# How much power does vavletrain use?

## Main Question or Discussion Point

How much power does valvetrain use?

I haven't been able to find a straight answer on this but when concerning the valve springs in particular, how much engine power does it use to actuate them? I was thinking the power used to push was used to push back as the lobe rolls over, so there would be no real power loss, but I am not sure. I found a formula one car page that says it uses 25% of the power at idle to run the valves, but that was pneumatic valves.

If there is a loss, how much does spring pressure use? As in, comparing a 50lb spring to a 60lb spring.

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Mech_Engineer
Gold Member
The power loss is due to friction or other unrecoverable losses in the valve train. You're right that just compressing the springs doesn't really account for the power loss because they push back once the lobe rolls over; however the weight of the springs does increase friction.

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I just wonder how much power it uses. You would think the camshaft would get really hot if it wasn't for oil cooling it.

Basically I have 2 identical engines, but one has 12 more valves and the springs are 10lbs stiffer. So one engine has 12 valves and the other has 24. Not only does the 24 have 12 more springs but it has 10lbs more on each spring. Do you think it would account for a 2mpg difference?

S_Happens
Gold Member
As in most cases, there is no "straight" answer. It depends on what type of valvetrain (OHC or OHV) and the specific components. I've never run across anything where people have made much progress in predicting the losses (except in cases where existing data is already available), although I have seen examples using a Spintron machine to measure the actual losses.

Personally I'd start googling "Spintron" and your specific application to see if there's any data that's already been recorded to get you in the ballpark.

With all the zillions of things happening in an engine all at once, that all affect each other, there is no way we could realistically answer the question.

"Could this account for a 2mpg difference".

If your question is, are the vavles solely responsible for this change, the answer would be no. If the question is are they a contibuting factor, the answer is yes. How large of a contributor would require a full investigation.

Ah I see. Thanks for the help.

AlephZero
Homework Helper
With all the zillions of things happening in an engine all at once, that all affect each other, there is no way we could realistically answer the question.
That isn't really true, because engine designers do exactly this type of calculation to work out how strong the camshaft needs to be, how much oil flow you need to cool it, and so on.

However you won't get the right answer just from the stiffness of the springs, because there are very high accelerations on the moving parts (hundreds or maybe even thousands of G) so large forces are required to move even small parts like cam followers.

That isn't really true, because engine designers do exactly this type of calculation to work out how strong the camshaft needs to be, how much oil flow you need to cool it, and so on.
This doesn't make sense unless you thought I was answering the question 'how much power does the valve train use'. If you'll look at the post again you'll see that I was responding to was:

Do you think it would account for a 2mpg difference?
Which you cartainly can't do by looking at the valve train alone. As you've not only increased spring stiffness but you've introduces twice as many valves, which will alter basically everything from intake, mixing and exhaust.

I don't know how much, but the valvetrain does make a difference. Some higher-performance cars, for example, use roller cam followers to try to reduce the friction. Oil is obviously absolutely crucial, as the forces required to accelerate the valves at what is probably thousands of Gs are quite high.

Also, I noticed engineers try not to put in a high "safety factor" for how strong the valve springs are. If the engine red-lines at a certain rpm, you might be able to go only a few hundred rpm more before the valves start to float and result in engine damage. If you make the springs any stronger than they need to be, you end up paying too high a price in efficiency.

I wonder why there are no rotary valves (at least I'm not aware of any) being developed.

You'd have to guess that it would vastly reduce the size of the head and eliminate the need for a camshaft whilst getting rid of this problem. I suppose it would make valve overlap difficult.

S_Happens
Gold Member
I wonder why there are no rotary valves (at least I'm not aware of any) being developed.

You'd have to guess that it would vastly reduce the size of the head and eliminate the need for a camshaft whilst getting rid of this problem. I suppose it would make valve overlap difficult.
Something like this?

http://www.coatesengine.com/csrv.html [Broken]

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