Q2 i would expect a richer lambda as with a lower VE compression will be down on cyl 1 and so not a commplete burn.
Q3 i would also expect a higer reading as temp of combustion will be lowered and so the the burn may not be complete

Q3 - is that not basically EGR. so on modern engines that utalise EGR for better emmissions EGR would surely creater a leaner burn, as there will be less oxygen in the cylinder compared to a completely scavanged/fresh charge.

but that is assuming that the mixture is adjusted to suit EGR mixture. if it isnt then it would be richer than 14.7:1

i cannot do any calcs on this sort of thing as im far from clever enough but that would be my verdict with my understanding on the subject.

Hi
Good points. I guess it depends on how much the VE has to drop before incomplete combustion occurs.
Cheers
Gavin :)

VEMs Authorised Installer / Re-seller. K head kits now available!

WB/EGT gauges. Click here for customers write-up

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My Mini build diary

I agree with Gavin that it will all show stoic. However as James_H mentions, Q3 is similar to EGR but your question mentions that the cylinder is fed with a stoic mixture. If it instead said that it was fed with fuel corresponding to stoic for a 90% VE air volume then is would be richer than stoic.

Of course this all assumes that ignition timing is correct and there is no miss.

Jean

http://www.jbperf.com/

I'm thinking carburetor for Round 1, rather than injection.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Well I just got mine to start on a carb (and dizzy)....

And I do mean just.... three fundamental mistakes, one of which fried a (conventional) electronic dizzy module.

Luckily I had a spare.

So, although it now runs, there is no way that I'm going to hook the two widebands up to it tonight.

The problem with "temporary" wiring is making mistakes, and I don't want to make any more mistakes with the expensive bits.

Fresh head tommorow....

Schrödinger's cat - so which one am I ???

So, do we conclude that the answer is 14.7:1 for all three?

If so, then I must conclude that the variations in AFR between the inner and outer cylinders from my first datalog with a carb is purely due to mixture distribution.

Graph from Log attached.

Attachments:

16.47kb

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Ok,

how about this,

assuming the inner and outer cylinders have a different VE they will have different compression ratios giving different burn characteristics.

I don't thing the different afrs between the centre and outer pairs are due to mixture distribution, as the outers are the leaner ones, which seems backwards to me.

say it is a VE issue could it be down to the siamesed ex ports?

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

CR will effect your ignition timing and obviously BMEP but not your AFRs.

AlexF

While ignition timing will not affect AFR it may affect the reading from a wideband O2 sensor. I'm not entirely clear on why that is but I assume it has to do with the sensor position and how much of the combustion happens in the exhaust.

As for Paul's graph, I would also say that the difference is entirely due to mixture distribution. And since it shows the inner cylinders being richer I would say that the main culprit is wall wetting since any evaporated fuel from wall wetting will be more likely to go to the inner cylinder in a 3:1 ratio. That also suggests that something similar is happening with the injection setup.

So with the upcoming siamese code able to fuel cylinders independently, it should be easy to confirm that since the standard MS2/extra code does have a transient fueling (acceleration, deceleration) based on wall wetting behaviour. If the fuel distribution is corrected in a steady state but there are still discrepancies in the AFR during those transients then it should be obvious if it is caused by wall wetting or not.

I may have to do something to the standard code to allow different transient corrections for wall wetting but I'll need to study this because up to now I have paid very little attention to this aspect.

Jean

http://www.jbperf.com/

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

with a lower VE on the inner two cylinders would you tend to get a less effective scavange? so more exhaust gass left in the cylinder after the exhaust stroke?

if you would then this could cause the inner cylinder to run richer as the exhaust gasses would contaminate the fresh charge. meaning that less oxgen is in the mixture so the feul isnt being used effectivly so giving you a richer mixture?

do you have EGT readings? or cylinder temps? EGT reading would be abit hard i suppose because of the siamesed center cylinders...uhmm if the cylinders run a reasonable amount cooler could you assume that there is EGR taking place in the cylinder?

uhmm just thinking about this abit further..as its carb readings the mixture is already set by the carb by the time its in the inlet manifold. so theoretically EGR isnt going to make any difference as the exhaust gasses will be taking place of MIXTURE rather just air.

this would point to the fact that your correct and this is down to mixture distribution.

or am i thinking about this too much and EGR would still be effective on a carb?

Me too, well start testing, not "resume", but I think I understand what Jean is trying to do for us.

I'm just about to post my first (very basic) test in the other thread...

Schrödinger's cat - so which one am I ???

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

so all in one post i taught myself what you were thinkin! :S not bad going! hahah!

http://www.jbperf.com/

Exxxcerlunt

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."