Polo 1,2 3-Cyl. - WPS 500 Pressure Question

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terbo
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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by terbo » Mon Apr 23, 2018 4:58 pm

Oh, ok...that would be a bummer, because the Waveform at idle
looks ok in sight of the Pressures.

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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by Steve Smith » Mon Apr 23, 2018 11:55 pm

Thank you for the posts and what a challenging subject to discuss.

There are so many phenomena’s to cover here but I will go through them as I understand them
Please wade in and correct these theories as you see fit as I have chewed the fat with these questions for some years. Many thanks to Mark Stammers here who allowed me to chew his ear some time ago on one such subject.

1. Why does peak pressure cylinder decrease at WOT as the engine accelerates? (Approx. 5800 rpm here)

2. During this cylinder pressure decrease phase (at WOT) the exhaust back pressure in the cylinder increases to near 2 bar where a pressure gauge mounted in the front O2 sensor boss (pre cat) reads only 100 mbar (1.5 psi)?
COMPRESSION OVER VIEW.jpg
Compression overview
This is a real phenomenon and not a limitation of the pressure transducer

Essentially everything takes time, as the RPM increases you eventually reach a point where the engine volumetric efficiency decreases.

We are visualizing the physical volumetric limitations of this engine.
It simply cannot breathe fast enough as you have proven looking at the gauge in the exhaust system simultaneously with the WPS in the cylinder.

Assuming the engine to be 100% Volumetric Efficient (VE) which it never will be, we should be seeing max airflow at approx. 55 gm/sec assuming this is a 55 PS engine “rule of thumb”

In reality this will be between 80 and 88% VE and so if we have good peak MAF figures (approx. 48 gm/sec) at such high rpm, then theoretically the engine is breathing OK and the only restriction on the exhaust side is the limitations within the cylinder.

Here is a typical example of an engine breathing correctly yet appears to have approx. 2 bar exhaust back pressure in the cylinder at WOT yet the actual exhaust back pressure is approx. 350 mbar measured at the O2 sensor. (This 2 bar cylinder pressure during the exhaust stroke at high RPM seems to be a characteristic value!)
BACK PRESSURE.jpg
Peak cylinder pressure during exhaust stroke
Here is another example with MAP and MAF confirming peak airflow and atmospheric pressure within the intake manifold (i.e. WOT) during cylinder pressure reduction phase & peak cylinder pressure during the exhaust stroke at 2.2 bar
MAF MAP CYLINDER PRESSURE.jpg
MAF MAP Cylinder Pressure
The next consideration is why do we have non uniform compression towers at high rpm and why does peak cylinder pressure shift in relation to crankshaft angle?

Below we have peak cylinder pressure at approx. 115 degrees of crankshaft rotation after the missing teeth at 5700 rpm
TDC SHIFT HIGH RPM.jpg
Peak pressure at high RPM
This is in stark contrast to our uniform compression towers at idle speed (860 rpm) where our peak cylinder pressure reflects a more definite TDC compression stroke occurring at 80 degrees of crankshaft rotation after the missing teeth
TDC AT IDLE.jpg
TDC Peak cylinder pressure at idle speed
I believe this to be caused by a combination of the expansion of the gasses due to the heat generated from the rapid compression and once again “time”.

Going back to first principles, at 5700 rpm we are going to compresses the cylinder rapidly with a rapid increase in temperature.

The properties of such compression and expansion of gases may well reveal why the compression towers are non-symmetrical (Any research you can provide here would be most helpful)

Let’s not forget as the compression falls (expansion stroke) the cylinder is never fully discharged of gas/air due to the exhaust back pressure (VE limitation)

This in turn will produce a pressure differential during the intake stroke where the pressure in the cylinder is greater than the atmospheric pressure in the intake manifold (The reverse of what we require) hence a reduction in peak pressure due to insufficient cylinder fill.

Coming back to your glowing catalysts, in my experience this has been down to a lean mixture/dramatic air leak or fuel quality

Any feedback would be most interesting here

I hope this helps, take care……Steve

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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by Autonerdz » Tue Apr 24, 2018 12:35 am

This is a real phenomenon and not a limitation of the pressure transducer

Essentially everything takes time,
Yup. The reason high RPM WPS tests have no real diagnostic value has to do with the physics of pressure wave propagation to the sensor. At some point the pressures in the cylinder are changing faster than the pressure wave can reach the sensor.

This is the reason we see peak pressures get later as RPM increases past a certain point. Not because that is actually happening.

On a good sealed cylinder, pressures will peak near TDC on a stock engine. If there is a leak, depending on the rate of the leak, peak pressure occurs early (before TDC). Tests must be done at lower RPMs for valid results.

terbo
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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by terbo » Tue Apr 24, 2018 7:07 pm

Wow, thank you for these Infos.I coukd not get my head around that.

I Will have to look into more infos on WPS Tests.
But makes total sense so far to me.

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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by Technician » Wed Apr 25, 2018 8:18 am

One of the really good things about this forum is you never stop learning and understand something new from an old idea. What I mean by this is; back at college we learned about volumetric efficiency (VE) and cylinder scavenging and when CATS were introduced to us back in the 1980's we were educated about their effects on back pressure in the exhaust systems, we were told about software programming to balance the effects of the CATS fitted (my words) thirty odd years have pasted now, but I must take my hat off to Steve, I never thought about exhaust back pressure increasing to around 2 bar because of the cylinder design and the effects of (VE). In the professional tuning world the guys with the dyno's etc must have seen things everyday engineers would never have dreamed of seeing!

After your roadshow finishes I think you guys should start to think about designing and preparing some evening seminars and going to market with your skills and training, I'm sure it will pay dividends :D

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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by Autonerdz » Wed Apr 25, 2018 8:08 pm

Normally compression pressures follow intake pressures. This capture was made before the WPS existed.
Attachments
intcomp.jpg

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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by pinpointdiag » Fri May 04, 2018 9:52 am

Hi this thread is very interesting. I have a wps 500 and this explains so much. This is a fantastically powerful tool.
Lots to learn though. So thanks for all the useful information. Ill Have to read it all a few times.

The polo Looks to have a fuel trim fault though. I looked at one a few months back and also had my WPS on it.
suspecting the usual stretched timing chain causing incorrect manifold vacuum. causing it to run rich.
However the valve timing looked spot on.

The fault turned out to be the fuel filter. It has the pressure regulator in it.
There were three possible part numbers listed at the local motor factors. They could not tell which one it was
so sent me all three.

When they arrived two were clearly wrong as the regulator was interchangeable not built in, the third one looked the same.
But then i noticed it had 4bar stamped in to it. The one on the car looked like it had been there for some time.
On closer examination it had the number 4 stamped in it. The pressure i measured on the car had been 3.8bar.
I then got the exact part number from VW and got the factor to cross refinance it. It was not one of the ones they had sent,and was not listed for the vehicle. The factors are one of the uk.s largest motor factor chains and the filters were a quality after market brand.
Got the correct filter and it was stamped up 3Bar. I wonder how many times this will happen.

The tech data i had used said 4 bar and had added to the confusion.

Correct filter fitted and car fixed.

The car had been bought at auction buy a trader so we don't know how long it had been like that.

HTH alan

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Re: Polo 1,2 3-Cyl. - WPS 500 Pressure Question

Post by terbo » Tue May 15, 2018 9:09 pm

Just noticed this one was not completed, i never get a Notification
from the Forum, don´t know why.Anyways....

The Problem with this Car "Glowing Cat" was solved with a new Engine ECU.
Don´t know what went Bananas in that thing, but with a used one
the car drives nice as before but no more extrem Heating in the Cat....

But i learned much from this thread....

Thx
Chris

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