Honda Power Unit Hardware & Software

[63l8qrrfy6]

Three layers might have to be used then to effectivley lengthen the bar. Takes up more space though.

You don't have to lengthen the bar..

Let's assume that mass doubles and lift halves when moving to a double valve.

In order to maintain the same natural frequency you need to preserve the stiffness to mass ratio. Effectively you need to double the stiffness of the 2 valve bar.

Torque is stiffness (torsional) x angular displacement. Since you've doubled the stiffness but halved the lift (angular displacement) the torque has not changed. The radius of the shaft can remain the same as the shear stress in the bar is not a function of length. You still have to achieve the increase in stiffness which you do by decreasing the shaft length.

The other thing to consider is the change in angular acceleration. For a simple harmonic cam profile the max acceleration is given by lift x w^2. Halving the lift would halve the angular acceleration. Since the mass has doubled and T=I*a , the torque requirement can again be shown to remain constant. Of course cams use significantly more complex profiles and peak accelerations occur on the flanks rather than at max lift but I think this is a reasonable assumption.

I wasn't halving the lift because i was assuming full advatange is taken for more flow in a race car application. So for arguments sake i kept the lift the same. For example when BMW moved from two valves to four valves per cylinder the valve lift did not reduce. Remember the valve diameter is smaller to fit two valves in the same space so the lift would be just about the same more or less.

So. I assumed the same acceleration. The moving mass maybe be not double, but more than 1.5 times the mass of one valve maybe? So the spring has to be stiffer by that at least. To get the required stiffness i was saying it is better to have a longer bar instead of only just bigger diameter or thicker walls. The spring will be under less shear stress.

Remember that at 0.25xDia lift the courtain area equals the valve area - maintaining the same lift would be completely useless. Instead lift has to be reduced proportionally with valve diameter.

[PhillipM]

That doesn't account for blocking effects from turning the flow though, reality is you'd open them further, generally your lift is piston clearance limited on an F1 motor.

[63l8qrrfy6]

Of course, maybe you'd open up 60-70% of the original lift. But then the mass increase is not 2x either - i can argue it's 1.6-1.7x so the conclusion doesn't change.

It is pedantic for a back of the envelope kind of calc.

[godlameroso]

Why does it feel like the discussion is shifting to port velocity/cylinder filling and the need for long duration low lift vs high lift short duration or a compromise of the two?

And that's not even considering exhaust valve requirements.

Good CFD is so critical here, otherwise it would be a nightmare to churn out the thousands and thousands of cam profiles that you'd have to test. There are a lot of systems that depend on the exhaust energy. Your combustion process has to have all these in mind, and finding the right valve timing is probably the most closely guarded secret.

The secrecy is made necessary because since there is no variable cam timing or lift profiles, an advantage here is essentially permanent. However it is a moving target, as it must be matched to all the supporting hardware it all evolves in tandem.

Which leads me to the regulations, whenever you see a specific number, like max voltage, or max compression ratio. You can bet at least one team is flirting at the edge of those numbers, and are politically attempting to cement their advantage by preventing another team from outdoing them.

[PlatinumZealot]

Remember that at 0.25xDia lift the courtain area equals the valve area - maintaining the same lift would be completely useless. Instead lift has to be reduced proportionally with valve diameter.

Here is another poster's view on the topic of valve lift...

Belatti,

Valve lift in a high speed race engine is defined by kinematics and not simply flow requirements. The flow coefficient of a poppet valve peaks approximately when the valve lift is about 25 or 30 percent of the valve diameter. So purely in terms of flow gains, there is no benefit to using more lift than that.

But most racing engines use much more lift than flow conditions require, for kinematic reasons. Using greater valve lift permits a cam profile that will achieve that max flow condition for a greater period of time with a fixed duration. Or in other words, you get to the condition of max flow quicker after the point of intake valve opening, if you employ more valve lift.

Consider this very simplified example. Say your valve needs 8mm of lift to achieve its maximum flow coefficient. If you use a cam that gives 8mm of lift, the first 4mm of valve lift are used to accelerate the valve away from its seat, and the last 4mm of valve lift are used to decelerate the valve to its point of max lift. If this cam has 300deg total duration and a simple harmonic motion, this means you would achieve a max flow condition 150deg after the valve open event.

But say you used 10mm of lift for that same valve with the same 300deg duration. The 10mm cam would achieve the max flow condition (ie. 8mm of lift) after only about 120deg after the point of valve opening, which is much quicker. Understand the difference?

As for your specific question about the actual lift dimension, I can't answer that.

Regards,
Terry

[roon]

Nice information there. I didn't know such a mechanism exists. The torsion bar is in two peices. One half enveloping the other. Smart. Would be interesting to see how it works for paired valves.

Similar to the suspension torsion bars in motorsport applications. There's a rigid outer housing/sleeve with the torsion spring inside. I'm not sure how much the outer sleeve is relied upon to flex. The bar within is doing most of the twisting. They aren't quite "halves" in this sense.

The nicest thing about the torsion spring is undoubtedly its lack of resonances. Recip' mass shouldn't be a problem if the finger also serves as a cam follower. Packaging would be the main downside.

I have a solution. Roll the bar into a helical shape. Rotate it 90* and place it atop the valve.

[63l8qrrfy6]

Remember that at 0.25xDia lift the courtain area equals the valve area - maintaining the same lift would be completely useless. Instead lift has to be reduced proportionally with valve diameter.

Here is another poster's view on the topic of valve lift...

Belatti,

Valve lift in a high speed race engine is defined by kinematics and not simply flow requirements. The flow coefficient of a poppet valve peaks approximately when the valve lift is about 25 or 30 percent of the valve diameter. So purely in terms of flow gains, there is no benefit to using more lift than that.

But most racing engines use much more lift than flow conditions require, for kinematic reasons. Using greater valve lift permits a cam profile that will achieve that max flow condition for a greater period of time with a fixed duration. Or in other words, you get to the condition of max flow quicker after the point of intake valve opening, if you employ more valve lift.

Consider this very simplified example. Say your valve needs 8mm of lift to achieve its maximum flow coefficient. If you use a cam that gives 8mm of lift, the first 4mm of valve lift are used to accelerate the valve away from its seat, and the last 4mm of valve lift are used to decelerate the valve to its point of max lift. If this cam has 300deg total duration and a simple harmonic motion, this means you would achieve a max flow condition 150deg after the valve open event.

But say you used 10mm of lift for that same valve with the same 300deg duration. The 10mm cam would achieve the max flow condition (ie. 8mm of lift) after only about 120deg after the point of valve opening, which is much quicker. Understand the difference?

As for your specific question about the actual lift dimension, I can't answer that.

Regards,
Terry

It doesn't matter. Let's say for argument's sake that the original valve has 2xDia lift. The reasonable assumption is that the smaller double valve will also have 2XDia lift. This means roughly half the lift of the original valve.

[godlameroso]

Reducing the lift increases flow velocity to a point?

[hurril]

I don't get the valve lift talk that long quoted paragraph mentions. Why would both the lower lifting and higher lifting cam use 4mm as intro and outro? That means that they have different accelerations / angles of attack (I am not familiar with the terminology.) Is there a reason why the lower-lifting one could not use the same angle of attack as the longer lifting one uses?

[godlameroso]

I don't get the valve lift talk that long quoted paragraph mentions. Why would both the lower lifting and higher lifting cam use 4mm as intro and outro? That means that they have different accelerations / angles of attack (I am not familiar with the terminology.) Is there a reason why the lower-lifting one could not use the same angle of attack as the longer lifting one uses?

Camshafts are a fascinating subject

here are the camshafts Honda used on their 2006 engine, you can learn a lot about the valve train behavior by looking at the lobe profiles.

[63l8qrrfy6]

I don't get the valve lift talk that long quoted paragraph mentions. Why would both the lower lifting and higher lifting cam use 4mm as intro and outro? That means that they have different accelerations / angles of attack (I am not familiar with the terminology.) Is there a reason why the lower-lifting one could not use the same angle of attack as the longer lifting one uses?

That is purely to show that you get peak flow for a longer duration by increasing lift beyond the value that gives the minimum discharge coefficient.

[gruntguru]

Surely lift will more or less halve with 2 valves so displacement will also halve.

Nothing in the discussion led me to assume that, rather I assumed the simple case of two springs for two valves vs one spring for two valves.

If you want to compare 2v to 4v heads on the same engine - OK the lift will reduce somewhat (not halved because valve diameter won't halve) and of course the valve mass will reduce (significantly since mass is proportional to somewhere between the 2nd and 3rd power of the valve diameter) - all-together a very complex proposition to try and make assumptions about.

[gruntguru]

The nicest thing about the torsion spring is undoubtedly its lack of resonances. Recip' mass shouldn't be a problem if the finger also serves as a cam follower. Packaging would be the main downside.

I have a solution. Roll the bar into a helical shape. Rotate it 90* and place it atop the valve.

Nice solution - and you re-introduce the resonances and lose the main benefit of the torsion bar valve spring.

[Tommy Cookers]

iirc Panhard also used torsion bar valve springs in production cars
leaf springs have been used
and hairpin valve springs were used in some aircraft aircraft and motorcycle engines

(unlike coil springs) the above types avoid the combination of substantial bending and torsional loads

[hurril]

I don't get the valve lift talk that long quoted paragraph mentions. Why would both the lower lifting and higher lifting cam use 4mm as intro and outro? That means that they have different accelerations / angles of attack (I am not familiar with the terminology.) Is there a reason why the lower-lifting one could not use the same angle of attack as the longer lifting one uses?

That is purely to show that you get peak flow for a longer duration by increasing lift beyond the value that gives the minimum discharge coefficient.

But I don't get how that's comparing like for like. The 4mm intro/ outro is what makes the difference unless there's some other factor that the text _should_ contain instead.