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Valve Spring Power Consumption

Valve Spring Power Consumption

Valve Spring Power Consumption

(OP)
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?  

RE: Valve Spring Power Consumption

You are missing measured data and data from (sensible) simulations.  It's impossible to directly measure the power consumption via a with/without test (as you would with A/C) so the numbers are often based on FITA.  The snake oil guys use this all the time.

There are software tools out there that can be used to estimate camshaft power losses.

- Steve

RE: Valve Spring Power Consumption

I have a hard time believing that the net work done is not closer to zero.

There are software tools out there that can be used to estimate camshaft power losses.

You will find that the software also has trouble believing that the net is not closer to zero... (which seems the only reasonable conclusion, given the logical consequences of several alternative hypotheses - consider, for example, the temperature that the springs would have to reach if asked to dissipate 100hp in their usual environment)
 

RE: Valve Spring Power Consumption

I think the OP was talking about valvetrain total power consumption, not the springs themselves.  Unless your post was one of your more obscure jokes (which I do enjoy smile)

- Steve

RE: Valve Spring Power Consumption

well, perhaps he can clarify.  I've certainly seen it claimed on the internet that the springs themselves consume terrific amounts of power (and so we should all go with rotary valves, etc).
 

RE: Valve Spring Power Consumption

Ideally springs themselves absorb zero power. Increasing the spring rate and/or preload increases the power absorbed in the valvegear due to friction and so on in bearings and sliding surfaces. All of the energy used to compress the valvespring itself is regained as it expands, except for some tiny proportion abosrobed in internal damping of the material.

  

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Valve Spring Power Consumption

(OP)
The question in my original post was in fact related to spring losses only.  It was a topic that I had heard from some very respected engine builders through the years, and quite frankly took their word at face value (I know, question everything).

I was looking at a desmondronic valve train, when I decided to calculate these purported  losses myself.  I found that yes for the scenario I was looking at, that yes, nearly 100 HP was required to compress the springs, but then quickly realized that power was returned to the system on spring closure.  Duh!

If I had not held those builders in such high regard, I would have probably done this exercise years ago.  

RE: Valve Spring Power Consumption

usac24,

The valve springs themselves do not have any real power loss.  They are elastic devices that acquire strain energy when compressed, and release that strain energy when relieved.  The mechanical losses in the valvetrain are primarily due to the friction created at all of the sliding contact interfaces.  Sliding contact occurs at the cam/follower interface, the rocker arm pivot point, the rocker/valve tip contact, the pushrod/rocker contact, the valve stem/valve guide contact, etc.  

The magnitude of the force produced by the valve spring should not be any greater than that required to resist the instantaneous inertia forces acting to separate the cam follower from the cam working surface.  Since the valvetrain friction losses are due to inertia forces, a desmodromic system, is in practice, no more efficient than a conventional valvetrain with springs.

As a component of total piston engine friction losses, valvetrain losses are not too significant.  The majority of engine friction losses are due to piston rings and piston side thrust.

As a crude estimate, let's say you have a total spring force (from all the springs in the cylinder head) of 1000 lbs at any given instant in the engine cycle.  That 1000lbs of spring force is producing a friction at the cam/follower interface, which is .75 inches from the cam rotation axis.  Assuming a coeff. of friction of 0.03, that would produce a torque of 1.88 ft-lbs.  At a camshaft speed of 2000 rpm (ie. half of engine speed for a 4-stroke engine), that would amount to only 0.72hp.  Not a big deal.  My numbers may be off, but I'm sure you understand the principle.

Regards,
Terry

RE: Valve Spring Power Consumption

Would it be true to say that at low rpm the losses from the valvespring are minimal as the spring returns energy on the rebound but as higher rpm levels are aproached the spring returns less of the rebound energy as the follower movement aproaches the accelleration rate of the spring rebound. Energy is taken up accelerating the upper half of the spring mass. Beyond the point of equal accelerations you then have a valve float situation and nill energy return by the spring.?

Malbeare

RE: Valve Spring Power Consumption

Have I missed something.

I thought friction and inertia were very different things and I always attributed a significant amount of the valve train power losses to each and a very small amount to differences in the energy taken to compress a spring vs that returned.

Regards
Pat
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RE: Valve Spring Power Consumption

perhaps another factor not usually picked up on is the amount of energy required to knock the exhaust valve off its seat  
, after all there is about 50 to 100 Psi gas pressure depending on throttle position, residual pressure left in the cylinder after the expansion stroke. a 35mm valve has 962.5 square mm
If you just run the head or valvetrain with an electric motor and measure the amps then this factor is not shown.
malbeare

RE: Valve Spring Power Consumption

as mentioned earlier, there is software which can fairly accurately calculate such losses (including those due to opening against gas pressure).  Valdyn from Ricardo is a package I've used in the past, and I was generally very satisfied with its ability to predict the behavior of the system (seating velocities, onset of spring surge, onset of separation, loss of chain tension, excessive chain force, etc).
 

RE: Valve Spring Power Consumption

Quote:

Since the valvetrain friction losses are due to inertia forces, a desmodromic system, is in practice, no more efficient than a conventional valvetrain with springs

This is not what the desmo boys say.  They say that conventional valvetrain power requirements are proportional to speed (friction dominant), whereas desmo requirements are purely inertial.  So there is a crossover point.

- Steve

RE: Valve Spring Power Consumption

Absolute control is a given.  Lower power consumption looks like it could be an unexpected (possibly even counter-intuitive) advantage.

- Steve

RE: Valve Spring Power Consumption

I know that I can turn the cam in my Ducati with index and thumb maybe 8# of pressure.
Try that on something else and you will reach for a mechanical advantage.
I was able to speak to some of the Ducati engineers at a world rally, and they both admitted that desmodromic started out because of inferior valve springs, but today it is a marketing point more than anything else.
I also remember a project with Pontiac that was about harmonic dampening in springs the unrelated find that there is a power loss as heat developed in the spring.

Cheers

I don't know anything but the people that do.

RE: Valve Spring Power Consumption

yes, of course there is a small loss...  even more of one if you intentionally design the spring with an internal spring to rub against (to reduce surge).
 

RE: Valve Spring Power Consumption

In a very high output engine, the valve springs can be noticeably hotter than the oil, but not so much that they smoke or boil the oil, at least not in my experience which is based on observation but not actual measurement.

Re the difficulty to turn a cam against valve springs. While it is difficult to turn against the spring as they compress, it is also diffcult to hold he cam back as the follower goes over the nose and tends to drive the cam indirection of rotation.

Regards
Pat
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RE: Valve Spring Power Consumption

The inertial forces for a fixed displacement system will increase as speed squared, so the power absorbed in friction will increase as speed cubed, if the coefficient of friction stays constant which it won't.

So on a power absorbed basis (not why they are used) their advantage wrt to springs should dissappear at higher speeds.



 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Valve Spring Power Consumption

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

- Steve

RE: Valve Spring Power Consumption

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

RE: Valve Spring Power Consumption

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)?   

RE: Valve Spring Power Consumption

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.

RE: Valve Spring Power Consumption

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

RE: Valve Spring Power Consumption

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
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RE: Valve Spring Power Consumption

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

RE: Valve Spring Power Consumption

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).
 

RE: Valve Spring Power Consumption

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).
 

RE: Valve Spring Power Consumption

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.

 

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