Is a positive displacement supercharger better for the log?

C

CobraSix

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okay, I know it's a moot point for me with the aussie head, but I was thinking about this the other day.

The big disadvantage of the log is the leaning out of outer cylinders due to fuel starvation right?

Well centrifigal chargers (super and turbo) compress the air internally and then push the compressed air into the log. It is still conceivable that there will be some boost loss in the ends of the log as the charge air expands to all the corners.

Now look at PD superchargers. THey do not compress the air internally, but rely on pumping more air into the plenum then can be used at the time, thuse creating pressure and compressed air/fuel mix that is then feed into the cylinder. Since this system relies on the restriction of flow, and basically fills the plenum, there should be even pressure through all cylinders.

Now I know I'm simplifing the fluid dymanics some, but this does pose the question, is the PD charger better in theory, however slight?

Slade
 
Nope. "Pressure is distributed evenly throughout a fluid;" that includes air. If the discharge pressure of the blower is the same, the pressure profile throughout the log will be the same, regardless of blower type.

What the PD blower will do for you is give you ful boost throughout the entire RPM range of the engine whereas the centrifugal types increase boost as RPM increases.
 
I know the fluid dynamics rules...(boy do I remember that class), but that even distribution is until ideal circumstances...ie: Not real. There will be high and low pressure areas within a fluid depending on the fluid flow and restrictions and such. Remember, charge air on an engine is not a static fluid, it is a dynamic fluid.

Now, I'm not saying one is better or worse then the other...just curious about debating.

Slade
 
Excuse me while I think out loud.

By definition, a fluid fills its available space. A compressible fluid fills that space with constant pressure, until you start to move it. Without the positive pressure from the compressor (turbo, blower, etc), the air is moved by vacuum. With the positive pressure, the air still moves by vacuum, relative to the positive pressure of the blower. The difference being that with the blower, there is greater pressure available to be absorbed by the system (friction losses). Also, the pressure drop from the base of the carburetor to each cylinder must be equal. Since the loss per volume of air is less to the middle cylinders, it must flow more air to provide the same pressure drop as the rest of the cylinders. Therefore, the middle cylinders will always get more air, regardless of the pressure at the base of the carb. It just has less distance to travel.

So, based on simple things, the same inefficiencies are still there and we would expect that, relatively speaking, the end cylinders will still be starved proportionally. However, with the increased flow, items like valves can become the limiting restriction.

I would expect to see the outer cylinders still starved for air, but as you approach a critical flow, the cylinders would get closer and closer.
 
It was good to hear you thinking aloud - it gave some plain-English interpretations of events. The ratio of log to runner is obviously of merit, and turbulence must play a relevant part in distribution. Look at the Oz turbo setups - they tend to have a large plenum and short runners. The HP some of these mills produce doesn't leave much room for mistake in A/F settings!

Whether you modified the intake by restrictors or cam design, it would be one serious modelling exercise! :shock:

Regards, Adam.
 
weeds":1ja993tf said:
Excuse me while I think out loud.

By definition, a fluid fills its available space. A compressible fluid fills that space with constant pressure, until you start to move it. Without the positive pressure from the compressor (turbo, blower, etc), the air is moved by vacuum. With the positive pressure, the air still moves by vacuum, relative to the positive pressure of the blower. The difference being that with the blower, there is greater pressure available to be absorbed by the system (friction losses). Also, the pressure drop from the base of the carburetor to each cylinder must be equal. Since the loss per volume of air is less to the middle cylinders, it must flow more air to provide the same pressure drop as the rest of the cylinders. Therefore, the middle cylinders will always get more air, regardless of the pressure at the base of the carb. It just has less distance to travel.

So, based on simple things, the same inefficiencies are still there and we would expect that, relatively speaking, the end cylinders will still be starved proportionally. However, with the increased flow, items like valves can become the limiting restriction.

I would expect to see the outer cylinders still starved for air, but as you approach a critical flow, the cylinders would get closer and closer.
Click to expand...

Very good, except remember, positive displacement blowers do not build pressure internally, they rely on the engine not needing all the air it is providing, so pressure builds as the air builds up. To me, that means that pressure distribution will be even in the plenum.

With a centrifugal (that actually compresses air) you will have some pressure losses as the charge flows through the intake. I guess it may be a minor point, since the wastegate will just pump the outlet pressure up based on MAP, but then there is the function of where is the MAP gauged from, so the issue is still at hand.

I really don't know the answer...this is really just a thinking out loud question, since the forum was quiet.

Slade
 
Here's my ramble:

It doesn't matter where the blower builds its boost (except for efficiency reasons, which centrifugal wins over roots). Internally the centrifugal always has a higher pressure which drops when it hits the outlet. There are multiple restrictions in the motor which add up to cause the final intake restriction. As you say, a positive displacement blower doesn't compress internally. This doesn't mean much because the restriction it faces is immediately outside the housing (well, right where the rotor breaks its seal from the housing). At that point you're building pressure. From there, the miniature pressure 'spike' will move out into the much larger area of the housing and farther down into the manifold, dispersing as it goes because of the larger volume, all at the speed of sound.

A couple things to remember. Air pressure changes at the speed of sound. Vacuum is pressure lower than atmospheric, but that doesn't really mean anything. Air always moves from higher pressure to lower pressure. Whether that's 20psi into 10psi or 1psi into 10inHG. 10inHG vacuum is 9.784psia when measured from 1 atmosphere (defined 14.696psi). 'Vacuum' is an almost useless measurement.

MAP position is always an issue. Pressure in the port is always going to be lower than elsewhere in the system (air is moving the fastest = lowest pressure). Plenum should be the highest pressure.


-=Whittey=-
 
Remember that pressure is really just the tool that moves the air. The more pressure you have, the more flow you can have given consistent conditions. The pressure is what is required to move the air against the resistance to flow. Keep that in mind.

Set the flow boundry at the base of the carb. From the flow perspective, it does not matter where the pressure comes from or how it gets there.

The compressor will provide only the amount of air the engine will take - conservation of mass, unless it exits through a wastegate. One may have the advantage as to when the pressure comes in or how much pressure it will provide, but assuming that the pressure (and flow potential) is equal from the PD blower to the centrifugal blower to the turbo, the flow will be the same. Once you get past the compressor itself, how it compresses is no longer part of the equation either.

Whittey posted while I was typing. Good points.

I do not know if it will apply here, but the terminal velocity of air is equal to the speed of sound. No matter how much more pressure you stick in there, it will not move any faster. Pressure and the speed of sound are related, so more pressure, it can move faster. But only until it reaches the speed of sound.
 
Air (gas) can definately move faster than the speed of sound. Hypersonic flow just has a whole lot more rules than your typical subsonic flow.


-=Whittey=-
 
When I was doing flow modeling for the discharge piping of pressure safety and relief devices, our client was very specific about things like this. One of their opinions, and you never questioned their opinions, was that air could not travel faster than the speed of sound in that situation. That situation was usually a high pressure source, through a valve, through piping and exiting to atmosphere. I used to have a paper written by one of the safety vavle manufacturers (Anderson Greenwood, I think) that detailed this phenomenon. IIRC, the pressure drop through the discharge piping was just enough to provide Mach numbers less than 1. It seemed to counter the information in Crane 410 at the time, but I was just a junior engineer at the time. I may have to go hunting.

Whittey, any information that you have about that one would be appreciated.
 
I'll go look in a bit.

I looked at Crane 410: pg 1-9
"The maximum possible velocity in the pipe is sonic velocity, which is expressed as:
Vs = sqrt (kgRT)"

But again, it refers to exit velocity.

I'm still learning.
 
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