Boost level and torque curve

[Lithium]

1 minute ago, thx78 said:

I don't understand anything anymore.

As someone who has been playing with turbo things for some time now, be prepared for this to not be the first time feeling this way   

I'm busy as atm, but if no one else covers the things I'm realising are worth mentioning to you then I will when I have a chance as it seems like I overestimated your knowledge on how turbos and wastegates work.  Otherwise I recommend having a look around how wastegates and boost control work, really anything on understanding the general mechanism of boost control as it will help you find the answers to the questions you're looking for.  Your initial question jumps some fundamentals.

[Kinkstaah]

1 hour ago, thx78 said:

so this is an anomaly??? I thought it was a voluntary increase

It could be. It might not be. It is impossible to know without context by asking the owner or the tuner on the day and knowing what they were doing/not doing/attempting to do.

You said earlier this is hard to understand because to your mind, a turbo is at full speed when it hits its spike. This is not true. The turbos actual speed is defined by how much air is being forced through it via the exhaust, unless you control it. The spike you are seeing at ~whatever RPM it 'spools' at, is where boost control is starting.

If there were no boost control the turbo absolutely would be spinning much faster at 7000rpm than 3000rpm, every single time, on every single engine. Boost control keeps the boost controlled within the limits you ideally want. If it were uncontrolled you would have two scenarios

1) You have a turbo that hits peak RPM and CFM (not boost) at the redline of the engine. This would work, but most people want more boost earlier.
2) You have a turbo that hits peak RPM well before the redline of the engine (say, 3500), and you explode the turbo by redlining the engine. (say, 7000rpm).

If you don't want exploding things, or lag, you design a turbo system to come on when you want the boost to be useful, and then not overspin itself into oblivion by using some form of boost control, to control the boost pressure accordingly.

[GTSBoy]

Wait until @Lithium posts a compressor map and tries to explain what's going on.

There is a strong possibility that the OP lives in a digital world and is not comfortable with analogue machines. A turbo is just about as analogue as it gets, with plenty of non-linear behaviours added in on top. Most of us who think we know how they work are actually only getting by on 2nd and 3rd order mental models that abstract away from the actual physics and bring it back to the sort of kindergarten level concepts we can hold in our heads. This is important when you need to hold 10 such concepts in your head at the same time. You need to reduce the complexity of the individual concepts to allow them to be simultaneously held and manipulated. Too much complexity in the base models makes it very difficult to make them work with each other mentally.

[Lithium]

5 minutes ago, GTSBoy said:

Wait until @Lithium posts a compressor map and tries to explain what's going on.

I was actually going to try and dig out a datalog with turbine speed and EMAP haha

[Lithium]

Right.  I'll try my best at a concise "bring you up to speed" on stuff that may be missing here - obviously open to questions or further input from others as relevant.

Here's a datalog from a responsive turbo setup with electronic boost control being used a bit, to keep it simple I've marked 3 points of interest.  All of these charts are on the same time scale on the X-axis, so you can reference what engine rpm is doing in the top graph, boost and EMAP (exhaust backpressure) in the second graph, and turbo rpm in the bottom graph.

A) The turbo gets it's power from exhaust gas, and pumping air takes work.  As a result you can't just spin a turbo with a fixed amount of exhaust energy and expect it to keep spooling - the bigger the turbo (ie, the more air it can push) the more exhaust energy you need to drive it.   The most obvious ways of getting more exhaust energy are by adding displacement, adding boost, or adding rpm - but as you add any of these the turbo also needs to do a bit more work so there is a big balance of these things needed to even get to your target boost and sometimes that's not even possible.

What you can see in step "A" is that there isn't enough engine rpm or boost to reach the level we want, so this is where "lag" is.  The dyno run continues and rpm increases, which gives enough energy to increase boost, which helps spool  the turbine speed up even more - so you can see that the rate that the boost (and turbo speed) are starting to ramp up faster than the engine rpm is, so turbo is really starting to wake up as the graph gets closer to point B....

 

B) At this point we've been able to reach the boost that is actually desired at this point.   To stop the boost from going further than this the wastegate will open and bypass gas past the turbine, meaning it doesn't continue accelerating at the point it was before but instead carry on at a more progressive rate which matches what the engine needs.   The wastegate will have a spring in it which is rated to a specific pressure where it will start bypassing, but electronic boost control (managed by the ECU) can adjust how much pressure the spring sees in order to allow some tunability on how much boost the wastegate actually sees, and therefore how much exhaust it bypasses.  

The tune in this case stops boost from ramping up HARD at around 21-22psi just before 4000rpm, then as the rpm continue it allows boost to continue up to around 25psi higher in the rpm.   You can see the turbo speed fairly steadily increases through the rpm to ensure it's keeping up with the increasing airflow demand due to the engine speed being higher and boost being pretty steady.   If the boost dropped off after a point then you may see the turbo speed level off or even drop.

 

C) You can see that despite the boost pressure staying pretty flat here, the exhaust pressure is steadily increasing and at this stage has overtaken boost pressure.   This isn't unusual, and is largely as a result of the increasing energy needed by the turbo to pump more and more air to suit the needs of the engine as it revs out further.   There are a bunch of variables in regards to how much back pressure there will be on a given turbo etc, but its one of the factors we manage when sizing and tuning a turbo setup.   When exhaust back pressure starts exceeding boost pressure you will eventually start seeing signs that the turbo is running out, the engine gets less keen to make more power and it gets harder to raise boost further.

In this case it's a fairly acceptable compromise for the power level (around 630kw on a 3litre engine with full boost by 4000rpm), but you'd not want to push it a lot harder than this.  The maximum speed rated for the compressor wheel on this setup is around 125,000rpm so you can see its starting to get close on that side as well - I feel like this kind of illustrates some of the turbo related things we both decide on how far to push, and are also limited to how far we can push depending on the parts combination.

Hope this helps more than it confuses things

 

[thx78]

I studied the principle of wastegate to begin with. so if I understood correctly. the moment when the turbo "spool" is where the boost control begins. When the target level of "psi" is reached the wastegate opens to regulate the exhaust pressure passing through the turbo and thus control its speed and the rate of boost

[thx78]

the boost controller allows you to adjust the opening of the wastegate (which only has one preset spring) I'm going to see how it acts on it

[Dose Pipe Sutututu]

6 minutes ago, thx78 said:

the boost controller allows you to adjust the opening of the wastegate (which only has one preset spring) I'm going to see how it acts on it

A stand alone boost controller will not give you the control you need, unlike a modern ECU.

Your boost will always naturally target the wastegate's opening pressure first, your controller then will allow you to add more boost as required.

[GTSBoy]

20 minutes ago, thx78 said:

he moment when the turbo "spool" is where the boost control begins.

Well.... yes and perhaps no. It depends on what you mean by "spool".

For most of us, the point that we would describe as where the turbo is "spooling" is the point where the wheel speed gets high enough for it to start making some boost. This is coincidentally around about the point where it starts making noise - hence the "spooling" sound.

If that is what you mean, then no - the wastegate should still be shut at this point. The boost will still be way below the point where it should start opening.

If, on the other hand, you mean "spool" as "reach full boost", then yes. At the point where the boost has reached target, then boost control has already started. The wastegate is already open, and has been for some time. Some short time, definitely, but still, some time.

If you have no boost controller - just the wastegate actuator connected to the boost source, then you have a mechanical system working as a pressure balance. There is pressure on one side of the wastegate actuator's diaphragm from the spring, and pressure on the other side coming from the boost measurement location (the "source"). This is not a digital thing. The wastegate does not stay shut until the boost pressure reaches the spring "pressure". The spring will start to compress as soon as you start to apply any pressure onto it. This can be controlled somewhat by adding pre-load into the spring, but you cannot add enough preload to make it into a digital switch behaviour. The wastegate will crack open and start to leak exhaust out (and therefore not though the turbine) well before you reach the target boost pressure.

Electronic (and some mechanical/pneumatic) boost controllers will act to prevent the boost source applying pressure to the actuator until just before you reach target boost, thus preventing the wastegate from creeping open. And some boost controllers will apply boost pressure on the spring side to further push it shut. And this can be be necessary because the exhaust pressure in the manifold also pushes on the wastegate valve and tries to open it and you cab get it leaking even without it being connected to the boost source.

[thx78]

ok, I understand much better how we can control the boost and increase the number of psi before opening the wastegate

[thx78]

this explains the possibility of several maps with different boost levels