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Eaton_corners
| | Posted on Saturday, September 19, 2009 - 11:45 pm: | 
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Not trying to be a smart-alec, but in a four stroke engine the cam turns 1/2 the speed of the crank. |
Blake
| | Posted on Sunday, September 20, 2009 - 12:06 am: |
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That would be the perfect valve train. But I doubt that the rebound of the valve spring returns any energy to the engine.
It most certainly does. How can it not?
And Eaton is correct, the cams spin at half engine speed, not twice. (Valves open/close just once for every two complete revolutions of the engine)
Dentguy,
Let's do a mental experiment. Let's assume we have two identical engines, with identical cam profiles for, but one is conventional and one uses a Desmo valve actuation scheme.
Power equals force times distance divided by time (LB*FT/s).
The force required to resist inertial loads and close a valve is the same whether it is closed via spring or desmo finger.
At top engine speed, just before onset of valve float for our conventional valvetrain engine, the entire capacity of the valve spring is being consumed just to close the valve and just keep the shim/bucket/finger-follower barely in contact with the cam. If the engine goes any faster, the spring cannot close the valve fast enough and a gap opens between cam and finger-follower/shim/bucket. Follow?
So in that case, the spring is producing the exact same average force over the same distance and in the same time interval as the desmo actuator would see.
So the power lost would be identical.
In the Desmo engine, that power comes from the engine.
In the conventional valvetrain engine, that power also came from the engine, but it was stored in the spring during the valve opening event.
if it takes the same amount of power to open the valve as it does to close the valve at max engine speed, then compared to the Desmo engine, the conventional valvetrain engine takes twice the power from the engine itself for valve opening but no power from the engine itself for the valve closing as that is provided by the stored energy in the spring.
But the total is the same.
Make sense?
It's just an energy equation equality is all.
Again, we assumed the same valve opening/closing profiles and the same valve mass and engine speed, and we are looking at the engine operating at/near the rev limit.
Fun stuff. I'd put it into equation form, but that would take some time, and I'm lazy.  |
Blake
| | Posted on Sunday, September 20, 2009 - 12:08 am: |
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The only difference is in the valvetrain inertias and how much power they take. Does the spring contribute more inertia than the Desmo fingers? I suspect they are not too much different. |
Dentguy
| | Posted on Sunday, September 20, 2009 - 01:49 am: |
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Good stuff Blake. Was hoping you had some test/fact stuff, but good stuff either way.
About the top engine speed stuff. I agree with the case/scenario that you spoke of, but we know they don't run at top RPM all the time and you're assuming (fair to assume) that the springs are providing just enough to barely keep them in contact at top RPM.
Now take all that and add in the extra friction/pressure on the camshaft/followers/cam bearings from the high spring pressure (anytime it's at all on a lobe) that the Desmo doesn't have. Also causing more heat and wear from that pressure.
The Desmo valve train doesn't have a big advantage like it did years ago (when speaking of valve float), due to valve spring advances. A lot of the valve float problems back in the day were caused by spring resonance. Modern progressive springs don't resonate doing away with that cause of valve float.
I do think the Desmo system is cool. Even cooler when you have a head off and spin that cam pulley around with your finger tips.
Oh, thanks for responding to my post. I appreciate it. |
Backcountryme
| | Posted on Sunday, September 20, 2009 - 01:58 am: |
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My bad guys. I had my thinking cap on backwards. Dent, I like your thinking though. I guess I like the Desmo idea because it is different. And I like different. But I know the problems we had with cars. Again, it was a pushrod motor so things were quite different with the addition of roller lifters, pushrods and such. Lots of slop in those to setup a valve float problem real quick. But my question is, wouldn't it take more "power" to open a valve with a spring then it would to move the cam followers on a desmo? Maybe not at high RPM, but in the lower range? |
Blake
| | Posted on Sunday, September 20, 2009 - 03:21 am: |
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I agree, more friction overall with the spring system.
The spring does put energy back into the engine though. Can't violate the rules of physics.
For the record, I really like Desmo driven valves too.  Too cool. |
Crusty
| | Posted on Sunday, September 20, 2009 - 06:11 am: |
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Actually, desmodromic valve actuation was originally used to overcome the problem of valve spring breakage. I believe that it was Mercedes who first used it back in the early 50s, but I'm not positive. Anyway, metallurgy has advanced enough that that isn't the problem any more.
The reason that a pushrod actuated engine is more prone to valve float is the reciprocating mass of the lifter, pushrod, rocker arm and valve. That's why overhead cams are more efficient at high RPM.There's a lot less mass between the cam lobe and the valve.
Do remember the epic battles between Colin Edwards and Troy Corser in World Superbike? Edwards was on the V-twin Honda, while Corser was on the Ducati. Both engines made comparable power,but the Desmo Duck had no advantage over the sprung valve Honda.
Personally, I don't like desmo valves. There are more moving parts in the system which means there are more opportunities for something to break. It's also more of a pain in the ass to get properly adjusted. |
Vagelis46
| | Posted on Sunday, September 20, 2009 - 07:11 am: |
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The only advantage desmo has is that it might allow "extreme" cam profiles, as they say. Also Ducs use belts to drive the cams which are lighter than hy-vo chains. But the heads are heavier with the desmo.
Ducati is not always right. Sometimes they skrew up, trying to keep their tradition and missing the point. Just look what they have done in motoGP, where by keeping a 90degrees engine they have an ill handling bike.
If sprigs are OK for IL4s spinning at 15000rpm, it is great for the 10-12000rpm of the V-2.
At 1000cc, the stock Duc 999 were never faster than the stock Rotax of the Aprilia RSV. Where was the desmo advantage then ??? |
Jimidan
| | Posted on Sunday, September 20, 2009 - 01:13 pm: |
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Posted on Sunday, September 20, 2009 - 12:08 am: Edit Post Delete Post View Post/Check IP Print Post Move Post (Custodian/Admin Only)
The only difference is in the valvetrain inertias and how much power they take. Does the spring contribute more inertia than the Desmo fingers? I suspect they are not too much different.
So, if that is true (and I liked your explanation as to why you thought it was), do you care to speculate why AMA/DMG limits Ducatis to 850cc in Daytona Sportbike class, while allowing the other twins 1125 and 1000cc? |
Imonabuss
| | Posted on Sunday, September 20, 2009 - 02:48 pm: |
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Racing heritage, Ducati's power claims, and a more radically tuned engine stock. But if Ducati had shown up and proved to be slow, AMA/DMG would have given them concessions. They would have done it for KTM, but KTM only came to one race and got 34th or something. They'd have needed a lot of concessions (BTW they were allowed to run a full fairing on the 990, for those who don't know). Look what AMA did for Aprilia; by the end of the year Chaz's bike had tons of exotic stuff on it and was quite fast. Much faster than the Buells, but Chaz kept chucking it, (Sorry, couldn't resist) |
Fresnobuell
| | Posted on Sunday, September 20, 2009 - 04:02 pm: |
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by the end of the year Chaz's bike had tons of exotic stuff on it and was quite fast. Much faster than the Buells,
I know they got a new rear wheel, but I don't recall anything else. Maybe they should have gotten an exotic shifter, or at least one that didn't fall off or malfunction (twice in the same weekend.)
I don't think the Aprilia was faster than the Buell, no way. |
Anonymous
| | Posted on Sunday, September 20, 2009 - 04:47 pm: |
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By the end of the year, there was a carbon-fiber airbox and a lot of titanium on Chaz's Aprilia, as well as a Marelli Racing ECU package with traction control that would cost you over $10,000 if you bought it at retail. |
Dentguy
| | Posted on Sunday, September 20, 2009 - 05:13 pm: |
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I don't think the Aprilia was faster than the Buell, no way.
I agree.
By the end of the year, there was a carbon-fiber airbox and a lot of titanium on Chaz's Aprilia, as well as a Marelli Racing ECU package with traction control that would cost you over $10,000 if you bought it at retail.
All within the rules right? |
Anonymous
| | Posted on Sunday, September 20, 2009 - 07:45 pm: |
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The Aprilia was 10MPH faster on radar than the Buells at Road America. That's a fact. And the Apriilia was the fastest bike at New Jersey. The Yamaha were also faster than the Buell at New Jersey at the end of the straights.
All the factory guys take radar guns to the end of the long straights to find out. The AMA often used a radar gun near the start finish lines on tracks, but the speeds there do not represent the fastest area of the track.
The Buell explodes out of the corners and on short straights, but don't confuse that with top speed. So, different performances at different tracks.
Yes, but the rules were changed during the year to allow them to run the stuff. The Buell was the only bike penalized during the year, and it became obvious the penalties were working except for the fact Danny wouldn't give up and gave every race 110+%. |
Fresnobuell
| | Posted on Monday, September 21, 2009 - 12:25 am: |
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Good point Anony. Too bad these facts can't be illustrated for the fans somehow. It could help the bias against the Buell. |
Aeholton
| | Posted on Monday, September 21, 2009 - 07:45 am: |
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Good point Anony. Too bad these facts can't be illustrated for the fans somehow. It could help the bias against the Buell.
I've observed that facts don't really mean a lot to biased individuals. Typically bias comes from feelings, not the understanding of facts. Haters will be haters, regardless. |
Dentguy
| | Posted on Monday, September 21, 2009 - 08:47 am: |
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"The Buell explodes out of the corners and on short straights, but don't confuse that with top speed. So, different performances at different tracks."
I'll take explodes out of corners and on short straights any day over top speed. To me, that is faster. |
Blake
| | Posted on Monday, September 21, 2009 - 10:58 am: |
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Hey Jimi,
>>>So, if that is true (and I liked your explanation as to why you thought it was), do you care to speculate why AMA/DMG limits Ducatis to 850cc in Daytona Sportbike class, while allowing the other twins 1125 and 1000cc?
I think Imonabus did a better job of answering your question that I could have.
Let's hope that in due time that we will be saying the same about a Buell middleweight twin. |
Geforce
| | Posted on Monday, September 21, 2009 - 12:30 pm: |
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Thanks for the helpful inside info there Anon. We always appreciate these visits.
A couple of questions for you if you get time regarding the 1125RR if you can shed a little light on it for us without doing any damage. If not, we understand.
1. Is there a projected or planned HP goal for the RR and, can you possibly give us a window for that? EG: 160-170 RWHP, 170-180RWHP
2. How are the larger radiators working out in comparison to the standard size/positioning? (They look fantastic if you ask me but I was curious to know how they came about.)
Thanks for your time, and again thanks for the info and insight. |
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