|This article is of interest to the following WikiProjects:|
This entire article
This entire article looks like it has been written by a bunch of children--all of whom have bias and are attempting to prove the other side wrong. This article needs to be re-written in a more professional tone and informative purpose.
Current engineering methods
Just a few notes: In the article it says that most cam profiles nowadays are created using linear-jerk schemes. This is not quite the case nowadays, most software packages now have polynomial schemes where users can control points on every derivative level. Another popular option nowadays is the user of Bezier splines on the acceleration curve. The reason for this is that manipulation of a series of linear jerk lines still creates excitation in the higher derivatives.
Most conventional valvetrains nowadays use finger-followers, not flat-tappet mechanisms. The reason is that the use of a roller better returns the spring compression energy (snap forward). A second advantage is the ability to use a hydraulic lash adjuster at the finger pivot instead of inside the tappet (does not to be as robust because it is not under high acceleration)
That image is not an accurate depiction of desmodromics. There is no positive closing action with one cam lobe. There should be a second cam lobe with a complementary profile to close the valve.Seasalt 15:15, 23 September 2006 (UTC)
Better clean my glasses. Could see it tonight.I was wrong, but the second cam is hard to c 4 me..Seasalt 12:16, 24 September 2006 (UTC)
The cams are there, but the animation is incorrectly done, as the closing cam doesn't actively close the valve...
- It looks to me like the cam knocks it closed and then the pressure differential holds it closed. It makes sense. You just need the valve closed long enough for the pressure differential to build up. --Gbleem 15:17, 22 April 2007 (UTC)
"How it works" section
Is not a description of how it works but the Ducati patent on a way of changing the shims by sliding the opening rocker to one side. —Preceding unsigned comment added by 126.96.36.199 (talk) 21:46, 16 June 2008 (UTC)
These sections make very specific statements and claims which should backed up by citations (i.e. "high speed photography work..." etc)
Otherise, it gets removed. Thanks. Izaakb 20:28, 21 January 2007 (UTC)
The Advantage/Disadvantage sections is turning into a discussion of springs, this article is just a discussion of desmodromic valves. I am thinking these sections are not useful, they certainly should not be 60% of the text. Maybe restricted down to one paragraph each at the most. Izaakb 03:53, 22 January 2007 (UTC)
Desmodromic valves are used so rarely that very little research has been done into them. Indeed Ducati are the only company to have any success with them, and hence it is really only their technical department that has done any work on the system. Hence citations are hard to come by. What the article says is entirely correct. The proposed deletion would only remove another of the few sources of information on desmo systems. (Izaakb, i would have thought that as a motorcycle historian you would appreciate this!) Inginero990 00:28, 25 January 2007 (UTC)
My opinion of the information is irrelevant -- Wikipedia requires that the claims or information be verifiable and cited. If citations are "hard to come by" then the work is not verifiable and thus, cannot be posted on Wikipedia. If you need an explanation of this policy, please refer to this Wikipedia:Verifiability. In short:
1. Articles should contain only material that has been published by reliable sources.
2. Editors adding new material should cite a reliable source, or it may be challenged or removed by any editor.
3. The obligation to provide a reliable source lies with the editors wishing to include the material, not with those seeking to remove it.
Izaakb 04:09, 25 January 2007 (UTC)
(Comments by Dan Grinde 22 May 2009) In 1967 I began engineering school with metallurgy and soon motorcycle racing. Ducati kept the desmo as a badge of design and manufacturing prowess. Even their 250 cc mid-1960's models employed bevel-gears and shaft(no chain)to drive the overhead cams. Roller-bearing cam followers and tappets (mostly for pushrod systems, especially in auto engines) reduced the friction and wear problems in valve trains for other manufacturer. STP, castor oil, and soon after synthetic lubricants reduced the mechanical advantage of desmos. Even so, high RPM motors, especially air-cooled, were vexed by exhaust valve failures.....clearance from valve to seat, a mere .010 inch, would soon vanished due to heavy spring pressure and high temperatures.
Worse, exhaust pressures forced any oil away from the contact area of the valve guide and steel stem. Favorite cures for the air-cooled motors of aircraft, Porsche cars, and a few bikes....bronze valve guides and my favorite...sodium-filled exhaust valves.
I know how to adjust the valve lash for Jaguar, Alfa Romeo, Ferrari and a few Saabs. For a Desmo, I will hire a maestro."4 squiggles" Dan G —Preceding unsigned comment added by 188.8.131.52 (talk) 08:21, 23 May 2009 (UTC)
Removed uncited sections
I removed the two uncited "Advantages/Disadvantages" section. Editor Jobstbrandt has not offered to post citations to the work he has referred to which is counter to Wikipedia policy (i.e. Wikipedia:Verifiability and Wikipedia:No original research).
I've communicated with the editor several times asking him to post citations, but he has refused.
And just to make it clear to everyone, I have no problem with the content of the sections (except perhaps the "mental blindness" comment) save for the fact that the information is uncited opinion. Izaakb 22:17, 25 January 2007 (UTC)
I see that much of the editing I did was not noticed. The "mental blindness" referece was removed early on and citations were added as I could find them on the web, much of the subject not being common knowledge. I would like to resubmit the article under a new title such as "Desmodromic drive, then and now" or something similar. It is, after all, the technical competition conventional cam, flat tappet, and spring return valve drive against which desmodromic valve drive is measured.
Jobst 20:56, 27 January 2007 (UTC)
Please read the very last sentence of the "Disadvantages" section. The section ends in mid sentence. "Reasons for not using a desmodromic approach are increased maintenance and valve noise which can be uncomfortably loud in engines with four or more" cylinders? valves per cylinder? Remain nameless (talk) 20:02, 7 July 2008 (UTC)
Why would an engine maker bother with a desmodromic system? What are the disadvantages of springs? Is there an appreciable lag in closing time when a spring is used? What problem was the desmodromic system designed to solve?
Well it solves the problem of choosing springs because you would want springs that are light enough that they do not create huge amounts of drag at low rpm but stiff enough that it closes the valves at high rpm. when going into the high revs like bikes do, sometimes vlaves don't close fast enough and that robs power and then the piston can hit the open valve and just blow up the engine. thats basically what desmo valves solves. Finbar Canavan 03:49, 15 May 2007 (UTC)
- A spring valve requires more work by the engine to open the valve than a desmo valve. A desmo "helper" spring has about 7-8 lbs of pressure whereas a traditional spring poppet valve may have over 100 lbs of sprung tension. Consider that with a 100 lb spring, the cam shaft, when turning, must overcome 100 lbs of tension per spring (a 2 valve engine will have two springs per cylinder). That robs HP from the engine, since the cam shaft is turned by engine work.
- Okay, that makes sense. Is the drag from the spring significant enough to merit this, though? I suppose it must be. Also, wouldn't the spring put all the energy it absorbed back into the camshaft as it extended again? Slight losses from friction, obviously. Aside from that, everything makes sense. A stiffer spring would be needed to close the valve quickly, but is undesirable due to 'robbing' HP from the engine. Has this been documented somewhere, with statistics? That kind of data would make a great addition to this page, if available. Phasmatisnox 03:42, 16 May 2007 (UTC)
- I don't know where it has been documented, I am sure Ducati has some data on it. The best way for you to see how spring pressure affects the cam is to take a head and turn the cam by hand. Cam lobe profiles often do not permit the spring to "put back" the energy into the camshaft because the lobe shape dictates the timing of the valve opening and closing, thus, the lobe is always working against the spring tension. If you have a desmo head, you only have spring tension (and only about 7-8 lbs of it) when you turn the lobe into the opening rocker, when the closing rocker begins to move, there is no tension whatsoever, just the weight of the valve! Conversely, the 100 lb spring is always pressing against the cam lobe, first it is working against the opening lobe and second it is pressing against the turning lobe, but it cannot "speed up" the engine or add back HP, it is just riding the curvature of the cam lobe.
Disputed section: Disadvantages
I opened this discussion section to debate/dispute what I find to be inaccurate statements throughout the Disadvantages section. I have put those statements in quotes followed by my reason to debate their accuracy.
1. "Since those days, lift, velocity, acceleration, and jerk curves for cams have been modeled by computer to reveal that cam dynamics are not what they seemed."
- The design of camshafts has improved due to computer modeling, but cam (or more correctly, valve) dynamics have been studied for years, even before computer modeling.
2. "With proper analysis, valve adjustment, hydraulic tappets, push rods, rocker arms, and above all, valve float, became things of the past...without desmodromic drive."
- Valve float and valve bounce are absolutely not "things of the past"; modern designs have raised the valvetrain limitation on engine rpm, but that limitation still exists and valvetrain dynamics is still an important part of engine design.
3. "Today most automotive engines use overhead cams, driving a flat tappet to achieve the shortest, lightest weight, and most inelastic path from cam to valve, thereby avoiding elastic elements such as pushrod and rocker arm."
- Most automotive engines today use overhead cams but still use rocker arms to actuate the valves. The use of OHCs removes the weight and flexibility of pushrods. The tradeoff is that they make the cylinder head(s) taller which can create problems packaging the engine into smaller engine compartments. Also, rocker arms are still used since 1) the designer can put a hydraulic lash adjuster on one end to eliminate valvetrain noise and lash adjustments, as well as 2) they enable the valve lift to be greater than the cam lobe height. OHCs can be direct-acting (straight line through cam-tappet contact point, tappet, and valve stem) though this adds even more height to the engine and does not negate the requirement of the mass of the tappet.
4. "With such cams, that mostly do not look like the ones "artists" formerly designed, valve noise (lift-off) went away and valve train elasticity came under scrutiny."
- Cams still look like cams, regardless of whether they were designed on a computer or by trial-and-error.
5. "Today's cams have mirror image (symmetric) profiles with identical positive and negative acceleration while opening and closing valves. An asymmetric cam either opens or closes valves more slowly than it could, speed being limited by Hertzian contact stress between curved cam and flat tappet from accelerating the mass of valve, tappet and spring."
- Symmetric cam profiles have been the most common since the first camshaft were made. If we allow the presumption that valve opening and closing speed is limited by contact stress between the cam and tappet 1) the mass of valve, tappet, spring (if present), and rocker arm (if present) and 2) both normal and tangential (friction) forces and stresses must be taken into account.
6. "Maximum valve acceleration being limited by cam-to-tappet galling stress, is governed by moving mass and cam contact area. Rigidity and contact stress are best achieved with conventional flat tappets and springs whose lift and closure stress is unaffected by spring force, both occurring at the base circle where spring load is minimum and contact radius is largest. Curved (lever) tappets of desmodromic cams cause higher contact stress than flat tappets for the same lift profile, thereby limiting rate of lift and closure."
- Maximum valve acceleration is limited by several structural and metallurgical constraint: friction between cam-tappet, tappet-valve, and valvestem-valveguide can all cause galling or erosion of parts. The forces required to accelerate the valvetrain masses cause the increased friction which eventually overcomes the material constraints of the valvetrain parts, guides, and seals.
- How is valve lift and closure stress possibly *un*affected by spring force?? The only time spring force will not contribute to closure stress is at the one (extremely high) rpm where valve inertia and valve spring force combine into a valve closing profile that exactly matches the cam profile. At all other points - both higher and lower - valve springs will contribute heavily to cam-tappet contact stress.
- Even "flat" tappet actually have a curved face. This curvature - along with a slight beveling of the cam face - causes the tappet to spin, enabling a rolling contact. Sliding contact between cam lobe and tappet would quickly destroy both. Also, the flat face of the tappet restricts the shape of the cam lobe. A faster opening (and closing) cam lobe profile requires a smaller diameter tappet to prevent geometric interference.
- Rather than spinning flat tappets most modern engines use a rolling wheel to eliminate sliding friction between cam lobe and tappet. This wheel can be incorporated into the valve tappet or into the "finger follower" (which is a type of rocker arm). Rollers could be used in desmodromic valvetrains to minimize frictional stresses. However, rollers do have the disadvantage of increasing the mass that must be accelerated.
7. "With conventional cams, stress is highest at full lift, when turning at zero speed (engine cranking), and diminishes with increasing speed as inertial force of the valve counters spring pressure, while a desmodromic cam has essentially no load at zero speed (in the absence of springs), its load being entirely inertial, and therefore increasing with speed. However, its greatest inertial stress bears on its smallest radius."
- This statement is incorrect:
- - With conventional cams the force at the cam-tappet contact point is highest at the highest valve-opening acceleration because at this point the cam is having to apply force to move the valve-spring-tappet mass in addition to the valve spring force trying to hold the valve closed. On the cam lobe closing ramp the contact force is lower because part of the valve spring force is being used to move the valve-spring-tappet mass.
- - With a desmodromic valvetrain the valve spring force is entirely removed: the cams only apply forces as needed to accelerate the valve. This means that the conventional-looking cam applies force during the accelerating phase of valve opening and the decelerating phase of valve closing. The second cam per valve is essentially the inverse of the first and applies force to decelerate the valve during the opening movement then accelerate the valve closed. These forces are relative to the inertial mass of the valve, rocker arms, and lash-adjusting mechanism.
8. "Desmodromic valve drive was often justified by claims that springs could not close valves reliably at high speed and that the forces caused by suitably strong springs exceeded what cams could withstand. Since then[when?], valve float was analyzed and found to be caused largely by resonance in valve springs that generated oscillating compression waves among coils, much like a Slinky. High speed photography showed that at specific resonant speeds, valve springs were no longer making contact at one or both ends, leaving the valve floating before crashing into the cam on closure.
For this reason, today as many as three concentric valve springs are sometimes nested inside one other; not for more force (the inner ones having no significant spring constant), but to act as snubbers to reduce oscillations in the outer spring.
An early solution[when?] to oscillating spring mass was the mousetrap or hairpin spring used on Norton Manx engines. These avoided resonance but were ungainly to locate inside cylinder heads. Today, Formula One racing engines use gas springs that have no resonant parts, their working parts having an insignificant mass compared to the force of their compressed gas.
Valve springs that do not resonate are progressive, wound with varying pitch or varying diameter called beehive springs from their shape. The number of active coils in these springs varies during the stroke, the more closely wound coils being on the static end, becoming inactive as the spring compresses or as in the beehive spring, where the small diameter coils at the top are stiffer. Both mechanisms reduce resonance because spring force and its moving mass vary with stroke. This advance in spring design removed valve float, the initial impetus for desmodromic valve drive."
- Outside of the first sentence this discussion of valve springs is completely unrelated to the subject of the article. If that is not sufficient reason to remove it then let's discuss its dubious statements:
- - When we discuss valve float in poppet (mushroom-head) valves we usually mean valve bounce, that is, the loss of valve control due to "bouncing" off the valve seat at the end of the valve closing motion. In even direct-acting OHC valvetrains there will be valve bounce; this is evident as there is nothing except the valve spring to hold the valve closed. One problem with valve spring resonance is that at resonant frequencies the spring doesn't hold the valve closed when it needs to. To extend the engine's operating rpm range one must raise the valvetrain's resonant frequencies. This can be done by increasing the spring rate or lowering the valvetrain effective mass. Desmodromic valvetrains positively control valve position throughout the valve opening and closing, eliminating valve bounce and float until we reach rpms and valve effective masses where the (slight) flexibility of the rocker arms becomes relevant.
- - I say "effective" mass because what matters is the mass that is *moving*. A low moment-of-inertia rocker arm can have lower effective mass than a tappet; this is because the whole mass of the tappet moves with the cam. Compare this with the rocker arm where the most massive part is the fulcrum, which only rotates around a rocker shaft.
- - Double and triple valvesprings have much higher spring force than single springs, as can be seen by perusing any aftermarket valvetrain catalog. It is true that the designer can choose to have inner and outer springs with different force and therefore different resonant frequencies. Designers can also specifically include a valve spring damper for the same purpose.
- - *All* springs have resonant frequencies. In fact even rocker arms have some flexibility and hence have resonant frequencies. This is why rocker arm designers strive to make their arms super-stiff (higher resonant frequency) as well as lower effective mass. Beehive springs are normally used because they are more narrow at the moving end and so have lower effective mass. They also compress farther before bottoming out.
- - Formula 1 racing engines use pneumatic valve springs because the compressed gas has extremely low mass. Their designers still need to be concerned with the mass of the valve and the valve piston which seals the gas cylinder, so at some rpm they'll still resonate. It's just that that resonant frequency is so absurdly high that other engine failure points are of more concern.
- i would agree with this description on corrections of valve train/springs... read any book relating to cylinder heads and/or valve-trains by author (and engine guru) David Vizard alll his research and practice supports the arguments raised here in favor of the disputer. the information should be looked at and edited as needed.
- one note: Beehive Springs do the best at minimizing resonant frequencies at high engine speed due tho their shape and wire gauges that make up the spring. this is because, as a Bee Hive Spring is cycling, its resonant frequency constantly changes and good (expertly matched) cam/valve choices can be made so that none of (most of...) the resonant frequencies of all said parts may never all match at one given point. Therefore they may cancel out most of the probllems that face a traditional spring style or the dual spring style which thrys to due what the beehive style does by using two springs with different resonance.
- Some of the added weight of additional springs, as well as higher rates (poundage) is a problem for high revving motors when compared with the Bee Hive design, as it does the same thing but with less mass, as well as, less spring rate poundage as would be compared to a dual spring or stiffer single spring.
- Any one interested should check out the aforementioned author David Vizard's How To: Build Horsepower as well as David Vizard's How To:Port and Flow TEST Cylinder Heads you can also check his webpage to clarify anything in dispute at http://www.motortecmagazine.net — Preceding unsigned comment added by 184.108.40.206 (talk) 22:00, 28 May 2012 (UTC)
i'm guessing control. i think the etymology section (which should be deleted) is describing the etymology of desmos (in modern greek the word control derives from knot) rather than describing the etymology of desmodromic, which has only ever meant "controlled path". it is a common mistake to assume that the meaning of a word is some fanciful combination of the old meanings of the root words — Preceding unsigned comment added by 220.127.116.11 (talk) 07:24, 26 August 2011 (UTC)