Quote:
Originally Posted by Daox
I read most of the articles Chaz posted. I don't think we can say variable lift technology 'virtually eliminates pumping losses'. It sounds like they've just moved the restriction from the throttle body to the intake valve. Something still has to control how much air gets in, and they're still using the intake valve as a restriction. Its definitely an improvement, but I just don't think we can say pumping losses are virtually eliminated. I think reduced is a much better word.
The only exception I can see is if they're using something similar to Toyota's atkinson-like cycle (used on the Prius/Camry hybrid) where the intake charge is partically exhausted out of the intake valve.
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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).
Luckily, if we're willing to give up low end torque, cam phasing alone with Atkinson cycle (aka high duration intake cams) can get us pretty low throttling losses. Say you have a 285 degree duration intake cam. If you retard the intake valve opening point to say 10 degrees after TDC (the higher the rod/stroke ratio the better to reduce pumping losses here), the intake valve closes at 295 degrees after BDC. This means you're bleeding off ~60% of the intake charge! Thus to idle you only need to have maybe 50kPa vacuum instead of 80, and when cruising on the highway you only need 20kPa or less vacuum.