Quote:
Originally Posted by Dimman
The B in BMEP means that is measured in the same way that the torque figures used to derive it are, on an engine brake dyno. BMEP and MEP are not the same.
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Uh that doesn't change anything. I was assuming brake all along.
It is a derived figure from torque and displacement. Because displacement and torque are typically given directly it's better to just use that instead of some contrived pressure figure that doesn't actually mean anything. Again, when I hear "specific torque" in my head that immediately makes sense. When I hear "mean effective pressure" I have to first see that this is a unit of work per rotation of the engine, then work backwards and apply some silly conversion to get torque per unit displacement, all for nothing.
Anyways, you were talking about reduced friction. Again, per unit displacement. The pistons have a smaller circumference but there's also less displacement. The stroke is smaller on the 2GR which is more friction, but the bore is much larger so the piston rings have less friction per unit displacement. With the same stroke, friction (and cooling loss) is greater per unit displacement because the area/volume ratio is higher. So the 2GR actually has an advantage there. Of course the heavier pistons increase friction elsewhere but it can't be directly compared like that.
Also I suspect that inlet speed works a bit differently with a direct injector. I said a long time ago that perhaps D4-S engines don't have variable lift because the highly even distribution of fuel makes it not necessary to achieve satisfactory combustion efficiency. Afterall, the lower the inlet speed, the less pumping loss (granted, it is a relatively small amount of energy in the first place).