Brake booster venturi tube: Theory of operation
10-15-2013, 11:51 PM
#16
Nordschleife Master
Do you think that the little orifice has air drawn out of the hose by the column of air passing by it....creating a vacuum at low intake manifold vacuum conditions or it just a pressure equalizer, so that there is not excessive vacuum on the booster under high intake manifold vacuum conditions?
10-16-2013, 09:48 AM
#17
Rennlist Member
Rob's original question was "how does the venturi work" and that is the query I addressed initially - he did not ask "what does that sub system do" - that is another matter altogether.
The point I tried to open up for discussion is what does it actually do. I think it is reasonably obvious that it permits [some?] vacuum to be applied all the time so indeed it helps to ensure that our leaky vacuum systems have a method of being kept "topped up" when the throttle is open, partially or fully for extended periods as what happens at idle is largely irrelevant.
As there is no continuous air flow in this circuit and leaks are small, then this system is, I suspect, designed to ensure that if [as frequently happens] the passive vacuum system loses seal somewhere, it thus ensures that the vacuum on the servo is maintained as it is probably designed to evacuate more flow than the amount trying to leak into the servo from the failed auxciliary component probably regulated by [small] flow through the check valve . I trust you will have noticed that if one of the vac acutators fails you immediately lose the flappy but not your brakes?
The question that needs to be asked is if this venturi system is purely to protect the vacuum of the accessories as Dr Bob postulates, then why is it connected to the servo and not directly to the passive vacuum system?
To go back to Greg's comment the hose is connected on the boot as it has to be inbetwen the butterfly and the MAF so that air flow metering [thus fuel] is correct at idle conditions. Reverse flow is not possible due to the pressure gradients and would make no engineering sense I can think of.
The restriction orifice Greg refers is probably there to ensure that the maximum amount of air that can flow through this system is controlled such that it can do what is needed without being excessively large and thus upsetting idle speed by totally bypassing the ISV.
What tends to confuse people is the difference between a "Venturi" and the "Venturi effect". A venturi is simply a device that accelerates the velocity of a flowing stream by means of a tapered restriction [as opposed to an orifice with square edges]. Such a device can be used to measure the amount of gas flowing by having pressure tappings at the correct positions- the differential pressure generated being meassured at specific points and thus used to infer flowrate.
In our case what we actually have is correctly referred to in Chemical Engineering terms as an "Eductor". This uses the Venturi effect to develop an area of low pressure [a partial vacuum] to induce a secondary flow into the device. Thus the device has to be correctly engineered so as to develop a certain amount of vacuum to thus educt the required amount of fluid. Such devices can be used for gases or liquids. A good example of a gas liquid system is what happens in the carburettor on a motorcycle. A good example of a liquid/liquid eductor system is the self-baling mechanism on a small boat. The device on our 928's is obviously gas/gas.
If as Dr Bob posultates, the device is purely for the auxiliaries then why did they plumb it to the brake booster system? They could have done this to the other side of the little check valve separately from the brake booster if it was not intended for the braking system.
The reason this intrigues me is because of a problem I recently had on my Cayenne Turbo S. These things have what appears to be a secondary vacuum system but it is driven by electric vacuum pumps and I have been tying to understand why.
This all makes me wonder if when force feeding the 928 something is being done that has some negative impact on the brake system. If such were the case it obviously cannot be anything too dramatic but a marginal type of thing?
I am trying to reason out what we see based on the kit provided- one has to assume that Porsche did not do what they did for no good reason whatever it may be. Dr Bob may well be right but I have a feeling there is more to it than just that.
Regards
Fred
The point I tried to open up for discussion is what does it actually do. I think it is reasonably obvious that it permits [some?] vacuum to be applied all the time so indeed it helps to ensure that our leaky vacuum systems have a method of being kept "topped up" when the throttle is open, partially or fully for extended periods as what happens at idle is largely irrelevant.
As there is no continuous air flow in this circuit and leaks are small, then this system is, I suspect, designed to ensure that if [as frequently happens] the passive vacuum system loses seal somewhere, it thus ensures that the vacuum on the servo is maintained as it is probably designed to evacuate more flow than the amount trying to leak into the servo from the failed auxciliary component probably regulated by [small] flow through the check valve . I trust you will have noticed that if one of the vac acutators fails you immediately lose the flappy but not your brakes?
The question that needs to be asked is if this venturi system is purely to protect the vacuum of the accessories as Dr Bob postulates, then why is it connected to the servo and not directly to the passive vacuum system?
To go back to Greg's comment the hose is connected on the boot as it has to be inbetwen the butterfly and the MAF so that air flow metering [thus fuel] is correct at idle conditions. Reverse flow is not possible due to the pressure gradients and would make no engineering sense I can think of.
The restriction orifice Greg refers is probably there to ensure that the maximum amount of air that can flow through this system is controlled such that it can do what is needed without being excessively large and thus upsetting idle speed by totally bypassing the ISV.
What tends to confuse people is the difference between a "Venturi" and the "Venturi effect". A venturi is simply a device that accelerates the velocity of a flowing stream by means of a tapered restriction [as opposed to an orifice with square edges]. Such a device can be used to measure the amount of gas flowing by having pressure tappings at the correct positions- the differential pressure generated being meassured at specific points and thus used to infer flowrate.
In our case what we actually have is correctly referred to in Chemical Engineering terms as an "Eductor". This uses the Venturi effect to develop an area of low pressure [a partial vacuum] to induce a secondary flow into the device. Thus the device has to be correctly engineered so as to develop a certain amount of vacuum to thus educt the required amount of fluid. Such devices can be used for gases or liquids. A good example of a gas liquid system is what happens in the carburettor on a motorcycle. A good example of a liquid/liquid eductor system is the self-baling mechanism on a small boat. The device on our 928's is obviously gas/gas.
If as Dr Bob posultates, the device is purely for the auxiliaries then why did they plumb it to the brake booster system? They could have done this to the other side of the little check valve separately from the brake booster if it was not intended for the braking system.
The reason this intrigues me is because of a problem I recently had on my Cayenne Turbo S. These things have what appears to be a secondary vacuum system but it is driven by electric vacuum pumps and I have been tying to understand why.
This all makes me wonder if when force feeding the 928 something is being done that has some negative impact on the brake system. If such were the case it obviously cannot be anything too dramatic but a marginal type of thing?
I am trying to reason out what we see based on the kit provided- one has to assume that Porsche did not do what they did for no good reason whatever it may be. Dr Bob may well be right but I have a feeling there is more to it than just that.
Regards
Fred
10-16-2013, 10:17 AM
#18
Nordschleife Master
Fred --
Thanks for correcting me on the point where the venturi jet pump is connected. I think my cruise control experience must have related to the general vacuum system and boosted engine, not this venturi jet pump.
My guess would be that the venturi jet pump is there to make sense that the brake assist always has vacuum, even if the throttle is open. For example, suppose that the throttle sticks open. Now, without the venturi jet pump, there might be no break assist. With the throttle stuck open, I'd like to have the brake assist available. Even excluding the cases such as stuck throttle, I think it's nice to have a consistent brake feel regardless of how long one has been driving WOT.
Getting back to the general vacuum system, what kind of electric vacuum pump does Porsche Cayanne Turbo have? I might want to install that, if it's small.
Best, Tuomo
Thanks for correcting me on the point where the venturi jet pump is connected. I think my cruise control experience must have related to the general vacuum system and boosted engine, not this venturi jet pump.
My guess would be that the venturi jet pump is there to make sense that the brake assist always has vacuum, even if the throttle is open. For example, suppose that the throttle sticks open. Now, without the venturi jet pump, there might be no break assist. With the throttle stuck open, I'd like to have the brake assist available. Even excluding the cases such as stuck throttle, I think it's nice to have a consistent brake feel regardless of how long one has been driving WOT.
Getting back to the general vacuum system, what kind of electric vacuum pump does Porsche Cayanne Turbo have? I might want to install that, if it's small.
Best, Tuomo
10-16-2013, 11:20 AM
#19
Rennlist Member
Fred --
Thanks for correcting me on the point where the venturi jet pump is connected. I think my cruise control experience must have related to the general vacuum system and boosted engine, not this venturi jet pump.
My guess would be that the venturi jet pump is there to make sense that the brake assist always has vacuum, even if the throttle is open. For example, suppose that the throttle sticks open. Now, without the venturi jet pump, there might be no break assist. With the throttle stuck open, I'd like to have the brake assist available. Even excluding the cases such as stuck throttle, I think it's nice to have a consistent brake feel regardless of how long one has been driving WOT.
Getting back to the general vacuum system, what kind of electric vacuum pump does Porsche Cayanne Turbo have? I might want to install that, if it's small.
Best, Tuomo
Thanks for correcting me on the point where the venturi jet pump is connected. I think my cruise control experience must have related to the general vacuum system and boosted engine, not this venturi jet pump.
My guess would be that the venturi jet pump is there to make sense that the brake assist always has vacuum, even if the throttle is open. For example, suppose that the throttle sticks open. Now, without the venturi jet pump, there might be no break assist. With the throttle stuck open, I'd like to have the brake assist available. Even excluding the cases such as stuck throttle, I think it's nice to have a consistent brake feel regardless of how long one has been driving WOT.
Getting back to the general vacuum system, what kind of electric vacuum pump does Porsche Cayanne Turbo have? I might want to install that, if it's small.
Best, Tuomo
We seem to be thinking along the same lines- having identified several potential reasons for its being.
As for the the Cayenne's vacuum system I still do not have the bigger picture - all I understand at the moment is this may be something unique to the Turbo variants. A plastictube failed [the pipe cracked] on a push on union and I got a red alarm on the dash- "brake booster failure" so I was not going to bugger around with that. It turned out they had to rip out a few things to get at it the problem and of course there are two sub system- cost me $500 in total. I could easily have fixed the failed component for virtually nothing had I had full prior knowledge of the problem .
I did have the chance to look at the pumps [if that is the correct term for them]. I would say they are about the size of a decent fist clenched- kind of like a jar of peanut butter type of thing.
If you go to the cayenne section of Renmlist someone linked me to a post aabout the problem and I believe that shows the vac pumps. Like everything Porsche though I bet they cost a bunch.
Regards
Fred
