Diesel injection effects on fuel rail pressure sensor

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Diesel injection effects on fuel rail pressure sensor

Postby Steve Smith » Fri Oct 11, 2013 3:51 pm

Hi Guys from Steve Smith at Pico Technology. I wanted to throw this out for discussion as it came up in a conversation this week regarding, "What effects can be seen on a diesel common rail fuel pressure sensor in relation to injector operation?"

A loaded question I know and this is why I thought it would make for a good discussion. I will be carrying out the following tests myself but if anyone has any info or theories I would much appreciate any feedback.

The idea was to identify a potential failing injector by looking at common rail pressure characteristics. I replied this week to a guy working with agricultural vehicles with the following:

Regarding a “sticking injector needle”, is it safe to assume this has then produced a misfire or a lumpy over fuel condition?

My first port of call would be serial data looking at fuel trim figures per cylinder. Forgive me if this is not a feature with your machine, but each injector will have a fuel trim value or feedback value which the computer has applied to each cylinder to maintain engine balance/efficiency. One of the ways in which it achieves this is to constantly monitor (like you have mentioned) the frequency of the output signals from the cam/crank sensor for irregularity due to misfire etc. The ECU has the advantage of being able to monitor this constantly over continual running long before the imbalance will be detected by the driver.

Using the scope in the same fashion is possible but only if the misfire is regular and apparent when testing. There most certainly will be a visible change to the shape of the waveform from the crank sensor during a misfire (if large enough and present), This could then be linked to injector operation using either the voltage probe or current clamp in order to identify which is the offending injector. Using Channel A on one of your injectors as the trigger and using Channel B as your crank sensor signal, look at the pattern acquired to see if your injector signal lines up directly with the irregular pattern in your crank signal. Here you will then have found your offending cylinder, if not choose another injector until the two patterns align.

On a further note, the fuel rail pressure sensor may also be another port of call but one which I have not evaluated yet? The theory is that there has to be an effect on the fuel rail pressure sensor when the injectors open, and if all injectors are delivering correctly then the effect on the common rail pressure should be seen as equal on the scope. However if one injector has a sticking needle, there could well be a characteristic visible in the signal output from the fuel rail sensor. Again link this signal to an injector using the method above and this could help to pin point the offending cylinder/injector

To conclude

Channel A use voltage or current clamp on an injector as a trigger to capture a waveform (Look at waveform for irregularity compared to a known good injector)
Channel B monitor crank signal for irregularity (Look for change of frequency of crank signal relevant to misfire)
Channel C monitor effect on fuel rail pressure sensor output. (Look for irregularity in pattern compared to known good pattern or against cylinders with no misfire)

I hope this helps and any feedback you have would be much appreciated.

Thanks for reading this guys and when I have some results I will post them for viewing

Take care.....Steve
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Re: Diesel injection effects on fuel rail pressure sensor

Postby KimAndersen » Sat Oct 12, 2013 9:45 am

Hi,

Now that you have brought this interesting subject up - became I curious to see if i could find any information regarding diagnosing a sticking injector needle on the web.

After a little search - did i find this website.http://www.auto-testers.com/index.php?id=strona_edia They have some good information regarding the subject you are looking for.

I haven´t used there product so i cant say any good or bad about this product.

Seen with my eyes it´s a oscilloscope. As a Picoscope owner this is a good news as we all know how good the Picoscope are, when it comes to capture such signals live.

Judge for yourself whether this is useful information or not. :wink:

Regards
Kim
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Re: Diesel injection effects on fuel rail pressure sensor

Postby Valhalla » Sun Oct 13, 2013 10:21 pm

Hi Steve,
Elsewhere on this forum is a thread for piezo current measurement for injector diagnostics. Now, strictly speaking, this is not quite what you were driving at in your post (I think) because this is trying to use an electrical charge characteristic to measure an electrical failure (piezo stack failure) rather than pure and simple hydraulic sticking. It does work to some extent, but you have to be careful.
As alluded-to in your post, you are better looking at cylinder contribution (through flywheel velocity) for a low/high injection quantity on one or more cylinders, however your are more likely to find that the scan-tool data for the ECU making that same correction is much more useful that whatever you can derive from the Pico (a rare thing, but for a good reason), so unless you can switch-off the cylinder balancing function in the diesel controller, then you are better using the scan-tool information. The Pico information for a "low" or "high" cylinder would take a huge amount of processing to be as good as most diesel controllers, but you would also have to know how the balancing function was switched against each engine operational domain (early Bosch functions only operate near or at idle, for example) and whether the data is kept from one drive cycle to the next, how it is used (if at all) at higher engine speeds and loads, etc. etc.
Also in this forum are a few threads on using rail-pressure to establish a sticking needle. Yes, you can do this, but you have to be really careful to not paint every engine with the same brush! How the fuel rail is laid-out can make a huge difference to the way that the RPS signal will fluctuate from cylinder to cylinder, even on a good engine, and also the way that the pump is set to compensate the fluctuations can make a big difference. So you could end-up with an answer for one engine type, which is useless for another.
All the best with your work!
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Re: Diesel injection effects on fuel rail pressure sensor

Postby Robski » Mon Oct 14, 2013 7:23 am

I'll stick to doing it 'Frank Sinatra' style !

BUT if Pico were to write in an RPM math channel(s) it would be useful to capture a CKP wave to observe RPM drop sync'd with an injector event.
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Re: Diesel injection effects on fuel rail pressure sensor

Postby Steve Smith » Mon Oct 14, 2013 9:46 am

Hi guys, this is great input and thank you so much. What has started as technical support question has now moved to a live case study. I hope to get to work on this issue with a donor vehicle here at Pico or at home. It most probably will turn out to be next week as my evenings are free and I am sure these checks will raise as many questions as they answer. One thing for certain is that the oscilloscope continues to open up additional routes of diagnosis for all vehicle conditions.This enables us to support serial data from our scan tool with real operational information from our scope to produce additional evidence when replacing such expensive components like injectors.

Looking at the cylinder balance function, how good would that be if we could switch this feature off as you suggested? This would give us a level playing field to evaluate individual cylinder contribution. This has got me thinking again (which is dangerous I know) do we know if increasing fuel pressure using the active test feature of a scan tool prevents the cylinder balance feature from operating whilst we captured the scope traces mentioned above?

Again, this is on my to do list and I will feedback ASAP.

Thank you all again for your input and the additional questions running trough my head. Take care......Steve
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Re: Diesel injection effects on fuel rail pressure sensor

Postby Steve Smith » Mon Oct 14, 2013 9:51 am

Just clicked Rob. "MY WAY" ? Sorry....slow or what?
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Re: Diesel injection effects on fuel rail pressure sensor

Postby Steve Smith » Tue Oct 29, 2013 9:56 pm

Sorry about this, I have lost my last post and hope it finds its way to you tomorrow. If not I will try again tomorrow night!
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Re: Diesel injection effects on fuel rail pressure sensor

Postby Steve Smith » Tue Oct 29, 2013 9:56 pm

So Sorry I have taken so long to follow up this quest but it’s been a little busy (No excuses I know)
To cut a long story short regarding the effects on common rail pressure and Diesel injector operation, I have carried out 2 tests, the first of which on a 2007 BMW 318d. The plan was to look at the relationship between injector operation, common rail pressure, and engine speed to see it would be possible to determine the condition on of an injector or even confirm sticking/poor fuelling
Well so far so good by looking closely at the common rail fuel pressure alone it was possible to analyse and reveal every pulsation in the common rail from the HP pump delivery to the release of fuel via the injector. What was interesting to see was the increase in common rail pressure at the point of injection? (The reverse of what I would have expected). By filtering this signal and using the AC coupled feature there are some very revealing characteristics within the pattern at the point of injection that will contribute to evidence to support injector diagnosis.
I have included 2 BMW psdata files using the AC coupled technique with an induced misfire, along with a zoomed image with some helpful pointers.
I will post a similar set of results with a 2002 Mondeo. I sincerely hope this helps and would appreciate any feedback.
Attachments
MONDEO 2.0D NO.4 INJECTOR MISFIRE.psdata
(1.12 MiB) Downloaded 319 times
BMW FUEL PRESSURE OVER INJ-20131031-0001.psdata
(1.65 MiB) Downloaded 247 times
BMW FUEL PRESSURE OVER INJ NO2 CYL MISFIRE 20131031-0001.psdata
(2.21 MiB) Downloaded 273 times
CR PRESSURE SENSOR-29-10-2013 21-30-39.jpg
COMMON RAIL IMAGE
Last edited by Steve Smith on Thu Oct 31, 2013 10:53 am, edited 1 time in total.
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Re: Diesel injection effects on fuel rail pressure sensor

Postby taho » Wed Oct 30, 2013 1:09 am

steve smith wrote:What was interesting to see was the increase in common rail pressure at the point of injection? (The reverse of what I would have expected)... I sincerely hope this helps and would appreciate any feedback.

Hello!
It can be assumed that the peaks (increased voltage) is related to the fact that the injectors and pressure sensor connected to the control unit, the current to the injectors 15 amperes, and the voltage to 80 volts (peak). That is, probably 0.27 volt surge - the problem of stabilization (filtering) the power circuits of the control unit ... good luck.
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Re: Diesel injection effects on fuel rail pressure sensor

Postby Steve Smith » Thu Oct 31, 2013 10:57 am

Many thanks Taho for your input, it really does help. I will try to carry out the same tests over the next week or so using a separate power supply and earth independent of the vehicle at the fuel rail pressure sensor. I will then retest and view the effects on the fuel rail pressure sensor output again in relation to injector operation and will feedback ASAP. Many thanks again, take care.......Steve
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