Why DOHC and not SOHC ??

[oldcorollas]

I can see a rather obvious improvement to the cam over bucket design just by looking at it.
Make the bucket with a dual loop OVER the cam(like handles), but slightly to either side of the lobe. Then create 2 small closing lobes on either side of the main lobe.
Assuming the bucket can be affixed to the valve or collets, it'd work, I'd say fairly well if it could be made strong enough.
Lash might be a pain to worry about, but perhaps inertia will help out with the last few thou.

I'm just going to assume someone's thought of it, so why isnt it used?

you mean Ducati Desodromic??



umm.. if you make such a large bucket... a) how do you install it, and b) how does it decrease weight in the system? and you would need a helper spring to hold seat pressure. even the ducati desmo has a helper spring for seat pressure..

[myne]

That's more or less it - why isnt it used in more engines?
Patent? Not reliable? Minimal improvement? Cost?

[Nim]

That's more or less it - why isnt it used in more engines?
Patent? Not reliable? Minimal improvement? Cost?

How is that reducing weight or moving parts? A standard lash adjuster setup looks better to me.

[myne]

It removes the springs from the eqution.
Springs dont respond with the direct mechanical force of a rocker. They're elastic, they're slow, and thus have to be sprung significantly higher as the RPM requirements climb to compensate for their slow response. The stiffer the spring, the more energy is lost compressing them.
A rocker pushing it up has absolutely predictable results. The valve will climb at exactly the same relative timing at 40,000 rpm as it will at 1000rpm. It can also NEVER suffer valve float. Springs simply cant do that.

[Nim]

Oh I see... okay that's actualy a pretty good idea. You could attach a VTEC like system to it pretty easily too.

[Starfire]

That's more or less it - why isnt it used in more engines?
Patent? Not reliable? Minimal improvement? Cost?

I believe that all of the above are limiting factors. It also tends to make for quite large heads.

A full solenoid setup will give the best possible results for a 4 stroke piston engine as lift and duration are fully configurable across the entire rev range and you dont have to deal with the insane spring pressures of a high rpm head. The two things that wear/fail in a performance engine are usually valve springs and cam lobes.

The problem with solenoid setups at the moment is that the components required either dont exist or are very expensive and the control systems are not yet up to the job.

[JustCallMeOrlando]

That and the fact the current requirements on conventional 12V automotive systems are stupid high. From what I read car manufacturers are holding off till the higher voltage (40+) systems in the works.

[ViPeR_NiPPleX]

they could do a split 12v / 40+v system... would be bulky tho and if they used switch mode to step down 40+ to 12v, i think it would be quite unreliable with the load/current requirements

[oldcorollas]

That's more or less it - why isnt it used in more engines?
Patent? Not reliable? Minimal improvement? Cost?

i guess you've never seen a ducati sitting at the lights then?















with the rider looking down trying to decide if that much noise is normal or a problem

[Dimitri]

they use desmo on tvrs too.
engine designers have their own bias and influences, ofcourse they get breifed on specific capabilities that there designs must have, but as long as they can achieve whats required then therye going to stick with what they know.

with ther new toyota stuff i am guessing the designers are getting some pretty specific requests as far as efficiency is concerned.

i think thats why sometimes you will see a design that seems flawed or maybe not the best solution for the job. design breifs arent always going to reflect specific features that we would like to see in an engine, because we have (or rather think we have) differant focuses than car manufacturers.

i personally am into engineering over design, as alot of you, whether you know it or not, are also. and this to a lessor extent i guess could be said about engine designers, they will often opt to re engineer rather than re design.

i dont even know where im going with this any more, bah.

[oldcorollas]

exactly, toyota now must design for most efficient (as we all should anyway )
less losses = more power = less fuel used = less emissions.

honda is a good example of using the minimum amount of material to achieve a specific goal, which is why their outputs are so high, but do not respond well to "overuse". the 3S BEAMS as well as 2ZZ are good examples of using lighter parts (titanium valves) to reduce inertia to reduce spring size and thus losses, increasing power and reducing emissions

[Hen]

Just a quick note on hydraulic valves rather than cams. BMW offered a hydraulically operated valvetrain in one of its 3-series cars a few years back. All I can remember is the ads pointing out that the engine could do away with a throttle. Air intake was adjusted using the valve lift and timing.

Just had a look and Google turned up "valvetronic"

Hen

[oldcorollas]

not hydraulic as such, still uses a cam and springs... just variable thingy between cam and rocker

[Hen]

Oh crap. I'd read a bit on hydraulic valve operation, and when I saw the BMW thing I assumed they must do it that way. Back to my corner...

[river]

Hi,

You could always ressurect the sleeve valve. It was used with great success (after they ironed the bugs out of it) on aircraft engines in WW2. Also, I think there were a handful of sleeve valve car engines. However, I think IIRC that there is still some moving (rotating) metal that had to change directions, but I'm not sure if it had the same mass/inertia as valves.

seeyuzz
river