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Old 03-15-2013, 05:03 PM   #14 (permalink)
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Centroid of heat release is a better metric to use for combustion phasing than the crank angle of peak pressure. This is particularly true for common rail diesels were you can have multiple injection events in the same cycle (3-4 is not uncommon).

Consider the traces in the middle left of this graphic:

FYI, "AHRR" stands for "Apparent Heat Release Rate".

You can see that there's two "humps"--one from the compression and one from the combustion. A slight variation will make one or the other the higher peak and alter teh crank angle of peak pressure by 20 deg or so, even though the combustion hasn't really changed much at all. Infact, the irony is that very late combustion phasing (as in the middle right plot) cause the angle of peak pressure to occur earlier (at TDC from compression).

Also keep in mind that with modern common rail diesels, the combustion process is "shaped" via many injection events in the same cycle (3-4 is common, although I've seen more). It's not just a matter of squirt-it-in-let-it-go-boom anymore.
Diesel Dave

My version of energy storage is called "momentum".
My version of regenerative braking is called "bump starting".

1 Year Avg (Every Mile Traveled) = 47.8 mpg

BEST TANK: 2,009.6 mi on 35 gal (57.42 mpg):

Last edited by Diesel_Dave; 03-15-2013 at 05:08 PM..
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