Variable valve actuation
Variable Valve Actuation, or VVA, is a generalised term used to describe any mechanism or method that can alter the shape or timing of a valve lift event within an
internal combustion engine. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camlesssystems.
The valves within an internal combustion engine are used to control the flow of the intake and exhaust gasses into and out of the combustion chamber. The timing, duration and lift of these valve events has a significant impact on
engineperformance. In a standard engine, the valve events are fixed, so performance at different loads and speeds is always a compromise between driveability (power and torque), fuel economy and emissions. An engine equipped with a variable valve actuation system is freed from this constraint, allowing performance to be improved over the engine operating range.
Please note: the remainder of this article has not been written yet, although the categories have been defined:
Types of VVA System
Mechanical Variable Lift
Cam Summing Systems
Reciprocating rocker Systems
Lost Motion VVA Systems
Conventional engines operate with one or more camshafts controlling the intake and exhaust valves. As the valve timing is fixed it will not allow for optimum engine performance over all RPM ranges. Engine performance can be enhanced by varying valve open and close timing to suit the engine RPM.
Valve Control Techniques
This method of valve control is often found in camless engine designs. A proponent of this technology is Valeo, which has indicated that its design will be utilized in volume production by 2009.
In this design the valves are opened and closed and held open or closed by means of electromagnets.
Some of the problems which may be encountered with this methodology are:
- Deceleration of the valve once set in motion is difficult to accomplish. This difficulty is exacerbated by the greatly increased magnetic pull of the magnets as the valves approach the end of their travel (magnetic pull increases dramatically as the distance from the magnet decreases). 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 on its own will 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 speed changes, the valve and spring balance may be compromised 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.
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)
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 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.
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.
Valves that open and close in fixed times 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 “to 8,000 rev/min and beyond”. This figure was likely obtained under laboratory conditions. Real world engines using their devices with 7 ms valve open and close times may 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 currently 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.
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.
It has been said that some F1 engines use a cylinder of pre-compressed gas to close the valves pneumatically.
Cargine Engineering AB, a Swedish Company, has produced pneumatic valves and have fitted them into several different engines. One of these test engines is running in a test SAAB 2-5. [http://www.cargine.com]
Some links to explore
Effect On Engine Performance
Power and Torque
HCCI and CAI
Variable Valve Timing
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