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08 May 2010

Hydraulic vs bucket+shim valve adjusters

Been researching the various valvetrain designs out of a philosophical discussion on hydraulic adjusters not commonly employed in motorcycles (H-D engines are the notable exception). Fascinating!

I found out how hydraulic adjusters are currently used in various car engines and how elegantly they solve the problem of keeping valve clearances. I also read a bit about pneumatic actuated valves with cams/followers or with electro-hydraulic actuators and no cams. Clever!

Being uninformed about hydraulic valve adjusters, I did the right thing and researched a bit. I'll share a summary for those that, like me, would like to know.

Hydraulic valve adjusters are a clever solution meant to solve the problem of keeping correct valve tolerances at any engine/oil temperature. Incidentally the design led to having permanent contact between cam, lifter, pushrod, rocker arm and valve stem, making it quieter.

[I now realise that the more common cam followers+shims+buckets should equally have (more or less) permanent contact between all the parts. This happens because followers to shim clearances are filled by oil pressure that builds up underneath the buckets as the engine runs: makes sense and came from a very reliable source.]

This is an open loop feedback system in which pushrod operating travel changes by action of hydraulic lifters which, in turn, are influenced by engine pressure and/or temperature. Ingenious!

"Our" design is more on the style of "getting it right for the typical range of engine temperatures".

Two typical engineering approaches to the problem, both with their pros and cons. While it's easier to see the cons in the non-hydraulic shim+bucket style system, the hydraulic type is not without them:

Being an open loop system, it relies on correct information coming in from oil (density+type+dirt+volume=different pressure vs temperature curves) as well as integrity of the lifter itself (spring load). But even if everything else is kept, oil changes with wear and that affects operation of the lifters. Self adjusting valves gradually come out of adjustment at all engine temperatures. Then of course this is a more complex system and probably more expensive to manufacture. Hydraulic lifters are precision parts with very tight tolerances. It is hard to tell if the time it takes to get a valve significantly out of tolerance is so long that it becomes non-serviceable... I really don't know. Some people say yay other say nay.

Me, I would go for the hydraulic type, but I'm an engineer, not a business man. On that note, how about the desmodronic valve actuator system currently used in Ducati engines? Funky, eh?

Thanks for reading. Feel free to poke holes at it.

References were:

1 comment:

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