×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Why is 124% VE 'the limit'? What governs this?
4

Why is 124% VE 'the limit'? What governs this?

Why is 124% VE 'the limit'? What governs this?

(OP)
I haven't found any explanation why around 124% volumetric efficiency is the max you can apparently get in an engine.

RE: Why is 124% VE 'the limit'? What governs this?

I doubt it is a rule, more of an outcome related to practicalities of typical designs such as ratio of valve area to piston area, Mach choking, pumping losses, runner lengths and finish and so on. 115% is more typical.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why is 124% VE 'the limit'? What governs this?

4
There is an actual physical limit although I suspect 124% has been obtained empirically. The additional cylinder fill is supercharging courtesy of wave action and like any supercharging, it requires energy to pump that extra air into the cylinder. In this case the energy comes mostly from work done by the piston during the intake stroke to create a negative pressure wave, which reflects from the runner mouth as a positive wave, which arrives at the valve just before it closes. The limit is imposed by the amplitude of that negative wave. The descending piston is pulling air in through the intake valve. Lets say it can produce a pressure of 0.5 bar absolute in the port (this is close to reality in a high VE race engine). That will return as a positive wave of approximately 1.5 bar absolute. If the compression process is isentropic and this pressure could be transferred to the cylinder at EVC, the density will be 1.33 times atmospheric. Of course cylinder volume at EVC will be less than at BDC so VE will be less than 133%.

Further VE improvement by producing a negative wave less than 0.5 bar absolute would be problematic.
- the intake runner length is determined by the time required for the wave to return
- reducing the runner diameter to increase the wave amplitude will reduce cylinder filling during the intake stroke - offsetting the increased wave supercharge.

Some energy is added to the wave from the exhaust system during valve overlap but that is secondary and also has limits.

je suis charlie

RE: Why is 124% VE 'the limit'? What governs this?

(OP)
Thank you very much!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close