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Diesel knock at higher rpms and load?

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Guys, I have a 3.0l Audi TDI that I am running in a new aircraft that I and a team of guys have built. It's from a 2014 Q7. I have a Motec M141 that was initially configured by Motec guys here in Georgia. They put a base tune on it and got it running. I am now in the process of attempting to learn how to tune it. I have watched the Diesel tuning webinar as well as the one Motec offers so I have a decent understanding now on what I need to do.

The problem I have having is when I try to run higher pressures from the HPFP I get what seems like diesel knock at higher RPM and loads. A little background, my initial tune that the Motec guys over here set up was with the HPFP table maxing out at 95 MPa. I have a CP3 fuel pump that I know is capable of 200 MPa but when I try to bring the pressure up around 120 MPa the diesel knock comes in dramatically enough to reduce rpm by several hundred from say 3400 to 3000 in a second or so. This starts a cascading effect with the ECU backing off fuel and before anything can be done the rpm is down to 2000 without even moving the throttle. It makes quite a disturbing racket when this happens.

If I keep the HPFP below 110 MPa there is no problem. I have tried adjusting the injector linearization and that has helped a bit. I have also tried setting the pilot pulse so it stays throughout the complete rpm range. That seems to have had an improvement but I have not had a lot of time to test this change and I have not yet tried higher pressure than 120.

It's odd in that I can hear a different deeper note in the engine just before this happens and it can happen after about 5 seconds with the throttle and everything else pretty much constant.

Does anyone have any ideas what might be causing this and how I might resolve it other than capping the HPFP at 110 to be safe?

I will be posting a YouTube video tomorrow that shows it happening along with the Motec output. In the meantime I have attached a quick video showing where it crashes in the log.

Thanks to anyone that can help me solve this.


FYI, my YouTube channel is

Attached Files

A couple of things that jump out at me in this situation. First of all as you increase fuel pressure (assuming you're maintaining a constant fuel mass), the injector pulse width becomes shorter so you're essentially advancing the 50% burn point. You're also going to be introducing more fuel in a shorter time which means there will be more fuel introduced to the combustion chamber during the ignition delay which can make diesel knock more pronounced. You may find that as you increase fuel pressure you need to retard the injection timing - What is your timing at this point? Some additional pilot injection may also be beneficial here.

Secondly you're actually losing control of the HPFP in that log. You're targeting around 115 MPA but the fuel pressure climbs to 150 MPA and you can see the closed loop duty trying to reign it in. It looks to me like your feed forward table for HPFP duty needs some work.

Andre, thanks for your insights. I think we've discovered the root cause and it's that the lift pump I'm using is not providing the 4 bar of pressure that the HPFP requires. I'm upgrading to a FASS pimp capable of 70psi. I also have my injector return plumbing incorrectly going back directly to the tank rather than having back pressure from the HPFP as Audi requires it. These lack of pressures appear to be starving the injectors shortly into each run up. I'm going to resolve these issues and see how things are going there. Thanks.