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]

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

 

[ivymike]

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)

 

[SomptingGuy]

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 )

- Steve

 

[ivymike]

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).

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[pmtoo]

http://www.profblairandassociates.com/RET_Articles.html

Blair wrote a series of articles for RET magazine that discuss valveing. In one of them calculating power losses is throughly explained.

 

[usac24]

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.

 

[tbuelna]

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

 

[malbeare]

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

 

[patprimmer]

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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[malbeare]

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

 

[ivymike]

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).

 

[SomptingGuy]

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

 

[patprimmer]

I though the advantage of desmo was absolute control of the valve with no possibility of float or bounce.

Regards
Pat
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[SomptingGuy]

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

- Steve

 

[thundair]

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.

 

[ivymike]

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).

 

[patprimmer]

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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[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.