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

 

[ivymike]

Kyle - don't make too many Holiday Inn jokes- you're bound to remind me that I've only been home a few times in the last 5 months! Good to hear that I wasn't too far off base in suggesting that crank accel might be significant afterall. I'm also glad to hear that I'm not the only one who has used the skier analogy - although it makes my heels and toes sting to think about it.

 

[PFM]

All,

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

 

[ivymike]

as I've been saying all along, yes. the acceleration will affect cam jerk levels, jerk affects vibration & resonance speeds, vibration contributes to float.

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.

 

[strokersix]

ivymike,

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.

 

[magnograil]

You might also want to remember that "constant RPM" is not really constant. It is an average value over the measured period, either one or two revolutions. The crank rotational velocity increases with the power stroke and decreases throughout the rest. The flywheel absorbs the power stroke and delivers it thoughout the remaining cycles to reduce the rotational variation. Given that everything is elastic, depending upon where the cam drive is taken, it can see higher acceleration during part of the revolution at a lower "constant" speed due to torsional vibration. A heavy crank pulley is not a torsional damper but just a secondary flywheel. A damper would have to dissipate energy to control the vibration.
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.