Valve Float Due to rapid Acceleration
Valve Float Due to rapid Acceleration
(OP)
All,
I have seen in print an article discussing valve float due to rapid engine acceleration at an RPM below that at which valve float occurred under a slower acceleration rate. I have seen evidence of this in engines on track that used low gearing and hard acceleration. I believe I have seen results of this in some drag applications as well. I can see why this could be true intuitively, the reason or calculating the why is beyond my rusty calc skills. I read the post about ignition errors due to rapid acceleration and it prompted this post. If possible I would like an equation I could plug in a known valve float RPM, an acceleration rate and the predicted float RPM. Hey may as well wish BIG.
Thanks in advance.
PFM
I have seen in print an article discussing valve float due to rapid engine acceleration at an RPM below that at which valve float occurred under a slower acceleration rate. I have seen evidence of this in engines on track that used low gearing and hard acceleration. I believe I have seen results of this in some drag applications as well. I can see why this could be true intuitively, the reason or calculating the why is beyond my rusty calc skills. I read the post about ignition errors due to rapid acceleration and it prompted this post. If possible I would like an equation I could plug in a known valve float RPM, an acceleration rate and the predicted float RPM. Hey may as well wish BIG.
Thanks in advance.
PFM





RE: Valve Float Due to rapid Acceleration
Changes in ignition timing are due to torsional distortion of the camshaft etc.
Regards
pat pprimmer@acay.com.au
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RE: Valve Float Due to rapid Acceleration
Thanks for the reply but this is not due to tach lag, the data AQ does not miss much. The ignition changes are not due to torsional distortion either take a look at thread 71-107708. RPM is a very slow dimention "Minutes" as opposed to Micro seconds requied to accelerate an engine. If we exaggerate the problem to say the engine accelerates from 4000 rpm to say 8000 rpm in .1 seconds then the engine would see an acceleration rate greator than the average rpm at say 9000 rpm.
I am sure there is an integral in there somewhere to show the rate of acceleration is greator than the average RPM, should have stayed in school longer or used it more.
PFM
RE: Valve Float Due to rapid Acceleration
There is some class of drag racing that is getting pretty popular. It is based on a "street car" type car and the rules are fairly restrictive as cam lift and duration are limited.
Comp Cams is grinding cams with some really steep opening ramps to jerk the valve open to produce a larger valve opening event that than the lobe would produce.
RE: Valve Float Due to rapid Acceleration
I guess that you are saying that with an ECU, the rpm increase between sensor input and firing is enough to alter the timing compared to a steady rpm.
I can see that for ignition, but cams are mechanical, so the rate of lift or valve acceleration in directly proportional to RPM times lifter rise per degree of rotation. The only variable that can effect valve or valve spring inertia at a given engine rpm is vibration or stretch in the cam or cam drive, like a chain stretching, then catching up. This might instantaneously (like for a few degrees) alter cam rpm vs crank rpm. It is hard to imagine it is enough to make a measurable difference to valve float rpm
Regards
pat pprimmer@acay.com.au
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RE: Valve Float Due to rapid Acceleration
The pickup is only the rpm indicator and reference point.
This signal is sent to the ECU which then "calculates" the timing for the next cylinder to fire based on a number of inputs such as throttle position, infered airflow, temperature, Ox sensor input, air charge temp., throttle position etc. then returns a timed pulses back to the coil control circuit that fires the coil or coils depending on what kind of ignition system.
A loose timing chain usually causes the cam timing to retard sleightly and tends to reduce the low end torque a small amount.
As far as ignition timing, it can be reset to still fire the at any point in the crank rotation even if the cam does go retarded and drives the dist a bit later but this is what can be adjusted backwards to compensate (as long as there is adjustability).
RE: Valve Float Due to rapid Acceleration
I didn't say RPM sensor. I said sensor input, which i think is a general term. As far as I know, many sensors are input in pulses which are in a very short but finite time frame. At very high acceleration rates, the time between the last sensor pulse and the ECU calculation can give a different result as it will predict what is required for the engine when the signal was sent, and the engine will have changed speed in the meantime.
Even a tight timing chain can stretch and recover due to the elasticity of the steel or whatever it is made of. The stretch allows the cam to slow slightly as it retards, and the recovery requires the cam to accelerate to slightly faster than steady state as it catches up. The timing changes can be due to elasticity in any or all components in the cam drive train, including between the sensor point and the cam drive point on the crank.
I have never seen this, It is mere speculation to attempt to explain some unexpected results.
Also, if the engine is accelerating rapidly, it is probably at full throttle and in open loop. Also the time frame between measurements will be so close, that temperatures will not have time to change.
Regards
pat pprimmer@acay.com.au
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RE: Valve Float Due to rapid Acceleration
I did qualify my answer by referring to an EFI control system that does use the profile pick up as an rpm sensor and cylinder id refernce and is all it does.
I'm sure there are all sorts of variations that can influence the final ignition point between the first signal time before processing and the final point the coil fires.
On a standard ignition trigger signal too operate the final coil firepoint reguardless of how it is done; there are indeed a number of distortions that delay the final ignition time.
Have a nice day!
RE: Valve Float Due to rapid Acceleration
Norm
RE: Valve Float Due to rapid Acceleration
Thanks for the link, it is on the right path. I am not convinced that Pat is correct. I guess the answer may be in the instantaneous acceleration vs the average we call RPM. I have not been able to prove this (with math) one way or the other to my satisfaction. If we push my example to an unreal level like 4000 to 8000 RPM in .001 seconds then the rate of acceleration should be much greator than the average of 8000 RPM.
Again thanks for your help.
PFM
RE: Valve Float Due to rapid Acceleration
I am not convinced that I am correct either. It is just my thoughts on the matter.
Regards
pat pprimmer@acay.com.au
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RE: Valve Float Due to rapid Acceleration
Valve float occurs only when the combined mass of the valvetrain cannot remain in relative symmetry with the camshaft lobe. When the follower (rocker arm or lifter) cannot remain in contact (oil film contact eliminated here for the sake of discussion) with the closing ramp of the camshaft, float occurs. If the lifter or follower is hydraulically assisted, the increased clearance will cause the valve to either remain open until the lifter bleeds down, or hit a piston top if the clearance is too tight. If the lifter is mechanical with no hydraulics, valve float may simply result in a momentary loss of power, at which time the engine rpm drops back and the valve train regains symmetry. If the engine rpm is increased above the rated control rate of the combined valve mass, the pushrods or rocker assemblies may impact and bend or break. Been there and done that. I once motored an engine on a dyno with weak valve springs to demonstrate valve float at 3,000 rpm. No pistons, no induction or exhaust, only a crank, block, heads, camshaft with one cylinder and valvetrain. A strobe light worked well to show the effects.
The rate of acceleration should have no effect on the valve float, no matter how many iterations I look into. There is valve spring surge, oscillations due to the harmonics of the valve spring, and bounce when closing at high rates, but these are independent of rate of acceleration.
Franz
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RE: Valve Float Due to rapid Acceleration
The rate of acceleration you're referring to is the average rate of crankshaft angular acceleration, right?
The acceleration required to keep the follower in contact with the cam over the nose is critical to "valve float," of course. This acceleration rate is determined by the engine speed and cam profile.
The amount of jerk (rate of change of acceleration) required to keep the follower in contact with the cam nose (kinematic considerations) can have an important effect on valvetrain vibration. The jerk on the opening ramp can be particularly significant in some applications, and there may be others where jerk over the nose plays a role (can't say I've seen that though). Accelerating the crankshaft constantly will alter the effective jerk profile of the cam lobe, and I can't say offhand that the effect on valvetrain vibration will be insignificant. As you mentioned above, vibration of valvetrain components can contribute signficantly to "float."
RE: Valve Float Due to rapid Acceleration
Hi Franz. I cannot argue that it "may" but that there have been a couple of specific engines where I actually engineered in some float. One engine was a single cyl Briggs-Stratten for a racing mini-bike application back in the late 60's (yeah, we even raced those darn things). We had rules restrictions on valves and springs but a loophole on camshaft lobe profile. Rules specified only max lift. It took a few efforts but we actually gained a bit of top end without sacrificing too much reliability (the limiting factor to power was the rod strength window which was pretty narrow). I tried this on a couple of other engines with MUCH less success, however.
Generally I look at valve float much as I do the plague.
Rod
RE: Valve Float Due to rapid Acceleration
Rod
RE: Valve Float Due to rapid Acceleration
One thing else, if the valve springs develop RPM sensitive harmonic oscillations that come in phase or syncopation during the RPM range, I could concieve intermittent float. In that case, proper valve dampening coils should minimize the problem.
Franz
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RE: Valve Float Due to rapid Acceleration
You are right, I am looking at a snap throttle type acceleration, or a low first gear launch at the drags. I still see the potential for the motor to accelerate at a rate higher than the given RPM.
Thanks for the post.
Dave
RE: Valve Float Due to rapid Acceleration
RE: Valve Float Due to rapid Acceleration
Comparable in that you would use RPM to describe one aspect of acceleration, or less than one RPM if the acceleration is in a straight line, then it is feet per second, per second (toss in meters if you wish).
Franz
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RE: Valve Float Due to rapid Acceleration
Rate of acceleration on the angular velocity of the crank might cause some change to degree of torsional deflection in the crank and cam.
Load on the flywheel will vary according to rate of acceleration, as a greater percentage of engine power will be lost to internal inertia, thus reducing the power to the flywheel.
What have I missed?
Regards
pat pprimmer@acay.com.au
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RE: Valve Float Due to rapid Acceleration
Vibration of valvetrain components, entirely? As I mentioned above, jerk in the cam profile contributes to valvetrain vibration, and valvetrain vibration contributes to "float."
(If the kinematic acceleration of the cam hardware was all that you needed to consider, then valvetrain dynamics would be a much simpler subject)
RE: Valve Float Due to rapid Acceleration
Maybe I am misunderstanding what you mean by jerk, maybe that means I am a jerk, but I understood jerk to mean a sudden movement when compared to the speed of normal movement, and I would expect that to be caused by torsional vibrations in the cam itself.
Maybe it also means what Rod was talking about where a lobe opening rate is deliberately designed to induce some float over the nose and thereby increasing lift, but that would happen at a specific rpm, irrespective of acceleration of the engine.
Regards
pat pprimmer@acay.com.au
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RE: Valve Float Due to rapid Acceleration
RE: Valve Float Due to rapid Acceleration
Have I got it right this time.
Regards
pat pprimmer@acay.com.au
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RE: Valve Float Due to rapid Acceleration
The input has been great but I still see no clear answer. Let me push the example a bit further. I hope my math is good. Again we accelerate the motor from 4000 RPM to 8000 RPM but this time in .0070 seconds. The valve and spring package is good to 8500 RPM. My math says at 4000 RPM one engine revolution takes .015 seconds, at 8000 RPM one revolution takes .0075 seconds. Now if the motor starts at one engine revolution in .015 seconds and ends at .0075 but the total elapsed time is less than the time required for one engine revolution at the end RPM at some point the instantaneous RPM MUST be greator than 1 revolution in .007059 seconds. All the input on valve float etc.. are great it all happens in an instant. Now the example is extream but...... I hope it serves the purpose.
Thanks again.
PFM
RE: Valve Float Due to rapid Acceleration
4000rpm = 1rev/0.015sec
8000rpm = 1rev/0.0075sec
4000rpm/0.007sec = 571428rpm/sec
Assume you have constant acceleration from 4000rpm to 8000rpm over a period of 0.0070 seconds. The engine will turn 252degrees in the time during which it is accelerating (0.007sec). The max instantaneous engine speed in that time is 8000rpm, which occurs at the end of the acceleration period.
to do the math yourself, for constant acceleration rate:
crank position = (initial position) + (initial speed)*(time) + 0.5*(acceleration rate)*(time)^2
for the example above,
252deg = 0deg + (24000deg/sec)(0.007sec) + 0.5*(3428571deg/sec/sec)(0.007sec)^2
252deg = 0 + 168deg + 84deg
Pat:
The jerk I was referring to is inherent to the shape of the cam profile, but its value depends on both the cam rotational speed and the cam rotational acceleration. The jerk contributes to valvetrain vibration, I believe, by altering the force-vs-time input to the system (changing the harmonic content of the excitation).
TVs might be higher during acceleration, but I've never run a transient analysis to look at torsional vibrations so I can't say.
RE: Valve Float Due to rapid Acceleration
Just thought I'd add the following to my previous post. If we take your statement above, and translate it into more familiar terms, essentially what your math is saying is something like this: Driving down the road... At 60mph, one mile takes 1 minute. At 70mph, one mile takes 0.857 minutes. Now if the car starts at one mile in 1 minute and ends at 0.857 but the total elapsed time is less than the time required for the car to travel one mile at the end speed at some point the instantaneous speed must be greater than 1 mile in 0.800 minutes.
Does the above statement make the slightest bit of physical sense? Of course not- we know from experience that no matter how fast we accelerate, we're never going to go faster than the fastest speed we reach while accelerating, regardless of how quickly we're accelerating. A car that accelerates from 0-60 in 5 seconds doesn't magically go 90mph halfway in between, just because its speed increases by 60mph in less than the time it takes to drive a mile at 60mph!
RE: Valve Float Due to rapid Acceleration
Two---Soooo, being the jerk
Rod
RE: Valve Float Due to rapid Acceleration
Thanks for bringing out the 2X4, sometimes when my record gets stuck it needs a tap back to reality. The car deal got through my fog and made the point.
Rod, As for your cam and not floating the valves with a flick of the throttle well that is a whole new thread too, what if the valve floated just after the opening ramp and was not near the piston when it floated?
Ok, Thank you all for the help with this thread.
PFM
RE: Valve Float Due to rapid Acceleration
Then you'd have a REALLY odd cam profile.
Rod, I certainly can't come up with an actual situation from memmory where the camshaft angular acceleration would have had a big effect on valvetrain performance. I can't rule out that such a situation might occur given the right confluence of circumstances, though, because I know that the jerk of the cam profile can be very important, and I know that the jerk is related to both cam velocity and cam angular acceleration. Cam jerk won't ever cause separation in a kinematic situation - only cam acceleration can do it there - but cam jerk can definitely cause severe vibrations in a dynamic situation, where the various valvetrain components can vibrate significantly. I haven't done much with OHC engines, but I suspect they're much less sensitive to this than pushrod engines. All that said, I'm not certain that camshaft acceleration (within reason) will have any significant effect on cam jerk.
RE: Valve Float Due to rapid Acceleration
There is a useful form of valve float which I refer to as valve "loft" and it can improve performnce when used in a controlled manner.
Regards,
John Lawson
RE: Valve Float Due to rapid Acceleration
"It is simply not possible to accelerate the valve more rapidly on the way up to maximum RPM, than it is accelerated while operating at maximum RPM."
You state that with such conviction. Though IvyMike has given me an example I think I can live with I still have some issues I cannot resolve.
Lets move to a super extream example, 4000 RPM to 8000 RPM in .00001 seconds. Now if we apply Mike's formula to this all looks fine but can it be?
The formula would give a result that the crank did this in like 1 degree of engine rotation, yet it does not have an effect on valve opening, broken rods, pistons etc..
This question has been proposed to more than one engineer (not on this forum) and they were not able to prove or disprove the concept. That is why I tried it here.
Thanks again to all.
PFM
RE: Valve Float Due to rapid Acceleration
As to pushrod engines, you already know that is pretty much limited to the mini--- all the head work is now done by APT---it's just too late for me to start at the bottom of the curve again. $1500 will get a top notch head and I could waste that much just getting the ports to flow the same.
Good morning, John Lawson. As to the "loft" thing---as I posted above, I have some small degree of experience in grinding a cam lobe profile that enhanced that little phenom, albeit in the dark ages.
One thing I did learn---it is not just the acc ramp/opening side of the profile that causes the "good float", the closing/decel ramp was also a determining factor. As I said, this worked well on a 5 hp (to start with, anyway) Briggs-Stratton side valve engine and it did not work at all on a couple of automotive engines where I got the idea in the first place. This was all 1960's stuff and is totally out of whack today. I would never think of doing something like that on one of my 10,000 rpm engines now.
Rod
RE: Valve Float Due to rapid Acceleration
RE: Valve Float Due to rapid Acceleration
In a simplified (but technically correct in my opinion) linear model crankshaft motion will be superimposed on valve motion. This means that crankshaft acceleration will be added to valve acceleration.
Whether this is significant or overshadowed by other effects is open to question. I suspect that other effects such as those discussed above in previous posts are dominant.
RE: Valve Float Due to rapid Acceleration
Try this - draw an acceleration vs crk angle diagram for a single valve at a single engine speed. Note that for large portions of the cycle, valve accn = 0, regardless of what the crankshaft is doing. Also note that for zero crank acceleration, there is significant valve acceleration.
The valve acceleration diagram will look like a distorted letter M, with a positive hump followed by a negative-amplitude valley followed by another positive hump. The first hump is the opening flank (and ramp), the valley is the cam nose, and the second hump is the closing flank (and ramp). The amplitudes of the humps and valleys are determined by the shape of the cam, and are linearly proportional to cam velocity (prop to crk vel). Thus we can see that kinematic valve acceleration is proportional to (average kinematic) crankshaft velocity. A little calculus will show that the first time-derivative of the cam acceleration profile (cam jerk) is proportional to the first time-derivative of the crankshaft velocity (crank accel).
In the kinematic model above, separation only occurs when the amplitude of the negative valley @ the cam nose is greater than the max acceleration amplitude that the springs can impart to the valvetrain hardware. In other words, if |the spring force at maximum deflection divided by the effective mass of the components| is less than |the required nose acceleration|, separation will occur.
In a dynamic system, all of the valvetrain components are flexible and have mass, and all can vibrate individually as well as in a group. If you look at a dynamic valve acceleration trace, there will be a wavy pattern superposed upon the kinematic "M" pattern. The valves will open a bit later, and close a bit earlier (or later) than expected. Peak acceleration is no longer necessarily at the same point in time as it was in the kinematic model. The amplitude of the max acceleration is no longer directly proportional to crankshaft velocity either because the vibrations will increase and decrease as various vibratory modes manifest within the valvetrain at different engine speeds (although the underlying kinematic acceleration still is prop to crk vel).
These vibrations are driven by the force input to the valvetrain from the cam lobe, which is time varying (of course). The frequency content of the excitation will influence the amplitude of the response at any given engine speed. The frequency content of the excitation is influenced to some extent by the camshaft angular acceleration, which is why I don't want to rule out engine accel as a possible contributor to valve "float."
RE: Valve Float Due to rapid Acceleration
I don't think my statement above was quite correct - crank acceleration will influence cam jerk, but they're probably not proportional. (I haven't actually written out any equations and solved)
RE: Valve Float Due to rapid Acceleration
I think you do understand what I'm saying.
I'll restate for the benefit of those who perhaps don't:
The valve position, velocity, acceleration, jerk, etc. are tied directly to the crankshaft. Crankshaft motion is superimposed on valve motion. If the crankshaft is going faster then the valves are too. If the crankshaft accelerates then the valves see higher acceleration and lower deceleration in an additive manner. Likewise, crankshaft torsional vibrations are also superimposed on valvetrain motion.
Obviously there is a lot more going on with regard to vibrations, deflections, etc. but I believe my statements are technically correct.
RE: Valve Float Due to rapid Acceleration
I am not a cam designer, nor a rocket scientist but I did stay at a Holiday Inn Express last night. No actually I have been involved with valvetrain testing using the Spintron test equipment.
The acceleration in question is the engines acceleration rate which is variable even at W.O.T. based on load, gear ratio and power curve shape. If I remember right our T/A engines accelerated around 2500 rpm/sec in 2nd gear 1200 rpm/sec in 3rd 600 rpm/sec in 4th and 300 rpm/sec in 5th.
My experiences tell me the following;
If the valvetrain has a period of instability before maximum rpm, the engine can pass through it more easily the faster the engine is accelerating.
Loft is your friend. Its the landing that's the problem.
Bounce is what happens when the ski jumper lands on the flat. (BAD)
Piston to valve issues occur only at TDC overlap. The cam position at overlap has the exhaust just closing and the intake just opening. So valve bounce can only affect exhaust piston to valve clearance.
Later,
Kyle
RE: Valve Float Due to rapid Acceleration
The valve set-down can be a seperate issue also involving the flank set-down rate causing valve seat bounce.
The spring setup still has effects on this under different conditions.
One thing I have noticed on many different head designs is the possible tendency of the head material and design in the area of the valve seats contributing to the bounce issue with it's own 'ringing' reaction after inital hit by the valve head.
RE: Valve Float Due to rapid Acceleration
RE: Valve Float Due to rapid Acceleration
Do I feel a shift here? Strokersix I think you are with me somehow they add to each other. I think the math is beyond my old brain, too many years since those parts had to work.
Ivymike, is there room in one of those equations for engine acceleration to have an effect on valve float? I do not think it is by a large amount but I do believe it is there.
Thanks again for all the input.
PFM
RE: Valve Float Due to rapid Acceleration
Follower acceleration and crank acceleration don't add to each other. crank acceleration changes the rate of change of kinematic follower acceleration (valve jerk). Follower kinematic acceleration itself is only driven by cam rotational velocity and cam shape.
RE: Valve Float Due to rapid Acceleration
I think I see what you are saying.
valve velocity is related to crank position
valve acceleration is related to crank velocity
valve jerk is related to crank acceleration
I was incorrect in my previous post and I offer my apologies and thanks for the correction.
RE: Valve Float Due to rapid Acceleration
Resonance of the crank can be different during acceleration than at constant speed due to the non-uniform nature of the impulses applied renforcing higher harmonics. The cam is also driving the crank from the valve train loads, backward during lift and forward during closure. Inertia makes the lift driving loads higher than the closure.
A uniformly accelerating engine (ignoring torsional vibrations) can change the valve spring harmonics since the spring is being accelerated differently thoughout the lift cycle than with a constant rotational speed. The coils are initially accelerated slower at the start of lift and decelerated faster at the end which results in different reaction than rotating the cam at a constant speed.