Calculating vacuum pressure.....

[willwoll100]

..........created in a cylinder.

Ok how would you go about calculating the vacuum pressure created by the intake stroke in a cylinder during WOT?

Regards

Will

[derekthetree]

probably using PV=nkT for a change in the volume at a given temperature. might be a bit simplistic though

[willwoll100]

Hmmm how would this take into account piston velocity?

[Matra et Alpine]

Guess you're doing Physics at school derekthe tree
It is probably too simplistic cos the valves open while the piston is moving and accelerating and teh decellerating down so there is no simple single state calculation. On top of that the restrictions in flow in the valves, inlet manifold, filter etc all complicate it.

Depends what you want to do with it. Can you (will....) explain why you want to know the nuumber and how you hope to use it ?

[derekthetree]

Originally Posted by Matra et Alpine

Guess you're doing Physics at school derekthe tree
It is probably too simplistic cos the valves open while the piston is moving and accelerating and teh decellerating down so there is no simple single state calculation. On top of that the restrictions in flow in the valves, inlet manifold, filter etc all complicate it.

Depends what you want to do with it. Can you (will....) explain why you want to know the nuumber and how you hope to use it ?

uni please

and the piston had better be spherical
and the gas a monatomic one
and the temperature constant
and there be no gravity
and ....

[willwoll100]

I need to simulate a manifold vacuum pressure on a wind tunnel model. I've spoke to one other person who apparently works on the type of engines I need to simulate (E39 (I think) M5) he apparently thinks it's only a few millibar difference!!! This has only got to be a ball park figure by the way. So if I've got the piston speed, bore and intake valve area what do you think I could use to help calculate the vacuum pressure?

Thanks again

Will

PS Temperature can be taken as ambient

[Matra et Alpine]

Which side of the manifold ? Cylinder or outside air ?
The "wind tunnel model" has me "worried"
Trying to compute manifodl pressure at the free interface - ie the filter - is VERY complex as the vacuum peaks will be partially smooted in teh manifold and any chambers therein.

You're not in the simple calculation domain I'm afraid.
Real world example will be more useful than computed theoriticals

There are a few java claculators out their for piston velocity/pressure and flow rates etc. Suggest trying to find them ( I'll try to dig up one used long LONG time ago ) Can't guarantee that any of these will get you in teh ballpark, but hopefully will make the parking lot

[willwoll100]

Ok lets say that I'm neglecting the filter and just wanting to know what the pressure will be at the valve entrance, I'm not modeling the whole air intake just the body, intake boss and the elbow that connects to the boss. Here's a pic for you to have a look at.

AIR_INTAKE_ASSEMBLY3.JPG

[Matra et Alpine]

ah wait, are you wanting to calulate the pressure ie vacuum generated by the engine or are you not needing to model the flow characterestics of the duct ?? Still confused what and why sorry

[willwoll100]

LOL, yeah I need to calculate the vacuum pressure created by the engine

[MrKipling]

You could do that by figuring out the volume of air taken in by the engine, couldn't you? Surely a simple sensor could do that.#

I'd like to add that I'm basing these theories on nothing more than my small dose of common sense, so don't shoot me down!

[Knuto]

You can download Engine Analyzer at www.performancetrends.com

[willwoll100]

Have you used the analyzer before and is it user friendly as my head is battered enough what with learning CFD amongst everything else LOL

[Knuto]

It's a very userfriendly. You just need some geometry and properties of the engine. I just used it for fun and to learn a little about tuning engines. You should try it out.

[MrKipling]

That's a good site - that'll go in the mag next issue. Thanks for that one.