Camshaft friction

[091980]

Does anyone know if the torque needed to turn the camshaft increases with RPM?

[Billzilla]

Sure does, I imagine as the square of the rpm as well.

[091980]

Thank you! Exactly the guy I was expecting to have the answer. Your webpage told me to post it here first though.

Btw, you wouldn't happen to know roughly how much torque it would take to turn it on a 4age at, say, 1000 rpms?

[banana_socks]

The valve springs are goin to require a LOT more force to compress than just the pure friction of it rotating. This also means its not just a constant torque required to turn, it will be dependent on position. Why exactly do you need to know?

[Billzilla]

Btw, you wouldn't happen to know roughly how much torque it would take to turn it on a 4age at, say, 1000 rpms?

No idea sorry.

The valve springs are goin to require a LOT more force to compress than just the pure friction of it rotating. This also means its not just a constant torque required to turn, it will be dependent on position. Why exactly do you need to know?

Don't forget that there's also other valve springs also trying to push the cam in the direction it's moving, which helps.
They're quite hard to get turning but once they do the load levels off quite a bit (I think) then as the revs pick up the load increases again.

[091980]

Billzilla, what you're saying makes sense according to what I've found on the internett.

banana_socks, the reason why I need to know is that I have som ideas for a simple, bolt on vvt gear. But to come up with the ideal design I need to know what sort of varying forces can be harnessed to change the phasing of the cam. I don't know if it can be done, but I wan't the entire system to be contained inside the vvt wheel, so it can be swapped easily with no further modifications.

[banana_socks]

I don't know exactly how much return force the valve springs will supply. For example when you are at valve float there would be zero return force, and I would suspect in reality it would decrease as you approach this speed.
If you were able to get hold of the mathematical profile of the camshaft, then it would not be that difficult to calculate. Drag from hydrodynamic bearings is simple, the valve speed can be easily derived from the cam profile, and the corresponding force from the springs is easily calculated.

Alternatively you could approach it from the drive side of the system. The belt/chain has been designed to transmit that force without breaking, so if you can find the ultimate strength of this (easier with a chain) you can attempt to work out the design torque of the camshaft. Wouldn't be extremely accurate, but could probably get a ballpark figure.

[Billzilla]

I don't know exactly how much return force the valve springs will supply.

Exactly as much as it takes to push them down, though the up/down ratio varies quite a bit depending on the cam position. This is why the cam gear on a Subaru (opposed four) gets a hard time as the cam tries to bang itself back & forth as it goes around due to the valve springs working on the cam lobes, and why a V8 single-cam has a relatively easy time in that respect as there are far more springs and cam lobes to smooth it all out.

[andrew_mx83]

the cams in my 24v I6 turn pretty bloody smoothly by hand but i always notice the effect of the springs when i do timing belts on I4 engines so i imagine the flat 4 would be a real prick!
random tip: subaru cam gears are plastic (some anyway)


not that its any solid numbers for you but i had a 1.1hp electric motor trying to turn a 7m over once. it failed miserably, only lasting a few seconds before the elec motor cut out but when i dropped the cam belt it would run for much longer (say 20 seconds).

[091980]

I've been trying to figuring it out. To me it makes sense that the friction or rather load on the camshaft have to increase lineary. I've read somewhere that the friction from bearings decreases with engine speed for a good while before increasing again so i keep this force out of the question. So i guess while the engine is running, it must be moving the valves down that causes the increase in the torque needed to turn the camshaft. This is because the valves and other pieces must accelerate faster and faster with rpm. Thus the force acting on the valve mass must be bigger. I ignore the forces from the valvesprings since most of it is transferred back to the rotating camshaft when the valve closes. At least as long as there is four cylinders in a row. So as engine speed increases the the torque needed to accelerate the valves increases lineary. The power increases exponentially. So the major factor in determining the torque needed to turn the camshaft would be valve lift and the mass of the valve, bucket etc. I think this is also referred to as inertia friction...

This is of course just me trying to use my brain, so please correct me if this doesn't make sense.

Thank you.

[Supra967]

Not inertia friction, just inertia. This is to do with accellerating the mass of the actual cam shaft, it's size (mass and diameter) and it's accelleration (not just rpm). There's a lot going on with the valve train, a lot you have to get your head around. I don't think ignoring forces is going to get you the result you're looking for. Cam profile would play a major part too.

[Shakotan_Aaron]

Just curious as to why you are looking into this? for engine building purposes, some type of school paper, or just for general knowledge and understanding of the arthritic internal combustion engine?

I wonder if you could buy or make some kind of toque measuring device and use it simply on a head of your choice...?

My understanding is that under a constant rpm the only toque needed is to overcome the small amount of friction. The constant drive from the belt helps to smooth out any 'bumps' in the compression and return of the valve spring. However the valve spring cannot return all the energy borrowed from the cam lobe as it has the weight of the valve and also the friction of the valve guide that it must carry with it.

Actually the more i think about it, the more interesting (possibly nerdy in the auto world) it gets.

[Hiro]

The constant drive from the belt helps to smooth out any 'bumps' in the compression and return of the valve spring.

Remember that the drive from the timing belt itself isn't constant/smooth, there are surges in it coinciding with the power stroke of each cylinder. Granted the harmonic balancer should alleviate most of that, but since we're living in the frequency range of the valves opening and closing then it still should be considered.

[091980]

Thank you all for the replies! I really appreciate it.

First of all, the reason why I'm looking into it.

I have for a while wanted to develop a concept for vvt mechanism that can work for any engine, that is to say, a straight bolt on camshaft pulley where all the parts and the mechanism itself is conatined within the gear it self. This of course means that the mechanism would have to be powered by any forces wich can be harnessed inside the gear. In my head it would be a mechanical design able to phase the camshaft to smooth out the torque curve. To be able to use any force for this purpose it must change with the rpm of the engine. This is why I'm looking into this particular aspect. If I can come up with a cheap effective design, my plan is to make it into a business, but that is in the future. I'm currently an economics student so I have no resources or workshop to do any experiments, and that is why I have to rely on you fine people. My plan is to come up with a few possible designs and test them out in AutoCad or something else, and if it can be produced cheaply and people are willing to pay for this modification I would try to invest some money into it. I am a Toyota enthusiast so if this comes true I would first make it for engines like the 4age, 3sge and what not.

Reasons for looking into this:
To be able to use any particular force, it has to be reliant on the engine rpm. That is to say, it has to change with the rpm in a predictable way. The exact measures is not so importent. I need the information to deside wether to pursue a certain design or to leave it. My dream is to make a design which is cheap, versatile and a straight bolt on piece.

Of cource, this is all just in my head at the moment, so feel free to say whatever you want. If you think my ideas is plain out stupid, please let me know!

[allencr]

....Variable valve timing - Wikipedia, the free encyclopedia...

Any flyweight/governor setup would have to be prety heavy because of its small diameter & large force needed to change its position & keep it stable.
Everything old is new again.