Valve Float Due to rapid Acceleration 3

[PFM]

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

 

[patprimmer]

I expect any apparent change in onset of valve float rpm will be due to tachometer lag.

Changes in ignition timing are due to torsional distortion of the camshaft etc.

Regards
pat pprimmer@acay.com.au
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[PFM]

Pat,

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

 

[spdingtkts]

You know what is interesting is that Comp Cams actually grinds some cams to purposely do this.

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.

 

[patprimmer]

PFM

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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[1Bluegrass]

To reply to the ECU quote above; If fuel injection is the control method then no that is not quite correct.
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).

 

[patprimmer]

1Bluegrass

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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[1Bluegrass]

pat, i'm sorry you take it so specifcally about the sensor.
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!

 

[NormPeterson]

There might be something in

http://www.eng-tips.com/viewthread.cfm?qid=85349&page=4

] that you could use. The topic did wander off a bit from its title into mention of the effects of crankshaft rotational acceleration on piston acceleration.

Norm

 

[PFM]

Norm,

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

 

[patprimmer]

PFM

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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[franzh]

Even after reviewing the posts, I think the strictly mechanical action of valve float has not been addressed.
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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[ivymike]

The rate of acceleration should have no effect on the valve float, no matter how many iterations I look into

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."

 

[evelrod]

"...valve float may simply result in a momentary loss of power..."

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

 

[evelrod]

Sorry, guys. I ment to add that in several strobe tests of DOHC engines that I never found any deviation in profile from a change in acceleration rates. I got sorta wrapped up thinking of some of the crazy things I did when I was younger. Sorry.

Rod

 

[franzh]

The acceleration I am referring to is NOT the cam opening or closing ramp, but rather the opening post, that the rapid rate of acceleration (snap throttle) can induce valve float.
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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[PFM]

Franzh,

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

 

[ivymike]

...rpm and acceleration aren't comparable quantities, right?

 

[franzh]

I guess one would be measured in revolutions per minute, or degrees rotation per second, the other in degrees rotation per second, per second.

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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[patprimmer]

As I see it, inertia in the valve train vs valve spring tension is the sole cause of valve float. The inertia will not be effected directly by crank shaft acceleration, but only by rate of lift of the cam follower, which will be directly effected by the profile of the cam lobe and the angular velocity of the lobe. The angular velocity of the cam lobe is pretty much 1/2 the angular velocity of the crank, plus or minus very short term changes due to slack and distortion in the mechanism, including torsional vibrations in the crank and cam.

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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