Valve Spring Power Consumption

[usac24]

I've seen various claims of valve spring power consumption of 20-35% of gross power for 2-valve pushrod engines. In our own application, inline 4-cylinder w/peak horsepower of 335@6400RPM, I can calculate approximately 100HP used to open the valves. This was based on the work done from the cam base circle to the nose of the lobe.

However, isn't this same work performed in the opposite direction as the spring helps to drive cam on the back side of the lobe? I realize there will be some frictional heat loss in the spring, and valvetrain inertia will loft the lifter briefly after max lift. I have a hard time believing that the net work done is not closer to zero.

What am I missing here?

 

[SomptingGuy]

Yep, that's how the desmo boys see it too. Predicting where that crossover point is though is a fairly complicated business.

- Steve

 

[evelrod]

I think I mentioned somewhere along the line that 'once upon a time' a friend was racing go karts and needed a test fixture. We built a simple spring scale dyno that worked ok and later a setup to spin his four stroke head to ck for ??? whatever, float and such. We used an old Briggs from a small lawnmower, as I recall... Now, what I do remember is that it took a substantial amt of power to get the camshafts spinning but almost nothing to keep them spinning. Power consumption then, I would assume, would be higher at very low speeds? Inertia? Frictional losses?

I have always tried for the lowest spring pressure commensurate with the rpm needed. Mostly to reduce power losses in driving the setup and from the substantial heat generated in the springs (sorry Pat, I've seen springs get hot enough to 'char' the oil around them).

I sure wish I had been smarter in those days and recorded some of the crap we did.

Rod

 

[ivymike]

Now, what I do remember is that it took a substantial amt of power to get the camshafts spinning but almost nothing to keep them spinning.

Combo of Stribeck curve & overcoming inertia (camshaft+drive mechanism)?

 

[evelrod]

That certainly could be a part of it. More or other reasons?

http://www.sae.org/events/pfs/presentations/2005spikes.pdf

Isaac, it's been a looooong time since I opened a text book! I had this link on file from the ZDDP discussion.

Rod

 

[tbuelna]

GregLocock makes a valid point. As I noted, valve train friction losses are due to contact sliding motion at the various points of contact in the valve train joints. The magnitude of those losses are a function of instantaneous normal forces at the joint and the tribological conditions present.

I must correct my previous comment about spring versus desmo system losses being equivalent. A spring system incurs friction losses that are unnecessarily high at low speeds, due to the fact that the spring force characteristics are sized for high speeds. A desmo system does not suffer from this problem, thus it would have less friction loss at lower operating speeds. Having said that, a well engineered spring system should still have similar losses as a desmo system at high speeds.

The drawback of a desmo system, versus a spring system, is that a desmo system incurs valve train friction losses throughout the entire engine cycle, since the closing cam must continually apply force to the follower to keep the valve seated. A spring system does not have these losses since the spring keeps the valve seated. Thus there can be clearance between the valve and rocker when the follower is on the cam's base circle, and no friction losses are incurred during this period.

Finally, the friction characteristics at each joint interface can vary widely during a given engine cycle, depending upon the type of motion present. A joint with constant relative sliding motion (ie. cam-to-follower) will likely always be hydrodynamic, and thus will have a fairly low and constant Mu. A joint with oscillating motion (ie. a rocker shaft) will degrade to boundary conditions at its interface when the relative motion stops and reverses, and thus will have a higher Mu during this period.

 

[ivymike]

my memory is a bit rusty... but I didn't think that the cam-follower interface built much of an oil film.

 

[patprimmer]

If Desmos use a closing cam with zero clearance, there will be problems with changing clearance or interference as valve train component temperatures change.

As I understood them, they have a light spring to hold the valve closed or they use inertia and chamber pressure to close the last few thou and then hold the valve closed.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[tbuelna]

ivymike,

Any cam-follower contact, whether flat tappet or roller, would have constant relative motion with oil present. So it would easily create conditions for EHL contact. Even a very small amount of oil is satisfactory, since the EHL oil film at the contact area is only a few micro inches thick and only a few hundredths of a square inch in area. The reason such a small EHL oil film can support such large loads is due to the fact that the film pressures developed in that EHL contact can easily exceed 50,000 psi.

Regards,
Terry

 

[ivymike]

If memory serves, a single micro-inch (25.4 microns) would be 5x as thick as the oil film at that interface under many conditions (similar story with the ring-cylinder wall interface, where the oil film is in the neighborhood of 2-5um thick).

 

[ivymike]

this software, for example, uses a greenwood-williamson model to calculate contact pressures, etc., at the interface under light load, and a contact model of some sort for higher load scenarios. The GW model wouldn't be necessary if it was safe to assume that the interface was in a hydrodynamic regime all of the time (GW is used for the thicker side of boundary lube, I think, and uses a statistical surface representation to estimate the split between load supported by asperity contact vs. load supported by the lubricant film).

 

[ivymike]

ooops... THIS software:

http://www.swri.org/3pubs/brochure/d03/valve/valve.htm

 

[ivymike]

I looked this up, because I was starting to worry about my terminology:

http://www.stle.org/resources/lubelearn/lubrication/default.aspx

The thickness of the fluid film determines the lubrication regime, or the type of lubrication. The basic regimes of fluid film lubrication are:

Hydrodynamic lubrication – two surfaces are separated by a fluid film,
Elastohydrodynamic lubrication – two surfaces are separated by a very thin fluid film,
Mixed lubrication – two surfaces are partly separated, partly in contact, and,
Boundary lubrication – two surfaces mostly are in contact with each other even though a fluid is present.

Based on the above terminology, I think that it's not uncommon for the cam-follower interface to operate in the mixed lubrication regime under load (opening, for example), with occasional excursions to boundary lubrication (perhaps during closing?). Again, my memory is rusty on this stuff. I hope it'll eventually come back if I keep picking at it - preferrably before I go too far out on a limb.

 

[ivymike]

hey, the second page of this pdf makes me feel better that there's still something shiny under the rust:

http://www.obitet.gazi.edu.tr/makale/makale/internalcombustionengines/055.pdf