Physics of air flow...???

[Tim K]

I have been reading up on the threads on "Ram air" and cold air induction, drilling airboxes, etc. I was thinking about running some hose from the bumper opening to the intake for a home-made "cold air induction kit". According to several users, they have run 2.5" or 3" diameter hoses up from the bumper opening in the front of the vehicle with good results.

So now you are saying..."what the heck does this have to do with physics????" Well, here is my question:

If I were to place 2 funnels in the bumper slot each connected to a length of 2.5" (diameter) hose....then connected those hoses to a Y-connector, which in turn was connected to more 2.5" (diameter) hose running to the intake...how would the air behave? "ram air" is rumored to only work at high speeds, and cold air induction helps most when you are moving. So I was wondering if this fits somewhere in between. I am just not sure how the air would behave.

I have 3 theories:
Assuming that when travelling 40MPH, the air entering the tubes would be moving at 40MPH (essentially).

A) 2 tubes with 40MPH air meet at the Y-connector and are forced up the 3rd tube at twice their previous rate (granted friction and so forth would slow it down somewhat).

B) the Y-connector causes an "air bottleneck" and the 3rd tube will not flow any faster in effect slowing up the 2 intakes.

C) the air is compressed in the 3rd tube, thus getting twice the oxygen to the engine but at the same rate (40mph) as the intake.

If theory A is correct, this would be a great way to get LOTS of cold air to the engine....Any Physicists out there?

 

[Walrus]

Can anyone say, SCRAMJET, boys and girls?

First let me say that I am not a physicist (can't even spell the word...). I do have some thoughts on your theories, however.

Basically, regardless of how fast you move the tube through the air, without significant pressurization (which I don't think will happen at speeds your vehicle can travel), you'll not induce any airflow into the engine above what it calls for during its intake cycle(s)(lets face it, if you take a tube with one closed end and ram it through the air, you get bumpkus! The fact the engine still "draws air into it, will be tantamount to a closed tube and airflow will only occur in pulses of intake, setting pressure waves within the tube disrupting the airstream). You can; however, significantly affect the density by directing as cool as an airstream toward the intake as possible (as you proposed in your post). I know this is full of holes, but realistically, given the operating speeds and RPM of our vehicles, the best bet is a smooth, un-obstructed path. This offers the best results for normally aspirated engines...

I would propose the following... A duct as you have proposed, with an opening to vent excess air into the engine compartment to replace that airflow you will have obstructed by closing off the bumper intakes with the "funnels"... This vent will allow airflow to continue through the tube without the disruptive pulses, and will cool the ambient air around the intake as well. Just watch out for water, etc.

I did some quick, rudimentary calculations and determined the following:
At 55 mph, my explorer turns 2200rpm plus or minus. At this engine speed, the airflow in a 3" duct attached to my intake would be moving at roughly 54 mph! Intrestingly enough, almost the same speed as my forward speed! Of course, the determining factor is engine rpms... According to my calculations, 2200 rpm results in an air velocity of approx 54 mph thru a 3" duct. I may have mis-calculated so, guys, check my arithmetic! I used 3" duct as an example to keep the math simple, and was basing this on a 241.6 cuin displacement (4.0ltr). Obviously, a smaller diameter duct would have a corresponding higher velocity. I reported velocity in mph because it means more to me than feet/sec.

[Edited by Walrus on 12-20-2000 at 04:47 PM]

 

[Ru Dawg]

Well, physics, it's been a while but I give what I can. First of all, you are correct about the effectiveness of Ram-Air, somewhere over 130mph. Therefore, at lower speeds the engine will only "intake" what it naturally "sucks".

Anyway, as for your theories...it depends on the amount of air resistance on the receiving end. with little or no resistance you will get theory "a". Otherwise you will get the results of theory "b". Theory "c" is not quite possible, pressure can only be maintained in a closed system. In other words, in engine applications you will most likely see the results of theory "b".

Now, your idea will definetly get colder air into the engine, which should be effective for some added ponies and mpg's.

DISCLOSURE: I'm not a proffessional, these are my opinions/ideas/eduacated guesses based on my physics and engineering backround.

 

[shagg44]

i would have to say that the air would slow down considerably from your 40 mph, basically for the simple fact that the pressure is not going to be all that great. sure it will be moving fast, but it will be slowed down w/ friction, the 'coming together' of the two hoses and not to mention the filtration process. which leads me 2 my next point, how are you going to filter the air coming in?

same as everyone else, i'm not a pro or even near it, just my thoughts on the matter.

 

[Cameron]

Without making boost (by supercharger or turbocharger) you are limited by the volumetric efficiency of your motor.

Your motor would benefit from colder air being drawn into the motor so there is something to be said about drawing the air from outside the engine compartment. Colder air is denser and therefor would generate more horsepower.

If any of you guys (or gals) are gluttons for punishment, there is an interesting discussion on the effects of temperature, humidity and barometric pressure on horsepower production on the following web site:

http://www.webtripper.com/automotive/tech/

This guy is from MIT and is as big a geek as they come. My kind of people!

Peace

 

[FiremanRon]

I came up with 35 mph through a 3" duct. Part of that is because each cylinder only draws air on every other revoloution. Actually it would be less, because I assumed atmospheric pressure, not a vacuum.
The main thing is that you want as many air molocules as possible to reach the intake valves. This can be done by raising the pressure or reducing the friction. When the pressure is lower, there are fewer molocules (like manifold vacuum). When the pressure is high, there are more (like a super charger). If you can raise the pressure at the point whre the air enters the tube, that will be tranferred through the system, barring losses from friction. That is why many hood scoops face the base of the windshield: it is a place of high pressure, as the wind is being redirected over the roof. It may even be a better location than the bumper, since that is where all the NASCARs have their's. The way to find out is to use a manometer, which is a pressure gage that can measure very small differences in pressure. Whichever point has the highest pressure, is where you want to take air from. The differences at Explorer speeds are probably neglegable.
All of your theories have merit, but because of friction, "B" is closest to reality. Also, since friction is a function of the cube of velocity, the faster you try to cram the air through, the more friction it creates. Less restriction is much easier to obtain.

 

[ckrichard]

If you use the hood scoop and unless it is extramly tall, then at +40mph they loose almost all effectiveness. they can actually draw a vacum(depending on the car style and height of the scopes). The air at higher speeds does not flow directly over the hood. It creates an "arc" of air starting at the base of the hood and going to the windsheld. This is what makes cowl induction work so well. When the air strikes the windsheld it creates a high pressure area and the cowl takes advantage of this high pressure. This is very hard to show and understand with out the help of a picture.

chris

 

[mosinman]

Well, it sounds like a great way to get water into your engine. Also, no matter how gigantic your intake is, you are still limited by your MAF and throttle body. The solution to this problem is the supercharger!

 

[haktmal]

And that's when our theory of "Force Air Induction" comes in!

 

[RedBeard]

Originally posted by mosinman
Well, it sounds like a great way to get water into your engine. Also, no matter how gigantic your intake is, you are still limited by your MAF and throttle body. The solution to this problem is the supercharger!

My vote is for the supercharger, too! Don't forget the snorkle that goes to the roof!

-RB