Discussion and questions related to the course Diesel Tuning Fundamentals
First of all HP academy did a great webinar lessons on it: https://www.hpacademy.com/previous-webinars/135-introduction-to-vnt-turbo-boost-control-variable-nozzle-turbochargers/
But some thing about them are left out so here is some more on the how and why they use them.
VGT or VNT are some of the names used by Garrett, BorgWaner and Holset turbo for the variable geometrical turbochargers.
These turbo are used in support to the EGR system on diesel engines as they are able to control exhaust and boost pressure a lot better @ the same time compared to a conventional wastegate turbo.
Fast spool-up @ low RPM is not the main motivation to use them as they have more moving parts and therefore more expensive to produce and break down more often so if it where up the a OEM they like a conventional turbo al lot better and they can spool as fast as a VNT turbo as well as there inertia of the rotating parts are the same. You just need a smaller exhaust side and a big wastegate to make this happen. The hi flow characteristic of a VNT turbo are not that great eider so between them it’s a give and take on efficiency. Remember diesel engine are not build to run hi RPM.
VNT/VGT turbo are used in different running modes by the engine management systems depending on engine load status or regeneration of the DPF system and so on and some turbo like the Holset VGT can also be used for engine breaking as well closing the exhaust side on the turbine side completely creating exhaust pressure. As you can understand no fuel will be injected during engine breaking.
MAF or MAP as for mass air flow sensor or manifold air pressure sensor.
Do I need to run MAF with a VNT/VGT turbo? No! I run all of them on MAP as long as there is no EGR system active. If a EGR system is active it can be good to now what air mass is entering your engine not to reduce oxygen level to a point the engine will not combust very well so running a MAF sensor can be a solution al do last generation diesel engines also use the o2 sensor for reading left over oxygen as well and adjust the EGR system when needed.
I do not like MAF sensors as there reading becomes very inaccurate even up to 5% @ 0,5Kg/sec air mass and most stuff I run dos 0,8Kg/sec so it’s MAP and airtemp for me to calculate airmass. Maf sensor are always in a spot that it will limit things and they break down very easy if things get extreme with dust and water.
Some of us are compensating for short coming in there programming skills on how to build a diesel ECU telling the story there way as it fits them best. In general not a smart thing to do as we all want to get the max out of our projects and a mass air flow meter being mandatory to sense shortcoming no please and try harder to get things working please.
Big chance in VE possible with a VNT turbo? Well yes but that’s manly due to the fact that the boost control function or set-up is done incorrect. Common mistake is to close the vanes to far by setting-up the end stops for the turbo activator the wrong way. Also if the boost control strategy is done correct the ECU is able to calculated the correct activator position and yes this correct activator position thing can be done with a single feed forward map as well but in general I only use this feed forward map to kick the turbo activator to the most desired position for optimal boost/engine response and link it to the accelerator peddle sensor, not fuel mass demand as this can delay things and let the PID boost control loop take care of things hitting boost pressure target spot on, but that’s personal as I’m a control freak. LOL.
When you say you tune MAP based load are you using both intake and exhaust pressure model? I appreciate diesels won't have overlap like a high revving SI engine but they must still have some VE sensitivity to IMP:EMP?
No I only use manifold pressure/temp based and a Mullier diagram to calculate air mass in general and VE correction for a small part. As for EMP or drive pressure, not very interesting on a small engine as setting up the end stops on a VNT activator correct and some realistic numbers in your boost target map will not give any problems regarding exhaust pressure and a big difference in VE on low and part load will also not give any problems as fuel/stroke is less than the oxygen level to burn this fuel efficient and if you give the PID controller more free dome it will kick boost up in a split second after applying fuel again opening the VNT vanes a bit more reducing AMP so there should be a good balance there as well.
Also it’s common sense that somewhere between 2100 and 2500Rpm you want the VNT activator 85 to 100% open so it runs like a free floating turbo from that point and as your boost pressure drops chance is you exhaust pressure will go up from that point as well running the turbo out of it’s flow map. Remember boost pressure it self dos not make big numbers on air mass. I often tell people they running there turbo like a hair dryer way above what the turbo can handle running hi EGT and smoke and compressor discharge temp way above 300C centigrade so yes for those over boosting there engines reading EMP and making it part of the boost control strategy will build a very good boost control set-up.
On the 12,7L rally engines we use a EMP sensor but this is not part of any fuel or air mass calculation. It’s just there as part of the boost controller so I can reduce boost when EMP gets to hi and also gives a warning on the engine parameter display as there is a possibility something is wrong with the wastegate systems.
We do run engines with valve overlap to scavenge out any exhaust gas even injecting intake air into the exhaust system this way to spool the turbo as cold air will expand in a hot exhaust manifold but these engines are very sensitive to exhaust pulsation from the manifold so get it wrong and you end up with a load of EGR. A diesel engine pulsates a lot more if it comes to exhaust gas. 200 bar combustion pressure on a diesel and opening the exhaust valve will give a big bang to start with and what comes after dos not have lot of volume left but what if this big bang bounces back somewhere in your exhaust manifold as having no place to go fast chance is it will enter the last bit of a other cylinder exhaust stroke trapping exhaust gas inside the cylinder. We seen this mixing up a 5 and 6 cylinder exhaust manifold on a 6 line race engine dumping exhaust gas out of 4 cylinders into one side of a dual scroll turbine housing even making it sound as a sick V8 so I did know something was wrong but were to start?? Take it all apart and find out is all you can do.
Example of a Mullier diagram air mass calculation.
Air temp g/Kg M3 @ 1013mBar