Question about Zero float valve design in Desmodromic valve actuation.

[Lenny123]

Question about Zero float valve design in Desmodromic valve actuation.

Here’s a link below if anyone needs to refresh there memory on what’s a Desmodromic valve actuation is.

http://www.wheelsofitaly.com/wiki/index.php/Desmodromic_valve

My question is this, if this zero float valve system saves on horsepower just how much horsepower does it take to work the conventional poppet valve system? Say on a 6 cylinder engine each cylinder having 4 valves.
Also does anyone know of any research reports I can see or download on zero float valve systems for the internal combustion engine?

I very interested in learning about just how much horsepower can be saved using this system and what other benefits there are.

Thanks for reading.
LennyC

 

[GregLocock]

I don't know about benefits but I imagine your HC emissions at cold idle would kill any chance of using desmo valves in a first world country, on cars.

Desmo proponents often point out the high spring forces in a conventional valvetrain, forgetting that the spring stores energy, and back drives the system on the return stroke.

Heywood has a good section on friction losses in engines, which would include the valvetrain, if nobody else has answered in the interim I'll post more tonight. I vaguely remember measuring several hp for a 4 cylinder at 5000 rpm.

Basically if you can figure out how to seal at different temperatures, and in the presence of valve seat recession, then you might have a viable system.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[patprimmer]

@ Greg

Solid tappet on the opening cam and hydraulic on the closing cam?

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

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for site rules

 

[GregLocock]

Yes, that might do it.

Idle 20 kPa bmep, 5000 rpm 10 kPa, for a reasonably efficient valvetrain, worst maybe double that.



Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[BrianPetersen]

Ducati engines are not designed for zero lash. The closing cam pushes the valve to just short of fully closed (to avoid risk of mechanical jamming due to varying thermal expansion of the various parts) and a light spring takes care of the rest. Cylinder pressure during the compression and power strokes helps to hold it closed, too.

I don't see why this would have any effect on exhaust emissions. Their engines have to pass the European exhaust emission regulations for motorcycles, which are much more stringent than in North America, and Ducati to my knowledge have no plans of reverting to spring-closed valves.

 

[thundair]

X2 Brian
I had a chance to talk to the Ducati engineers back when they had the World Rally in Vegas and they agreed that at the time the Desmo outperformed the valve springs, but the Desmo was a good brand mark for now. With the modern springs cams are a lot more aggressive then a Desmo.
I broke a barrel bolt in my Ducati and when it was apart I was turning the cams with my fingers and a very light touch, when I realized if it was heavily sprung I sure it wouldn't turn by hand. I realize you get something back from a spring but there will be heat loses.

I don't know anything but the people that do.

 

[tbuelna]

Lenny123,

A desmo valve system is better in some respects and worse in others, than a conventional coil spring and cam system.

The desmo system has lower friction losses while the valve is being opened and closed, at most engine speeds, since the cam-to-follower friction losses are only as high as the valve inertia forces. On the other hand, a spring and cam system has friction losses that are higher than necessary at low engine speeds because the spring force (or rate) is optimized for high speeds.

Conversely, the desmo system has higher friction losses during the period when the valve is closed, since the closing cam and follower must always maintain a force on the valve to keep it seated. A spring and cam system does not have this loss, since the spring keeps the valve seated and no contact is required between the cam and follower. Thus it has no friction losses during this period.

Overall, the desmo system is likely slightly more efficient. But not normally enough to offset its greater cost and complexity.

With regards to valve float, this is not usually a problem for modern poppet valve engines, except at very high operating frequencies. But if you're truly concerned about valve float, I'd suggest that a pneumatic valve spring system would outperform a desmo system, and would be less complex.

Regards,
Terry

 

[patprimmer]

Instead of the hair-clip spring to close the tappet clearance, a conventional valve spring about as strong as the hair-clip design could close the tappet clearance portion of the valve closing and still leave tappet clearance to eliminate the friction to keep the valve closed.

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

http://eng-tips.com/market.cfm

for site rules

 

[JayMaechtlen]

Thundair-
"X2 Brian
I had a chance to talk to the Ducati engineers back when they had the World Rally in Vegas and they agreed that at the time the Desmo outperformed the valve springs, but the Desmo was a good brand mark for now. With the modern springs cams are a lot more aggressive then a Desmo."
I think something got lost there, if only my comprehension.
Way back when you were in Vagas, Desmos outperformed springs. The following statement suggests that the Desmo is kept for bragging rights and brand recognition?
Are current sprung systems quicker than old Desmos, or new Desmos, or ??

Jay Maechtlen

http://home.covad.net/~jmaechtlen/

 

[GregLocock]

Pneumatic springs are good for 19000 rpm on a 4 stroke. Steel springs have been used for 14000 rpm on a production engine. It looks to me as though desmo's advantage is at low speed, not high, which is fine, that is where efficiency really matters.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[dicer]

No matter what the design is, a conventional coil spring, ovate, bee hive, multi spring etc. will always have surge at various frequencys. That can add to valve train loading etc. I think desmo has a conventional spring beat as far as stability goes. A conventional valve springed valve can chatter ie bounce on the seat, also causing such things as valve head separation, seat face hammering etc.

 

[GregLocock]

OK, except that once you have a spring in the system, as the Ducati does, you get all the same baggage.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[BrianPetersen]

The spring in the Ducati system can only affect a very limited range of motion. The valve is physically prevented (by the closing cam) from opening more than whatever the valve clearance specification is (0.1mm?) The valve is physically constrained from bouncing around outside of the clearance between the opening and closing cams. Spring surge etc are much less relevant.