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Re: Exhaust valve bounce
Posted: Fri Nov 01, 2019 5:45 am
by ptuomov
MadBill wrote: ↑Thu Oct 31, 2019 7:25 pm
I haven't seen any definitive data showing that exhausts
do bounce higher or sooner than intakes. Since bounce can only occur after EVC and by then (40°+ ATDC) the piston will be out of reach of any still-connected ex. valve, gross signs of P-V contact can only be the result of float.
Float and bounce by the intakes leave more subtle clues and less immediate destruction, so judging only by the damage, it would be easy to conclude that exhausts are the weakest link due to sooner/higher bounce.
Also, typically, exhaust valves are lighter than same-technology intakes, plus commonly have less lift and more duration, both of which make the exhaust spring 's job easier than the intake's
For a turbo cross-plane V8 with short log-style exhaust manifolds, I think the exhaust gas pressures are a big deal. See the attached graph from Vannik's software. At IVC, the pressures help close the intake valve. At EVC, there's a nasty pulse interference in this cylinder at 5500 rpm right at EVC that fights against the valve closing:
.
Pressures.png
.
Normally aspirated and with headers, story might be completely different.
Re: Exhaust valve bounce
Posted: Fri Nov 01, 2019 2:32 pm
by David Redszus
At IVC, the pressures help close the intake valve.
At IVC, the pressures on both sides of the valve should be nearly equal.
At EVC, there's a nasty pulse interference in this cylinder at 5500 rpm right at EVC that fights against the valve closing:
During overlap, cylinder, inlet and exhaust pressures are relatively low and are changing with crank angle.
If we take the existing gas pressure at any given crank angle and multiply by the valve area, we can see the forces that will affect valve and spring forces.
Re: Exhaust valve bounce
Posted: Fri Nov 01, 2019 3:22 pm
by ptuomov
David Redszus wrote: ↑Fri Nov 01, 2019 2:32 pm
At EVC, there's a nasty pulse interference in this cylinder at 5500 rpm right at EVC that fights against the valve closing:
During overlap, cylinder, inlet and exhaust pressures are relatively low and are changing with crank angle.
Do you think this is also true for a cross-plane V8 twin turbo engine with log exhaust manifolds? If you do, what do you think went wrong in my simulation that created the above simulated pressure trace?
Re: Exhaust valve bounce
Posted: Tue Nov 05, 2019 10:15 pm
by ptuomov
I found this article on the topic interesting, since it is for direct acting bucket lifter:
http://www.profblairandassociates.com/p ... %20011.pdf
It does confirm that gas loads matter for the exhaust valve. "Static motoring" aka "Spintron" seems to work for the intake, but even for a normally aspirated engine the exhaust valve side is meaningfully different fired vs. motored.
Re: Exhaust valve bounce
Posted: Wed Nov 06, 2019 5:02 am
by Warp Speed
ptuomov wrote: ↑Tue Nov 05, 2019 10:15 pm
I found this article on the topic interesting, since it is for direct acting bucket lifter:
http://www.profblairandassociates.com/p ... %20011.pdf
It does confirm that gas loads matter for the exhaust valve. "Static motoring" aka "Spintron" seems to work for the intake, but even for a normally aspirated engine the exhaust valve side is meaningfully different fired vs. motored.
Actually, the intake side differs also under firing loads.
Re: Exhaust valve bounce
Posted: Wed Nov 06, 2019 5:09 am
by ptuomov
Warp Speed wrote: ↑Wed Nov 06, 2019 5:02 am
Actually, the intake side differs also under firing loads.
Yes, as the graphs in the article show. However, based on my reading, the difference is much bigger for the exhaust valve.
Re: Exhaust valve bounce
Posted: Wed Nov 06, 2019 7:50 am
by Orr89rocz
So trial and error for spring loads on exhaust side inturbo applications?
Re: Exhaust valve bounce
Posted: Wed Nov 06, 2019 10:15 am
by Warp Speed
ptuomov wrote: ↑Wed Nov 06, 2019 5:09 am
Warp Speed wrote: ↑Wed Nov 06, 2019 5:02 am
Actually, the intake side differs also under firing loads.
Yes, as the graphs in the article show. However, based on my reading, the difference is much bigger for the exhaust valve.
Depending on the stiffness of the actuation system. The instantaneous speed obeserved depends on the firing loads present, amd more the cam drive system than valve system actuation compliance.
Re: Exhaust valve bounce
Posted: Wed Nov 06, 2019 11:47 am
by hoffman900
Warp Speed wrote: ↑Wed Nov 06, 2019 10:15 am
ptuomov wrote: ↑Wed Nov 06, 2019 5:09 am
Warp Speed wrote: ↑Wed Nov 06, 2019 5:02 am
Actually, the intake side differs also under firing loads.
Yes, as the graphs in the article show. However, based on my reading, the difference is much bigger for the exhaust valve.
Depending on the stiffness of the actuation system. The instantaneous speed obeserved depends on the firing loads present, amd more the cam drive system than valve system actuation compliance.
I think you’re talking about the changes in angular velocity? How single cylinders, I4s, V8s, etc all are different and how to effects the camshaft profile?
I see a lot more thought put into this with the F1 and Sportbike engines then the traditional V8s, and never understood why, except for the RPMs they run.
I wonder how much of exhaust bounce on the side is because the crank just rapidly accelerated on the power stroke. Seems to me the exhaust lobe would need smoother dynamics, especially as the amount of cylinders decrease.
Re: Exhaust valve bounce
Posted: Wed Nov 06, 2019 12:20 pm
by ptuomov
Orr89rocz wrote: ↑Wed Nov 06, 2019 7:50 amSo trial and error for spring loads on exhaust side in turbo applications?
In my opinion, it'll always be trial and then maybe the error, too. Doing basic F=ma math from a known starting point normally aspirated and then adding incremental seat load based on the simulated exhaust back pressure at EVC got me reasonably close in the first trial. The valvetrain did what I expected it to do, to the extent I'm able to say what it actually did. So who knows.
Re: Exhaust valve bounce
Posted: Wed Nov 06, 2019 12:35 pm
by hoffman900
ptuomov wrote: ↑Wed Nov 06, 2019 12:20 pm
Orr89rocz wrote: ↑Wed Nov 06, 2019 7:50 amSo trial and error for spring loads on exhaust side in turbo applications?
In my opinion, it'll always be trial and then maybe the error, too. Doing basic F=ma math from a known starting point normally aspirated and then adding incremental seat load based on the simulated exhaust back pressure at EVC got me reasonably close in the first trial. The valvetrain did what I expected it to do, to the extent I'm able to say what it actually did. So who knows.
Overlay your cam profiles on the crankshaft velocity graph Engmod4t puts out. May have to do it in excel. Would be interesting to see!