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valvetrain forces
- Thread starter romychaj
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- Thread starter
- #3
Correct- but in a properly designed system valve tip force will never exceed valve spring force; so if you know peak lift values and you know spring characteristics, you know what the upper limit for all the forces are. Whether or not you need a more detailed answer than that depends entirely on what you're trying to do, which so far you have not specified.
- Thread starter
- #5
if you look at a graph of measured force vs. degrees you will see that what your saying might not be correct. why should the acceleration of the valve be limited by the decceleration available from the spring? i will dig up the graphs for other engines and try to post one here as an example.
Valvetrain dynamics is second only to combustion in terms of complexity. From the original post I'm going to wager you dont have a few years' experience specializing in the niche so would advise leaving whatever you're doing to someone who does. If you truly want to begin to learn the subject, SAE has several good texts.
enginesrus
Mechanical
The valve spring is not the only "force" to over come, the exhaust valve see's cylinder pressure in its opening phase(cam timing dependent). Depending on rocker ratio the push rod side can see more force than the valve side. Your going to have a difficult time trying to measure it, not impossible but not easy.
Mention of not exceeding spring force. Is that true in a condition of spring harmonics, or valve float?
Mention of not exceeding spring force. Is that true in a condition of spring harmonics, or valve float?
- Thread starter
- #9
yes, i dont have "a few years in specializing ..." but i can differentiate between decceleration and acceleration (the later easily being able to exeed max spring force).
so would anybody know where i can find a graph of valve tip force in a ls1 with stock cam (or similar engine)?
this is an example of a zetec engine (comparison of two cams):
so would anybody know where i can find a graph of valve tip force in a ls1 with stock cam (or similar engine)?
this is an example of a zetec engine (comparison of two cams):
romychaj said:
Think about the physics of what would happen at the point on the cam lobe where the valve acceleration changes sign during valve opening if the force applied to accelerate the valve is greater than the force applied by the spring to keep the valve tip in contact with the cam lobe.
This is called valve float, and it's bad. Usually really bad.
GregLocock
Automotive
Back in the day we had manual methods to develop cam profiles which obviously included checks on valve acceleration. Sadly neither of my two remaining engine textbooks cover it. I'd have thought perusal of the SAE papers and the IMechE collections on small engines would be a good place to start.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
In simple terms the contact force is the sum of three components:
1. Spring force. This is very simple to determine. This force varies in direct proportion to valve lift. When you rotate the engine very slowly, this will be the contact force.
2. Inertial force. This is directly proportional to valve acceleration which can be determined from the valve lift profile (2nd derivative of lift). This force is +ve (adds to the spring force) at low valve lifts and -ve at higher lifts. This force increases with engine speed.
3. Gas force. Usually less significant than 1 and 2. Difficult to analyse precisely without actual data on cylinder and port pressure. Varies with engine speed and load and is obviously different for intake and exhaust valves.
je suis charlie
1. Spring force. This is very simple to determine. This force varies in direct proportion to valve lift. When you rotate the engine very slowly, this will be the contact force.
2. Inertial force. This is directly proportional to valve acceleration which can be determined from the valve lift profile (2nd derivative of lift). This force is +ve (adds to the spring force) at low valve lifts and -ve at higher lifts. This force increases with engine speed.
3. Gas force. Usually less significant than 1 and 2. Difficult to analyse precisely without actual data on cylinder and port pressure. Varies with engine speed and load and is obviously different for intake and exhaust valves.
je suis charlie
The thing about gas force too is that it's highest when spring force and valve inertia are lowest... the relationship is almost purely inverse. If all you're looking to calculate is envelope forces it can be just about disregarded.
Of course we don't know what OP is really after so maybe it matters and maybe not.
Of course we don't know what OP is really after so maybe it matters and maybe not.
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- #14
what op is after: "... where i can find a graph of valve tip force in a ls1 with stock cam (or similar engine)?" (citation from above.) exactly like the graph above. the same for lifter/lobe would be nice. other parameters like measured acceleration, stifness, friction, float analysis or other cams welcome.
grunt, add to it: friction, contact angles rocker/pushrod and rocker valve.
swinny, did you see the graph i posted?
these are the most popular u.s. engines. such measurements were surely done thousands of times by gm and others. i would have thought this was "common knowledge".
grunt, add to it: friction, contact angles rocker/pushrod and rocker valve.
swinny, did you see the graph i posted?
these are the most popular u.s. engines. such measurements were surely done thousands of times by gm and others. i would have thought this was "common knowledge".
1400 N ~ 315 lbs.
Looks like like Crower hot rod 8500 rpm springs are only 174 lbs open.
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- #17
swinny, now you lost me.
looking at the graph, can you explain where the second 1400 peak comes from? i can see how the valve deccelerates when banging onto the seat, but how would that translate into contackt force? inertia would make the rocker tip lift, no?
there is no second peak here:
(doesnt upload, will try later.)
edit: uploaddoesn work.
looking at the graph, can you explain where the second 1400 peak comes from? i can see how the valve deccelerates when banging onto the seat, but how would that translate into contackt force? inertia would make the rocker tip lift, no?
there is no second peak here:
(doesnt upload, will try later.)
edit: uploaddoesn work.
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