round lobe cams 2

[PedroCG]

http://www.youtube.com/watch?v=inrBHHVVfng&feature=channel_page

hi, anyone knows how this works? are the valves piston like?

 

[PedroCG]

perhaps i was not very clear, the video explains it well enough. the rocker arms are actuated by a round lobe cam, this produces a movement similar to a piston, so how are the valves kept close during compression/combustion?

 

[ivymike]

There isn't enough info provided... but the mechanism as a whole must provide motion other than what you've described (or the engine wouldn't run well, or perhaps wouldn't run at all). It doesn't look like a crank-slider to me... there's something going on between the round cam follower and the rocker mechanism. There's plenty of info about desmodromic valve mechanisms on the internet.

 

[evelrod]

Desmodromic valve actuation has been around for over a century. My only experience with it is from a Ducati 750 and later a 900 of a friend. Neat setup and Ducks 'sound' great!

First hit on Google...

http://www.ducatidesmo.com/

Basic stuff.

Rod

 

[patprimmer]

It was use on looms to weave cloth even before the IC was invented.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[tbuelna]

The system is desmodromic and the valve motion is defined by an eccentric. The eccentric provides a relatively poor valve opening and closing motion compared to a conventional cam.

Mr. Decuir's claims about valvetrain friction are also incorrect. A cam and roller follower with spring closing would have the same friction (or possibly less) as this desmo system. The friction losses are mostly a function of the valvetrain inertia forces and the coefficient of friction at any sliding interfaces. The spring force only provides enough resistance to keep the follower in contact with the cam lobe.

Decuir's valve train components look like they are fairly heavy and would have greater inertias than a conventional valvetrain. So all other things being equal this desmo system would probably have higher mechanical losses.

Finally, although it's hard to tell from the video, this desmo system looks like it requires some force on the rocker arm to keep the valve closed. This means there is always some friction loss at each valve throughout the entire engine cycle. This is not true for a conventional valvetrain. When the valve is seated (ie. the follower is on the cam's base circle) there is no friction loss because there is a slight clearance between the follower and cam lobe. The exception being if there is some sort of hydraulic lash adjuster which does create a very small force.

You can see the details of this device in US Pat. application number 20080060596.

Go here

http://patft.uspto.gov/

 

[PedroCG]

i know some desmo systems

http://members1.chello.nl/~wgj.jansen/,

but most of them are ducati like, or cilinders/cones etc with a groove, this one was quite unusual. tbuelna thanks for the info, the system on the patent is a bit different, the one from the video has no gap, but if the working principle is the same i migth have invented a desmo of my own... with continuos VVT.

thanks for the replys guys, if someone has more info on these kind of desmos (round lobes, or crankshaft like) i'd be thankfull, dont want to invent something from the stone age.

 

[ivymike]

I hope the Geico cave men don't post here... they'd probably be offended by that remark!

 

[PedroCG]

why would they be offended? desmos are complicated stuff.
I meant something that was already out there for ages and was so undoable that not even a caveman could make it work.

 

[patprimmer]

Pedro
Old is not necessarily basic, simple or unsophisticated.

Pyramids, Colosseum, concept of democracy, the works of Archimedes, Socrates, Shakespeare, Galileo, Newton and da Vinci spring to mind.

I think the first 4 valve per cylinder twin cam st up was first used about 1912. Quite a few years before Toyota discovered the O2 advantage.

From old and probably unreliable memories, The Mercedes desmo I believe used a system where they used inertia in the valves to overcome the gap due to running clearance on the cam, then used cylinder pressure to keep the valve seated despite the clearance. I also believe that some use a light spring to hol the valve on the seat, especially at idle and cranking rpm for starting.


Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[PedroCG]

i know pat, i have acess to a junkyard, its more like a museum of old times... the kind of things i find there realy makes me think that given the technology of those days the stuff that was build was more "sophisticated" and the skill involved in machining, casting etc much greater.