The title makes this question sound worse than it really is.

I was thinking about dynamic compression and IVC and got to thinking. The reason for dynamic compression is because as the piston comes back up, the intake valve is still open and bleeds off pressure. As the velocity of the air increases due to RPM, would the momentum of the air pull in more air and bleed off less pressure creating a higher cylinder pressure? It makes sense in my head, Im just wondering if the momentum isnt great enough and the velocity slows down so fast that it really doesnt matter.

Just trying to better understand the concepts here.

Thanks,
-Stu

 

the real world cylinder pressure does go up with rpms depending on the engine. but yes it changes in a running engine. if fluid dynamics didn't cause a change the the throttle flap still would less resistance makes it easier to fill the cylinders better fill = greater pressure. sorry for the short vauge responce.

 

Hey Stu,
Look at a VE chart some time and pay attention to where it peaks. Should be at or near the torque peak and if you can also find some intake velocity information, you'd see that it increases right with VE.
Air has mass and you are correct in your 1st assumption... if the velocity of the charge is high enough, it wil continue to fill the cylinder even as the piston rises. It's not about "pulling"... it's about inertia.

That, in a small nutshell, is how VE of 100+% is possible.

As engine speed increases, it becomes a balancing act where trapped exhaust residuals reduce oxygen content.

-Mindgame

 

"In gasoline engines, unlike steam engines, the pressure in the cylinders is not some fixed, steady amount during the course of the power stroke. Rather, it varies tremendously, rising very rapidly right after the spark plug fires, then falling irregularly and more slowly over the remainder of the power stroke, finally dropping off to near atm. pressure after the exhaust valve cracks open." -Automotive Math Handbook

MEP (mean effective pressure) can actually be calculated roughly, if a dyno chart is handy. HP=PLAN/33,000 P being cylinder pressure in psi, L being length of stroke in feet, A being total area of the pistons in sq.in, and N being the number of power strokes per minute, which would be half the RPMs.
(When the equation is expressed in inches, it becomes HP=PLAN/396000.)
(Even simpler P*D*RPM/792000: P=PSI, D=displacement)

Like mindgame said, if you were to be able to look at a MEP chart overlayed on a dyno graph, MEP would hang right around with the torque curve all the way through.

Torque can even be calculated if the MEP is known, or backwards, Tq=(MEP*D)/151
MEP is kindof a measurable value, although not for the ordinary man. You would need very high tech ($$$$$$) equipment that can actually measure the extraordinarily fast pressure changes in the cylinder under operation.

But to get back on the topic of your question...knowing that the weight of air is about .0076 lbs/ft^3, if it is moving at a known speed, you would know what kind of acceleration (decel) would be required to stop the air, and go from there, seems complicated to me right now, but Im sure once Jon chimes in it will make more sense.

jon

 

Thx for comparing VE to that... it helps me understand VE better now. I knew 100+% VE was possible, but it wasnt exactly clear.

On the topic of velocity.... As far as I understand it, getting 100+% VE requires very good cylinder heads that can flow well enough and maintain velocity as well as having IVC that is timed perfectly to let in as much air without letting any back through the intake on compression. Velocity are RPM and port (cross sectional area and flow) dependent. The reason strokers with small ports are RPM limited is because the velocity gets too high (above mach .6) and gets turbulent, if I understand it correctly.

I know intake pulses are related somehow...

Care to elaborate on this?

(If Im bugging you too much, you can always tell me to shut up and buy some books... but they arent cheap, but neither are aerospace books )