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  #1  
Old 02-09-2015, 09:24 AM
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Relentless Relentless is offline
 
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Default What is a Pressure Recovery Plenum?

Getting many questions about my comments about "pressure recover plenum" so here is more detail on this effort.



There are fundamentally two areas that make the separate plenum concept
work well

1.) Temperature
2.) Pressure/Volume

Firstly, temperature.
In the standard arrangement we have four inlet tubes that are cast into
the engine oil sump. By design, this arrangement will subject the
incoming air to the to the oil temperature which at operating temp will
be in the region of 90deg C.
Using a standard day at 15 deg C, you can see that an engine that is
taking in air at around 50 deg C or more, is less dense, which means
less power. If we separate the induction so that we can draw in cool air
directly then we will obtain the benefit of dense air and with it the a
corresponding increase in power.

Secondly Pressure/Volume
The air box (plenum) diffuses the air, decreases the velocity and
increases the pressure. Therefore increased amounts of air can be
inducted with an air box (plenum) Using resonance tubes (intake tubes)
we are able to increase mid and high end performance.

Fundamentally there are two types of induction tuning, inertia and
resonance. Inertia tuning is the domain of the inlet tract length and
dia and the resonance by the air box (plenum), technically called a
Helmholtz resonator.
Cylinder filling and charging at a certain speed is dependant on the
system resonance frequency (which is dependent on the valve being open
or closed)

Without using such an airbox (plenum) we would not be able to tune the
system effectively.
When we designed the plenum we used mathematical optimization models and
CFD,it allowed us a close approximation to an optimized product.

To answer the question quickly:

The air box (plenum) diffuses the air, decreases the velocity and
increases the pressure. Therefore increased amounts of air can be
inducted with an air box (plenum) Using resonance tubes (intake tubes)
along with a significant reduction in air temperature, we are able to
increase mid and high end performance.

Latest version working with integrated EFii injector bosses:
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  #2  
Old 02-15-2015, 10:18 AM
rv7charlie rv7charlie is offline
 
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Default

Thanks for the post. Takes me back to the articles in Hot Rod Mag back in the 60's. Many A/C guys seem to have missed the memos from the car guys on how to make an engine work.

Couple of questions. Any attempt at bellmouths on the intake tubes? Also, it looks like the injectors are pointed upstream instead of downstream. Did you find this to work better, or was it a packaging issue? (Alt engine guys have found that pointing them downstream works better for them.)

Thanks,

Charlie
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  #3  
Old 02-16-2015, 05:49 AM
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1. Fundamental physics seem to suggest that a 35C air temperature rise, merely by passing through a naturally aspirated intake plenum warmed by the oil, is impossible.

2. The manifold is claimed to convert dynamic pressure to static pressure. What is the source of the dynamic pressure? If aircraft velocity, how is it different from any other manifold connected to an appropriate intake?

3. A resonator is tuned for one frequency, which means a particular RPM. Is this manifold tuned for cruise (CS prop assumed, say 2300~2400) or for max HP (the 2800~2850 seen in past posts)?
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  #4  
Old 02-16-2015, 06:12 AM
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Ooh, I like where we are going here!
Reaching now for the FD (fluid dynamics) textbooks....
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  #5  
Old 02-16-2015, 07:53 AM
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Bells on the intake runners have shown to boost flow up to 18% on the flow bench over a straight tube and probably more in the case of pulsed flow as happens in operation. You'll find almost every performance intake system uses these. Of course if the actual port flow is a lot less than the tube flow, it might not make much difference in hp.

The Porsche Indy Car engine from many years ago used reverse injector direction like this as it was found to improve power on methanol where there is a very large volume of fuel injected compared to gasoline. Throttle response and cold starting are likely to be compromised however but the proof is in the pudding with back to back tests of injector orientation.
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  #6  
Old 02-16-2015, 11:41 AM
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Default Pretty cool!

Can't wait to see the finished parts and some performance numbers!
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  #7  
Old 02-16-2015, 12:12 PM
BillL BillL is offline
 
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Quote:
Originally Posted by rv6ejguy View Post
Bells on the intake runners have shown to boost flow up to 18% on the flow bench over a straight tube and probably more in the case of pulsed flow as happens in operation. You'll find almost every performance intake system uses these. Of course if the actual port flow is a lot less than the tube flow, it might not make much difference in hp.

The Porsche Indy Car engine from many years ago used reverse injector direction like this as it was found to improve power on methanol where there is a very large volume of fuel injected compared to gasoline. Throttle response and cold starting are likely to be compromised however but the proof is in the pudding with back to back tests of injector orientation.
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  #8  
Old 02-25-2015, 12:46 AM
AC Aero AC Aero is offline
 
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1. Fundamental physics seem to suggest that a 35C air temperature rise, merely by passing through a naturally aspirated intake plenum warmed by the oil, is impossible.

2. The manifold is claimed to convert dynamic pressure to static pressure. What is the source of the dynamic pressure? If aircraft velocity, how is it different from any other manifold connected to an appropriate intake?

3. A resonator is tuned for one frequency, which means a particular RPM. Is this manifold tuned for cruise (CS prop assumed, say 2300~2400) or for max HP (the 2800~2850 seen in past posts)?

Response

I was asked recently to explain what was meant by the phrase “pressure recovery”, in context to my cold air box (plenum) for the AX50. It appears that the explanation was lacking and a few more questions have arisen. I will elucidate.

1.) Heat transfer: conduction, convection and radiation. Conduction is transferring heat energy by actual contact with a warmer object and conducting the heat through solid material. Convection is transferring heat by moving thermal energy. In this mode, a fluid transports the thermal energy from one location, where it picks up heat, to another location, where it gives up that heat. Radiant heat transfer involves a hotter surface emitting infrared heat towards a cooler surface where this radiant heat is absorbed.
For the heat transfer from the hot engine oil to the incoming air, we can use the “plane wall” method. provided we know the material and gas (air) properties it is easy enough to do. We can make the calculation a little more complex by taking into consideration flow conditions and frictional losses associated with the wall of the intake tubes. Once we have understood the mechanics of the fluid we can then take a look at the thermal effects and use Newton’s law of cooling. We know the flow in a tube is completely enclosed, therefore an energy balance can be applied to determine how the mean temperature varies with position along the length of the tube and how the total convection heat transfer is related to the tube inlet and outlet temperatures.
Heat soaked components, such as a standard sump with the induction tubes cast internally will be subject to a significant temperature increase. The example I used in my original post was to highlight the effect of temperature on the performance of an engine. Because of this condition, we chose to separate the air box from the oil sump. I can assure you that the math is sound.



2.) The pressure of a gas is a state variable, like temperature and density and any change in pressure during the process is governed by the laws of thermodynamics. Although pressure itself is a scalar quantity, we define a pressure force to be equal to the pressure (force/area) times the surface area in a direction perpendicular to it’s surface. If a gas is static and not flowing, the measured pressure is the same in all directions. But if the gas is moving, the measured pressure depends on the direction of motion. This leads to the definition of the dynamic pressure. Therefore when the gas exits the induction tube and enters the plenum a change takes place, velocity decreases rapidly and pressure rises. Thus, increased amounts of air can be available with the use of a plenum (air-box). When we examine in detail the standard sump with it’s integral cast induction tubes, joined in the center, one can see immediately that there is no plenum and therefore no possibility of a pressure recovery. Increasing the air velocity alone, is not desirable when it does not contribute to the harmonic tuning of the intake pulses in the intake manifold.

3.) In boxer and V type engines, charging is achieved by a combination or inertia and resonance charging. For your ref, the plenum you have seen on the post uses a modular system where we simple change the internal dictating to suit the performance characteristics of the particular engine. The picture on the post was a very early design concept and has change quite considerably since then.
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  #9  
Old 02-25-2015, 05:44 AM
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Weasel Weasel is offline
 
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Quote:
Originally Posted by AC Aero View Post
2.) The pressure of a gas is a state variable, like temperature and density and any change in pressure during the process is governed by the laws of thermodynamics. Although pressure itself is a scalar quantity, we define a pressure force to be equal to the pressure (force/area) times the surface area in a direction perpendicular to it?s surface. If a gas is static and not flowing, the measured pressure is the same in all directions. But if the gas is moving, the measured pressure depends on the direction of motion. This leads to the definition of the dynamic pressure. Therefore when the gas exits the induction tube and enters the plenum a change takes place, velocity decreases rapidly and pressure rises. Thus, increased amounts of air can be available with the use of a plenum (air-box). When we examine in detail the standard sump with it?s integral cast induction tubes, joined in the center, one can see immediately that there is no plenum and therefore no possibility of a pressure recovery. Increasing the air velocity alone, is not desirable when it does not contribute to the harmonic tuning of the intake pulses in the intake manifold.
I don't understand. How can converting some of the velocity pressure to static pressure (in the plenum) and then converting some of the increased static pressure (in the plenum) back to velocity pressure gain you anything?

I do understand harmonic tuning and I believe you can see gains with this but I dont see gains made in just converting from velocity to static and back to velocity unless it is to reduce friction losses in the overall system?
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  #10  
Old 02-25-2015, 06:31 AM
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Quote:
Originally Posted by AC Aero View Post
1. Fundamental physics seem to suggest that a 35C air temperature rise, merely by passing through a naturally aspirated intake plenum warmed by the oil, is impossible.

1.) Heat transfer: conduction, convection and radiation ... I can assure you that the math is sound.
Goodness. I'm a Professional Engineer (ME with 32 years engineering experience) just read this. You have a lot of words there but never connect the dots. I'm afraid that saying "I can assure you the math is sound" is not good enough.

All Newton's law of cooling says is the rate of heat transfer is proportional to the difference in temperature between the two materials. Great. But the total temperature rise has other factors including mass flow rate. (m*Cp*delta T) and in our case the mass flow rate is significant. Does any particular air molecule stay around that nasty old hot sump long enough to pick up much heat and therefore exhibit a meaningful temperature rise?

I did a quick velocity calculation based on a 3" diameter air duct and the air for a 360 cu in engine at 2500 rpm is moving about 60 mph. (180 cu in per revolution etc.) I doubt that the temperature rise is very much.

Perhaps instead of assuring us the math is sound you'll take us through the math?

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