Mudflap wrote: ↑10 Jul 2020, 14:38
Hoffman900 wrote: ↑09 Jul 2020, 22:17
The bore size is going to dictate the valve spacing and diameter, which is ultimately going to dictate the maximum diameter of the buckets. Valve cant plays a role too and I know some F1 engines had this.
You cannot increase velocity on these designs as I pointed out without running off the edge. This wipes out camshaft lobes. It's literally a physical constraint you cannot go past. However acceleration and the rate of acceleration (jerk) are still at play, and is dictated by design and material selections. You can tune fourier shape forces by spring shape and material selection, and in pushrod application; material, shape, thickness.
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In the real world, you can only make lifters / buckets so big due to physical constraints, and thus your maximum velocity will also be dictated by this. Pushrod engines with rockers and finger follower type applications, there is a multiplication effect, so you have to account for what the spring can handle and work backwards through to design the lobe.
This is going nowhere.
This whole velocity limited thing started off for the simple reason that in NASCAR they imposed bucket diameter and curvature. I can't stress this enough but it not an issue in the real world!
To convince yourself go and pick a NASCAR roller lifter cam profile with the highest velocity you can find and back-calculate the required bucket diameter to see if it is a reasonable dimension. Until then we are just arguing over how long a piece of string is.
I'll work out an example for you:
In the lift profiles you showed above the peak velocity is 0.008 in/deg. With sensible clearance the required bucket diameter is 0.97in. The NASCAR max diameter was 0.875 in. In NASCAR that would be illegal, in the real world to make the diameter 0.095in larger is an engineering graduate's morning warm up job.
To put this into perspective, the 1 inch 1966 Cosworth DFV tappets would have been more than adequate for this profile.
So dismissive...
Peak velocity is around .00745 in/deg. It's the purple line. Working backwards, this gives an edge clearance of .010". As I said before, they push the boundaries with these things, and they can because the tolerances, very strong CGI blocks, and no chamfer on the lifter edge.Since you love the real world so much, these are real world figures.
Also, this is at the lobe, not the velocity seen at the spring / valve which is a whole other matter. With a OHC bucket type valvetrain, this is 1:1, what you see at the lobe is what you get at the valve. NASCAR, with the FT camshafts, were using in excess of 2.37:1 rocker ratios. .00745" velocity at the lobe would be almost .01778in/deg at the valve.
Of course a 1" lobe would be adequate, and considering the vintage, it would probably need that margin due to tolerances. NASCAR builders did use 1" mushroom tappets up until they were outlawed in the 1980s. This was the maximum they could go as they already had to narrow the lobes to keep the lifters from fouling on each other.
A 1" bucket / tappet, using the same tolerances as that NASCAR engine would be .00855 in/deg.
Again, as mentioned above, with mushroom lifters of 1", you run out of real estate to the adjacent lobe. Real world.
On a OHC application, the bucket can only be so big before you foul on the adjacent bucket. Real world.
If you worked backwards checking velocity, a roller cam may have an effective tappet diameter of 50in+. A single bucket / tappet alone would be the size of the entire engine.
However if you look at acceleration, they are similar and since bucket / flat tappets are not acceleration constrained, they can reach peak velocity sooner. However, since the roller isn't velocity constrained, it can keep increasing to the point that the system needs to also slow it down to get over the nose. With faster acceleration, you can move the valve faster and spend more time slowing it down, thus increasing open area at the top of the lift curve.
If you look at the lobe profile from Honda's Third Generation Formula 1 engine, you saw maximum intake valve velocity of 15mm/rad (.0103"in/deg) and acceleration of around 55mm/rad^2 (.03779in/deg^2). This doesn't account for multiplication through the finger follower. An older NASCAR application when multiplied through a 2.37:1 ratio rocker arm, might only be .01778in/deg (velocity) and .00209in/deg^2 (acceleration). Pneumatic "springs" are nice. This gets back to my initial post in this thread.
PlatinumZealot wrote: ↑10 Jul 2020, 01:57
Hoffman900 wrote: ↑04 Jul 2020, 20:35
Again, the engine doesn't care how the valve is lifted. It only sees the valve lift profile.
What about.. valve train losses and durability? The engine must care about this. Or are you referring to the combustion only?
What I mean is you design the valve lift curve and work backwards through the valvetrain to determine lobe shape. If the geometry was such that the lobe had to be square to get the valve lift shape, then so be it (obviously this is a hypothetical scenario). You can really get a sense of this with OHC rocker valvetrains / finger follower. To get a symmetrical valve lift profile (which isn't exactly ideal, but for conversation sake), the lobe will be asymmetrical due to the varying rocker ratio. If you look simulated mass flow into the engine, you'll see the mass flow doesn't exactly follow the valve lift curve exactly, so that's a whole other matter.
PlatinumZealot wrote: ↑10 Jul 2020, 01:57
Hoffman900 wrote: ↑04 Jul 2020, 20:35
With the push rod system, there is some disconnect between the lobe profile and the valve profile through the rpm range, due to system stiffness, this is all measurable and compensated for, and in NASCAR's case, it is used to their advantage through controlled loft that gains open valve area with increasing rpm. Regardless, top pushrod racing engines are valve spring limited, not anything else.
Can you explain how this is done? Do they rely on some sort of valve float?
They use the pushrod to pole vault the valve as rpms increase. This can all be controlled so where it's done, how it's done, and how / where the valve lands on the lobe. In a typical pushrod valvetrain, valve open area (as expressed as duration) is typically lost as rpms increase due to bending.With controlled loft, they are doing their best to maintain the duration figure but using the pole vaulting to increase peak valve lift, thus producing a net gain in open area. Obviously this takes A LOT of engineering to do, and from the valve side, it's a bit scary as they run very tight P-V clearances.
PlatinumZealot wrote: ↑10 Jul 2020, 01:57
A lot of hondas and other japanese cars and even BMW used this sort of setup. SOHC with rockers. There is a tuner called Bisimoto (he tunes porsche now but used to tune honda) who capitalized on the light weight of the SOHC design, and once had the fastest "All motor" Hondas. He didn't care about indivually tuning the exhaust and intake cam because he could just build a custom cam, and he didn't care about variable valve timing of course... Sometimes I wonder why F1 doesn't use SOHC? Maybe because the want to avoid rockers? Though rockers are not necessarily that bad as you say.
Bisi had the fastest SOHC in drag racing, but most were using DOHC for obvious reasons and were much faster. They were just in other classes. He did good work on that, but DOHC makes tuning infinitely easier and cheaper as you don't need to have a new cam cut every time you want to change centerlines. Most serious blank sheet performance engines (F1, MotoGP) and hot production engines (Sportbikes, etc.) are all using finger followers for reasons I have mentioned in other posts. It's the best of both worlds - very stiff / low friction valvetrain and rocker multiplication (which gets around the limitations of the bucket diameter).
