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...

 

[EclipseTrbo420A]

Yes, that's what I should have added.
Today I was thinking, "This ain't exactly rocket science, what's so hard to understand?". Then I remembered my friend, Hank, who IS a rocket scientist. He's a retired engineer from the JPL in Los Alamos. He helped design the first succesful Mars probe. I called him and he agreed to look at the problem and give me his answer. I'm believing whatever he says. I've sent the email and I'm waiting for his response. Can we agree to agree on the opinion of a rocket scientist??? I'll post my question and his response.

Ill go with that, but It will be weird thinking Im right if Im wrong.

 

[JayRolla]

^^^^ Yea come on help the thread starter with his problem first. I say if the MBC works than keep it. Just take the MBC out during the boost leak test.

OOPs posted this and then saw there was a whole other page. OOPS

 

[donmagicjuan]

What the hell does a rocket scientist have to do with any of this? When in doubt, turn to cliches, I guess. I'm an M E major, and you guys are killing me!

 

[EclipseTrbo420A]

Take out your MBC during test. If your car HITS AND HOLDS your desired PSI, then run it. That simple.

 

[kenamond]

What the hell does a rocket scientist have to do with any of this? When in doubt, turn to cliches, I guess. I'm an M E major, and you guys are killing me!

My sentiments exactly. I'm an M.E., but I now write computational physics software at Los Alamos. The only way I'll be convinced one way or the other is doing the experiment myself, but as you've demonstrated, that wouldn't convince anyone else.

 

[kenamond]

Yes, that's what I should have added.
Today I was thinking, "This ain't exactly rocket science, what's so hard to understand?". Then I remembered my friend, Hank, who IS a rocket scientist. He's a retired engineer from the JPL in Los Alamos. He helped design the first succesful Mars probe. I called him and he agreed to look at the problem and give me his answer. I'm believing whatever he says. I've sent the email and I'm waiting for his response. Can we agree to agree on the opinion of a rocket scientist??? I'll post my question and his response.

Did you send him the link to this thread?

 

[donmagicjuan]

No, it only has to be slightly larger than the pressure set point. Slight difference, slightly open. Large difference, largely open.

This is completely backwards. Slightly open = much restriction, wide open = little restriction. For a constant flowrate, D/P is related to restriction, in laymen's terms.

 

[EclipseTrbo420A]

Guys I dont know, think about this.

The pressure is moving, its directional. Its not just filled with pressure then the flow stops like a boost leak test. The turbo spooling is different. Theres pressure and force. The air during a boostleak test is stagnant air that just builds on the ball slowly with no real force moving the pressure, its just building. Once the pressure rises we cut it off and leave no where for it to go. It will leak untill our desired boost level is in the system then close, in my case at 18PSI.

Lets go back to the 402 WHP room. If I hang a ribbon from the celing and pressureize the room it will stay completly still with the pressure on all sides, not because of the pressure on all sides, just because theres not moving force.

If I put a fan in the pressureized room and blow it on the ribbon it will move, It will move even though there is a room pressure of 18LBS theres a moving force exerted on the Ribbon. See what I mean>?

 

[donmagicjuan]

Guys I dont know, think about this.

The pressure is moving, its directional. Its not just filled with pressure then the flow stops like a boost leak test. The turbo spooling is different. Theres pressure and force. The air during a boostleak test is stagnant air that just builds on the ball slowly with no real force moving the pressure, its just building. Once the pressure rises we cut it off and leave no where for it to go. It will leak untill our desired boost level is in the system then close, in my case at 18PSI.

Lets go back to the 402 WHP room. If I hang a ribbon from the celing and pressureize the room it will stay completly still with the pressure on all sides, not because of the pressure on all sides, just because theres not moving force.

If I put a fan in the pressureized room and blow it on the ribbon it will move, It will move even though there is a room pressure of 18LBS theres a moving force exerted on the Ribbon. See what I mean>?

No, this is a conceptual error. Pressure is nothing more than force acting per unit area. It's a way of normalizing the forces acting within the system. There is no such thing as a flow component of force. If there is a given pressure in the system acting on the ball of the MBC, it makes no difference whether the air in the system is in motion or at rest. The force on the ball is the same. The only factor the flow introduces that we're concerned with here is the concept of a pressure drop across a component, which would not exist if the air were at rest. Rapid changes in flowrate can result in transient pressure spikes, or forces if you will, but we're talking about the operation of the MBC at a constant boost level, when changes in flowrate are relatively small.

 

[EclipseTrbo420A]

Can you explain your whole thing to me one more time please? Im sorry, if you dont want to its cool. This has been going on a long time.

 

[donmagicjuan]

Can you explain your whole thing to me one more time please? Im sorry, if you dont want to its cool. This has been going on a long time.

- Which part, the MBC operation or a specific concept?

 

[EclipseTrbo420A]

- Which part, the MBC operation or a specific concept?

The operation, please.

 

[donmagicjuan]

Well, here's the step-by-step thing I wrote up again. Was there anything in this that didn't make sense?

I got bored, so I constructed a theoretical step-by-step timeline to explain the operation of a ball-and-spring type MBC. I’m making the following assumption to simplify this discussion: the controller is set to maintain boost at 16 psi on an EvoIII 16G with a stock actuator setting of 8 psi.

Initial conditions: intake manifold pressure is at 0 psig, the throttle has just been fully opened, pressure in the actuator line is 0 psig, and the MBC valve and wastegate are shut.

Step 1: Boost pressure begins to rise, but it is still less than 8 psig, so the MBC valve is still shut – no surprises there.

Step 2: Boost pressure barely exceeds 8 psig. Since the actuator line pressure is still zero, the MBC valve cracks open. Pressure downstream of the valve attempts to equalize with intake system pressure, but as soon as it starts to rise, D/P across the valve drops and the spring shuts the valve. Therefore, pressure begins to bleed back to zero again. The pressure in the actuator line never reaches the level required to open the wastegate.

Step 3: Boost pressure is at 9 psig. Once again, the MBC valve opens, and pressure downstream of the valve attempts to equalize with boost pressure. When the actuator line pressure rises above 1 psig, the spring shuts the valve and pressure bleeds off. When pressure bleeds below 1 psig, the valve opens again and the cycle repeats. Thus, the valve will cycle to maintain an equilibrium pressure of 1 psig in the actuator line. At no point does the pressure in the actuator line rise high enough to open the wastegate.

Step 4: Boost pressure continues to rise between 9 and 16 psig. The MBC valve continues to cycle as outlined in step 3, maintaining actuator line pressure 8 psi lower than intake manifold pressure.

Step 5: Boost pressure exceeds 16 psig. Now the pressure in the actuator line is high enough to accelerate the actuator arm and open the wastegate. As the wastegate opens, D/P across the turbine is reduced, which reduces the output of the turbo, as we all know. Therefore, boost pressure is lowered. As this happens, the D/P across the MBC drops and the spring shuts the valve. Pressure once again bleeds from the actuator line and the wastegate is permitted to close.

Step 6: With the wastegate closed, boost pressure will rise again until the setting of 16 psig is exceeded, and the process repeats.

 

[Coup D E'Tat]

I can't believe this thread went on for 4 pages.

 

[EclipseTrbo420A]

No, this is a conceptual error. Pressure is nothing more than force acting per unit area. It's a way of normalizing the forces acting within the system. There is no such thing as a flow component of force. If there is a given pressure in the system acting on the ball of the MBC, it makes no difference whether the air in the system is in motion or at rest. The force on the ball is the same. The only factor the flow introduces that we're concerned with here is the concept of a pressure drop across a component, which would not exist if the air were at rest. Rapid changes in flowrate can result in transient pressure spikes, or forces if you will, but we're talking about the operation of the MBC at a constant boost level, when changes in flowrate are relatively small.

Your saying pressure IS force, I'm saying its a kind of force. Thats why the ribbon in a pressureized room can still be blown. Understand that?

Im going to read te step by step again now.

 

[kenamond]

Guys I dont know, think about this.

The pressure is moving, its directional. Its not just filled with pressure then the flow stops like a boost leak test. The turbo spooling is different. Theres pressure and force. The air during a boostleak test is stagnant air that just builds on the ball slowly with no real force moving the pressure, its just building. Once the pressure rises we cut it off and leave no where for it to go. It will leak untill our desired boost level is in the system then close, in my case at 18PSI.

Lets go back to the 402 WHP room. If I hang a ribbon from the celing and pressureize the room it will stay completly still with the pressure on all sides, not because of the pressure on all sides, just because theres not moving force.

If I put a fan in the pressureized room and blow it on the ribbon it will move, It will move even though there is a room pressure of 18LBS theres a moving force exerted on the Ribbon. See what I mean>?

If you put 18psi on one end of a vaccuum hose and leave the other end open, the air will leak out at the same rate whether that 18psi is generated by a centrifugal compressor (turbo compressor) or a positive displacement compressor (air compressor sitting in your garage or work shop). If the flowrate is large enough, sure, the turbo would be able to keep up better than the air compressor, but we're talking about a small hiss of air. When the air gets to the MBC, it knows nothing about what caused the 18psi at the other end of the hose.

When you floor it, boost raises, hits the specified boost level (let's use 18psi) and stays there the rest of your pull. Forget about everything leading up to 18psi for a moment.

The wastegate is obviously open to some degree, because boost is not raising, right? That means that there is 8psi in the WGA (how else is it being held open?). Okay. Now, we know from the boost gauge that the compressor is producing 18psi boost. The MBC has two sides: one is connected to the compressor and the other is connected to the WGA. So there is 18psi on the compressor end of the MBC and 8psi on the WGA end which gives a relative difference in pressure across the MBC of 10psi. We also know that the MBC has to be letting air by the ball, because if it weren't, that air in the WGA would leak out the vent hole and that 8psi would drop, and the flapper would close. But we know the pressure in the WGA is holding at 8psi, because boost is holding at 18psi solid and not going up. So put it all together. We have an MBC that is open. We have a pressure difference across it of 10psi. How could this be if the MBC opened at 18psi as you believe? You cannot refute that the MBC is open at 10psi differential, so someone needs to explain why it would be open at 10psi AND 18psi differentials in order to support your claim.

The ONLY way I can think it might be open at both 10psi and 18psi is if the MBC opens at an 18psi pressure difference (when you finally reach 18psi) and stays open as that difference drops to some much lower level where it finally seals up again. But there's a problem with an MBC that acts this way. What if you hit 18psi, the MBC opens, you run a bit at full boost, then you let off the gas a bit and the boost drops to say 10psi. The MBC is still open so you're exposing the WGA to 10psi, so it's still open. Now you get back on the gas, but since the WGA is still open, you can't increase boost, and you're stuck at 10psi. It just doesn't work out that way, so I still don't believe that theory.

 

[EclipseTrbo420A]

If you put 18psi on one end of a vaccuum hose and leave the other end open, the air will leak out at the same rate whether that 18psi is generated by a centrifugal compressor (turbo compressor) or a positive displacement compressor (air compressor sitting in your garage or work shop). If the flowrate is large enough, sure, the turbo would be able to keep up better than the air compressor, but we're talking about a small hiss of air. When the air gets to the MBC, it knows nothing about what caused the 18psi at the other end of the hose.

When you floor it, boost raises, hits the specified boost level (let's use 18psi) and stays there the rest of your pull. Forget about everything leading up to 18psi for a moment.

The wastegate is obviously open to some degree, because boost is not raising, right? That means that there is 8psi in the WGA (how else is it being held open?). Okay. Now, we know from the boost gauge that the compressor is producing 18psi boost. The MBC has two sides: one is connected to the compressor and the other is connected to the WGA. So there is 18psi on the compressor end of the MBC and 8psi on the WGA end which gives a relative difference in pressure across the MBC of 10psi. We also know that the MBC has to be letting air by the ball, because if it weren't, that air in the WGA would leak out the vent hole and that 8psi would drop, and the flapper would close. But we know the pressure in the WGA is holding at 8psi, because boost is holding at 18psi solid and not going up. So put it all together. We have an MBC that is open. We have a pressure difference across it of 10psi. How could this be if the MBC opened at 18psi as you believe? You cannot refute that the MBC is open at 10psi differential, so someone needs to explain why it would be open at 10psi AND 18psi differentials in order to support your claim.

The ONLY way I can think it might be open at both 10psi and 18psi is if the MBC opens at an 18psi pressure difference (when you finally reach 18psi) and stays open as that difference drops to some much lower level where it finally seals up again. But there's a problem with an MBC that acts this way. What if you hit 18psi, the MBC opens, you run a bit at full boost, then you let off the gas a bit and the boost drops to say 10psi. The MBC is still open so you're exposing the WGA to 10psi, so it's still open. Now you get back on the gas, but since the WGA is still open, you can't increase boost, and you're stuck at 10psi. It just doesn't work out that way, so I still don't believe that theory.

When you back off the throttle you lower consumption of air into the motor resulting in less flowing thrugh the turbine.

The result of that is now were pushing less boost. the second you back off and flow less than 18psi the MBC closes and the bleeder hole vents the pressure behind the ball and closes the wasegate. If you get on it again you now have a closed wastegate like you want and have to build 18PSI ON THE BALL untill it will open and deliver to the WGA again.

 

[kenamond]

Your saying pressure IS force, I'm saying its a kind of force. Thats why the ribbon in a pressureized room can still be blown. Understand that?

Im going to read te step by step again now.

There's a famous equation discovered by a guy named Newton. It goes F=ma. Force is the product of mass and acceleration. Rearrange it and you get a=F/m. This gives the acceleration of a body with mass m with a force F applied to it. More specifically, F is the sum of all forces acting on the body. If they are all equally distributed all over the body, the net force is zero, and the body will not accelerate. No acceleration means a body that is not moving will not move. In your ribbon case, if the room is at 18psi calm air, the sum of all forces acting on that ribbon completely cancel out, so the ribbon does not accelerate.

Now for the wind case. Moving air has energy in the form of kinetic energy. If that air runs into something and has to slow down or stop, that kinetic energy is converted into pressure. Bernoulli discovered that. This is what lets an aircraft wing generate lift (air going over the curved top of the wing has to speed up, so the opposite thing happens - the pressure is converted into kinetic energy to speed up the air going over the wing, so the velocity goes up, and the pressure goes down, and the top of the wing has less pressure than the bottom, and we get lift. A venturi in a carburetor is the same way to suck fuel in to the venturi. For the ribbon, the wind hits the ribbon and slows down and this increases the pressure on that face of the ribbon to maybe 18.01psi, but the pressure on the back of the ribbon is still at 18psi. This gives a net pressure difference. This causes the ribbon to accelerate (move) away from the wind.

 

[kenamond]

When you back off the throttle you lower consumption of air into the motor resulting in less flowing thrugh the turbine.

The result of that is now were pushing less boost. the second you back off and flow less than 18psi the MBC closes and the bleeder hole vents the pressure behind the ball and closes the wasegate. If you get on it again you now have a closed wastegate like you want and have to build 18PSI ON THE BALL untill it will open and deliver to the WGA again.

I demonstrated that the MBC CANNOT BE CLOSED at full boost with a 10psi difference across it. At full boost, you have 18psi on one end and 8psi on the other. That makes a difference of 10psi, but the MBC is still open.

 

[donmagicjuan]

Your saying pressure IS force, I'm saying its a kind of force. Thats why the ribbon in a pressureized room can still be blown. Understand that?

Im going to read te step by step again now.

The problem with your ribbon example is that you cannot make the approximation that it will behave as a rigid body, whereas it is a valid approximation for the MBC ball. Let's replace the ribbon in the example with a ping pong ball hanging from the ceiling. The ping pong ball can only move if the net force acting on it is not zero. The only thing in contact with the ball is the air. Forces on the ball due to the air are determined by the summation of an infinite number of point pressures acting on the surface of the ball multiplied by a differential area (yay, calculus!). The flow of the air in the room creates minor pressure differences in various areas of the room, which are significant enough to result in a nonzero force summation that accelerates the ball because of its low mass (F = ma). This is why I say there is no such thing as a flow component of force. However, if you were to replace the ping pong ball with a bowling ball, the flow wouldn't move it at all. Now let's return to the case of the MBC valve. The ball being held shut by the spring behaves much more like the bowling ball than the ping pong ball. The small changes in pressure due to the turbulence in the intake system will not contribute appreciably to the total net force on the ball.

 

[EclipseTrbo420A]

I demonstrated that the MBC CANNOT BE CLOSED at full boost with a 10psi difference across it. At full boost, you have 18psi on one end and 8psi on the other. That makes a difference of 10psi, but the MBC is still open.

This is where we disagree. Im looking at the turbo/IC system as a whole. This is so simple and you guys are making it harder than it needs to be. You block the pressure untill its where you want it, then it passes and so it doesnt get trapped behind the ball when the source is cut off, they made a pin hole, small as possible. Thats it. Im kinda tired now. Ill be here tomorrow dont work. Keep the thoughts comming guys.

 

[EclipseTrbo420A]

The problem with your ribbon example is that you cannot make the approximation that it will behave as a rigid body, whereas it is a valid approximation for the MBC ball. Let's replace the ribbon in the example with a ping pong ball hanging from the ceiling. The ping pong ball can only move if the net force acting on it is not zero. The only thing in contact with the ball is the air. Forces on the ball due to the air are determined by the summation of an infinite number of point pressures acting on the surface of the ball multiplied by a differential area (yay, calculus!). The flow of the air in the room creates minor pressure differences in various areas of the room, which are significant enough to result in a nonzero force summation that accelerates the ball because of its low mass (F = ma). This is why I say there is no such thing as a flow component of force. However, if you were to replace the ping pong ball with a bowling ball, the flow wouldn't move it at all. Now let's return to the case of the MBC valve. The ball being held shut by the spring behaves much more like the bowling ball than the ping pong ball. The small changes in pressure due to the turbulence in the intake system will not contribute appreciably to the total net force on the ball.

If what your saying is true...a puff, 1 puff then the source cut off, would move through the pressureized room forever and never die since its switching positions constantly with the pressure rather than the pressure getting pushed/or blown by the different force and then settling down.

 

[donmagicjuan]

If what your saying is true...a puff, 1 puff then the source cut off, would move through the pressureized room forever and never die since its switching positions constantly with the pressure rather than the pressure getting pushed/or blown by the different force and then settling down.

What? Pressure is not an object that can be pushed or blown! Actually, the flow in the pressurized room will eventually decay because of the intermolecular collisions taking place in the air. Since energy cannot be created nor destroyed, the end result is an overall increase in the temperature of the air in the room (average random molecular kinetic energy), but it's an immeasureable increase, and it's neither here nor there.

 

[kenamond]

This is where we disagree. Im looking at the turbo/IC system as a whole. This is so simple and you guys are making it harder than it needs to be. You block the pressure untill its where you want it, then it passes and so it doesnt get trapped behind the ball when the source is cut off, they made a pin hole, small as possible. Thats it. Im kinda tired now. Ill be here tomorrow dont work. Keep the thoughts comming guys.

Actually, I believe the opposite is true; it's more complicated than at first glance. I don't see why you disagree. This is taking a while, so slowing down won't hurt . Let's find out what you don't agree with in steps so that we can discuss each step. Tell me which of the following numbers you don't believe (I'm talking about the full boost case, not getting there):

1) The pressure on the compressor side of the MBC is at 18psi.
2) The pressure in the WGA is 8psi.
3) The MBC has two sides: one to the compressor, one to the WGA.
4) 18-8=10
5) The MBC is open.
6) Boost is holding at 18psi.
7) If air is not supplied to the WGA by the MBC, the 8psi will drop as air leaks out the vent hole.
8) The MBC is open at a differential pressure of 10psi.

 

[EclipseTrbo420A]

Actually, I believe the opposite is true; it's more complicated than at first glance. I don't see why you disagree. This is taking a while, so slowing down won't hurt . Let's find out what you don't agree with in steps so that we can discuss each step. Tell me which of the following numbers you don't believe (I'm talking about the full boost case, not getting there):

1) The pressure on the compressor side of the MBC is at 18psi.
2) The pressure in the WGA is 8psi.
3) The MBC has two sides: one to the compressor, one to the WGA.
4) 18-8=10
5) The MBC is open.
6) Boost is holding at 18psi.
7) If air is not supplied to the WGA by the MBC, the 8psi will drop as air leaks out the vent hole.
8) The MBC is open at a differential pressure of 10psi.

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

A block pressureizedized full of water with marble in it. For some reason now we have special powers where we can move the marble with out messing with the block or water physcially. Everthing is still, nothing is moving.

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.