NefMoto

Hello all. I compound turbo'ed my wife's B6 A4 1.8T a while ago. We've daily driven it now for 2 years just on 18psi and big turbo on wastegate (6psi).

I left it alone and now would like to start up again. I've considered a few different approaches to boost control and wanted some input from this forum.

I have a 2nd N75 (for 2nd WG) and MAP with integrated IAT (between turbos) all wired into ECU. ECU can control 2nd N75, but it's only open loop right now adjustable by unisettings (adaptation channel). ECU has access to 2nd MAP and IAT as well.

I could completely duplicate existing PID control functions or just run thru existing functions twice with 2nd set of maps.

I could set low pressure (LP=big turbo) target boost independent of high pressure (HP=small, stock turbo) target boost or have a pressure ratio vs RPM map of one turbo and extrapolate target boost for both turbos.

I'm concerned both kfdmix maps (1 for each turbo) will change based on what boost the other turbo is at, requiring an infinite number of kfdmix variations.

Just thinking out load.

Freudian slip?  ;D

Cool project, any pictures of the turbos?

How is the ecu controlling 2 separate n75's (separately)?

A few notes.

1. There needs to be only one boost target.
2. You must be using some sort of bypass for the small turbo, right? That is your switch point.
3. To control the big turbo you can just write your own code. I see no point in running ME7's (broken) algorithm twice.

I have source Keil files for a standalone boost controller within the ME7 ECU that I wrote.
You will need to adapt it to the ECU and adapt the switchover, but you could use it. It is full PID with a single pre-control map.

T3/T4 journal over the stock K03:

(http://creativeion.com/rey/vw/b6_bt/b6_compund_turbo_project_mock06_sm.jpg)

I've coded the ECU to use an unused PWM output based on an re-purposed adaption channel memory address.


1. 1 manifold (total) target + 1 LP target between turbos to keep both turbos as close to their efficiency islands as possible as oppose to one turbo doing more work than the other.
2. No compressor side by-pass. Intake air always goes thru both compressors. Large(LP) first, then small(HP), then IC, then manifold
3.  Any links to discussion on this broken opinion? Works good enough for me to try and utilize it and save me some custom coding work I can the use else where.

Thanks,
Rey

I'd love to feel what this car drives like. Compressor outlets Y'd together? Intake the same through a single Maf?

No 'Y'. In series. Intake -> MAF -> T4 -> K03 -> IC -> manifold

(http://creativeion.com/rey/vw/b6_bt/b6_compund_turbo_project_mock03_sm.jpg)

It's nothing special to drive right now. Only running stage 2ish power for reliability. Then tuning got put on back burner while we drove it for 2 years, mainly wife.

I love this project, can't advise on tuning strategies but I will follow this! I think k03 bypass control (exhaust) is a big factor in how much power this compound will make, so in that case there is not much pressure control needed after the t3/t4 is spooled? I mean you should be looking for flow, not for boost with this concept I would say.. My first thought is to open up all gates as soon as you have spooled the big turbo  :)

Nice work,is that custom welding? Or a kit? Would never get such fun in my transverse engine bay and the transfer box

Why don't you just run a manual boost controller? Apparently you get faster spool times

lol

Please explain, mechanically, how a manual boost controller can hold a wastegate closed tighter than completely closed.

If that was sarcasm, kudos :)

i would add a bypassvalve over k03 compressor.
Will be restrictive in topend.
And use big wastegate over k03 turbine.
using stock k03 wastegate will restrict airflow and cause overspeed.

Yes this setup is quite pointless this way. Won't make any more power than just the K03.
You need bypass on both sides. And there needs to be only one boost target.

K03 needs big external WG and in the intake you need a Y with a switchover. Either mechanically (look at Transporter 2.0 BiTDI) or electrically (vacuum) controlled (look at BMW 535d).

As long as the K03 can spin fast enough to generate ANY positive pressure, it will not be a restriction on the intake side. Exhaust side is a different story. However, this setup focused on simplicity, not peak power. Installing it is not much more than replacing downpipe and turbo inlet pipe.

Consider running in colder weather. Colder = more dense intake. Is there a point where ambient temp is so cold and air is so dense that K03 becomes a restiction instead of still pressurizing intake? No. Compressor takes pressue in front and multiplies it, regardless of front pressure. Instead of cold weather providing more dense pre-compressor air, the big turbo does. Same idea as elevation changes.

For the record, what is considered more that just K03 power?

If I want percise control of both turbos to maximize their efficiencies, I need two boost targets.

prj you've said twice I only need one, both times without explanation, but that's expected of you.


The big problem with this setup is the small K03 exhaust housing and wastegate. I haven't turned it up enough yet to see the problem, but I'm sure it will come up.

If I can see big turbo 22-24psi boost up top AND small turbo 20-22psi boost way down low, I'd consider this a success.

That's ~5-6psi from K03 + 16-19psi from T3/T4 up top. <- neither turbo breaking a sweat.

manual vs ecu boost control- what's the fun in having an ECU at my disposal if I don't use it as I want :)


The limited testing I've done has K03 still controlling final boost up to 18psi while big turbo is on wg at 6psi. I gave BT wg 10% duty with no other changes and K03 duty goes to 0% while slightly over boosting. I don't remeber exactly, but BT boost was maybe 12-14psi while K03 boost was 6ish psi at 0%.  I'll go back and look at logs.

K03 spools faster than stock.

Thanks,
Rey

Pressure is irrelevant. You will max out the compressor's flow.
This is incorrect. You use the small turbo only until the bigger turbo starts producing more boost than the small one.
You need only one boost target. You can change it on switchover of course.

This setup will not make noticeably more power than just the single K03.
Because the flow of the K03 with the open internal WG is limited. You need a much bigger WG.

Also, you will max the compressor flow of the K03, overspin and destroy it.

I honestly recommend looking at some OEM solutions instead of trying to badly reinvent the wheel.
What you are doing will not work.

So, what is 'noticeably more power than a single K03'? I guessing it's a little more than 'any more power than just the K03'. On a B6 A4 quattro that is. I'm having a hard time qauntifying this statements.

What are compressor maps used for again? overall flow or pressure ratios where flow is derived from a specified pre-compressor pressure?

Ideally small turbo will 'multiply' big turbo's boost, though slightly. So, there will be no full switch-over. Maybe you're thinking about sequential turbos?

Thanks,
Rey

Maybe once you get past the 'switch-over' hang-up we can revisit controlling both turbos with 2 targets. There's also part-throttle control.

There is nothing to get past.
I have tuned and engineered many compound turbo setups. You simply do not understand. End of story.
But go ahead. You don't seem to care.

You also don't seem to understand compressor maps. Look at the shaft RPM at high flow, but 1.0 pressure ratio.
You will exceed acceptable shaft RPM and destroy the turbo.

Some pressure will always be in the waste gate line on boost. because you can't get 100% duty cycle, anything above atmospheric pressure I would consider to be helping the waste gate spring. Basic Engineering

show us spooling logs with wgdc 95% and with unpluged wg line. im very interesting ^)

elrey, blowing a huge hot air masses thru tiny k03 isnt a good idea anyway
and what happens at the top end? id say there is a point when comp wheel produces more rotating power than turbine wheel.
thats really weird setup for me, just think about bypassing it

If there is ANYTHING causing your wg to leak during spool, you have a design problem that a MBC will not fix.

I am also concerned with the K03 flow capability. If it where 2 equally sized turbo's the second k03 compressor will add more pressure regardless of input, but if you try to cram all the air from the big turbo through the small one it may be like a venturi (and shaft speed will be crazy high). Think less about pressure and more about flow :)

 ??? why would I have high shaft speed but only 1.0 PR?

This whole idea about the small turbo being a restriction on the intake side is a VERY common misconception about compound turbos. The flow capacity of a compressor is dependent on /derived from the pressure ratio across compressor. P2/P1. Change P1 and flow capacity changes, be it by elevation, weather, or big turbo in front of it. Maybe consider mass flow vs volumetric flow capacity, where the big turbo is changing mass flow not volumetric flow.

Did some ppl miss the last post on the first page?

prj please describe or better yet share a link to any one of your many compound turbo projects also.

Were they sequential turbo systems or compound turbo systems?

The limiting factory here will be the total exhaust flow capacity thru the K03 turbine AND K03's wide open wastegate. That's something on which I doubt anyone has any hard data. Again, a compromise I intentionally made.

Is there ANY way we can get back to ME7 threory instead of compound turbo theory, which there are a lot better sources on the internet than both prj and me?


Thanks,
Rey

[quote author
=nyet link=topic=12095.msg100714#msg100714 date=1486107355]
If there is ANYTHING causing your wg to leak during spool, you have a design problem that a MBC will not fix.
[/quote]

Obviously my humour wasn't happening. I had issues with my waste gate bending open though on a k03 though :s

Why would I want to destroy my turbo by un plugging the waste gate line. :)

this thread makes my head hurt. at the end of the day if you cant bipass the ko3 it will be a restriction period. id have a large wastegate before the k03  to bipass the small turbo when the the large turbo comes on line with a mechanical switch over on the cold side so when the large turbo takes off a swing check or vacuum actuator closes off the small turbo outlet so it wont bleed back. series hot side parallel cold side otherwise pointless

Wouldn't it have boost creep if the waste gate wasn't big enough already ?

I haven't turned up the boost enough yet for that to be a factor. As I increase overall boost and/or have big turbo make more of the boost and thus exhaust flow, it may still show itself.

Because air needs to go through the turbo. Look at a compressor map.

I am done here. This is a waste of time arguing with someone who does not grasp basics of mechanical engineering and does not understand the concept of flow within an ICE.

I believe you are referring to the far bottom right of the compressor map for small turbo due to total flow of big turbo going thru small turbo. Is that what you are saying?

E.g. The flow coming out of the T3/T4 falls far right on the x-axis of the K03 compressor map, that right?

Yes like that. To visualize flow people often compare to water and pipes etc. Well, think of your k03 as that little water mill that you used to poor water on when you where a kid (or in my case the one I poor water on for my kid). The mill has to turn for the water to pass through. Lots of water will only go through as fast as the mill will turn. If you pressurize the water the mill will turn faster.
Thats why modern compound/sequential setups bypass air as soon as the large turbo starts producing boost. This can be easily achieved by means of a spring checkvalve that will open automatically as the big turbo spools up and you only need to worry about the exhaust bypass (that you can control with either a wastegate or valve).
Here is a picture of a system with this layout:

Only shortcoming of the water wheel analogy is that water can't compress. If it could, and you compressed the water to 2x it's original density, you would then be able to pass twice the amount of water past the wheel at the same wheel speed (ignoring temperature).  Stilling wondering if my previous post is what prj is referring to.

destroyin turbo? lol
no need to full wot run, only spooling difference
cmon
do it

I understand your point of view, I guess it depends if there is a direct relation between volumetric and mass flow rate or not. It is confusing that different units are used between manufacturers to describe compressor performance as well. In any case, my comments are only meant as help for your project to succeed.
 For boost control it may be sufficient to only monitor the large turbo pressure for control, the small turbo needs to open all gates as soon as useable boost is made on the large turbo in my opinion. No need for high boost if you can have proper flow, keep temps low and timing high :-)

Thanks. I'll say this when looking at compressor maps: they are corrected for sae or at sea level. Once you change P1, you have to recalculate/recalibrate the x-axis flow. The axis values will shift left or right depending if you are correcting for a higher or lower P1. The big turbo is increasing the P1 for the small turbo map. Single turbos, it's atmosphere or whatever pressure pre-compressor which may be lower due to filter restriction.

That's what I believe causes a lot of confusion around this topic.

As for small turbo boost control, what about the majority of driving conditions? e.g. partial throttle or even just spooling? I have to control the small turbo at some point even if it's not at the top-end WOT.

How about a more big turbo oriented control, you can use throttleplate angle for part throttle while the k03 is at 95% dc. You could run the hp gate from lp pressure to make it switch to bypass mode automatically and use lp Watergate for further control. Or if you can use 2 n75 you can keep k03 iwg control for low end and fix n75 duty to 0% from a certain pressure up and take over with lp n75 control?

I already have lost a few extended  tips off my billet turbine wheel,(boost leak over speed), I since turned them off completely and sent away for Re balance.

elRay is correct here.

The exhaust side 100% needs a bypass, the compressor doesn't.  However, the power available is less than if the larger turbo is used on its own.

The large turbo has compressed the air before it enters the small turbo, so its volume for a given mass is much smaller than it would be entering at atmospheric.

Rick

I will not argue that, but what about total area under curve / driveability compared to any one 'reasonably price' turbo?

Better for sure.  Have experienced it on 4G63 and Cosworth YB but with basic mechanical control.

Rick

From my experience in Diesel compound setups this is set up properly (Mainly working on 6.4 Powerstrokes).

It's a common misconception that the compressor is limited to some arbitrary flow.  Compressed air is entering the smaller (HP) compressor.  The small compressor is simply multiplying this incoming pressure, and further compressing it to a higher pressure.  Plot this out on a compressor map and you'll see it still falls in line. 

However, the engine would be limited by how much air can flow through the hotside of the HP turbo.  elrey has addressed this by running the wastegate open all the time on the HP turbo.

Diesel guys have been doing this for years without compressor bypasses.  I think people are mixing up sequential and compounding turbos.  In a compound setup the LP isn't "coming on line" like it does in a sequential set up.

Now I may be wrong, but I think the best way to go about this would be to limit the HP to a safe pressure ratio (say 2) and then control boost on the LP as you normally would conventionally

If I'm controlling both turbos WGs with 2 N75, 95% for small and big turbo at low end, and then xx% for big turbo at top end, why not just control both all the time???  0% to small turbo is still controlling it.

For small turbo, I can picture a kfdmix with 0% on the top-end. Right now, with 10% big turbo duty, small turbo duty automatically goes to 0% without me changing anything just due to PID control even with a ~95% kfdimx value.

The problem with making small turbo kfdmix 0% up top, is what if big turbo is not spooled yet? For instance when starting WOT already at high RPM or gear shifts? I would want small turbo to be 95-100% for the few seconds it takes big turbo to respool. So, a static kfdmix will not work for that situation.

I think I need an offset or factor map applied to small turbo's kfdmix output based on big turbo's current boost pressure.

Maybe that can be done with a second map and/or a threshold from LP boost before final HP n75 result is passed on. that would not be hard to code. for example if LP boost is less than xxxxmbar and load request is over xxx, replace HP n75 duty with 95% if you want it hardcoded or make a small calibration map from reqested load and actual boost that gives out a new dynamic n75 dutycycle.

Btw I hope you have the option to bypass the K03 hotside with a properly sized wastegate? That was what I was referring to with the mechanical control from LP turbo. Even in a K04 hotside, the exhaust has proven to be the real topend restriction and egt will rocket with large compressors. Can imagine that even with wide open iwg you will get backpressure and heat from the housing when that big compressor starts to pick up speed...

mcorr = m*sqrt(T2/T1)/(P2/P1)

mcorr = Corrected Mass Flow (new x-axis)
m = Actual Mass Flow

P1 = STP Reference Inlet Pressure
P2 = Compressor Inlet Pressure

T1 = STP Reference Inlet Temperature (*K)
T2 = Compressor Inlet Temperature (*K)

Course the pain is that not every turbocharger manufacturer's "corrected" conditions are the same... Hence why some measure in mass flow rather than corrected volumetric flow, since that depends on the environment.

(http://2.bp.blogspot.com/_eYNXA0gKwI0/TU9IV1zDk6I/AAAAAAAAAP0/4TbZvo7gvgc/s640/Garrett+GT20+Turbocharger+GT2052+Compressor+Map+Honeywell+450x600+www.TurbochargerSpecs.Blogspot.com.jpg)

In this situation - taking into account what I said above - if the engine is capable of consuming 25lb/min, if you make the compressor inlet temperature 40*C and pressure 2bar(abs) on the inlet, when you correct the mass flow assuming, it's equivalent to that turbocharger moving 18.42lb/min.

25*sqrt(313/288)/(2/1) = 18.42lb/min

I'm fairly sure I'm correct here... Issue being you run up against the choke line quite quickly at low PR - look at the trend of the compressor shaft speed lines, at higher airflow, even at lower pressure, you're tending towards higher and higher compressor shaft speeds and into choke...

this might be helpful understanding gasoline R2S system:

http://www.turbos.borgwarner.com/en/press/knowledgeLibrary.aspx

"Regulated Two-Stage Turbocharging for gasoline Engines"

That's a sequential system though.

no, please have a closer look,
this is your setup,
just the compressor bypass and a bigger turbinebypass at the K03 is missing

When one turbo is used for low-end only and big turbo is used for top-end only = sequential

Small compressor bypass = sequential. It may compound for a very short rpm range durning transition (switch-over), but it is a sequential (two-stage) system.

A full compound systen has both turbos contributing thru whole range.

This whole thread is off-topic.

You know what? It doesn't even matter if the system in question is sequential, compound, good design, or bad design. They all have 2 WGs that both require some 0%> duty <100% at some RPM range.

I have the opportunity and ability to control two wastegates across the full RPM range. I want to discuss strategies for a 2 N75 PID control using new code, but no one can get past the hardware. Isn't this a software forum?

 :(

What ideas do you have for control, maybe we can think along your plans or contribute our vision on the code? What you are doing is beyond most readers capability so the group of capable readers that are willing to think with you wont be that big. It is easier to comment with offtopic replies (whether they are helpfull or not).
I am just a beginner with custom code and have no experience with compound or sequential setups at all, so my advice may not be usefull at all. But the topic interests me and my first thoughts are that you can take a shortcut by having a conditional replacement value for the original n75, that value does not need to be static.
So, can you give me a description of control that you are looking for? Which wastegate needs to do what and when?

@ruan - I gather from your post that you agree with me generally that compound systems shift the small turbo's compressor map x-axis to the left. However, you doubt the T3/T4 - K03 combo still fall on the adjusted K03 map. Is that correct?

I haven't (I don't think) provided what PRs I plan to operate at nor have actual K03 - T3/T4 maps been referenced in your calculations. Unless you are suggesting the gt2052 is comparable to the K03

I cannot provide opposing calculations at this time, just stating the incomplete picture on which both our assumptions are based.

@TijnCU -

I was breaking the discussion up into two separate parts:
1) setting target boost (one target for each turbo, or more accurately, one target for big turbo and then total manifold target) A small turbo target could be considered by subtracting big turbo boost from manifold boost.
2) PID control

First thing I would like to mention is to not get stuck on only thinking about one long WOT run. Because you'll only consider 95-100% small turbo duty and xx% big turbo duty. As I mentioned before, small turbo may have to maintain 20psi at 5500 RPM for a short period after a shift or off then back on throttle up there. And that might not mean 95-100% duty on small turbo, but full PID control.

So, I can see a full range of duty % needed at anytime for both turbos.

I think the tricky part is each turbo's response to a duty% might change based on the other turbo current pressure, and it might not change linearly.

IF it did change linearly, I could see just 4 kfdmix maps, two for each turbo. One map for when other turbo is operating at wastegate only, and 2nd  map when other turbo is operating at peak boost. Then just interpolate between the two maps based on other turbos current boost/its peak boost.

Just like KFZW1/2 and fnwue.

Correct on the first count certainly. That was this morning and the coffee hadn't quite got my brain working in a sensible explaining fashion. The GT2052 example was merely an example to show on a turbo that's in the same ballpark area that is being discussed which conveniently has it's axis as corrected mass flow rather than volumetric - nothing else inferred from that.

Without going into specific values for what you're wanting to do... Compounding is great if you want extreme pressures, however from my experience there is quite a trade off in terms of where that pressure can be generated - the airflow you'll likely want to support at high engine speed will (even taking into account the above) be too great for the HP unit compressor.

This is why the OEMs use systems like R2S - because you can utilise that HP unit at low RPMs and provide great transient response, generate high pressures at mid RPM whilst utilising compounding, but bypass the HP unit so that you don't overspeed it... That's why compound setups are more popular on diesel motors IME.

Point of my post being:

- Mass flow vs corrected mass flow (or simply - volumetric flow) means given a higher inlet pressure you can move a greater mass of air through the compressor.

- Doesn't mean you'll always be able to move enough even with the above.

In terms of what the original thread is here for, fraid I'm about as much use as tits on a fish - many people with vastly more experience than myself on that subject. I've a bit of experience with how I might apply this in terms of a CR diesel, however there's so much knowledge here, I don't think my input would be hugely helpful! I just find reading stuff on the subject of ME7 fascinating coming at it from a completely different angle :)