in cylinder pressure waveform that I don't really understand?

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Julius Nolin
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in cylinder pressure waveform that I don't really understand?

Post by Julius Nolin »

The car in question is a volvo 2009 with a 5 cylinder 2.5 liter petrol engine.
Troubleshooted from another workshop and a new engine is needed.
I looked at it a bit and did a compression test.
warm engine but not full throttle.
(disabled-friendly = impossible to accelerate and brake at the same time)

cyl 1: 4,25 bar
cyl 2: 10,5 bar
cyl 3: 11 bar
cyl 4: 11 bar
cyl 5: 11 bar

I did a cylinder leak test on cylinder 1 and it blew up heavily in one of the spark plug wells.
(unfortunately, I didn't check the exhaust pipe as I thought it was obvious that it was a valve problem)

ok, so now it is already the case that it will be a lot of work to get the car running again and I think the car will end up scrapped. But then we have a problem to solve so we can gain some new knowledge.

I recorded the cylinder pressure waveform from cylinder 1 and 2 with the engine running and as far as I'm concerned it's both cylinders that have problems.

I have attached the data from both cylinder 1 and 2.

Can someone help me so that I interpret these correctly. or am I completely wrong about it?

MVH Julius
Attachments
Volvo_V70_2009_Petrol_20220822-0002.psdata
cyl 1
(2.16 MiB) Downloaded 209 times
Volvo_V70_2009_Petrol_20220822-0003.psdata
cyl 2
(22.94 MiB) Downloaded 178 times

Steve Smith
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Re: in cylinder pressure waveform that I don't really understand?

Post by Steve Smith »

Hello Julius, thank you for the post

I have looked at the waveforms and one of the complexities to tackle is differing cylinder speeds

Capture Volvo_V70_2009_Petrol_20220822-0002 has a single buffer at an engine speed of approx. 1400 rpm

Capture Volvo_V70_2009_Petrol_20220822-0003 has 11 buffers at varying engine speeds

I have near matched 2 compression towers at 1400 rpm from both captures but need more information

I assume the compression values were achieved using a conventional compression gauge?

You mentioned "I did a cylinder leak test on cylinder 1 and it blew up heavily in one of the spark plug wells."

Can I confirm that you detected an air leak from cylinder 2 spark plug well when testing cylinder 1?

Could you confirm all spark plugs were installed (inc. cylinder 2) when testing Cylinder 1?

Sorry for all the questions, I am trying to get a mental picture of the diagnosis to date

Take care....Steve

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Julius Nolin
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Re: in cylinder pressure waveform that I don't really understand?

Post by Julius Nolin »

Hi Steve,

Thank you for the nice answer and I am always equally impressed by your interest.

compression values ​​are achieved with a conventional compression gauge with all spark plugs out and throttle closed (it is not possible to press the brake and gas at the same time without major interventions).

When I did my "cylinder leak down test" on cylinder 1, I had the cylinder a few degrees into the expansion rate. Then it blew up out of cylinder 3 (if I remember correctly) and that is consistent with the air going out the manifold and into the cylinder which is on exhaust cycle.

the customer has agreed to submit the cylinder head for refurbishment. so I've attached a little gobit that shows what happened.
now the only question is why did it happen?
the engine rebuilders said it was common on these engines :/

think I made a mistake in not sitting down and having a proper plan when I started using my Pico. as you wrote there that there were two different engine speeds. it means that the values ​​cannot be trusted. Studies continue and I thank you so much for the reply.

p.s. is it of interest to get an in cylinder pressure waveform after the car is finished? d.s.
Attachments
20220826_124801.jpg

Steve Smith
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Re: in cylinder pressure waveform that I don't really understand?

Post by Steve Smith »

Hi Julius, thank you for the feedback and nice image too

There is always something satisfying about turning the head over to find your suspicions are confirmed, nice work

I now understand the diagnosis you describe and based on a 5-Cylinder engine with a firing order of 12453 I can see with both valves closed on cylinder 1 (one exhaust valve burnt/leaking) how your compressed air will be detected in cylinder 3

Based on the fault and the difficulty in matching engine/cylinder speeds, we can see from your capture (-002) cylinder 1 is low on compression, with inconclusive/late EVO event and leaning compression towers (Blue waveform) N.B. Was the pressure transducer zeroed prior to test?
Image 1 A
Image 1 A
Below I have included an Overlay chart that helps to explain your cylinder leakage test results
Image 1
Image 1
Scope view 1 compares cylinder 1 with cylinder 2 (Lime green waveform)

Scope view 2 has cylinder 1 only

Scope view 3 has cylinder 2 only

A leaking exhaust valve on cylinder 1 will most certainly influence other cylinders on every stroke.
Let’s take the power stroke of cylinder 1 (with a leaking exhaust valve) the pressure associated with combustion will be “felt” by cylinders 5 and 3 as their exhaust will be open

The compression stroke of cylinder 1 (with a leaking exhaust valve) will be felt by cylinders 4 & 5 (not to mention the effects of overlap too)

Now let’s think about cylinder 1 which is continually exposed to the exhaust manifold dynamics for every stroke. The possibilities are endless as are the influences to our cylinder pressure waveforms

A pressure transducer in the exhaust tailpipe may well have returned a clearer picture but hind-sight is wonderful thing and time is always against us

I hope this helps and yes please, can you save and share your known good capture in the RWL as this will help others somewhere

Take care…..Steve

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