finding peak piston velocity in terms of crankshaft degree
11-23-2004, 06:29 PM
#2
Registered User
Join Date: Feb 2000
Location: Bountiful, Utah
Posts: 1,248
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by Boost It!
How would one go about doing this? is there apoint where its always the fastest?
The change in direction at the top of the stroke and bottom of the stroke is where the Volicity is quickest..yes?? No??
Is this what your after?
Last edited by Hot Rod Hawk; 11-23-2004 at 06:31 PM.
11-23-2004, 07:36 PM
#3
Registered User
Thread Starter
Join Date: Oct 2004
Location: Costa Mesa, CA no more!
Posts: 236
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by Hot Rod Hawk
A shorter rod will cause the piston to travel within the bore at a faster rate [higher load rate]. A longer rod will slow piston velocity within the bore, and it will increase dwell time at TDC and BDC.
The change in direction at the top of the stroke and bottom of the stroke is where the Volicity is quickest..yes?? No??
Is this what your after?
The change in direction at the top of the stroke and bottom of the stroke is where the Volicity is quickest..yes?? No??
Is this what your after?
how is peak velocity slower with a shorter rod.
if both start at the same, and end at the same time- and one "stops" for a longer period time...
wont average speed be the same but peaks be different- with the longer having a higher peak?
is peak piston velocity right after and before TDC?
11-23-2004, 07:39 PM
#4
Registered User
Join Date: Feb 2000
Location: Bountiful, Utah
Posts: 1,248
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by Boost It!
how is peak velocity slower with a shorter rod.
?
?
Originally Posted by Hot Rod Hawk
A shorter rod will cause the piston to travel within the bore at a faster rate [higher load rate].
11-23-2004, 07:52 PM
#6
Registered User
Join Date: Feb 2003
Location: Mooresville, NC
Posts: 2,640
Re: finding peak piston velocity in terms of crankshaft degree
found this statement by rich...
but wouldnt you have that same point of velocity every 90 degrees after that?
im just trying to visualize it in my head(not an engineer or anything!)
im probably WAY off base here though!
Originally Posted by rskrause
With a 350 Chevy, switching to 6" rods from 5.7" ones will delay peak piston velocity
from 74.5° to 75.5°.
Rich
from 74.5° to 75.5°.
Rich
im just trying to visualize it in my head(not an engineer or anything!)
im probably WAY off base here though!
11-23-2004, 09:53 PM
#7
Registered User
Join Date: Sep 2002
Posts: 655
Re: finding peak piston velocity in terms of crankshaft degree
I'm thinking peak velocity of the piston occurs when the rod angle to the
piston is at the greatest point, and the crank angle is approaching 90
degrees with respect to the perpendicular of the cylinder bore.
At TDC and BDC, the piston is only moving tiny amounts as the crank throw
and connecting rod are inline with the cylinder bore (parallel)
piston is at the greatest point, and the crank angle is approaching 90
degrees with respect to the perpendicular of the cylinder bore.
At TDC and BDC, the piston is only moving tiny amounts as the crank throw
and connecting rod are inline with the cylinder bore (parallel)
11-23-2004, 10:19 PM
#8
Registered User
Join Date: Oct 2002
Location: Upstate NY
Posts: 2,931
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by Boost It!
How would one go about doing this? is there apoint where its always the fastest?
2) Max Piston velocity doesn't occur at 90 crank position with a finite rod length. Imagine a 1 mile long rod. It would have very little angularity on a 3.50 stroke engine throughout the stroke, so max. piston velocity would occur nearly at 90 and 270 degrees if TDC was 0 degrees. Now imagine a zero length rod. Yeah, it won't work, but the piston velocity would be constant at the velocity of the center of the rod journal.
3) It's a simple(?) geometric relationship of crank throw and rod length. The spreadsheet has been posted previously. Try searching.
4) When BMW revealed data on the P83 F1 engine they said peak piston speed was 45 m/s (@19200 revs) and mean was 25 m/s. That allows you to back into rod length once you figure stroke length.edited: Peak P/S was 40 m/s, not 45. Thanks folks for pointing out the error. 45/25 does couse some satrange engine parameters.
Last edited by OldSStroker; 12-01-2004 at 07:20 AM.
11-23-2004, 11:31 PM
#9
Registered User
Thread Starter
Join Date: Oct 2004
Location: Costa Mesa, CA no more!
Posts: 236
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by OldSStroker
1) Why do you want to know, other than curiosity? Average piston speed is usually a benchmark for engine durability or even "doablity". It isn't affected by rod length, where max. piston speed is.
2) Max Piston velocity doesn't occur at 90 crank position with a finite rod length. Imagine a 1 mile long rod. It would have very little angularity on a 3.50 stroke engine throughout the stroke, so max. piston velocity would occur nearly at 90 and 270 degrees if TDC was 0 degrees. Now imagine a zero length rod. Yeah, it won't work, but the piston velocity would be constant at the velocity of the center of the rod journal.
3) It's a simple(?) geometric relationship of crank throw and rod length. The spreadsheet has been posted previously. Try searching.
4) When BMW revealed data on the P83 F1 engine they said peak piston speed was 45 m/s (@19200 revs) and mean was 25 m/s. That allows you to back into rod length once you figure stroke length.
2) Max Piston velocity doesn't occur at 90 crank position with a finite rod length. Imagine a 1 mile long rod. It would have very little angularity on a 3.50 stroke engine throughout the stroke, so max. piston velocity would occur nearly at 90 and 270 degrees if TDC was 0 degrees. Now imagine a zero length rod. Yeah, it won't work, but the piston velocity would be constant at the velocity of the center of the rod journal.
3) It's a simple(?) geometric relationship of crank throw and rod length. The spreadsheet has been posted previously. Try searching.
4) When BMW revealed data on the P83 F1 engine they said peak piston speed was 45 m/s (@19200 revs) and mean was 25 m/s. That allows you to back into rod length once you figure stroke length.
i want to know out of curriousity, but not just for ****s and giggles.
Obviously, you want to relate camshaft events to the peak timing. Ideally, youd have your cam peak (if thats where the heads flow best) when the piston was at max velocity, no?
11-24-2004, 07:37 AM
#10
Registered User
Join Date: Oct 2002
Location: Upstate NY
Posts: 2,931
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by Boost It!
iObviously, you want to relate camshaft events to the peak timing. Ideally, youd have your cam peak (if thats where the heads flow best) when the piston was at max velocity, no?
I looked at the intake port velocity in an EA Pro simulation of an LT1 with a custom cam. The intake port velocity peaked just about 90 degrees ATDC. This didn't vary a lot from 3000 to 6600. The peak piston velocity is about 75 degrees ATDC, remember.
On this particular engine, peak velocity at 3000 rpm was about 300 fps, but it also had a -100 fps reverse flow as the intake closed late. By 6600 the peak was up to about 500 fps and there was no reverse flow indicated. This cam was designed to peak torque around 5000 and have a fairly flat hp peak from 6000 to 6500, so it was optimized for that. At 6400, when the piston velocity was zero at BDC on the intake stroke, the intake velocity was still over 300 fps!
I wish I could post the graphs.
My take is that because the air has mass and takes a finite time to accelerate, the air velocity always lags behind the piston position which is creating the pressure drop which gets the air to flow. The up side is that the air slows down slower than the valve closes at higher rpm and packs itself into the cylinder near intake valve closing point.
This is all NA, of course.
11-24-2004, 10:08 AM
#12
Registered User
Thread Starter
Join Date: Oct 2004
Location: Costa Mesa, CA no more!
Posts: 236
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by OldSStroker
I looked at the intake port velocity in an EA Pro simulation of an LT1 with a custom cam. The intake port velocity peaked just about 90 degrees ATDC. This didn't vary a lot from 3000 to 6600. The peak piston velocity is about 75 degrees ATDC, remember.
On this particular engine, peak velocity at 3000 rpm was about 300 fps, but it also had a -100 fps reverse flow as the intake closed late. By 6600 the peak was up to about 500 fps and there was no reverse flow indicated. This cam was designed to peak torque around 5000 and have a fairly flat hp peak from 6000 to 6500, so it was optimized for that. At 6400, when the piston velocity was zero at BDC on the intake stroke, the intake velocity was still over 300 fps!
I wish I could post the graphs.
My take is that because the air has mass and takes a finite time to accelerate, the air velocity always lags behind the piston position which is creating the pressure drop which gets the air to flow. The up side is that the air slows down slower than the valve closes at higher rpm and packs itself into the cylinder near intake valve closing point.
This is all NA, of course.
On this particular engine, peak velocity at 3000 rpm was about 300 fps, but it also had a -100 fps reverse flow as the intake closed late. By 6600 the peak was up to about 500 fps and there was no reverse flow indicated. This cam was designed to peak torque around 5000 and have a fairly flat hp peak from 6000 to 6500, so it was optimized for that. At 6400, when the piston velocity was zero at BDC on the intake stroke, the intake velocity was still over 300 fps!
I wish I could post the graphs.
My take is that because the air has mass and takes a finite time to accelerate, the air velocity always lags behind the piston position which is creating the pressure drop which gets the air to flow. The up side is that the air slows down slower than the valve closes at higher rpm and packs itself into the cylinder near intake valve closing point.
This is all NA, of course.
11-24-2004, 12:51 PM
#13
Registered User
Join Date: Oct 2002
Location: Upstate NY
Posts: 2,931
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by Boost It!
now my question is- why 75* ATDC and is it like that for every computer
I'll try a F1 BMW engine when I get a chance.
Reread this and see if it helps visualize what's happening:
"Max Piston velocity doesn't occur at 90 crank position with a finite rod length. Imagine a 1 mile long rod. It would have very little angularity on a 3.50 stroke engine throughout the stroke, so max. piston velocity would occur nearly at 90 and 270 degrees if TDC was 0 degrees. Now imagine a zero length rod. Yeah, it won't work, but the piston velocity would be constant at the velocity of the center of the rod journal."
11-24-2004, 03:24 PM
#14
Registered User
Join Date: Dec 1969
Location: Phila., PA
Posts: 1,150
Re: finding peak piston velocity in terms of crankshaft degree
I jsut did some quick math using the only geometry I remember from middle school- the Pythagoreum Theorum (sp?). You know.... A squared + B squared = C squared. Solve for the piston location when the rod and crank throw are at 90* to eachother.
For a production 350...
A = 1/2 the stroke length (3.48 / 2 = 1.74")
B = rod length (5.7")
C = *the result* how far up the piston (wrist pin centerline) is from the crank centerline
let's see..... this goes here..... that goes there........................... carry the one........... sqare root is.................
Piston wrist pin will be 5.96" above the crank centerline when the crank throw is 90* to the rod. Thank you, thank you. I'll be here for your entertainment all week. Now for my next amazing trick....
At TDC using the same scenario the wrist pin will be 7.44" above the crank centerline (1.74 + 5.7 = 7.44). At BDC the piston will be 3.96" above the crank centerline (5.7 - 1.74 = 3.96).
The "halfway" point between the two would be, obviously, 5.7" above crank centerline (when the crank stroke is 90* to the bore centerline). But the point at which the crank stroke is 90* to the ROD, must happen before that...... and (amazingly) my math shows that. 5.96 > 5.7.
Now, I don't know how to reverse-calculate crankshaft angle at that point, but I'm sure somebody does (and I'd love to know). So instead I went downstairs and mocked up the whole mess with some wire cut to length and a t-square. Then I broke out my protractor and measured the crank throw angle relative to the centerline of the bore at that point.
IT'S A MIRACLE!!!!!! I'm measuring somewhere close to 75* ATDC is where the rod goes 90* to the crank throw. More that that, a longer 6" rod doesn't make much difference in that angle. The rod length is HUGE compared to 1/2 the crank's stroke and so a little .300" bump in rod length doesn't make any noticable difference on the crank angle at which the rod is 90* to the crank throw.
I'm going to take a nap. I have a math hangover now.
For a production 350...
A = 1/2 the stroke length (3.48 / 2 = 1.74")
B = rod length (5.7")
C = *the result* how far up the piston (wrist pin centerline) is from the crank centerline
let's see..... this goes here..... that goes there........................... carry the one........... sqare root is.................
Piston wrist pin will be 5.96" above the crank centerline when the crank throw is 90* to the rod. Thank you, thank you. I'll be here for your entertainment all week. Now for my next amazing trick....
At TDC using the same scenario the wrist pin will be 7.44" above the crank centerline (1.74 + 5.7 = 7.44). At BDC the piston will be 3.96" above the crank centerline (5.7 - 1.74 = 3.96).
The "halfway" point between the two would be, obviously, 5.7" above crank centerline (when the crank stroke is 90* to the bore centerline). But the point at which the crank stroke is 90* to the ROD, must happen before that...... and (amazingly) my math shows that. 5.96 > 5.7.
Now, I don't know how to reverse-calculate crankshaft angle at that point, but I'm sure somebody does (and I'd love to know). So instead I went downstairs and mocked up the whole mess with some wire cut to length and a t-square. Then I broke out my protractor and measured the crank throw angle relative to the centerline of the bore at that point.
IT'S A MIRACLE!!!!!! I'm measuring somewhere close to 75* ATDC is where the rod goes 90* to the crank throw. More that that, a longer 6" rod doesn't make much difference in that angle. The rod length is HUGE compared to 1/2 the crank's stroke and so a little .300" bump in rod length doesn't make any noticable difference on the crank angle at which the rod is 90* to the crank throw.
I'm going to take a nap. I have a math hangover now.
11-28-2004, 11:24 AM
#15
Registered User
Join Date: Jan 2001
Location: houston, Tx
Posts: 293
Re: finding peak piston velocity in terms of crankshaft degree
Originally Posted by OldSStroker
4) When BMW revealed data on the P83 F1 engine they said peak piston speed was 45 m/s (@19200 revs) and mean was 25 m/s. That allows you to back into rod length once you figure stroke length.
(45 m/s)/(25 m/s) = 1.80
To get that ratio of max piston speed to average or mean piston speed on 4 inch stroke BBC I would have a 3.70 inch rod! That would net me say at 7000 rpm a max piston speed of 8385 FPM and a mean piston speed of 4667 FPM for a ratio of the same 1.80 of max vs mean piston speeds.
I think that F1 data is a misprint!