Quote:
Originally Posted by serialk11r
Say you had a variable lift system that controlled the rocker ratio rather than using a secondary rocker to vary lift and duration together. That would essentially be giving the engine independent throttle bodies at the valves.
This would still reduce pumping loss because 100% of the air under vacuum is contained within the cylinder and will thus return more energy to the system upon compression. With independent throttle bodies, the throttle is leaking air through all the time vs. only during the intake stroke, so it doesn't have this benefit since the throttle valve must be closed further to reduce pressure to compensate for the "leakage". With the typical plenum and single throttle body setup, you essentially just have a consistent reduced pressure atmosphere after the plenum, and a quick PV diagram lets you see why it doesn't recover as much energy on compression.
The "best" solution is considered to be fully variable duration late intake valve closure since it has the least pumping loss, but it is the most difficult to implement. Early intake valve closure is easy because the systems that reduce lift basically "waste" a portion of the cam lift, all you need for that is an extra rocker that can engage at varying "heights" from the cam's center, but the extra valvetrain mass increases friction, especially at higher engine speed (which is why BMW does not use Valvetronic on the S65 and S85).
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Nissan and Toyota do use higher rpm engines though. BMW systems are groundbreaking but they are never simple like the Japanese who later improve upon their ideas.
What do you think about electromagnetic controlled valves (magnetic actuators)? I think it is feasible and could completely eliminate the mechanical linkage (camshaft/springs/etc). The ECU would only then have to control the electric pulses. The only downside I see is what happens if the engine loses electric power then you might run into bent valves as they might not return to their previous position. I guess you could use capacitors to store enough energy to limp the car to a complete stop while still having valve movement.
EDIT: Here is a cool little article that someone combined sources from into one answer:
http://wiki.answers.com/Q/Working_of...valve_actuator
Then all we would need is magnetic bearings for the crankshaft and we could almost eliminate the need for oil except for piston lubrication.