quote:

ORIGINAL: pat6674u

I would most certainly flunk this class. I hope you're a mechanic or nuclear engineer or something. You may have missed your calling if not. Great info by the way.

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ORIGINAL: KnotBand

quote:

ORIGINAL: pat6674u

I would most certainly flunk this class. I hope you're a mechanic or nuclear engineer or something. You may have missed your calling if not. Great info by the way.

not quite sure who you were refering to, but I do have a degree in mechanical engineering and I am a licensed in the state of Texas.

I might not know much about cars (self admitidly), but when you break it down into theory of how it works, I can understand that stuff....

I know why it works, but now how it works.

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ORIGINAL: LX200

Knot.... well... I am not sure your analysis is correct either. An engine is a positive displacement device. The piston moves down, creates an opening of a FIXED volume, and air can flow in to fill this volume. The ACTUAL cubic feet of air moving into an engine is based on the RPM's of the engine, and the total volume displaced with each RPM. For our 4 liter engines, each RPM displaces 4 liters of volume. Each cylinder gets a shot at sucking in 4.0L/6 or 0.66 Liters of air during a single revolution (I think.... maybe it takes two revolutions for all six cylinders to get a shot at intake??????).

Now, at any RPM, the amount of energy that can be produced is based on the amount of fuel that can be combusted in the cylinder during the combustion step. The only thing the fuel can use is oxygen molecules, so, at any FIXED RPM (and hence a fixed ACTUAL volume of air moving through the engine), the only way to get more power is to have more oxygen molecules in the volume of any cylinder. This happens when the air density entering the cylinder is more dense, and hence has more molecules of O2.

Now, why did I do the Rankine temperature calc. Rankine is a "thermodynamic" or absolute temperture scale. This means that it can be used in a thermodynamic "equation of state". The old equation

PV = nRT

is such an equation of state. this equation can be rearranged to read

n = P*V/R*T

it can further be rewritten as

n/V = P/R*T or Density = P/RT

where n is the number of mole of gas, in this case air. Moles is just another measure of matter. V is the volume, so n/V becomes density.
Now, to see how many more moles of air you get when you change the Pressure or Temperature, you can write two equations for two different states

Density1 = P1/R*T1 (oh ya, R is just a constant)
Density2 = P2/R*T2

Then to see how much the density has changed due to the two conditions divide the first equation by the second..... you get....

Density1/Density2 = P1*R*T2 / P2*R*T1

Now since R is in the numerator and denominator, and P1 = P2 (i.e. we are looking just at temp changes) we end up with

Density1/Density2 = T2/T1

As long as you use a ratio of THERMODYNAMIC temperature scale temps, this works.... Fahrenheit temps DON'T work in this equation. So, my previous analysis stands. Air that is 30% cooler contains about 10% more molecules of air for a given volume. Hence 10% more oxygen molecules entering the cylinder and ready to be ignited.... and the potential to make 10% more power.

From the website http://www.straightdope.com/mailbag/mtempscales.html
"As for whether just one temperature scale can be used for all purposes, in principle the answer is yes, provided it is an absolute scale (i.e., one whose zero point is the absolute zero of thermodynamic temperature). Certain physical relationships (such as the equivalence of heat and work, rates of chemical reactions, the ideal gas equations of state, the kinetic theory of gases, and the laws governing heat transfer) depend on multiplying or dividing temperatures. You won't get correct answers if you use a scale with an arbitrary zero point. Therefore around 1860, two absolute temperature scales were developed: the Rankine scale, based on the Fahrenheit degree, and the Kelvin scale, based on the centigrade degree. Note that the SI unit of temperature is called the "kelvin" (not "degree kelvin"). The freezing point of water is 273.15K and 491.67Ra"

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ORIGINAL: LX200

I am also not sure the assumption that there is a very minimial effect from the shield is correct. It the car is just sitting still, with a general convective type of air flow under the hood, there will be a fair amount of mixing. BUT, when a car is moving forward, there is a high rate of airflow moving from the front of the car, through the engine compartment, down under the car and out the back. This tends to sweep air across the engine and out the back. Now, with all the crap under the hood, there will be all sorts of turbulent flows, but, in general, the air flow is from front to back. This is why air filters are best up front. I believe this airbox could see air 10 or 20 degree less than the general air swirling under the hood....

But... I have no way to prove this. But, what the heck... lets figure it out. I am going to pick up a couple of thermometers, place one on one side of the airbox, and one on the inside of the airbox, and we what we have. This will be interesting....

Oh ya... you qouted first law of thermodynamics in your analysis. You used the closed form of the equation, and this is an open system. Also, the first law is for non reacting systems. An engine is a chemical reactor, so you need to add terms to account for chemical reaction in the system, and work the system does on the surroundings, so the old m*Cp*dt is not sufficient to analyze an engine.

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ORIGINAL: stangman53

Does anyone have any documentation on WHP for different CAI'S I have a mac and I am not happy with it I have a slight bog on takeoff and parkway entrance form a roll. I was either thinking about the Demolet or the C & L. Any response would be helpful.

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ORIGINAL: LX200

"just because you add 30% more air molecules"

WHAT!!!!! Adding 30% more air and 30% more fuel will have a HUGE effect on the engine. If it didn't, a supercharger would have no effect on an engine. The only reason you use a Supercharger is to jam more O2 molecules into a fixed space, so you can spray more full in, combust the fuel, generate more gas for expansion, and have this expanding gas do work against the piston. By your logic, a supercharger would not have an appreciable effect on engine performance. Yet, in reality, it has a massive effect. Adding colder air with more molecules achieves the same thing.... more O2 molecules in the cylinder and hence more fuel can be added, and more power made. This point CAN'T BE DEBATED. IT'S FACT.

Your heat analysis totally misses all the heat associated with the chemical reaction of combustion... you know, heat of reaction? This is why exhaust is so hot, not because of a temperature rise associated with Polytropic compression. The chemical reaction releases massive heat, causing the gases inside the cylinder to expand and push the piston out, doing work. NO analysis of an engine can be done without considering the KEY step which is combustion.

You state.... "now, if you want to look at it as density = #of moles/volume, fine...yes the higher the density, the larger number of moles for a constant volume, but I don't know one engineer that deals with moles, and the last time I used them was in chemistry class. "

Well, that's because you hang out with mechanical engineers, and don't deal with chemical reactions, which is what is occuring during the combustion step. I'm a chemical engineer and EVERYTHING involving reactions and mass transfer is done in moles. Moles are the accounting used in chemical reactions. AND, my analysis was looking at a RATIO... so who cares if it's moles or mass... it's a ratio, units of measure cancel in a ratio, remember? That's why Cp/Cv is a unitless constant. It's a ratio.

And, adding 30% more water to the ocean would have a huge effect. Not use why you use the example of adding some minute percentage of water to the ocean. I don't get the connection.

I'm done discussing this one...

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ORIGINAL: LX200

Listen pal, go back and read the thread from the start. YOU were the first one who outright stated that what I had said was WRONG. And guess what, it wasn't wrong. Then you dropped little cuts about "my little ideal gas law" when that was precisely the thermo concept to prove my point (again, supported by web source so you can learn some thermo). From minute one of this thread, you've set yourself up as the expert who totally refuted what I said. Then your other cutting comments like "no engineers I know use moles" as if my use of moles instead of a mass unit of measure somehow means I don't know as much as you or your mechanical engineering pals (and mechanical engineers at my company, who design multi-million dollar centrifugal compressors use moles all the time). Then you told me that I can't use the Rankine temperature scale to calculate a ratio of temperatures to find the effect on density (further insinuating I don't know what I am talking about) , COMPLETELY proving you lack even the most basic understanding of thermodynamics. Absolute temperature scales are taught in the first week, you must have missed this class. I even cited a web source that explained what an absolute temperature scale is, so you can brush up. Your blabbing about Rankine just being Fahrenheit with larger numbers made me laugh out loud. If that's all it was, why do you think it was developed??? Becasue many years ago somebody who understood thermo (unlike yourself) realized that it would be beneficial to have an ABSOLUTE temperature scale with degree increments that lined up with the Fahrenheit scale. Go back to thermo class? I can teach it.

So again, review the thread from your first response, and maybe you'll see you were actually the first one to suggest I didn't know what I was talking about on multiple accounts, and refuted me by quoting the adiabatic, closed system form of the first law (Q=m*cp*dT), as if that meant something. So Knotband, if you want to tell me I don't know what I'm talking about, do your homework, because you were wrong on every point you made.

If you had approached this topic by saying "Hum, that's interesting, and I think this is what you are trying to say, but I have another view" this would have been an entirely different thread. But you downright said I don't know what I'm talking about, so you can expect a response in kind. I think if anybody reads this whole thread they'll realize YOU don't know what your talking about. I've actually been nice up till now. Most of your messages are incoherent babble designed to mask the fact that you screwed the pooch on your first response to me and were totally incorrect in your assessment of that post.

Now I'm done talking to you about this. Personally, I don't think you know what you're talking about and you don't think I know what I'm talking about, so lets just drop it.

Go ahead, take your last shot. I'll give that to you. But also, if you've worked on any bridge designs in your area, please let me know which ones... I'd like to plan alternate routes.

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ORIGINAL: toosober

CAT FIGHT!