machten wrote:The other thing that comes to mind after some more consideration,
____ I actually appreciate that you (for one) seemingly comprehend & understand the matter which I've pointed-out in this thread, and have also bothered to further think-out any possible flaws in my conception of it.
" is that the exhaust valve needs be held closed "enough" during the induction stroke when there is negative pressure in the cylinder, so either the light spring needs to be sufficiently strong to resist, or in the case of "no spring", you probably do need enough of the closer lobe to hold the exhaust valve closed for "enough" of the induction cycle. "
____ This possible issue consideration is even better conceived than your last, and it's another issue-point which I've already-before considered & dismissed. ...
__ When I first realized that there seemed to be no actual need for the closing-lobe's track-face once the rocker-arm's follower had tracked past it's closing-ramp section, that was with the consideration that Ducati still retained the service of valve-spring type springs (probably only because that type of closure-spring was already tried-&-true & proven as reliable for mass-production).
However for starting-purposes, the weaker rocker-spring has also proven itself sufficient to keep the valve seated even after the perfectly-set closing-clearance (between the closing-lobe's upper-most track-face & it's follower) has loosened-up past it's intended zer0-setting. _ So-thus that (extended)- section of the closing-lobe is then no-loner able to provide sealing-assistance anyhow, and-thus the rocker-spring is then certainly left without that particular assistance with the valve sealing-job. _ Yet even so, the kick-starting process doesn't suffer any.
__ That point made, now onto the next...
" Are you thinking Bob that the portion of the closer lobe you've nominated as remaining is sufficient to do that? "
____ No, not entirely,, as that (rather skimpy) remaining section probably doesn't continue-on to cover the duration-point between 90-degrees ATDC & BDC (during the intake-stroke).
__ However, under racing conditions (when an engine is push-started, rather than kicked-over), even a rocker-spring could be dispensed-with as unneeded.
And that's because there's still ANOTHER factor which tends to keep the exhaust-valve closed ! ...
__ At higher engine-revs (above kick-starting RPM), especially with wilder-cams (like the DESMO-cams), the 'momentum' of air-flow is not to be so much assumed as being merely a relatively weak force, (as it's that particular force alone, which is depended upon to allow a wild-cam to better fill the cylinder at higher RPMs !).
That air-flow momentum-force is another factor which helps to close (and hold closed) the ex.valve...
So when the piston's intake-stroke is creating negative-pressure within the cylinder (of a RUNNING engine), the intake-charge was already waiting (behind the intake-valve) in an overly-positive state of pressure, so-thus THAT volume of air really WANTS to fill the cylinder, (and helps to push the ex.valve shut),, whilst the momentum of the mass of the exhaust-gasses,
(which had just been pushed-out and thus isn't interested in stopping & going BACKward into the cylinder [especially when there's another opening with air that's more than glad to do so instead]),
creates a vacuum which strongly tends to suck the ex.valve closed (just as a reed-valve would certainly do), and until the main mass of that expelling ex.gas has escaped the exhaust-plumbing, it's sucking-force only tends to pull the ex.valve shut !
So (except at the lowest engine-speeds when there's relatively little air-momentum), it ought to be understood that the cylinder's desire to become filled is seconded-ordered with respect to the ex.gases wishing to continue on their way out (through the ex.pipe). - (If there was no exhaust-pipe/plumbing, THEN the sucking-effect would instantly disappear,, and probably THEN the ex.valve WOULD indeed be left inclined to become [temporarily] pulled back-open, just-as you had speculated as possible).
But we do know that (at higher engine-speeds) the momentum of the ex.gas must be strong enough to pull a (normally free-sliding) ex.valve shut, cuz that's the very-same force which causes 'overlap' to be as effective as it's known to be !
" I'm thinking if one was to implement Bob's closing lobe theory
there could be good reason to consider having different (smaller than stock) closer lobe durations for Inlet and Exhaust lobes. "
____ This post of yours has now lead me to a new thought, (which I need to further think-out, to become absolutely sure of),, as it seems that a performance-gain might possibly also arise from that suggestion.
__ UPDATE - The new-thought which Kev's posted-wording (inadvertently) inspired, is concerning what BENEFICIAL, (as opposed to merely neutral), side-effect could possibly result from such proposed running without normally restricted-control of the valves...
__ Well we know that the purpose of valve-springs, and also that of the closing-lobes, is to prevent 'valve-float' ... but what-if not only the return-springs were removed, but also that greater-bulk of the closing-lobes as well ? - With just the closing-ramp left remaining intact !?
Normally when the valve has reached the maximum lift-height of the opening cam-lobe, the springs & closing-lobe's track-face then hold-down & restrict the valve from further advancing to any higher-point than that dictated by the opening-lobe's designed-height ! _ So what if that (normally)- set restriction was completely removed ?
Well then as the revs climb higher, such a less restricted valve would then likewise be launched harder and thusly become more apt to open-up & outward to an even higher-point. - (And of-course we're aware that when the valve-head is raised further out of the way, increased air-flow is then the result.)
The ADDITIONAL available valve-lift would sort of work like a reed-valve - becoming more aptly to increase as RPMs built-up higher,, and-so this would be the beneficial-effect of valve-float, (without it's negative-effect, since the closing-ramp would still prevent THAT aspect).
Of-course the available increased lift wouldn't be much more than just 2mm, since the closing-cam track-face is minimumly raised above the shaft, when near the point of max.valve-lift.
And rocker-springs would need to be required so as to make-sure a gap doesn't develop which could possibly allow the C-clip retainers to escape. -
(So THAT's the only possible reasoning for the original inclusion of the extended closing-lobe track-face - [for in the event that no closure-springs are employed], that THEN there's SOMETHING-else to make-sure those valve-keepers can't develop any chance of squeezing-out from their intended location.)
" ...and at this point I publically declare I'm getting beyond my depth... "
____ But you've been doing so well, and it seems that you're the only one who has wished to come-out & play.
" Given their relative mass and distribution from the centre of rotation of the camshaft, you'd think they have some small effect like a mini flywheel. "
____ Yes there's THAT effect, (neither good or bad),, but that's certainly also a source of some amount of added vibration (that's no-doubt considerably stronger than that of a cell-phone vibe.ringer-function).
" I also wonder how much of a benefit it would be to reduce the mass of the lobes. There must be some benefit, or they wouldn't have drilled the holes in the lobes. "
____ Probably done merely to reduce the off-center weight that's being flung-around inside there.
" I'm almost sorry to say, I've enjoyed thinking about it! "
____ Yep, as it's fairly good brain-food for building-up it's muscle-lines.
Dukaddy-DUKEs,
-Bob
