Quote:
Originally Posted by roflwaffle
What I'm saying is that it doesn't matter if the throttle or the valve limits air flow into the cylinder, either way the engine will see pumping losses of the big kind. By that I mean the ones associated with having the cylinder not fill with gases all the way. EGR in modern gasoline engines allows for pretty decent efficiency above 30-50% throttle, but below that, since the cylinder can't fill up with air, BSFC gets progressively worse. Here's a fairly new gasoline engine, and here's a heavy duty diesel. Notice how the diesel is fairly efficient over the entire range of power that it makes, which the gasser isn't until the torque it's making/air it's pulling in is sufficiently large. Those low load losses with gassers are what I refer to when I'm talking about big pumping losses. Little pumping losses are volumetric efficiency at different engine speeds. So, while changing the lift may help flow at certain engine speeds, for a few percent increase in efficiency, changing load via gearing, or other methods, can change efficiency much more than a few percent.
That being said, changing the lift after changing the gearing may result in way more benefit since changes in VE are magnified at high load, at least that's what I've read. |
OK, I've got you now. One of the reasons that a diesel has better volumetric efficiency is that it runs all the time at WOT. So there are much fewer pumping losses caused by a throttle plate, and as you point out it is all from the valve in that case. In a gas engine, we have both the throttle plate(s) and the valves. So, would this change make any difference? I suspect it will, but not that much. (Obviously, there is some relation flow-wise between the two, and if we have any engineers on this thread maybe they can teach us the equations.) BTW I have already taken care of low-hanging fruit like gear ratios, etc. so I am at the point where I can try tweaking a few things, or else I just yank out the entire motor and either rebuild it to high-mpgs standards, or replace it with something else. This is more time and money and I am not going there now. This mod is also very easy to implement and totally reversible, so no harm done and we learn something -- if I do it.
Is there anything in common with this mod, and the Miller cycle? In the Miller, IIRC, the intake is kept open way longer so some reversion occurs.