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Discussion Starter · #1 ·
Any good tuner, or good tuning book, can tell you that there is a difference between static and velocity pressure; it's nearly intuitive. But how do these properties arise? The answer is the core of fluid dynamics ...

What is pressure in the first place?

First a quote from NASA's Glenn Research Center website:

"From the kinetic theory of gases, a gas is composed of a large number of molecules that are very small relative to the distance between molecules. The molecules of a gas are in constant, random motion and frequently collide with each other and with the walls of any container. The molecules possess the physical properties of mass, momentum, and energy. The momentum of a single molecule is the product of its mass and velocity, while the kinetic energy is one half the mass times the square of the velocity. As the gas molecules collide with the walls of a container, as shown on the left of the figure, the molecules impart momentum to the walls, producing a force perpendicular to the wall. The sum of the forces of all the molecules striking the wall divided by the area of the wall is defined to be the pressure. The pressure of a gas is then a measure of the average linear momentum of the moving molecules of a gas"

http://www.grc.nasa.gov/WWW/K-12/airplane/pressure.html

Simply put, the pressure is the sum of the force imparted against the area of measurement by the momentum/kinetic-energy of the particles within the fluid.

Now for temperature and the speed of sound.

--- The speed of sound is equal to the square root of [the ratio of specific heats for the fluid multiplied by the gas constant for the fluid multiplied by the absolute temperature]. (An important property of air to know is that, below mach .3, its compressability effects are neglidgable; it's treated just like water or oil, but with different constants.)

Quickly one can see the link between temperature and the speed of sound given the other two variables are constant in any one particular gas; the speed of sound in a fluid is approximately, though not quite, the average linear speed of the particles within the fluid. Thus, you cannot use the fluid itself to propell the fluid ahead of it beyond of the local speed of sound without increasing the temperature. (Except in specially shaped rocket-style nozzles.)

Another important note is that temeprature is, in a sense (please don't get pedantic as I realize that average kinetic energy does not have direction), a vector quantity much like pressure; if the flow is parrallel to the measurement the average impact energy against the sensor will be smaller and the reading lower. (Thermometers measure the average kinetic energy imparted onto them, or lattice energy in some cases like most crystalline solids.) The difference is that people will simply say that the temperature is lower parrallel to the flow. IMHO this is a mistake on the GRC website; clearly, while the actual temperature will not be related to direction, the temeprature measured will be closely tied to the relative direction to flow of the fluid and thus the average impact intensity of the particles contacting the sensor.

So how does this create misconceptions?

Well, now that we all understand the exact nature of pressure, we can see that a problem arises: What happens when the fluid is moving quickly?

Since, at a given temperature (average kinetic energy), the particles in the fluid with have a constant average speed, if the overal-velocity of the fluid is high in a direction parrallel to the measuring area, less of the impact-force is directed towards the area of measurement.

Say what???!!

If you mount your MAP sensor, or it is already mounted, parrallel to the flow through your intake manifold you will not get an accurate measurement of the fluid density without calibrating the measurement.

An extreme example exists wtihin the turbocharger's compressor housing; the fluid is accellerated at choke flow to the speed of sound. At that speed there is nearly zero pressure tangental to the flow: if you were to place a pressure sensor in the housing at the diameter of the impellor tips you would measure very little pressure. The job of the scroll in the housing is to slow the cumulative velocity of the fluid down and convert the velocity pressure into static pressure with roughly the same average kinetic energy; the efficiency is largely related to the amount of vorticity generated when doing so. (Vorticity in a fluid usually disperses into heat.)

So, how the heck can I apply this to my car?!

There are a couple methods I can think of which would allow you to get more meaningfull approximations of fluid density and flow in your engine.

1. Mount the MAP sensor pointing towards the flow (such as right below the throttle on a T7 car where the flow points towards this point); this will allow it to measure "total", or often refferred to as "dynamic", pressure, instead of just static pressure. As the flow increases, even if the static pressure drops, the MAP sensor will now read higher and give you a good idea what amount of flow is going into your engine. The downside? Intake velocity is not necessarily linear; fortunately, if you mistakingly locate it where the flow is minimal, you will read a high static pressure because of the low cumulative velocity and that will give you sufficient information.

2. Use a MAF sensor which senses "dynamic pressure" via the cooling effect on the hot-element. It's not really any better than a well-designed MAP sensor, but it usually comes in a specially shaped housing which, as long as the intake is stock or well made, will have a similar flow-shape irrespective of flow-velocity. That means, unlike a manifold mounted MAP sensor, it does not usually need to be re-calibrated as the engine is modified. (Unless you modify the air-filter, and thus the airflow-shape into the housing .. or the housing itself, such is the case in Subaru tuning.)

It's the flow, not the size, that counts.

I believe low static-pressure to be desirable. The reason for this is that the low turbo efficiency arises from the conversion of velocity pressure, which is what the impellor actually generates, to static pressure. Why not keep as much velocity pressure as you can? Energy is lost in the conversion back and forth.

The idea that high boost pressure is better is, IMHO, fallacious as it usually means high static boost pressure and high associated pumping force; high air-mass/combust is desirable while high static pressure only results in excessive pumping losses because there is a greater vector-componant in the direction back towards the impellor, or engine in the case of exhaust componants.

The ideal setup would allow for the intake velocity to be high, but not cause excessive turbulence or resistance. (IE, be very smooth and have a low flow resistance value despite small diameter-sizing.)

The ideal setup would be a well-made compressor housing, low-resistance-but-low-crossection intercooler and induction system, high-flow camshafts/ports, and a high-flow exhaust system which does not convert too much of the velocity pressure at the ports back into static pressure. (The static pressure will be "seen" as "backpressure" by the engine and the stock manifolds on the T7 cars generate a great deal.) Also, the wastegate would be setup to be able to divert both velocity and static pressures; most can only divert static pressure as they are arranged parrallel to the flow. They are also usually arranged at a region of high flow where the static pressure is low which is very bad for turbo-control. (Not good.)

Tuner-Claims and Larger Pipes/Intercoolers.

Ever heard some tuner claim that by installing a larger pipe here or there that they have increased the boost pressure?

The problem is that they never specify static or dynamic pressure. Sure you can make the pipe bigger and reduce the fluid velocity; that will make the static pressure go up even though no additional air has entered the engine.

Induction Systems ...

Slow intake velocity, as we have established, can be a bad thing. Excessively high intake velocity which generates an unstable boundary layer or accellerates the air abruptly can also be bad. So what's good and what's bad in terms of design?

Good:

- Intake velocies @ peak flow LESS than Mach .3 wherein the boundary layer, all other things equal, is stable.

- Smooth large inner-radius bends; having a sharp inner-radius abruptly accellerates the fluid near the inner-wall as it must turn quickly. It also generates significant net vorticity because the inner gasses take a much different path than the outer gasses. Most tuners just put a larger pipe on it and call it a good solution; it is not.[/b] More gentle bends, and large-but-flattened bends actually solve this problem nicely and will reduce the dynamic pressure drop across the bend. (Most manufacturers only measure static pressure drop, which is less meaningfull.)

Bad:

- Bends with sharp-inner radii which generate something similar to air-craft wing-stall as the boundary layer pulls away from the inner wall.

- Obstructions to flow which require lower overal flow velocity to reduce resistance to flow. A poorly designed intake side to an intercooler is a good example; it can make the air have such a hard time entering the tubes that you need lots of them and a low velocity to reduce resistance.

- Excessively large piping which totally wastes all that velocity pressure your turbo generated and converts it to static pressure. Since the air is accellerated to a great linear velocity in the turbocharger there is no reason to slow it back down unless necessary for engine-function; slowing it down only allows more of the particles to "push" back against the turbo impellor and create resistance.

Dynamic Pressure and Volumetric Effiency.

Even I have been guilty of criticizing someone for having an ungodly high VE. The problem is that at high RPM the fluid velocity is so high that HUGE airflow can be generated at relatively low static boost pressures. Because VE is the "all encompassing" compensation factory, and because it uuuusuually doesn't go above 100% at peak torque, when someone quotes an airflow value which exceeds 115% at peak POWER it seems almost beyond belief.

The problem is that VE assumes you're measuring dynamic, and not static, pressure when calculating it; this rarely happens as I know of no MAP sensors which measure dynamic pressure.

So Why Post All of This?

Most people probably won't even read it to the end, but I thought it would be a useful resource for the ever-increasing number of self-tuners out there in the Saab universe.

This is information that 99% of the tuners themselves (with some exceptions) do not even know in its entirety; feel privalidged as you may come to understand this particular aspect better than many people who tune for a living!

I hope that, whether read or not, it is appreciated.

I may add more as things go along ...

Adrian W~

p.s. If you think some of these misconceptions are bad, you should see the ones relating to the Lorentz Transformation in Special Relativity and the ones relating to aerodynamic lift around an airfoil!
 

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Ok...
As I see it (in my simplistic way)
you have airflow to make a car go...
the more air the more fuel the more hp
The issue that you are talking about is how you measure the number of air molecules so that you can add the appropriate stochiometric amount of fuel (+ a bit extra) to create the right and most efficient bang which creates the power

Both of the techniques derive the amount of molecules by calibration... the problem being when you move outside it calibration ..these measurements are, by their very nature ,as you have so eruditely described, essentialy non linear in their nature so straying off their map strange things can happen...

I believe that these things can be tied down by equation and measurement but imho the empirical approach is the practical way to go and as long as you look at the results of the combustion process
egt /co/co2/power (and these can be measured relatively reliably )you can infer how close your predictions are
 

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Discussion Starter · #3 ·
The advantage to MAF sensors is that the measurement is more linear because, in effect, they measure "dynamic pressure". Also even GM V8's of 350+ hp use the same calibration as the 2001+ T7 cars.

Adrian W~
 

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Now hear this


Every time i read you guys continued investigations into engine management theory it feeds my own desire for knowledge .

Yeah , this is excellent data and im going to save it and soak into it sometime in the future.

Im starting to see there are a huge array of parameters affecting turbo performance and especially related to ve. Any change in hard or software promises a "hall of mirrors" effect in performance / driveability and most often would be negative unless carried out with a supreme understanding of the complete organic beast as per some specialist tuners (not naming any in parhicular)
 

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Adrian W>

Impressive.

One question only, why don't you sell this info to all the 99% tuning companies that don't know what they are doing and earn allot of money?
Or start a on tuning company and make even more money?

R/Sasse
 

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Discussion Starter · #6 ·
Originally posted by sasse:
[qb]One question only, why don't you sell this info to all the 99%  tuning companies that don't know what they are doing and earn allot of money?
Or start a on tuning company and make even more money?

R/Sasse [/qb][/b]
Firstly, I did NOT say they "didn't know what they were doing." But I CAN think of a tuner or three who did not reason to take fluid dynamics before starting tuning. That does NOT mean they are "bad tuners" or "don't know what they are doing". Ok?

Also I am already consultant to at least one Subaru tuning company. (The leader of which has not taken Fluid Dynamics btw, and I don't hold it against him. ) I'm helping them out with designing some headers for both the EJ205 and EJ257. The big problem with pulse-tuning turbo-headers is the local speed of sound; it's both quite high, requiring a long tuning length, and variable dependant on EGT. I'm working on making a really cool 4d map that would allow us to literally "see" the pressure at the exhaust port at various EGT, RPM, and tuning length. That should make pulse-tuning much easier as we would have a clearer "direction".

The other problem, specifically with the EJ motors, is the assymetrical exhaust ports. Some Subaru tuners claim to have "equal length" headers, but the assymetrical ports make the length from valve-to-collector, which is the important measurement, unequal. Not only that, but the front exhaust ports come out at an angle, and all the headers made so far come out straight. That creates a sharp bend at the port which is undesirable.


Sorry for the off-topic. I tend to get a bit carried away with this stuff ...

Adrian W~
 

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I have to say I understood a lot of this; words like, "the", "and" and "but" we not problem for me. But my ignorance notwithstanding - it all impressed the hell out of me.

Keep it up guys, and don't apologise for being a specialist and/or clever!

K
 

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Hi, interesting I think. Perhaps this is too obvious. But fluid dynamics, compressors and internal combustion engines have been around for a long time now; surely any tuner who's working with turbos will know all of this and more?

I did electronics at college, so this isn't my bag. But this is basic physics isn't it?

At this stage of development of the piston-engined turbo-charged engine any advances are going to be more incremental and subtle than basic design like impeller diameter and pipe size and shape. The computer simulation, incidentally, is a well known finder of these point-by-point steps forward.

I'm not a tuner. I'm not a mechanical engineer. But there are a lot of better educated people in the world than I and I suspect that a lot of tuners fall into that category.

My point? Nothing new here - yet.

Just my personal opinion. Not negative or offensive I hope.
 

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wow, i keep seeing apologies and i just want to say, "Thanks!"

the wealth of knowledge and information available through the community here is incomprehensible!

sure, some of the topics are far beyond my understanding, but I consistently see the information presented in such a way that I leave here, every day, knowing that I have learned something more.

keep up the good work (dont mind my off-topic comment) and happy Saabing!
 

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Discussion Starter · #10 ·
Originally posted by hunt.dogshome:
[qb]Hi, interesting I think. Perhaps this is too obvious. But fluid dynamics, compressors and internal combustion engines have been around for a long time now; surely any tuner who's working with turbos will know all of this and more? [/qb][/b]
I work with a few of the Subaru tuners; they know how to get lots of horsepower out of an engine, but their focus is mainly pragmatic, rather than scientific. "If it works ..."

The main misconception I see is with the term "Volumetric Efficiency" and how it is used. I've been guilty of this myself. Even the "good books" on turbocharging, Like "Turbochargers" and "Maximum Boost" will use the term VE. Both books are still good reads.

Ok, hypothetical example here ...

We have an engine, like a T7 engine, which runs a certain amount of air-mass/combustion. Let's also say, for argument's sake, that the temperature is constant at one load point and at our pressure measuring point. Ok?

Setup 1: Car runs saaaay 13 psi of static pressure through our 3" inlet pipe.

Setup 2: We reduce pipe size to 2.5" while keeping everything else equal. Now the static pressure has dropped, while the dynamic pressure has remained the same.

Looks ok, right? The problem is when you try to use the term "Volumetric Efficiency" to explain what happened.

Most calculators and equations to size turbochargers use static pressure and Volumetric Efficiency. So when we ran our smaller pipe, these equations, such as those in both of the major turbocharging books, will give us a higher Volumetric Efficiency.

Now, have we actually filled a higher % of the cyllinder's swept displacement with intake gasses? No ... and that's the problem. It can dramatically throw off the calculation and our understanding of what's happening.

As another example, a stock 230 hp B235R will read rouuuughly 7-8 psi of boost through the MAP sensor on the manifold at 121*F intake temperatures and 5,900 RPM. Despite that "low" reading, the flow into the engine on that same datalog was 26.36 lbs/min, which is a LOT of air.

At 121*F and 5,900 RPM and 8 psi of "boost" the engine would have a VE of 116%!
I can assure you that, with stock cams, valves, turbo, intake manifold, exhaust system, and ports, this is NOT actually happening. We're not, in reality, filling 116% of the cyllinder's swept volume ... not even close.

This merely illustrates the effect of high fluid velocity on static pressure.

Adrian W~

p.s. Know what's REALLY amusing? In "Turbochargers" by Hugh MacInnes, he actually states that "Static Pressure is not affected by the velocity of the gas." Yeah ... why not tell Bernoulli that.
Or an Aerodynamicist, for that matter, who relies on it, in part, to generate lift for his aircraft.


Better yet, the "Venturi Effect", the very basis for carburettion, is the direct result of a reduction in static pressure due fluid velocity.
 

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Thanks for taking the time to reply (very well by the way).

I'd sort of assumed that everyone would know about this sort of thing - I come across it with industrial orifice plates for flow measurement, pneumatic control and gas emmissions analysis.

Back to your original "why post?" For me; keep it up, makes for interesting reading. If I gain 20HP in the 900 in a couple of weeks, that's a bonus
 

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Discussion Starter · #13 ·
Originally posted by gorper:
[qb]Bernoulli isn't getting you from New York to London; Coanda is:[/qb][/b]
While the "correct theory" on lift is generally reffered to as "lift by flow turning", the Bernoulli principle plays a large role in the lift itself. While it is perhaps not the more important of the two processes going on, saying it has no affect on lift, or that it does not contribute to lift, is not correct. For Bernoulli lift not to play a role it would have to cease to exist; the velocity of the fluid over the foil is much higher than under ... it is impossible for there not to be at least some Bernoulli lift.

I don't mean to imply that the "flow turning" is not correct, because it is indeed quite accurate. But I do not believe it is accurate to say that the Bernoulli principle is not playing a role here.


What IS NOT correct is the idea that the air over the top of the wing wants to "meet up" with the air at the bottom. But that was proven wrong back in the 40's and can be read about here: http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

NASA has a very good series of pages on the matter. As it was NASA (at the time dubbed NACA) who in-part invented the air-foil AND discovered the "shed vortex" which led to the theory of the "trapped vortex", I would consider their oppinion worth noting. Also, of course, the work of Ludwig Prandtl was critical to this matter.

Check out their foil-sims as well and note the air-velocity around them: http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Some more NASA GRC pages ...

Lift from flow turning: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

Shed Vortex (didn't see it on the page you showed, maybe I missed it): http://www.grc.nasa.gov/WWW/K-12/airplane/shed.html

Good stuff. There are a number of other good resources at the Glenn Research Center's website.

Adrian W~
 

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Respectfully, if a prof at one of the US's top A&A programs, and whose field is computational fluid dynamics as applied to aeronautics, and whose research has been funded by firms such as Northrop, Rockwell, and Boeing, tells me that the reason why a plane flies has to do with the viscous nature of air "bending" over a curved surface and not its increased velocity/pressure drop over a wing, and you tell me that, yes, Bernoulli does, in fact, play a role, whom should I believe?

As an aside, I agree that NASA's website is, indeed, helpful, especially for people like myself who are not engineers. However, those pages are simply an educational outreach program for a K-12 audience (that's ages 5-18 for those unfamiliar with US educational jargon); associating their content with 1930s NACA research makes no sense to me. Besides, NACA *developed* air foil designs; they did not invent them. Perhaps I misunderstood but, as I read your post, you imply that they invented the wing!

As another aside, if you are interested in high-level, online discussion of performance-oriented, automotive engineering theory and design, a topic about which you appear to know alot, then I recommend checking out http://www.corner-carvers.com. Do be forewarned, however, that it's quite unlike any other car forum I've seen on the internet. You can get a good sense for what I mean by that by reading this: http://www.corner-carvers.com/wiki/index.p...20For%20Newbies.
 

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Discussion Starter · #15 ·
Actually I e-mailled Scott, whom seems like a really intelligent and very nice fellow, and this was the response (purely for informative purposes, I hope it isn't too blunt):

"Adrian,

I appreciate your comments. The paper targets non engineer/science
people.
In teaching a freshmen survey course to non-engineers I've
found
that the shed vortex goes way over their heads. I started thinking of
alternate ways to explain lift and decided to revert to the simplest
principles, Newton's three laws
. Much to my surprise at the time, I
could
go far without specifics. For example, I can infer the power curve
without
using lifting line theory. Students I deal with can visualize
downwash,
but have a harder time when I say things like circulation and bound
vortices. I do talk about wing tip vortices with lots of pictures.
That's
easier to get across, but the connection to lift is over their heads.

So, while the discussion you present below is necessary for our
engineering
students, I find it is a little too abstract for the lay
person. Nevertheless, I appreciate your interest and comments on the
paper.

Scott"

Here's a quote from another page on the NASA site:

"When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force."

http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

Adrian W~

p.s. Thanks for the forum links. NACA did not invent the air-foil, but they did invent many of the shapes commonly used today. I'm actually good friends with an old NACA/NASA engineer. Here's the NACA site search result showing all the reports with NACA Airfoil listed (some very good reading, check out their aircraft engine research; the theories on knock are beyond what is known today by the SAE): http://naca.larc.nasa.gov/index.cgi?meth...ds=NACA+Airfoil
 

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Possibly going off at a complete tangent, but the best thing that's been on telly for a while (apart from the Titan landing) was Burt Rutans X-prize documentary.

Mach ??? in a plastic airplane!

Rubber-powered rockets!

Tilting wings for re-entry!


What we need on this forum is Burt, his test pilot (purely because he is a nutcase) and half the design team from Cassini-Huygens.

5000HP? 170PSI? 500CuFtMin? 0-60 in 1? Give us a month.....

Forget meshed grilles, let's have ceramic heat shields
 

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Discussion Starter · #17 ·
Originally posted by hunt.dogshome:
[qb]Forget meshed grilles, let's have ceramic heat shields    [/qb][/b]
That's what I'm talkin' 'bout right there! Yeeehaaaw! Now if only we could get him to post here about Saabs ...

Adrian W~
 

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Q. Why is there a general ICE behaviour that cylinder 1 gets hotter than any of the other cylinders?

A recent personal discussion covered this point as it was an observation by someone in the field of Saab tuning so I am casting my thoughts around as this unsolved problem now interests me.

With respect for the interest and determination of a practicable scientific understanding, as with contributions from all representing a multi-disciplinary approach, I would like to open on 'emergent' behaviour considering the preceding fluid dynamics analysis and the 'extreme' velocities of the fed gasses in high and hyper performance engine solutions has touched on this subject albeit as a problem for students new to vortex mechanics.

Concepts that I am aware of in some similar fields of kinetic study are consistently based on the principles of harmonics i.e. even the study of electron flow through a material at the extreme small scale end of matter demonstrates 'macro' behaviours known as 'phonon propagation', a wave-like energy movement in the frequency of sound in some very surprising media.

Not being a specialist in any way, the principle of example as mediation appears to me simplistic and un-restricted by any factor, save that energy tries to balance out across a given system; therefore in this case of a hot primary cylinder some active principle, rather than close proximity to the turbo mounting, is being observed.

I am looking at the position of high frequency standing waves within the block -who said they had to be limited to the frequency of sound; why not typically at an infra-red frequency (distribution)? So as this propagation model represents a constant flow of energy, it also represents a ‘hot spot’, or general area of elevated temperature as the energy density is so much higher.

I don't want to get so far off the ground, but "harmonics like music knows no boundaries", helps me to stretch my wings, in case I need them.

The only question remaining is how would such an effect be observed, demonstrated and manipulated to suit the optimisation of thermodynamics within ICE design constraints?

This is primarily why I value a multi-disciplinary environment in problem solving. The object isn't to be right, but to reveal where 'right' may be located. IMHO, actual discovery behaviour is down to informed good fortune and intra-team support.
 

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Yes, obviously just so Andrew, but the observation is in context of a professional Saab tuner with broader experience of the discrete problems which remain unsolved despite taking the list of sources of heat, or lack of it, into account.

This is I think what makes it interesting?


But I will continue to ask the same question as you for good reason! I also meant to say "Well done Saabscene membership!"
 
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