Types of valve actuators used for camless engines?

Answer 1

To answer your question I have written a discussion on Valve Control Techniques which includes some information on non conventional cam trains.

VALVE SPRINGS

Valve springs, though used in all conventional valve systems, are the primary limiting factor on engine speeds, and generate many unpleasant effects, such as harmonics and valve float. Without them there is relatively little friction in the valve train.

The current industry push to create a camless engine may be missing the point. The springs appear to be the problem, not the cam shafts.

Most of the systems described below still utilize springs with all their incumbent problems.

ELECTRO MECHANICAL

Electro magnets are used to open and close the valve and hold it in position once moved.

This method of valve control is often found in camless engine designs. A proponent of this technology is Valeo, which indicates that its design will be utilized in volume production in 2009.

http://www.valeo.com/automotive-supplier/Jahia/pid/1317

Some of the problems which may be encountered with this methodology are:

- Deceleration of the valve once set in motion is difficult to accomplish, and inadequate slowing down of the valve can cause significant deterioration of the valve seat and other parts. Utilizing springs to effect valve deceleration limits the engine to lower speeds and may still not effect a gentle landing of the valve on its seat at all engine speeds.

- Springs utilized in this type of system may require very careful balancing with the valve movement in order to achieve gentle valve seating at differing engine speeds. As the springs deteriorate or the engine RPMs change, the valve mechanism may become unbalanced and ultimately lead to failure.

- The electromagnets will draw a significant amount of electrical energy, which may require a higher capacity alternator, which will in turn reduce the potential fuel efficiency of the engine.

- A powerful computer coupled with complex fast-acting control circuitry and devices will likely be necessary to control the valves in real time.

HYDRAULIC

A piston driven by hydraulics is used to open the valve and compress a conventional valve spring. The valve is closed by the spring. The valve open and close is affected by valve mechanisms controlling the flow of hydraulic fluids to and from the hydraulic cylinder.

This type of valve control has been advocated in the search for a camless engine. Sturman Industries, which incorporated its design into a large truck engine a number of years ago, is a proponent of this technology. (The truck did the hill climb at Pikes Peak)

http://www.sturmanindustries.com/main/hydraulicValveActuation.htm

Various methods have been explored to utilize hydraulic mechanisms to move the engine valves. Some claim to be successful at low engine speeds, but few claim to achieve that goal meaningfully at the higher RPM requirements of passenger vehicles..

Hydraulic systems suffer from 2 inherent problems :

1) The faster a liquid is moved , the more it tends to act like a solid. A fast-acting hydraulic system to activate automotive valves at the speeds required in passenger vehicles could require immense pressures, with all the incumbent problems, including the additional energy requirements of the hydraulic pump. Even if higher engine speeds were achieved, valve movement would likely be abbreviated and not fully follow the desired or optimum lift schedule.

2) Temperatures can vary seasonally over a wide range. The hydraulic medium could change viscosity as the temperatures change, which could cause variances in the system's performance which may be difficult to control.

Utilizing valve springs to assist the hydraulic system may also prevent the engine attaining higher speeds.

In order to achieve gentle valve seating, hydraulic systems must be carefully controlled. This control may require the use of powerful computers and very precise sensors.

STEPPER MOTORS

Stepper motors are used to open and close the valves.

This methodology of valve control has previously not been successful in camless engine design due to the limited RPM range inherent in the design.

The following link indicates that some measure of success has been achieved and a camless design for a medium to slow revving engine may be feasible.

http://www.powertrainltd.com/camcontec/pdf/2005-01-0772_Presentation.pdf

Valves that open and close in fixed times, not tied to the engine speed, cannot optimize engines running at differing speeds and importantly severely restrict engine speed. This is because the degrees of rotation for the valve events increase as the engine speed increases to the point where they are no longer practical.

Powertrain claim in their documentation at the above link that their device operates the valves at 8,000 RPM with a 7mm lift. I don't dispute this claim obtained under laboratory conditions but anticipate that real world engines using their devices with 7 ms valve open and close times will have useful speeds limited to well under 6,000 RPM, perhaps even under 5,000 RPM (7 ms at 8,000 RPM is 336 degrees of rotation which is unworkable in passenger cars). The engine may be able to run at higher RPM by limiting the lift and hence shortening the valve open close time but will most likely have a lower power output than is achieved at a lower RPM due to the lessening of breathing capability.

PNEUMATIC

Systems utilizing pneumatics to drive the engine valves would in all probability not be feasible because of their complexity and the very large amount of energy required to compress the air.

I understand that some F1 engines use a cylinder of pre-compressed gas to close the valves without valve springs while using conventional cam shafts to open the valve.

VARIANTS ON CURRENT CAM TECHNOLOGY

DESMODROMIC

"The specific purpose of the desmodromic system is to force the valves to comply with the timing diagram as consistently as possible. In this way, any lost energy is negligible, the performance curves are more uniform and dependability is better." Quote from Ducati.

This form of valve control utilizes camshafts but not conventional valve springs. A long- time proponent of this technology is Ducati, which has successfully used this methodology in their motorcycles.

http://www.ducati.com/

This methodology removes the valve spring limitation on engine RPMs. However, other factors such as the use of rocker arms to open and close the valves may have a limiting effect on performance.

A desmodromic system does not normally provide for positive seating of the valves, as they are for the most part held closed by gas pressures in the cylinder. The exception could be the exhaust valve, which may be partially open in the "closed" position during the intake stroke.

OTHERS

There are a number of mechanical valve operating systems based on traditional cam technology. Many rely on an offset gear to slow the cam as it opens the valve (and speed it up when it is not opening the valve), thereby prolonging the valve-opening duration. This together with cam phasing provides a limited form of variable duration. An example of this technology is the K series engine produced by Rover.

http://www.sandsmuseum.com/cars/eli...engine/vvc2.pdf

Problems with this methodology generally stem from the complexity and size of the units, the slow opening and closing of the valves, and the limited available variation in valve opening and closing times. Additionally, the valves may have to be lifted higher than their theoretical optimum height in order to achieve optimum gas flow.

Some examples of this methodology are only provided for the inlet valves and others are limited to lower RPM ranges.

John