Electro Servo Valve: Reasons Why Not Used?

[wolram]

A DOHC engine has the valves controlled by two servo motors, an engine speed encoder controls the valves possitions via electronics, this is an obviuos solution to four stroke ice valve control, what are the reasons why it is not used?

 

[Danger]

I don't quite understand the system that you're referring to. If the valves are controlled by servos, why does it have cams? Anyhow, if you mean servos instead of cams, I can't envision them being fast enough. Or do you mean that the servos control the cam profile?

 

[OgreOwner]

Electronics versys mechanics

Mechanical devices preceded electronic devices by quite a few years. Before electronics could keep up with the needs of an internal combustion engine, cams and pushrods had become such a mature technology that it is considerably cheaper to use them than to use an electronic solution. As the price of the electronics becomes lower, that may change. In one of the recent issues of Home Shop Machinist there was a serial article on building an engine with valves controlled by solenoids.

Karl Peterson

 

[Danger]

Hey, Karl;
This is the first time that I've seen your name, even though you seem to have a few previous posts. Welcome aboard! Does that Home Shop Machinist article refer to helinoids or regular solenoids? I've never even heard of the publication before.

 

[wolram]

The idea, (a first step) is to drive the cam shafts with servo motors, this would allow the cam shaft cycle to be maped, the profile of the cam would be less important, (it could be just an off center circle), the servo motor could ramp up or down at the required times to give maximum conrol of induction and exhuast phases.

 

[turbo]

Interesting idea. This would allow the inlet and exhaust valves to be timed differently, and that could result in performance gains over DOHC arrangements in which the cams are in lock-step with the crankshaft and the ignition curve is varied to provide optimum power at various rpms and loads. If a desmodromic valve actuation system were incorporated, that would eliminate valve float and greatly eliminate cam/follower wear due to the friction caused by valve spring pressure.

 

[wolram]

Turbo, the next step would be to do away with poppet vales, imagine a hollow tube sealed at one end with a port machined into it, the open end would either lead to carb or exhaust, this tube would run in the cylinder head casting and be driven at half engine speed via servo motor, the tube would be ceramic coated and close tolleranced, there may be some leakage while the engine is new but like the sleve valve engine would improve as carbon deposits are built up.

 

[turbo]

Baby steps, Woolie! Multivalve heads are very well-devolped now, and can provide a very efficient distrubution of air-fuel mixtures. I think you've got a really good idea, though, since servo-driven camshafts in a DOHC application can allow the intake and exhaust valves to be timed differently. This would allow the engine to be tuned for maximum mileage, maximum performance, or a mix of these, with a flip of a switch. If you need extra valve opening overlap to achieve high exhaust scavanging rates at high rpms, you can make it happen. You could probably also make it possible to specify the RPM at which maximum torque is developed.

 

[wolram]

Turbo, the problem i have is servo motor size and power requierments, i guess the electronics could be minimalized once the cam (proifiles) are found
to the best advantage, i just worry that electrical power saped from the engine would overcome any advantage.

 

[turbo]

Turbo, the problem i have is servo motor size and power requierments, i guess the electronics could be minimalized once the cam (proifiles) are found
to the best advantage, i just worry that electrical power saped from the engine would overcome any advantage.

With a desmodromic valve actuation system, there would be no drag imposed by the friction caused by the rocker arms/cam followers overcoming the force of the valve springs. The electrical drain on the system would likely be paid for by the losing the mechanical drag of a conventional valve train. You could also contemplate a mechanically-driven DOHC system with electronic advance/retardation.

 

[wolram]

Turbo, to the best of my ability i will follow this idea, once i have found a suitable engie to modify, the problems of a tyro inventor will be many, so i hope i can ask questions and maybe share any advatages gained.

 

[Danger]

Good luck with it, Woolie. Sounds interesting. I really don't think that the flow characteristics of a 'sleeve valve' system would be practical for an engine, but you should check with Clausius.

 

[wolram]

Good luck with it, Woolie. Sounds interesting. I really don't think that the flow characteristics of a 'sleeve valve' system would be practical for an engine, but you should check with Clausius.

What do you think of coupling the servo motor to an indexing gearbox to
provide cam dwell ?

 

[Danger]

I'd pretty much need a much more elaborate description, or preferably a sketch, to form an opinion. I can't quite envision the mechanism.

 

[brewnog]

Woolie, I think the reasons your original idea would not work are due to the speed at which such rotary servos can actuate. Obviously such a system would provide endless opportunities for variable timing and lift.

The favoured method of actuating this kind of system is to do away with the camshafts, but to have linear solenoids operating the valvegear. Currently, such actuators are horrendously expensive; those on the experimental engine I've seen cost something like £18,000 each.

Nice idea though, I'll keep a lookout for any advances (pardon the pun).

 

[brewnog]

You could also contemplate a mechanically-driven DOHC system with electronic advance/retardation.

Yes, and with variable lift I suppose too.

Have you seen anything like this? The only thing I'm aware of is the Ferrari-style arrangement whereby the camshaft moves along its axis through different lobe profiles.

 

[denni89627]

Yes, and with variable lift I suppose too.

Have you seen anything like this? The only thing I'm aware of is the Ferrari-style arrangement whereby the camshaft moves along its axis through different lobe profiles.

I've never heard of this but it sounds very interesting, and promising. It would be a good way to achieve max economy and max power for a given engine. I suppose when you "get on it" the computer would push the camshaft so the valves ride off a different part of each lobe. How is the cam driven? A chain with enough slop to allow for the movement along its axis? I guess it wouldn't have to move too far, quarter inch or so at a guess.

 

[brewnog]

It'll be chain or belt driven, but probably uses a splined arrangement to allow the camshaft to slide relative to the cam gear; a chain with 'slop' would be misaligned and not capable of maintaining correct timing.

The lobe profiles vary from left to right, continuously, providing a mild profile at one end and an aggressive profile at the other. I would imagine the movement is 20mm or so.

Interestingly, googling the subject returned a physicsforums thread which several of us had posted in. Happy days.

 

[wolram]

It'll be chain or belt driven, but probably uses a splined arrangement to allow the camshaft to slide relative to the cam gear; a chain with 'slop' would be misaligned and not capable of maintaining correct timing.

The lobe profiles vary from left to right, continuously, providing a mild profile at one end and an aggressive profile at the other. I would imagine the movement is 20mm or so.

Interestingly, googling the subject returned a physicsforums thread which several of us had posted in. Happy days.

Hi, brewy, if my idea works there will be no need for a cam, yes i know every thing under the sun has been tried before, but i think the only thing that will kill this idea is unequal thermal expanssion, (this is stage 2 ) of my idea not the servo drive, part of the idea is my (valve) only moves when needed.

 

[MedievalMan]

Guys, I did a project for my 4th year capstone in EE on a electrically actuated valve system on a small IC engine.

The main limitation we had, was not control of the solenoid/valve assembly (which position control might be tough, but velocity control is definitely needed to ensure a softer landing, or even in more advanced stages, fit exactly a certain CAM profile.)

Our main problem was power: even with the best solenoids we could buy off the shelf, with 2 car batteries in series (24V), it couldn't bust out enough oomf to open the valves quick enough to even reach idle speed (~1000 RPM).

The designs used in R+D by the car manufacturer's use custom designed actuators, with a solenoid for opening, and a solenoid for closing. They probably use some mean permanent magnets or tightly wound coils to get lots of oomf without making the solenoid/valve assembly huge (as the weight of the solenoid/valve is an issue.)

For someone who's been doing this for years, see:
http://rbowes1.11net.com/dbowes/

David Bowes EVIC engine.

 

[MedievalMan]

Mechanical advantage might help.

However, I think at low engine speeds, it will help give you enough force to open the valves consistently (as the gas force opposing opening the valve varies with many different factors -- to do this right, you probably need to model this force or at least get an accurate maximum value for it).

However, at higher speeds, the mechanical advantage would be limiting the speed of your valve assembly.

 

[brewnog]

Our main problem was power: even with the best solenoids we could buy off the shelf, with 2 car batteries in series (24V), it couldn't bust out enough oomf to open the valves quick enough to even reach idle speed (~1000 RPM).

The designs used in R+D by the car manufacturer's use custom designed actuators, with a solenoid for opening, and a solenoid for closing. They probably use some mean permanent magnets or tightly wound coils to get lots of oomf without making the solenoid/valve assembly huge (as the weight of the solenoid/valve is an issue.)

Which is precisely why the solenoids alone in the Lotus development engine I toyed with cost over £18,000 each!

Thanks for that link Medieval Man, I'm sure Wolram will mess his bed when he sees it.

 

[brewnog]

Oh, and I think you'd need an absolutely huge amount of mechanical advantage to contend with peak cylinder pressures with the kinds of solenoids you're talking about!

Just read yours was an EE project and not an ME project; fyi a typical automotive engine will easily hit 15-20MPa peak cylinder pressure. Fag packet calculations: With a valve diameter of, say, 25mm, you're looking at nigh on 10,000N peak force, which should cause a warm glow of admiration amongst you for the valvetrain designers, let alone whoever got the solenoid system to work nicely. (Oh, and that's 10,000N a few thousand times a minute!)

 

[MedievalMan]

Well, our project still worked, kinda.

It did what we proposed to do: with some solenoids, we got the engine to idle speed a few times, but barely.

The gas forces are quite complex.. I'm not sure how you would design around them. The easiest way, barring physical limitations, would be a speed/position feedback of each of the solenoids, such that more power is applied if the valve doesn't move.

That's going to be very costly though...

My research right now is basically getting position/speed/load torque information from a dc servo drive equipped only with a current sensor, using a variety of methods, including state estimation -- to use with an electric power steering system, to improve it's fault tolerance.

Theoretically, then, there might be a way to get the velocity of the solenoid knowing it's model, and the current through it. I'm not sure how well such state estimation algorithms would work with such a high bandwidth system (noise might become an issue here.)