You are on the right track.
Yes, cam lobes are designed with ramps at both ends of the lift curve.
The opening ramp is somewhat steeper than the closing ramp, and mechanical
lifter ramps are taller than hydraulic lifter ramps.
With mechanical lifters there has to be clearence in the system to allow
for temperature expansion of the parts involved, and to control the seating
velocity at all clearences, which constantly change during engine warm up
and at different engine loads ( read: exhaust gas temperatures), the ramp
has a "constant velocity section. That means that if you look at the lift
curve graphically, the constant velocity part is a straight line. At each
end of the ramp is a blend into the base circle at one end and into the
lift curve proper at the other end.
The seating velocity of the valves are controlled to a maximum of around
one meter per second to avoid noise and excessive wear.
If , for some reason, a valve seats off the ramp, there will be a clicking
noise from the valve hitting the seat too fast. This noise is often called
“tappet noise”, and happens either due to high clearence in the system, or
caused by a soft (airated) or collapsed hydraulic tappet.
Really hard seating, which is followed by a bounce back off the seat, will
happen at high speed when the spring is not capable of keeping the parts in
contact with each other during the lift cycle. The tappet can also leave
contact with the cam lobe, and when it comes crashing back into contact
again, bounce in the system can happen as the parts compress and relax.
To better understand what can happen, keep in mind that the load the spring
exerts on the valve train parts does not nicely follow the load
v.s.compression curve at high speeds.
When the valve starts to open at high speed, the coils closest to the
spring retainer get compressed first, and inertia causes a delay in the
force getting transferred to the rest of the coils.
This causes some of the coils to compress more than they would have at a
slower speed, and when that message gets transferred back, they relax
some. This sets up a “standing wave” in the spring and the coils “surge”.
It is sort of like an echo bouncing between two canyon walls.
This will cause a “wave” superimposed on the load v.s.lift curve.
At the low points in this “wave” the load of the spring is reduced, and
when this happens when the spring is trying to stop the opening motion and
reverse it to close the valve ( maximum negative acceleration), the load
may become insufficient, and there will be separation.
And, yes, with double springs the “standing wave” in the two will have
different frequencies, so it tends to keep the resultant combined load of
the two more even.
Sorry I went a little long with this, but there may be some out there that
will say: “Aha!”, and the rest can just hit “DELETE”!
From: ex jag email@example.com
Date: 16.01.2007 13:16:24
Subject: Re: [xk] Unleaded Fuel in XK’s
In reply to a message from Ottman0401@aol.com sent Tue 16 Jan 2007:
I don’t think the impact load on the valve as it closes is that
significant. If you think about it, the valve is already closed by
the time combustion occurs, so the combustion pressure would not
affect the impact on the valve seat. Secondly, the cam lobe is
designed with somewhat of a ramp to set the valve down on the seat.
The cam lobe in the XK engine is designed with a very steep ramp,
so the valve would fully open and close at a much more rapid rate
then your basic automobile engine.
You know what, forget everything I’ve said. I’m not an engineer
either, so I don’t know what in the heck I’m talking about, but it
does make sense…Doesn’t it?
The original message included these comments:
Obviously, the peak pressure depends on how hard you push on the gas
I do not know the specified valve closed spring load, however, a
number would be around 100 pounds, or less.
The spring load and gas pressure add up for total load of the valve on
the> > seat.
Does anyone care to figure out the impact load as the valve closes?
Am I the only non-engineer on this list? Interesting reading though.
Regards, Otto M.
ex jag, '66 E-type S1 4.2, '56 XK140dhc
Denison, TX, United States
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