First look sensor signal delay

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thoker
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First look sensor signal delay

Post by thoker »

Hello everybody,

I ordered a pressure pulse / first look sensor an attached it to the fuel pressure regulator on my 1992 Audi S2 Coupe, to check for fuel delivery problems.
I used the injector current of the third cylinder as sync, but the first look sensors signal did not match to the injector events as I thought it would.

The sensor came with a 1.63 m long hose with a diameter of 5 mm.
I replaced it by a smaller hose (0.18 m, diameter 6 mm) and repeated the measurement.
On the one hand, the signals amplitude decreased (I would think related to smaller gas mass in the hose), on the other hand the signal measured with the small hose had a time delay of about - 4.9 ms.

The difference of the hose length is about 1.63 - 0.18 = 1.45 m.
Taking sonic speed with 343 m/s into account, I get a time difference of 4.2 ms between both signals, which is near the actual time delay of 4.9 ms.

Is my assumption correct?
If it would be, the same calculation would be necessary when measuring gas pulsations at the tailpipe (sonic speed would change depending on gas temperatur and gas composition).

Thank you
Best regards.
Attachments
Comparison_hose_length_firstlooksensor.psdata
(13.54 MiB) Downloaded 347 times
Last edited by thoker on Sun Aug 30, 2020 7:54 am, edited 1 time in total.

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PicoKev
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Re: First look sensor signal delay

Post by PicoKev »

I have never seen a genuine First Look sensor supplied with a hose anything like you describe.

It comes as a kit like this https://www.picoauto.com/images/uploads ... _NO-CD.jpg

Where did you get yours from?

Kev.

muttnjeff
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Re: First look sensor signal delay

Post by muttnjeff »

Some suggestions or observations

An attached wave form is helpful

Some background on what the problem is and what diagnostic process or strategy you are using or used to get to this point

If fuel delivery is a concern why not check fuel pressure with a gauge

I’m assuming your using the sensor on the vacuum port of the regulator?

More info you supply the better the help you can get in return

liviu2004
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Re: First look sensor signal delay

Post by liviu2004 »

Must be a ditex one. This should be addressed on a ditex forum, not here!

thoker
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Re: First look sensor signal delay

Post by thoker »

Thanks for your response.

I attached a waveform to my first post, it didn't work at the first time - sorry for that.

@muttnjef:
I thought it would have no role, where the pressure comes from, depending on the question of time delay, using a longer hose on a pressure sensor.
But yes you are right, I am using a ditex pressure pulse sensor, attached to the fuel pressure regulators vacuum port and want to compare the drop of fuel pressure, when the injeactors open.
Global fuel pressure is fine, my fuel pressure gauge (pressure transducer) is not sensitive/ fast enough, for measuring the drop of fuel pressure when an injector opens.

@liviu2004:
Don't understand your concern. In my opinion, the behaviour of the signal is not sensor related, but simple physics. Furthermore I combine a ditex sensor with a picoscope - hard to find the right forum for this exact combination :wink:

I did another test yesterday:

- Disabled ignition, so the engine cranks without starting
- Disabled four of five injectors by unplugging them
- Measured injector current of the remaining injector
- Measured pressure pulse at fuel pressure regulator while cranking
- With this method I can be sure not to have any superpositions / pressure vibrations from other injector events in the pressure signal

Did this two times (long and short hose on first look sensor) and compared the resulsts:
-> When measured with long hose, the pressure signals responses about 4.6 ms later, compared to the measurement with the short hose

Would love to see a similar test, done with a WPS.
Attachments
Comparison_hose_length_injector_event.psdata
(23.02 MiB) Downloaded 327 times

liviu2004
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Re: First look sensor signal delay

Post by liviu2004 »

@thoker, fine you use such sensor but would be nice to put facts on the table. Otherwise one could understand you refer to pico products. No nice in my eyes.

muttnjeff
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Re: First look sensor signal delay

Post by muttnjeff »

Unless you are using the sensor reading as a measurement to a specification that is set by the manufacturer of the engine I don’t think a calculation is of any concern, reason being the pressure pulse sensor (regardless of brand) is used to help identify irregularities is pressure/vacuum pulse waveforms.

Now it’s possible I do not use it to the full extent of its capabilities, so I’m sure there may be more thoughts on your original question.

thoker
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Re: First look sensor signal delay

Post by thoker »

@liviu2004:
This was not my intention, sorry for that. I just thought about it the following way:
If the time span it takes, until the measured medium made its way from the source (e.g. pressure regulator) to the sensor, isn't way shorter than the cycle time of interest (e. g. injector events), you have to consider the signal delay, for not assigning a misfire to the wrong cylinder for example.

If the response time of the sensor is much shorter than signal frequency and time delay of the measured medium, it shouldn't matter which sensor you are using.
In my case, a longer hose changes the time delay significantly, therefore I am assuming, the response time of my first look sensor is negligible.

Does anybody have the chance to do similar measurements with a pico WPS or first look sensor to proof my theory?

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Fat Freddy
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Re: First look sensor signal delay

Post by Fat Freddy »

In my case, a longer hose changes the time delay significantly,
Would you have any data files to show that? If not, you've given me an excellent idea.
Does anybody have the chance to do similar measurements with a pico WPS or first look sensor to proof my theory?
Yes, I have. It's on here somewhere and it was a thread about valve overlap. I used a homemade first look sensor against the WPS looking at pressure waves in the exhaust manifold (not fuel pressure but still reaction to pressure change). It has the crank angle sensor as a clock.
That thread is the reason I'd like to see proof of 'significant' delays in signal time to hose length.

Here: topic10481.html?&p=33523&hilit=valve+overlap#p33523

thoker
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Re: First look sensor signal delay

Post by thoker »

Hi Fat Freddy.

Yes, please have a look at "Comparison_hose_length_injector_event.psdata", some posts above.

Imagine an 8 cylinder engine at 3.260 rpm, than 4.6 ms is enough to assign a misfire to the wrong cylinder.

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