Explanation of MBC leak on boost leak tests

[burntheblobs]

Can someone explain to me how this ISN'T a boost leak? I have it set to 12psi, and it will leak heavily when there is about 1psi in the system. Everyone always says it's totally ok, but why? A leak is a leak in my mind...

 

[toojung2die]

Did you send him the link to this thread?

Yes, he's got the thread. I may get a Master's thesis in response instead of the simple answer I was hoping for. I trust Hank, he's has a lot of practical experience in many fields and he loves cars too. After our initial discussion I'm getting the feeling that we're all right to some degree. He's talking about all the variables...

 

[donmagicjuan]

As soon as we move that marble, there will be a current in that water....no low pressure zones behind, just the pressure now moving or flowing, because of a different force to get it to move.

Matt, before you dig yourself any further into this hole, I'm going to stop you right here. Moving an object through a fluid creates a pressure difference across the object. The difference in pressure results in a nonzero force exerted on the object which opposes the direction of motion and is proportional to the object's speed. You might have heard of this force by its commonly used nomenclature (drag). To move the object at a constant speed you must exert an equal force on the object in the direction of motion. Current results when the higher pressure of the surrounding fluid attempts to equalize with the lower pressure of the fluid directly behind the moving object (fluid flows in a direction from high pressure to low). Pressure is not an object that can be thought of as moving or flowing. It is the fluid that is flowing. Hydrostatic pressure is nothing more than the quantity of force exerted by the molecules of the fluid upon one another per unit area. It is a quantity, as is speed. You wouldn't talk about a car's speed physically traveling along a freeway-- it's the car that moves, and there is a speed that corresponds to each point along the car's path.

 

[kenamond]

2.) A room whos pressure source is 18PSI will not stop filling at 10. There is force sperate than pressure.

Okay. Good. You disagree with #2. Let's discuss it.

You mention the number 10. I said 8. So first, why did you say 10?

I am willing to rephrase #2 and say that the pressure in the WGA is AT LEAST 8psi, because that is what is required to open the flapper, and since we are at a steady boost of 18psi, the wastegate must be open.

So, what pressure do you think is in the WGA at this point? I'm sure we can agree that it's not greater than 18psi, but it is not lower than 8psi. Therefore, our WGA pressure is between 8psi and 18psi. Hopefully, you don't fear a minimum amount of algebra here. Let's call this pressure P. That means I have to modify my list (the new list is below). Notice that I added #4.5.

With those changes, do you have any other doubts about the list?

1) The pressure on the compressor side of the MBC is at 18psi.
2) The pressure "P" in the WGA is between 8psi and 18psi.
3) The MBC has two sides: one to the compressor, one to the WGA.
4) 18-P=DP where "DP" is the pressure difference between the compressor and WGA ends of the MBC.
4.5) DP is between 0psi and 10psi (if P=8, DP=18-8=10, and if P=18, DP=18-18=0).
5) The MBC is open.
6) Boost is holding at 18psi.
7) If air is not supplied to the WGA by the MBC, the differential pressure DP will drop as air leaks out the vent hole.
8) The MBC is open at a differential pressure of between 0psi and 10psi (the difference will be whatever DP in #4 is).

 

[toojung2die]

A preliminary reply and question from my friend the engineer, followed by post #128 by Mack has led me to concede that my original understanding of the MBC/WGA relationship is flawed and therefore incorrect. +1 point for both Mack and Don for their patience and persistence!

What Hank wrote to me was:

If you want the solenoid to actuate at 18psi, that is what the check valve should be adjusted to. That is if you want the solenoid to Slam open! Do you?

I realized that is NOT what I want the WGA to do. It has to modulate the intake pressure at increasing and deceasing engine/ turbine rpms. His question threw a monkey wrench into my thought process and caused to to have to re-evalulate from the start. The wastegate flapper is going to spend much if not all of it's time partially open. In order for this to happen the MBC will have to be sending the WGA a boost signal of less than the 18psi we've been using in our examples.

Stick me with a fork, I'm done.

 

[EclipseTrbo420A]

A preliminary reply and question from my friend the engineer, followed by post #128 by Mack has led me to concede that my original understanding of the MBC/WGA relationship is flawed and therefore incorrect. +1 point for both Mack and Don for their patience and persistence!

What Hank wrote to me was:


I realized that is NOT what I want the WGA to do. It has to modulate the intake pressure at increasing and deceasing engine/ turbine rpms. His question threw a monkey wrench into my thought process and caused to to have to re-evalulate from the start. The wastegate flapper is going to spend much if not all of it's time partially open. In order for this to happen the MBC will have to be sending the WGA a boost signal of less than the 18psi we've been using in our examples.

Stick me with a fork, I'm done.

Ok, hes arocket scientist and I am not. But why is that we complain of little cracks around the flapper or we see the flapper being slightly open a huge problem for spool. I mean "we", as tuners. Full spool would take forever to come if the flapper spent most of its time open, thats a fact. I shimmed my actuator 1 washer just because the wastegate was cracked open alittle and I took for ever to spool. Now, did I miss read what you wrote there? You made it sound like the flapper is almost always cracked open and that a big no no for spool.

Now he asked if we want the wastegate to slam open, and the answer is yes. We want it to hold all the pressure the turbo is blowing untill our desired PSI, and when and only when that is reached, we want it to slam open and apply the pressure that was stuck at the ball on the WGA. Thats it. Ask you friend what his answer would have been if you said we wanted to MBC to slam open. Let me know what he says.

 

[toojung2die]

Now, did I miss read what you wrote there? You made it sound like the flapper is almost always cracked open and that a big no no for spool.

Yes, I'm only talking about after the turbo has spooled up during WOT. The WGA will be open varying amounts while modulating the boost around the desired intake pressure. A little above 18psi, a little bit open. A lot above 18psi, it'll be wide open. I see the flaw in my reasoning, now I will be re-reading the previous posts to understand the other reasoning.

 

[EclipseTrbo420A]

Yes, I'm only talking about after the turbo has spooled up during WOT. The WGA will be open varying amounts while modulating the boost around the desired intake pressure. A little above 18psi, a little bit open. A lot above 18psi, it'll be wide open. I see the flaw in my reasoning, now I will be re-reading the previous posts to understand the other reasoning.

Okay. I see what you are saying, but if thats the case my boost gauge should be jumping up and down 2-3 psi each direction at wot? If theres acutuation in the flapper the boost changing slightly will result from that. My boost gauge hits and holds.

 

[toojung2die]

Okay. I see what you are saying, but if thats the case my boost gauge should be jumping up and down 2-3 psi each direction at wot? If theres acutuation in the flapper the boost changing slightly will result from that. My boost gauge hits and holds.

Right, and the reason it hits and holds is what I was forgeting. This is a closed-loop feedback system. If I'm only thinking about the point at which the wastgate opens I can explain it with my first argument. I now see it is not as simple as I was looking at it. For the boost to stay steady there is a delicate balance between how much the wastegate is open and how much pressure the compressor is making, each constantly acting upon and adjusting the other. For this balance to happen there has to be a way for the wastegate to be partially open. If it's acted on with more pressure than it's designed for it can only be fully open. Now I see the wastegate cannot be only fully open or fully closed, it has to be finely adjustable to hold a constant boost. For the wastegate to be partially open it has to be seeing a pressure less than the 18psi you see on your boost gauge.

 

[donmagicjuan]

Right, and the reason it hits and holds is what I was forgeting. This is a closed-loop feedback system. If I'm only thinking about the point at which the wastgate opens I can explain it with my first argument. I now see it is not as simple as I was looking at it. For the boost to stay steady there is a delicate balance between how much the wastegate is open and how much pressure the compressor is making, each constantly acting upon and adjusting the other. For this balance to happen there has to be a way for the wastegate to be partially open. If it's acted on with more pressure than it's designed for it can only be fully open. Now I see the wastegate cannot be only fully open or fully closed, it has to be finely adjustable to hold a constant boost. For the wastegate to be partially open it has to be seeing a pressure less than the 18psi you see on your boost gauge.

Well put, toojung2die. This is a great point that had previously escaped me, although I don't know why.

 

[steve]

It's possible for a system to hold a steady state boost level with a wastegate that is only open or close. It just oscilates on and off at the point where it maintains the boost level.

Steve

 

[donmagicjuan]

It's possible for a system to hold a steady state boost level with a wastegate that is only open or close. It just oscilates on and off at the point where it maintains the boost level.

Steve

I would agree with this if the actuator were able to physically cycle fast enough for there to be no visible fluctuation in a boost log, but after observing how slowly the actuator returns to its closed position after being isolated from a pressure source, I have to disagree. After all, the rate at which the actuator shuts the wastegate is not controlled by the MBC valve, but rather the size of the relief hole in the MBC downstream of the ball. Since we agree that the design intent is to make this hole as small as practicable, it doesn't seem to me that it would ever be able to flow enough air to enable this type of control.

 

[toojung2die]

It's possible for a system to hold a steady state boost level with a wastegate that is only open or close. It just oscilates on and off at the point where it maintains the boost level.

Steve

I think if that were happening my boost gauge would be oscillating like my autometer a/f ratio gauge when it's in closed loop.

 

[EclipseTrbo420A]

I think if that were happening my boost gauge would be oscillating like my autometer a/f ratio gauge when it's in closed loop.

Only if you were to change the force moving the pressure back and forth, in this case the throttle. Point is at WOT the MBC see what you have it set to and is open, if you back of the force driving the pressure it will drop below 18 and the WGA/flapper will lose its pressure source and close.

 

[toojung2die]

I've come to the conclusion that this is a complex question and simple answers will not suffice. I have been convinced that if I want my intake pressure to hold at 18 psi (or whatever) it is not a simple matter of setting the MBC to open at 18psi.

This how I presented the question:

Here's the problem:

We have a pneumatic solenoid that's designed to actuate at 8psi. We want the solenoid to actuate at a higher pressure than it was designed, 18psi in this example. We put an adjustable ball and spring check valve between the pressure source and the pneumatic solenoid. There is a tiny bleed hole in the check valve to release trapped air and allow the solenoid to return to rest when the pressure source stops applying pressure and the check valve closes.

What this drawing represents is a turbocharger boost control system. The pressure source is the turbocharger compressor. The pneumatic solenoid is the turbo's exhaust waste gate actuator. When the boost pressure hits the desired psi the actuator opens the waste gate bypassing the exhaust gases around the turbocharger turbine and the turbocharger compressor produces less pressure or "Boost". By adjusting the pressure at which the check valve opens we can modulate the boost pressure.

You must be logged in to view this image or video.

Here's the question: We want the pneumatic solenoid to actuate at 18psi instead of 8psi so what psi do we set the check valve to open at? At what pressure should the check valve open so the solenoid actuates at 18psi?

Here's the last word from my friend, Hank:

OK, here is my problem. I don't know enough about the fine points of how a Turbocharger works. I have a general knowledge but NOT an in depth knowledge of the dynamics regarding pressures at all places in the system. All I can do is look at the psi differentials and make a conclusion based on the different pressures. I forwarded your introductory email to a very smart X JPL engineer who did Aero Dynamics at the lab. His response was to say he was not equipped to answer the question. I don't have knowledge of the dynamic curve of the actuator and how it responds to different pressures. I don't see it as a regulator. I thought it is an actuator. The word solenoid to me means to me that it is either OFF or ON. I don't see it as a regulator. From what you are telling me it does the same thing as the diaphragm unit of an automobile cruse control system. I am sorry but I don't understand and know enough to help you. Hank
By the way, A leaky ball valve could account what your friend who sets the pressure adjusted lower than the 18 psi.

PS-Can't prevent Image from appearing twice?

 

[luvmygst]

I say we just change the name from Manual boost controller to Manual Boost Regulator. There has to be some regulation(movement) of the Ball and the watsegate to maintain a desired boost level. I never thought I would see a 5 page post on the operation of a MBR I just had to throw my .02 in.

 

[EclipseTrbo420A]

I say we just change the name from Manual boost controller to Manual Boost Regulator. There has to be some regulation(movement) of the Ball and the watsegate to maintain a desired boost level. I never thought I would see a 5 page post on the operation of a MBR I just had to throw my .02 in.

Ahhh you brough it back! Last word from me.... Yes there is movement. The Pressure pushed the ball and spring to move back. Thats the movement. It takes 17lbs to move that ball back. tighten it up, it will take 18. even tighter will take 19.