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What is a Pressure Recovery Plenum?

Relentless

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

xkpoq1.jpg


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:
v7uknq.jpg
 
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
 
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)?
 
Ooh, I like where we are going here!
Reaching now for the FD (fluid dynamics) textbooks....
 
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.
 
Two Words

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.

Vena Contracta
 
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.
 
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?
 
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?

miracle_cartoon-M.jpg
 
I hate to beat up on AC Aero but I agree with Bubblehead and Weasel.
I only read Blah, Blah Blah and I am only a HVAC duct guy.
 
I can assure you that the math is sound.
I wish I had a dime for every time a design department told a production department that.

The math may be sound, but it may still be wrong if the real-world measurements can't support it. I, too, have doubts that you could measure a 35 degree rise through the intake on a standard Lycoming. My gut feel says maybe 5 degrees.

Our cabin heaters may pick up 30 degrees or so, but that's after running the cold intake air over your exhaust pipes, which are running a fair bit hotter.
 
Compare apples to apples, standard Lycoming horizontal intake (A1A, B, C, and D, A2a, B, and A3B6D, etc), vs the Ace intake.

Unlike the updraft sump originally intended for a carburetor application, the Lycoming horizontal sump does not run the air intake passages through the hot oil. The oil is segregated in an upper volume open to the crankcase, while air is contained in a separate lower plenum. The two volumes share a divider wall, and nothing more. There is some heat transfer through that divider wall, but it is very limited in the quantity of heat that can reach the intake air. Examine the illustration below. The vast majority of the air enters the plenum at the front and goes directly to one of the four intake tubes, without ever going near the hot divider wall.

That large lower volume is also a resonator, with all that implies. Note that there is no requirement to flow air through a resonator, and there may be disadvantages to doing so, like much more hot surface area in contact with the intake flow.



Also note the tuned length intake tubes, with bellmouths.



(Following photo credits to Charlie Kuss)





 
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"Let's you and him fight"

I love technical disagreements; it can (sometimes) lead to being better educated. :)

So let's say the temp rise in a conventional Lyc manifold is 5 degrees instead of 30.
What's the power loss percentage at sea level full throttle, with 5 degree hotter induction air?
Any difference in percentage lost at full throttle at altitude?

What if it's only 3 degrees?

I could see that it might make no significant difference for normal operations, but would it be more significant in racing?

Charlie
 
Dan,

Not all Lyc horizontal sumps are made that way. I've got one off a parallel valve engine (rear horizontal induction) that has the untuned tubes in the oil chamber, just like the updraft sumps. No plenum; they just converge to join the intake line from the carb/servo flange.

Don't know if it makes a significant temp difference, but I do know that the tubes are at least partially submerged in oil.

Charlie
 
Dan, Not all Lyc horizontal sumps are made that way. I've got one off a parallel valve engine (rear horizontal induction) that has the untuned tubes in the oil chamber, just like the updraft sumps. No plenum; they just converge to join the intake line from the carb/servo flange.
Don't know if it makes a significant temp difference, but I do know that the tubes are at least partially submerged in oil.

Sure. Rear induction sumps came both ways, plain or tuned, with plenum (rear induction on left):



It would make a significant temperature difference...but the OP was claiming 35C, 95 Fahrenheit in most of the USA. No way.

You want irony? Even with a Super Chilly Willy sump the entire intake system (plenum, intake tubes from sump to cylinder, everything) is bathed in 200F air, assuming the baffles are any good. It's one of the little differences between a dyno and a cowl, and it kinda puts perspective on that terrible 200F oil sump.
 
You want irony? Even with a Super Chilly Willy sump the entire intake system (plenum, intake tubes from sump to cylinder, everything) is bathed in 200F air, assuming the baffles are any good. It's one of the little differences between a dyno and a cowl, and it kinda puts perspective on that terrible 200F oil sump.

Oh, Dan--------there you go bursting another bubble. Gonna have to change your name from potato man to straight pin:rolleyes:
 
Any difference in percentage lost at full throttle at altitude?

What if it's only 3 degrees?

I could see that it might make no significant difference for normal operations, but would it be more significant in racing?

My super duper handy Lycoming power chart says that you lose about 1% power for every 10C (18F) rise in temp. Altitude and MAP are not relevant. If you achieve 50C reduction in temps you'd gain about 5% in power.

Also, the temperature advantage of intake tubes that are away from oil is not independent of the outside air temp. Unfortunately, this helps you less the hotter out it is, when you need more HP the most. Temperature transfer is occurs faster the higher the temp delta. If it's 115F out, and your oil temp is 190F, then you have only a 75F delta, so you will get less rise in your intake air temp than if it was -20F out and you have a 210F difference. Thus, on those hot and high days, this helps you less than on the super cold days when you already had an advantage.
 
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"I hate to beat up on AC Aero but I agree with Bubblehead and Weasel.
I only read Blah, Blah Blah and I am only a HVAC duct guy."



Ha! Funny. I got the same ringing in my ears. I'd like to see the air film coefficient they used in the heat transfer calcs. Probably would like to see them using velocity pressures in the air flow calcs, rather than dynamic. But what do we know?
 
All others bring data

What about the simple expedient of _actually measuring_ the air temp at the intake scoop and in the manifold just outside the intake valve. Two thermistors, one drilled hole in an induction tube. Can't be that difficult to do. Apparently it's only been calculated and not actually measured, so I must be missing something here in my armchair, and this must be complex rocket surgery we're talking about.

Or measure the after-sump temp at the carb inlet if concerned about possible Venturi and fuel evaporation effects. Don't they make carb inlet temp probes? I know measuring the air temp in the FAB ain't a challenge.

-Stormy
 
I love technical disagreements; it can (sometimes) lead to being better educated. :)

So let's say the temp rise in a conventional Lyc manifold is 5 degrees instead of 30.
What's the power loss percentage at sea level full throttle, with 5 degree hotter induction air?
Any difference in percentage lost at full throttle at altitude?

What if it's only 3 degrees?

I could see that it might make no significant difference for normal operations, but would it be more significant in racing?

Charlie

oh- oh- oh - - I can answer that - - it is just the proportion of the ratio of the temperatures - - either Kelvin or Rankine!! Absolute temperatures

Yes the direction is correct, but the range is way off. It will be small, but has some effect.

Example of the limits of heat transfer - I took the exhaust of a wankel engine - 1800F and put a 1 ft (12 inches) long water jacket around it. Flowing 60F water through it. So - and 1740F temperature difference. What was the temperature drop of the exhaust - - ?? 100 deg F (55C). Not much. This is a data point without the eye rolling discussion of boundary layers, heat transfer coefficients, turbulent flow, thermal conductivity, and equations.

So with oil at 180F and intake air at 60F - A difference of 120F what do YOU think? Not much, but it is a larger area, and the residence time is longer, but the surface heat transfer coefficient is much lower because of the velocity.

The cool intake benefit is real, and is a good thing, but lets keep it in perspective.

Just a quick comment on resonators, etc. All pulse and resonance with air is a spring mass system. Mass of the air and compressibility. Consider that with a piston in a chamber with a tube attached is a hemholtz resonator. It just has an intake valve in the system. Easy to get 120% volumetric efficiency, BUT only in a small rpm range without a variable system. Not as small as you might think, but will vary with temperature. Again as an issue of quantification, these short tubes can have an optimum diameter/ length, but at a fraction of the wavelength the resulting benefit will not be huge.

All of these claims (resonance, temperatures etc) would be best discussed with data for this forum. Real money requires real data.
 
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What about the simple expedient of _actually measuring_ the air temp at the intake scoop and in the manifold just outside the intake valve.

Oh, it's been measured. Had a note from a friend yesterday, who measured the carbed sump temperature rise (no plenum, intake runners in the oil) so long ago he can't remember exact numbers.

I'll have three measurements for the 390's stock tuned inlet sump (see post 13) in the near future...OAT, intake plenum exit, and just short of the head (with thermistors, as you describe), in flight, not in a (relatively) cool dyno room.

Anybody want to join me, maybe learn the rise in a non-plenum Lycoming sump, and in one of the aftermarket cold air sumps? You'll need:

Two (or three) National Semiconductor LM34AH-ND Temperature Sensors:

http://www.digikey.com/product-detail/en/LM34AH/NOPB/LM34AH-ND/182346?cur=USD

Three-conductor wire. 20 or 22 gauge is fine. Shielding not necessary, but the shield mesh can provide physical protection

Self-adhesive (heat melt glue lined) heat shrink tubing, ¼” and smaller

Good digital voltmeter or voltage panel display

Solder tools, a fuse holder, and connectors as required (current draw is very low)

The goal is a temperature probe on the end of a wire, which can be re-located anywhere under the cowl as desired.

The output of the National Semiconductor LM34AH-ND is read with an ordinary hand-held digital multimeter or a digital voltage display. The voltage corresponds to temperature, 10mV = 1 degree F. Example: Meter says 2.5 volts. 2.5V is 2500mV. 2500/10 = 250F. Whatever the meter says, just move the decimal point two spaces to the right and you have temperature.

Only three connections, aircraft power, ground, and sense. Connecting to the avionics bus so the sensor is “on” with flight instruments is fine. Meter negative and probe ground should both be connected to the aircraft single point ground bus.

Solder the LM34 to the ends of the three-conductor tefzel shielded aircraft wire. Insulate each lead connection carefully as you go, then cover the entire end with a short length of ¼” adhesive heat shrink. Leave the cap of the LM34’s can uncovered. Drill a hole in a manifold tube, insert the LM34, silicone into place. Go fly with the voltmeter in the cockpit.

Here's one being used to measure the in-cowl temperature in the area of a variable exit door servo:

 
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The goal is a temperature probe on the end of a wire, which can be re-located anywhere under the cowl as desired.


Rather than single point temp measurement, we should be using averaging elements, and airflow should be measured with a flow station.
 
Interesting. There are some misconceptions here though. A pressure recovery chamber is a chamber where pressure is recovered, not just a plenum chamber.

More precisely you convert dynamic pressure to static pressure. Converting static pressure to dynamic pressure is fairly easy. It involves accelerating the flow which can be done easily with small losses. Converting dynamic pressure back to static pressure is much more difficult. It involves decelerating the flow, which is difficult to do without inducing turbulence (losses). These losses can never be recovered as pressure, they are lost forever. All you have done is to increase the entropy, which basically means you have converted usable energy (directed velocity) into unusable energy (turbulence - chaos).

In more practical terms a pressure recovery chamber must gradually decrease the flow velocity without inducing turbulence. The aft section of a venturi is a typical example of pressure recovery device. The diverging angle must be less than 7-10 degrees for this to be effective.

venturi_flow_meter.gif


The plenum chamber has other qualities, like working as a discontinuity for mass- and transient tuning of the inlet pipes.

In any case, it is worth looking at the amount of eventual pressure available to be recovered. The dynamic pressure is p_dyn = 1/2 * rho * v?. The static pressure at sea level is 101325 Pa.

At 100 kt this will be 1750 Pa, or 1.7% increase
At 200 kt this will be 7000 Pa or 6.9% increase

With an optimal diverging element, you can expect to recover 90-95 % of that, which at 200 kt is indeed a fair increase in pressure. With a less than optimal diverging element, none of that pressure is expected to be recovered, at max about 20-30%, but only if the flow velocity after the diverging element is approximately the same as it is in front of the diverging element. This is definitely not the case when the air enters into a plenum chamber.


The other alternative here is the inlet of the plenum chamber is positioned at the stagnation point of the aircraft, but, the inlet flow into the chamber is much less than the velocity of the aircraft. This means the pressure recovery plenum simply works as a pitot measuring the dynamic pressure. In this case, talking about pressure recovery is a very odd thing to do, because the pressure is already "recovered".
 
Absolutely, and for me, largely thanks to Dan as he provide great and valuable info that is science based.

Hey, thanks, but let's not forget Dr. Svingen, and Bill Lane (who spent his entire career in engine design/development), and all the other really smart people who inhabit these parts.

Me? I'm just smart enough to know how badly educated I really am.
 
So... does anyone other than Relentless, ACAero, and rcpaisley have a viable product ready for market? I see a lot of textbook lectures in this thread but no proof of real world products. Help us out here if there are some that we should know about.
 
Another question....

Why hasn't the original poster responded to any of the sound discussions potentially refuting claims made in the OP?

I do love the dialogue and wealth of information that is provided in a thread after claims are made of substantial advances. What irks me is that when claims or advances are challenged, there is no rebuttal with supporting evidence by those claiming such.

Keep the great info coming!

DP
 
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