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Fuel rail pressure control on Commonrail diesel engines

Diesel Tuning Fundamentals

Discussion and questions related to the course Diesel Tuning Fundamentals


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As with so many things related to modern diesel injection systems not a lot of information is out there so copy paste settings and misunderstanding the true working on systems can be a common thing among diesel tuners.

The basics if you want to run an after marked diesel ECU system splitting things up a bit for better understanding.

Fuel rail or accumulator that hold fuel under pressure also known as the Commonrail.

3 things will happen in this rail that are very significant setting up rail pressure control.

1 fuel pressure drop while injecting hi quantity diesel.

2 fuel pressure spike due to injector closing. Remember diesel fuel has a mass as well.

3 fuel pressure wave travelling inside the hi pressure fuel system

If your able to take a hi speed pressure reading, pressure on small diesel Commonrail system can fluctuate up to 200Bar and in order to compensate some manufactures place orifices in the fuel rail or other parts of the fuel injection system to dampen this a bit but in most cases the ECU will read these pressure fluctuation in full so some things need to be done. More about this later on.

Fuel metering valve or fuel inlet metering valve on the hi pressure fuelpump.

In general there are two types. Most are NO or normally open so you need to apply power on them to close them. Other type less uses is the NC or normally closed valve so with this type you need to apply power in order to open the valve. Take note Commonrail systems with a NC valve on the fuel system often do not use a safety overpressure valve on the fuel system so things can become dangerous controlling fuel pressure as a NO valve.

To make things a bit more complex there are fuel metering valve that are very happy to take every position between the open and close position with a minimum correction on linearisation needed but there is also the type of fuel metering valve that are pulsating and there for runs on a lot lower PWM/Hz number. Trying to run these @ hi frequency PWM will not work with a possible chance of damaging them as they will get to hot.

Also take note not all PWM outputs on aftermarket diesel ECU are the same a those used on the factory ECU so it’s not uncommon that the fuel metering valve gets sticky on some ECU models while dialling in factory ECU settings. You can call this copy paste diesel tuning.

Reading fuel pressure form the Commonrail and get a decent reading out of it needs input filtering on the ECU. Will not start a conversation on how to do this as it’s to dependant on the fuel system it self all I can say about it we program our ECU systems with a engine load table to changes sample rate an filter intensity dampening out harmonics and this we compare with unfiltered input data from the fuel pressure sensor to optimise the fuel pressure reading before we use it as a feedback for the PID loop that controls the fuel metering valve/fuel rail pressure.

Setting up the PID loop and feed forward table for rail pressure control. Where to start???

Well read all of they above and get this working well. Well for most this will be impossible and to be fair some so called professionals on ECU systems will struggle as well ending up with a compromise putting most of there effort into the feed forward table fuel pressure control after intense data logging digging out some PWM effort numbers to populate the feed forward tables setting low number on proportional gain PID loop to avoid fluctuation. All I can say get the hardware working correct and sort out a good input filter for the fuel pressure sensor and 80% of all work is done. So only the last 20% will be PID loop and I personally use the feed forward table to kick the fuel pressure up and down and let the PID loop sort out target fuel rail pressure setting more aggressive gain numbers. Some of the fuel systems we use are extreme and proportional and integral gain 3D tables for the PID loop will make it possible to set different gain levels for under/overshoot as for very aggressive engine speed control for example where fuel demand can jump from 100% to zero very fast.

We also have some drivers that are tap dancing the accelerator peddle and a common solution to this is to delay the toque response action from the ECU giving the driver less control over the engine but the PID loop for fuel pressure control is able to keep up. With a bit more effort on PID control fuel pressure control your able to gain on that very aggressive toque response from a diesel engine without massive un/overshooting fuel pressure target and yes this is next level engineering refining the ECU control strategy and must admit every fuel system has different demands that can not be set with a standard set of functions/tables the easy way. Not doing this you could say the control functions on the ECU are not partially good @ any thing other than being a compromise on things that will work well for most of us unless you planning to go extreme.

Common mistakes made.

Fuel pressure control and injector fuel flow map. Again good input filtering on fuel pressure will avoid large changes in fuel metering of the injectors creating pressure fluctuation on it self and the PID loop for fuel pressure control will react on this. You can call this a secondary close loop error. There is no point setting the final sample rate for fuel pressure reading faster than what the fuel system can handle and same goes for PID loop speed as well. Thing is dos your aftermarket ECU have these settings and can they be used and are these options recommended to be used??

Slow oscillation of the fuel pressure under load. Most common problem and it can even been seen on some diesel training webinars. Integrator windup/down? Set lower gain numbers and look for pressure jump in relation to control effort by the ECU. If so big chance is you fuel metering valve get sticky so chance PWM frequency and look for improvement.

In this youtube movie your able to see the result of lowering a gain level on fuel pressure control but it’s still jumping if you look closer so it turned out PWM frequency was not set according to the documentation so we chance it able to set more aggressive gain levels on the PID loop.

[url=https://www.youtube.com/watch?v=7-lp6SU945M]https://www.youtube.com/watch?v=7-lp6SU945M

To sum things up 80% is understanding the hardware and dialling in the correct numbers into the ECU and 20% is ECU related as for PID loop stuff.

As last. Air or vapour in your fuel system is bale to expand after the fuel metering valve pressing unwanted fuel into the hi pressure pump chamber in the fuelpump unable to get good control on your fuel system. Something to look for in the first place with unwanted fuel pressure control problems.

First off i want to say thank you for taking the time to write out all this and sharing your knowledge of the common rail fuel system operation. I know this takes time and effort to, just know you are going to help many people with this post including myself. There are few people who have your level of knowledge in this field, let alone are willing to share it. So thank you, I look forward to having some good conversations in the future on topics like this.

Thanks Ruud.