Dynamic Compression Ratio

[Whittey]

I was reading this webpage:

http://cochise.uia.net/pkelley2/DynamicCR.html

And the two blue bulleted points made me pause for a second. He says:
The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine

Either one can be explained in the opposite direction. For the first, wouldn't a sufficient amount of boost raise your DCR above your SCR? Say you had a motor that displaced 1cid. Say also that the combustion chamber was 1cid. That would be a 2:1 compresion ratio. If you added 14.7psi of boost, you would have 4:1 compression, which is more than its SCR. Yes? No? (always assuming perfect world, of course).

As for the second, does not the dynamic compression increase as rpm's go up? Say for instance you have a 4" stroke and a real short rod. If the intake valve closes 20 30 degrees ABDC, the incomming air has X amount of time to turn around and escape the cylinder at 1000rpm. If you're spinning your motor at 2000rpm, the air has half the time to turn back around and exit stage left. Does this make sense or am I missing something here?


-=Whittey=-

 

[ivymike]

I think you're missing the first paragraph of that article, where he defines DCR as follows:

Dynamic Compression Ratio...uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder

Perhaps it would make more sense if you called it kinematic compression ratio, or something along those lines. The only way DCR as defined above will change while running is if the intake valve timing changes or the combustion chamber volume changes. DCR can't be bigger than SCR, because they're both based on the kinematic engine geometry. SCR is Vol.BDC/Vol.TDC and DCR is Vol.IVC/Vol.TDC . There is no way that Vol.IVC can be greater than Vol.BDC, assuming that nothing wierd is going on with the engine geom.

To answer your questions (in case the above didn't):
No, DCR as defined above does not care what the pressure of the air in the cylinder is, so no amount of boost will make a difference.

No, volumetric efficiency doesn't affect DCR either.