Far and away, the most common way to open the exhaust and intake valves in vehicle engines today is via a lobed camshaft (or two). It seems to be a very mature, proven technology that works pretty darn well.
It’s not without its drawbacks, though. Changing the valve timing with engine speed is generally considered to be a desirable thing, but it’s awfully hard to do when a camshaft is doing the opening and closing. I realize that there are techniques out there for doing so (VTEC comes to mind), but they all sound horrendously complicated. Not that they don’t work, but…
Has anybody ever tried forgoing the camshaft entirely and using individual, computer-controlled actuators on each valve? I’m thinking something along the lines of computer-controlled ignition timing, which is not really all that exotic anymore.
I’m guessing that nobody has been able to make this work without some kind of cost, size or reliability problems, as it seems like the natural way to go. Can some knowledgeable soul enlighten me?
I know nothing about this but I would think that it would take a mighty big servo to open and or close the valve. I would also hate to think of the consequences of missing the hard real-time schedule this would require. Imagine if your timing belt broke as often as Windoze gave the blue screen of death.
In the IEEE Spectrum last month, there was an article about power systems for cars. The current 12 V systems do not provide enough oomph for certain tasks and they are looking for ways to boost the voltage.
Two of the reasons for raising the voltage are:
combination starter/alternators to eliminate one of the components. (which I personally think is cool)
and
tada! electromechanical valves. If you have ever tried compressing a valve spring you will find out that it takes a lot of force. I guess the current electrical systems just do not supply enough power, so until the electrical systems are improved, there will be no electro-mechanical valves.
I think a French-made Forumla 1 car had computer-controlled pneumatic actuated valves, which were powered by an air compressor that ran off an engine belt. I think it was a Renault, but can’t find a reference. If my memory is not totally crazy, it was very unreliable in practice and expensive and complicated to boot.
Back in the early 80s Cadillac made the infamous V8-6-4 engine. To improve gas mileage the engine could shut off 2 and then 4 cylinders. It used a (very complicated) electro-mechanical system. I think it was a non-interference engine and held the valves open while shutting off the fuel injectors.
I say infamous because they weren’t reliable at all. Most people just had the system disabled.
I thought the spring was there because the camshaft can only push the valves, not pull. If a servo could do both the pushing and pulling, could we get rid of those springs?
I can see no viable alternatives in the foreseeable future. The camshaft (BTW the “lobes” are called cams and that is why it is called a “camshaft”) guarantees that the valve absolutely, positively, opens on time. The job of closing the valve can be done by the spring but as RPMs increase the spring is less effective. The higher the RPMs, the stronger the spring has to be (remember F=M*A?) and there is a limit even to that as the mass of the spring also counts. Also, closing the valve at such high speed with a spring has an unintended side effect: valve bounce. For this reason, high performance, high RPM motors often have no springs and have a positive open and positive close system where a cam opens the valve and another cam closes it using a Y-shaped thingy that pulls the valve shut.
If you’re talking about desmodromic valve systems, I only know of a couple motorcycles (Ducati) and maybe one car that ever had them. I don’t know of any Formula (15,000 rpm) or other race car engine that uses a positive valve closing system, including some Japanese motorcycles that can break 20,000 rpm. Would it be possible to provide links or references to some better examples, or just how often or how many motors have these?
A friend showed me an article in a car magazine about a rotating valve system for engines. No cams, no springs, no rockers. The valves are lobes on a shaft which rotates them past the cylinders.
I’ll see if I can find more specific information. It looked pretty promising.
Peace,
mangeorge
If you have ever tried compressing a valve spring you will find out that it takes a lot of force. I guess the current electrical systems just do not supply enough power…
I thought the spring was there because the camshaft can only push the valves, not pull. If a servo could do both the pushing and pulling, could we get rid of those springs?
The Valve springs provide Valve Seal. The tighter the spring, the better the seal.
