Question Regarding Excessive Valve Stem Length and Effect on Dwell

[JuryRigger]

I have a question regarding what prospective effect a very long (between 4-6") valve stem could have on dwell. To be more specific; I am looking at a setup where I would be running a .3125" poppet on a .25" throat; with an 1/8" stem (which would be reduced slightly for airflow within the valve-which is of no consequence to this question) against 3000 PSI. If I am understanding the old threads on the subject; my belief would be that the additional mass of the stem would require a higher hammer energy to create the required lift; and a reduced hammer weight (essentially minus the weight of the stem) to keep the dwell down to keep efficiency at a tolerable level.
Do I seem to be on the right track? I have not done the actual hammer weight/stroke calculations yet; as that gets messy due to a "swinging" revolver type hammer being integral to this potential design...
Thoughts much appreciated;
Jesse

[rsterne]

If the poppet mass (stem plus head) becomes a significant percentage of the hammer mass, then the hammer will slow more on impact, leaving less RESIDUAL energy and momentum available to create lift and dwell.... Think of the poppet and hammer travelling together after impact and you will get the idea.... In a typical PCP you might have a 50 gram hammer and a 2 gram poppet, which is only 4%.... Increase the poppet to 10 gr., and that is now 20% of the hammer mass.... Assuming the momentum stays constant (some is lost in decompressing the seal material), instead of the hammer slowing ~4%, it now slows ~18%.... so you have about 14% less momentum (and 25% less energy) to produce lift and dwell.... Lift is proportional to residual energy and dwell is proportional to residual momentum....

If you keep the spring length, rate and compression the same, you should lose power with the (much) heavier stem.... As always, changing the hammer mass changes the ratio of lift to dwell, but you have complicated matters by having the poppet (stem) mass a larger percentage of the hammer mass.... In this example, if you increase the hammer velocity by about 14% at stem contact, you should be back to the original condition, I think (providing the hammer mass stays the same)....

Bob

[JuryRigger]

Thanks Bob, that mostly confirms what I was thinking; though I did miss considering the effects of the now-remaining residual energy/momentum on dwell and lift as separate issues... The result of the hypothetical scenario you played out confirms what I thought to be the correct solution to regain correct dwell and lift for this unique situation (assuming the hammer is weighted correctly to begin with)-hammer velocity. That was what I meant by needing increased hammer energy, I used the wrong term of energy when I was really thinking speed. I need to actually get a ballpark figure together on what this poppet could weigh, and compare that to the ballpark hammer weight necessary to achieve my target power.
You have helped me understand how to figure this out (hopefully) by myself; many thanks! 
Jesse

[rsterne]

Note that your suggestion of reducing hammer mass to increase the velocity will reduce the momentum, and hence the dwell.... while NOT increasing the hammer energy.... Hammer energy (and hence lift) is purely a function of spring force and the distance available for acceleration.... Changing hammer mass changes the momentum (and hence dwell), but not the energy (both measured before contacting the valve stem, of course)....

Changing the poppet material can change the amount of energy lost by the hammer while it is decompressing the seat material.... Harder materials waste less energy, leaving more residual energy and momentum to open the valve....

Bob

[JuryRigger]

Bob, I was not thinking of dropping the hammer weight to increase velocity with the same spring energy (which indeed would lower the momentum); I was wondering if the hammer mass might need to be reduced due to the weight of the stem. Like you have shown in your example; the heavier stem would rob the hammer of around 25% of it's energy; which is lost to accelerating the stem-leaving too little residual energy to accomplish lift. However, since the momentum taken from the hammer is conserved by the stem (14%); is it possible that, since the heavy poppet now has significant momentum of it's own, that when you increase the hammer velocity by 14% (to gain back the lost energy), that you could have then increased the total momentum too much, throwing the ratio of lift-to-dwell off?
I suppose that I am being overly concerned over that possibility; since momentum increases linearly with velocity; and energy with the square... So getting the necessary increase in energy probably won't increase the momentum too much anyway.
The fine details of this will just have to be ironed out through experimentation-using the normal formula to get close with the knowledge that increased hammer strike will be necessary.
Thanks again;
Jesse

[rsterne]

I always work on the assumption that the hammer and poppet (including the now heavier stem) will travel in contact with each other.... There can be little argument that the hammer/poppet combination will slow, because if the momentum remains constant, if the mass increases 14% (compared to with the lighter poppet) then the velocity must decrease by the same 14%.... The energy, however, must decrease, because the velocity factor is squared.... Energy = momentum x velocity / 2.... That factor of 2 may well mean that the energy loss is not 25%, because the increase in total mass makes up for some of the velocity loss, I think, once you include the stem mass.... Neglecting the energy lost by decompressing the poppet material, this should mean a decrease in energy and lift (possibly about 7 %? ), but you may be correct that since the total momentum is unchanged the dwell would remain the same....

If you need the original lift, then the lost energy must be replaced by increasing the spring force and/or travel.... If you add enough spring energy to get back to your original residual energy (but with the heavier stem), then you should get back to the original lift.... However, as you point out, since the total mass (hammer plus poppet) has increased you MAY then have increased residual momentum, and therefore more dwell.... To bring the ratio of lift to dwell back into balance, a lighter hammer MAY help.... That would be reasonable, since without changing anything else (pressure, flow rates, poppet mass, spring rate and travel, etc.) if you fit a lighter hammer you still have the same energy, but less momentum.... Since you increased the spring energy to get your original lift back, a reduction in momentum from this new condition (via a lighter hammer) would seem reasonable....

Trying to guess the magnitude of how these (opposing) things interact is likely to lead to the wrong conclusions, IMO.... Trial and error is a much simpler approach.... If I had to guess (and using the example at hand), reducing the hammer weight by half the increase (1/2 of 8 gr. = 4 gr.) in stem weight (total mass now 46 + 10 = 56 gr.) .... and then increasing the spring energy (by about 3.5 %? ) to get back to the original lift.... might be close.... The "half the mass" is purely a gut feel, W.A.G....    .... kind of a "two steps forward, one step back" approach....   

Bob