Individual cylinder valve timing errors

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Individual cylinder valve timing errors

Postby Steve Smith » Fri Jun 30, 2017 3:49 pm

Everyday day we learn something new and last week was certainly no exception at the AMT Automotive Boot Camp held in Holland.

My role was to deliver a 45 minute presentation to 10 groups over 1.5 days where I met likeminded technicians all hungry for something new.

To cut a long story short, my presentation focused on WPS In-Cylinder measurements of a petrol engine where the valve timing could be manually adjusted to demonstrate the diagnostic capability of the pressure transducer.

The engine used was the PSA “TU” 1400 cc unit where the valve clearances were adjusted as follows:
Cylinder 1 valve clearances correct (Channel A Blue)
Cylinder 2 excessive intake valve clearance (Channel B Magenta)
Cylinder 3 Excessive exhaust valve clearance (Channel C Lime)
Cylinder 4 Exhaust valve with zero valve clearance - Held slightly open (Channel D Black)

The captures were taken simultaneously using 4 x pressure transducers with the intake partially restricted in order to produce a well-defined intake pocket during. (This helps to identify the exhaust valve closure and intake valve open points)

4 x WPS.jpg
4 X WPS


The psdata file below has the waveforms captured from the vehicle above during cranking. The Reference Waveform and Delay feature of PicoScope has been used to align the waveforms to enable like for like comparisons. Pages 30 and 132 will explain these features here: https://www.picoauto.com/download/docum ... ide-en.pdf

TEST 14 INTAKE LOOSE BLOCK AND BREATHER HOSE BLOCK.psdata
PSDATA File with 4 x WPS inc REF Waveforms
(9.14 MiB) Downloaded 58 times


Here we have a screen shot of the reference waveforms aligned together. The colours of the channels were changed to help with clarity when they are overlaid

4 X WAVEFROMS OVERLAID.jpg
Waveforms overlaid


I think we can see how the activity midway between the compression peaks (valve overlap) has changed for each waveform.

The Blue waveform is taken from cylinder 1 with no valve clearance issue and so forms the benchmark for all another cylinders to be compared against. We are often asked “What should good one look like?” and often there is no answer as there simply is no technical data.
In which case we can refer to the Waveform Library or refer to the vehicle in hand where we have a known good cylinder. It is rare to have 3 cylinders with different valve timing errors and hopefully one cylinder will be performing correctly. The phrase “Compare Apples with Apples” springs to mind

Cylinder 1 (Blue) valve event approximation:

CYLINDER 1 APPROXIMATION.jpg
Cylinder 1 valve event approximation


Interesting how there does not appear to be any valve overlap around 360 degrees as the pressure waveform falls negative prior to 360 degrees suggesting the exhaust valve to be closed and inlet open! (Food for thought)

Cylinder 2 (Magenta) valve event approximation

CYLINDER 2 APPROXIMATION.jpg
Cylinder 2 valve event approximation


What stands out for me here is the retardation of the inlet valve open event (IVO) given the deep negative pressure formed in the cylinder between 360 and 540 degrees.
What intrigued me was the inlet valve close event (IVC) being advanced as the compression starts to rise far sooner than cylinder 1 (Blue) shortly after 540 degrees. Valve duration is shortened.

The point to note here is that excessive valve clearances result in a retarded opening event and an advanced close event.

I mentioned earlier that we learn something every day, here is what I picked up from the Boot Camp crew. I was asked the question, “How can you determine the difference between excessive valve clearance and a shifted camshaft lobe?”

I have to admit I did not know the answer until we discussed the scenario, in a nut shell: Your valve timing will change for the valve in question (inlet or exhaust) but the duration will remain the same. (Unlike excessive valve clearance)

The second question came soon after “Why is the compression higher on cylinder 2 with excessive valve clearance?”

I originally thought that the compressions may have been uneven before my adjustments as I did not check, but one of the delegates highlighted how the compression stroke has increased due to the early closure of the inlet valve!
He went onto discuss how this was a technique they used during the winter months with fixed duty diesel engines where the compression is raised to assist with cold starting. (Brilliant) I guess this is fine for fixed duty engine and for this engine during cranking but may introduce combustion anomalies at higher engine rpm and load.

Notice also how the expansion pocket negative pressure between zero and 180 degrees is greater for cylinder 1 (blue) due to the fact the peak pressure at zero degrees is lower.

The expansion pocket is formed as the piston descends the cylinder during the Expansion stroke whilst the inlet and exhaust valves remain closed.
Given we are starting from a lower peak pressure (cylinder 1 blue) it’s no surprise how the negative pressure will be greater given the initial peak pressure was lower than cylinder 2

“We live and learn together”

Cylinder 3 (Lime) valve event approximation

CYLINDER 3 APPROXIMATION.jpg
Cylinder 3 valve event approximation


Here we have the reverse of cylinder 2! The exhaust valve opening event is regarded and the close event advanced. This can be seen by the positive pressure formed between 180 and 360 degrees.
This pressure drops immediately, prior to 360 degrees when the intake valve opens. Notice how the intake and compression events are identical between cylinder 1 (blue) and cylinder 3 (lime) as the intake valve clearances are the same

Cylinder 4 (Black) Exhaust held slightly open

CYLINDER 4 LOW COMPRESSION.jpg
Cylinder 4 low compression


Not really sure you a need pressure transducer in the scenario above as low compression is conclusive regardless of using a transducer or compression gauge. (Unlike the other cylinders)
Any feedback here would be most welcome as to whether we could identify which is our offending valve. (We know the exhaust valve is held open with this cylinder)

I have highlighted my concerns which are non-uniform compression towers, a reduced expansion pocket, and what looks to be a retarded opening of the intake valve between 360 and 540 degrees
I have racked my brain why the waveform looks like this (between 360 and 540 degrees) and my gut feeling is the piston is required to travel further down the cylinder before any negative pressure develops. Remember the exhaust valve is held slightly open but the piston seems to be able to pull a negative pressure (overcoming the cylinder leak)

Moving onto the final demonstration which surrounded cylinder leakage.

With 4 transducers connected to 4 cylinders an opportunity arose to introduce compressed air into a cylinder with closed inlet and exhaust valves, (But which one?)

CYLINDER LEAKAGE.jpg
WPS Cylinder leakage tap


Rotating the engine by hand whilst monitoring the cylinder pressures on a slow time frame reveals which cylinder to leak test:

VALVES CLOSED.jpg
Closed valve test


From the waveform above we can reveal not only which cylinder has closed valves but also in which direction the piston is travelling. A positive pressure indicates the piston is rising on the compression stroke whilst the negative pressure indicates the piston is falling on the expansion stroke.

Locking the engine in this position allows for the pressurization of cylinder 2

CYLINDER LEAKAGE 2.jpg
Cylinder 2 leakage


So here we have the cylinder pressurized to 4.1 bar where the pressure tap is closed at 13 seconds. The time taken for the pressure to decay in the cylinder was approximately 26 seconds. I guess the question now: “is this a good cylinder?”
Given there were no compression issues with this engine we must assume this is a typical value. Once again we need to compare apples with apples as I don’t believe there is such “decay time” data out there. The Waveform Library could be a huge resource is we adopt such a technique.

If this were a faulty cylinder, then the decay time would decrease accompanied with an air leak into the crankcase, exhaust, intake or an adjacent cylinder. Here then another WPS located in any of these locations would provide evidence of the source of the leak.

The vehicle used during this test utilised a MAP sensor. With the ignition on and the intake manifold blocked we could monitor MAP sensor voltage for any fluctuation in proportion to the cylinder leakage, (If the intake valve were the offending component)

All in all there are a number of things to discuss here as to the accuracy of valve open and close events but comparisons with non-offending cylinders is key to reveal anomalies

I hope this helps to reveal the possibilities of detection when it comes to shifted camshaft lobes, lifted camshaft, worn followers or poor/depleted valve seats

There is a great forum post here topic10481.html that digs deep into valve event approximation called “The search for overlap” Lots to read, digest and discuss for sure.

To close here, some typical values for valve open events for engines with fixed valve timing:

EVO 50-60° BBDC
EVC 5-15° ATDC
IVO 0-10° BTDC
IVC 50-60° ABDC

Now as for VVT that’s another discussion all on its own.
Take care…..Steve
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Re: Individual cylinder valve timing errors

Postby STC » Fri Jun 30, 2017 11:37 pm

Hi Steve.

Thank you for putting that together - will come in very handy one day.
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Re: Individual cylinder valve timing errors

Postby victor2k » Sat Jul 01, 2017 3:46 pm

Hello,
Thank you for this new tests.
For the cylinder leakage test can be used a pressure leak kit(connected in series) as a reference and maybe the final result will be added in Pico Automotive? :)
And related to the closed valve test you must be sure about the constant angular velocity.
Best regards
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Re: Individual cylinder valve timing errors

Postby Fat Freddy » Sun Jul 02, 2017 8:53 am

Another excellent presentation Steve and appreciate the effort that goes into doing them. I'm still chewing them over but with regards to:

I have racked my brain why the waveform looks like this (between 360 and 540 degrees) and my gut feeling is the piston is required to travel further down the cylinder before any negative pressure develops. Remember the exhaust valve is held slightly open but the piston seems to be able to pull a negative pressure (overcoming the cylinder leak)


I believe it's due to piston speed down the cylinder lowering the pressure faster than valve can leak, which is why the pressure rises again as piston speed decreases.
The image does not fit perfectly but I think one has to remember that because crank speed changes considerably during the cycle, rotation degrees is not going to match time. For example at 1006.7 the crank will be way past the 470* indicated as its just past the fastest part of the stroke.
Just my thoughts.

Piston velocity.jpg


HTH
FF
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Re: Individual cylinder valve timing errors

Postby Steve Smith » Mon Jul 03, 2017 11:45 am

Thank you for the feedback and thank you FF for the image above.

I just had one of those "penny drop" moments when I studied the image and the relationship between crank angle and piston velocity

There is so much to consider here with the 4 stroke cycle. "Nothing is easy" thats for sure.

We also have to me mindful of the effects of neighbouring cylinders upon the cylinder we are measuring.(Thinking here about engine running)

I had the luxury here of 4 x WPS attached to 4 cylinders measuring simultaneously during cranking. Basically the engine was just one big air pump.

In the real world we measure each cylinder individually at idle speed. The dynamics then change with regards to airflow and temperature.

A neighbouring cylinder with a seeping intake valve has the potential to change the shape of the cylinder waveform we are measuring.

This could lead us to conclude the cylinder under test is the offending cylinder when in fact it is purely "feeling" the effects of a neighbouring, offending cylinder.

More to consider no doubt, take care......Steve
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Re: Individual cylinder valve timing errors

Postby jamesdillon » Wed Jul 05, 2017 8:18 pm

Hi Steve

I am always amazed by the detail and sometime confounding information garnered by the WPS in cylinder.

I did a little case study a while back concerning a real world example of varying compressions and valve timing on an engine running fault.

I posted the link here http://www.aftermarketonline.net/Technical/2014/29871-/Under-pressure for anyone who is interested.

Perhaps an idea for a Pico Quad WPS kit, or is it just too soon to write to Santa?.

Cheers

James.
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Re: Individual cylinder valve timing errors

Postby Fat Freddy » Thu Jul 06, 2017 4:43 am

Still playing 'Devils advocate' here. :)

Re "Cylinder 2 excessive intake valve clearance (Channel B Magenta)"

I'm not convinced about the reasoning behind “Why is the compression higher on cylinder 2 with excessive valve clearance?”

And the theory
"I originally thought that the compressions may have been uneven before my adjustments as I did not check, but one of the delegates highlighted how the compression stroke has increased due to the early closure of the inlet valve!
He went onto discuss how this was a technique they used during the winter months with fixed duty diesel engines where the compression is raised to assist with cold starting. (Brilliant) I guess this is fine for fixed duty engine and for this engine during cranking but may introduce combustion anomalies at higher engine rpm and load."


That is not comparing apples with apples. EDIT: Yes the compression stroke has increased but the cylinder filling time, therefore efficiency, has been reduced. This theory may work on an engine that does not use a throttle plate. I feel it falls a loooong way short of apples.

My thoughts are, assuming the firing order on this is 1,3,4,2 and not knowing the exact set up or how much the excessive the clearance is, so I may be completely off track here.
Prior to the cylinder 2 event we have the cylinder 4. Cylinder 4 is a leak into the system. (I think you know where I'm going :twisted: ). I don't think with this leak into cylinder 4, the piston is going to have the capability to pull the same vacuum on the inlet manifold as a non-leaking cylinder, In effect we have doubled the throttle opening. . So when number 2 inlet valve opens there is a far bigger pressure differential, than on other cylinders, between cylinder 1 and the inlet manifold when cylinder 1's inlet valve opens. Therefore cylinder filling efficiency is a lot higher.
In plain english, the fault with cylinder 4 is effecting cylinder 2.

Just a thought Steve and I could well be wrong. What do you think?

Another thought. This time regarding:

I also have some worn inlet cam files which I may put up. Unfortunately diesel but may be worth sharing.

Cheers
FF
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Re: Individual cylinder valve timing errors

Postby jamesdillon » Thu Jul 06, 2017 8:22 am

Hi FF

Your assumption regarding the engine drawing on a closed throttle may not be the case.

Some engines, during cranking, hold the idle air control valve wide open, thus the incoming air flow circumnavigates the closed throttle via the idle air 'loop'.

Cheers

James.
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Re: Individual cylinder valve timing errors

Postby jamesdillon » Thu Jul 06, 2017 10:16 am

Here's an image from our Oscilloscope Masterclass training course showing valve clearance and valve timing relationship as mentioned above..

valve clearance effects.jpg
Valve timing affected by valve clearance
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Re: Individual cylinder valve timing errors

Postby Fat Freddy » Thu Jul 06, 2017 2:36 pm

jamesdillon wrote:Hi FF

Your assumption regarding the engine drawing on a closed throttle may not be the case.

Some engines, during cranking, hold the idle air control valve wide open, thus the incoming air flow circumnavigates the closed throttle via the idle air 'loop'.

Cheers

James.


Hi

That is correct. But in the case Steve has presented we know the throttle/idle circuit is sufficiently closed as it has created a vacuum with in the inlet manifold. What we don't know is the pressure within the inlet manifold and how it has been affected by the riding exhaust valve during the previous intake stroke and therefore volume metric efficiency. Or weather ABDC the still partially open inlet valve is letting air in or out the cylinder during such a slow rotational speed. Also we don't know any thing about the diesel engine.
That is why I don't think the theory is comparing apples with apples". There are to many variables.

I would like to have the test done with out the leaking exhaust valve during the previous intake stroke. Sorry Steve :) :wink: . I think there is a possibility the higher cylinder pressure was the result of a higher inlet manifold pressure not because the compression stroke was longer. Or possibly a combination of both.
As I said 'Devils advocate'.


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