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Home > Technical Chat > VEs AFRs and Widebands

Paul S

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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.

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Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."


joeybaby83

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

your exam starts.....now!

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Rod S

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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.

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evolotion

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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 :)

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Paul S

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On 7th Dec, 2008 evolotion said:
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 :)


That is part of Round 3 *smiley*

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Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."


Bat

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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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Paul S

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On 7th Dec, 2008 Bat said:
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 :)


Definately witchcraft.

Thanks for the proper answers Gavin. I'm not sure that you are correct about 3. But then you might be.

I'm about to cut a fresh head with bigger valves on the inner cylinders to test that it is the VE and not something else.

Let's have some more real answers, then we can start Round 2 *smiley*

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."


PaulH

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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

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James_H

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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.


Bat

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

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jbelanger

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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

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Paul S

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

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Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."


Rod S

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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 ???


Paul S

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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.


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Joe C

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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?

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Paul S

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On 8th Dec, 2008 mini13 said:
Ok,
assuming the inner and outer cylinders have a different VE they will have different compression ratios giving different burn characteristics.


I did not think that the CR would affect the wideband reading, but a good point to consider.

On 8th Dec, 2008 mini13 said:
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.


I can see the logic that the fuel will be pushed to the outer cylinders in the manifold bend.

However, I think that it is a little more complex. As the fuel is pushed to the outside of the bend, it gains velocity. So in the bend, the air is travelling slower than the fuel. At the exit of the bend the fuel is still travelling faster than the air. The only way it can do this is to flow in a spiral pattern. So the air moves down the centre of the port and the fuel spirals around the outside.

So instead of a straight laminar flow in the port, it is actually spiralling and could favour either inlet valve.

On 8th Dec, 2008 mini13 said:
say it is a VE issue could it be down to the siamesed ex ports?


I dont think so with the full 360 degree phasing.

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."


AlexF2003

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

AlexF


jbelanger

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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

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Paul S

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On 8th Dec, 2008 jbelanger said:

So with the upcoming siamese code able to fuel cylinders independently,


I'm intruiged as to how you are going to do this with just the two injector drivers, but I can't wait to resume testing.

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Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."


James_H

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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?


Rod S

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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...

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Paul S

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On 8th Dec, 2008 James_H said:
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?


There are two possible causes of a low VE. Firstly, poor scavenging of the exhaust gases. Secondly, even with good scavenging, the air cannot get into the cylinder.


On 8th Dec, 2008 James_H said:
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?


I don't think so because you are taking in the correct proportion of air and fuel.

On 8th Dec, 2008 James_H said:
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?


I'm logging EGTs & CHTs, but the results do not make much sense with the AFRs so far apart. I don't think that the centre cylinders will run hotter because of the siamesed port. There may be twice as much heat passing through the port but it is at the same temperature.

On 8th Dec, 2008 James_H said:
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.


Correct.

On 8th Dec, 2008 James_H said:
this would point to the fact that your correct and this is down to mixture distribution.


Thank you.

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Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."


James_H

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


jbelanger

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On 8th Dec, 2008 Paul S said:

On 8th Dec, 2008 jbelanger said:

So with the upcoming siamese code able to fuel cylinders independently,


I'm intruiged as to how you are going to do this with just the two injector drivers, but I can't wait to resume testing.

The injectors will not be wired the same way and you lose staging (but I have a solution). You have one injector in the first port on one driver (cylinders 1 and 2) and one injector on the other port on the second driver (cylinders 3 and 4).

Then even if you fire both ports at the same time in the semi-sequential manner, using the cam sensor, you know which cylinders you're fueling: 1 and 3 in one engine rev and 4 and 2 in the other engine rev. When you fire 1 and 3, the first driver will have a longer pulse than the second driver, and when you fire 4 and 2, the second driver will have the longer pulse.

Now the solution to get back staging is to have 4 drivers. This will be done by making a small modification to the MS2 card to wire the CPU pins used for PWM to unused pins on the card. It will also require the addition of 2 more injector driver circuits. And of course, it will need the code to do it. The drawback will be the loss of PWM so low impedance injectors will require some specific drivers. Fortunately, I already have a board that has 4 injector drivers that are made for such a purpose: see here. I you have high impedance injectors then using 4 simple TO-220 drivers such as the VNP5N07 will suffice (see here).

I won't have the code for the 4 injector drivers in the first upcoming release but it should not take too long to have it. It shouldn't be needed for the first tests anyway because I don't think you'll be using staging right from the start. When I have the code for the 4 drivers, I'll post pictures showing the needed modifications and wiring.

Jean

http://www.jbperf.com/


Paul S

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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."

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