Quote:
Originally Posted by youbetcha
I think you'll agree that when you have three cylinders working instead of 6, you have half the displacement available each revolution. And I agree that you need to fill them up with approximately the same amount of charge (the same work is being done).
So the question is, what is actually different and why is it more efficient? It sure looks like you need the same amount of air and fuel! Well ...
Keep in mind that the amount of time the intake valves are open for 3 cylinders is only about 1/2 the available time that you have with 6 cylinders. The throttle HAS to be open wider to accommodate the increased airflow requirement (about the same total airflow in a full engine revolution but greater bursts occurring in less time). Remember you have half the displacement to fill but the same amount of air.
The reason for doing this is that a "mostly" closed throttle creates much lower cylinder pressure which reduces the efficiency of the charge. Opening the throttle increases airflow when the cylinder is filled and also the cylinder pressure, increasing the efficiency.
Other technologies exist to increase cylinder pressure, like turbocharging, of course, which work in high airflow situations, but the purpose of VCM is to increase cylinder pressure at very low throttle.
The spring action of the air in the closed cylinders is a zero sum, by itself this doesn't contribute nor take away from the operation of the other three cylinders.
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Wow! And I thought I had a pretty good idea how VCM worked
before I read that. Thanks for the excellent information/education.