Air intake

[wingman]

I am investigating the optimum angle of incidence for the airbox of a Go Kart (for example a 100cc engine).

The geometry is regulated and is roughly as follows:



I am trying to model it and run it in FLUENT. Even in 2D this is more complicated than I thought.

Can anyone suggest what angle (eg 0 or 45 degrees) might be better for optimum mass flow, and perhaps some reasoning behind it - eg the pressure will be affected by angle.

Any ideas welcome.
Regards, wingman

[red300zx99x]

I would ask myself which is more important, the angle of attack into the plenum or the throat of the plenum to the engine. But then again you never know

[asphodel]

I thought 90 would the best to get the highest static pressure inside the plenum. Are you able to change any geometry?

[Monstrobolaxa]

Well.... 0 degrees will give you minimum pressure and 90 degrees will give you maximum pressure like asphodel said.....though didn't you mean dynamic pressure asphodel?

[tempest]

Well i am studying hydraulics, which is basically fluid flow. and if you model air as a low viscosity fluid ( do you?) then 90 degrees will give you both higher dynamic pressure and lower turbulence from the edge of the intake. For further reduction in losses (called head in civil engineering) try curving the pipe into the surface to get a sort of bell end. It will lower your loss here by about 10 times, although Im not sure how important this will be due to the low viscosity and desity of air.
[red300zx99x]I would ask myself which is more important, the angle of attack into the plenum or the throat of the plenum to the engine. But then again you never know[/red300zx99x]
well, both. I guess you want to make the flow through to the carby as smooth as possible (large diameter, low velocity)to keep the intake losses down and then have it go nuts with turbulence (small diameter) to mix the fuel properly through the air. what do you aero guys think?

[tempest]

another thing, keep that airflow as straight as possible, your airbox that you drew makes the air move to much inside it i think. it is too restrictive

[red300zx99x]

Don't know the end result of what your are trying to accomplish, but one must remember the 3 ways to make HP, and there are only 3 of them. Mechanical eff, thermal eff, and volumetric eff. By making the throat to the engine of small diameter you move the volumetric eff curve down in rpm range, sounds like a small engine so this would be a bad thing. In general smaller diameter or longer runners are better for big torquy motors, with the oppisite true for smaller high revving HP motors. But as with anything in engineering it's all a compromise, the problem is finding the combination to suit your needs. Most cars run will 1sized runner, while some newer cars ie Honda's, BMW's, and those great red cars have multiple stage or varible runners in order to reduce this compromise to gain a fatter torque curve. The smaller diameter throat to the engine to increase turbulence is would be one way to look at the thermal eff problem, but the current thinking is, probably dependant on application, get the volumetric eff good and where you want it then create turbulence for mixing withen the cylinder. Not sure what all is possible with your allowed specs, but this is what we practice and it seems to work rather well for our applications. Oh yeh, the ram air effect only works but so well until you start cutting into the other eff, damn compromises

[red300zx99x]

Another thing the optimum angle for air to enter the cylinder is straight down. Since the geometry is fixed how do you plan to change the angle of attack, engine mounting? or are you allowed to change the throat?

[West]

Can you explain why small diameter intake or longer runners produce more torque? I understand that w/ high rev motors you need short runners to make sure the air gets into the engine faster, w/ the big diameter getting more air, but I don't understand the opposite.

[red300zx99x]

hope this helps

http://auto.howstuffworks.com/question517.htm

[wingman]

Thanks for the replies... I should fix a few things.

tempest wrote:

For further reduction in losses (called head in civil engineering) try curving the pipe into the surface to get a sort of bell end. It will lower your loss here by about 10 times

This has been done, but I just didn't draw it in.

tempest wrote:

another thing, keep that airflow as straight as possible, your airbox that you drew makes the air move to much inside it i think. it is too restrictive

The geometry of the pipes are all fixed... although I'm not sure why said geometry was chosen.

red300zx99x wrote:

Another thing the optimum angle for air to enter the cylinder is straight down. Since the geometry is fixed how do you plan to change the angle of attack, engine mounting? or are you allowed to change the throat?

To change the angle, the whole airbox is basically angled down, and a curved pipe is used to the engine.

Look at my crappy diagram -

[red300zx99x]

Work on that curved pipe, that is where your hp will come from. Maybe shorter, longer, smaller in diameter or vice versa, but whatever it's design is very important. You will gain more here then optimizing of the ram air effect

[CFDruss]

Would you like a hand with your fluent setup

[wingman]

Would you like a hand with your fluent setup

sure thanks... I'll email you soon.