Guru... The eponymous guru....gruntguru wrote: ↑Thu Nov 22, 2018 6:21 am
Notice the 50% burn angle (a number often quoted by researchers and called "CA50") ends up being very similar (about 7* ATDC) at different AFRs, ignition systems and burn rates. Likewise CA50 stays about the same for different engine speeds (bottom image). CA50 is clearly a preferred metric for "people who really know" and is obviously one that is more reliable than PCP as a predictor of MBT.
@SS and @godlameroso.
Any talk of "cylinder pressure occurring at optimum crank angle/geometry/mechanical advantage" is rubbish. Work done during the power stroke is the integral of PdV. It doesn't matter what the mechanism below the piston does. If a more favourable mechanical advantage is able to extract more work early in the stroke, there will be less work extracted later in the stroke - period!
CITATION: Attard, W. and Blaxill, H., "A Gasoline Fueled Pre-Chamber Jet Ignition Combustion System at Unthrottled
Conditions," SAE Int. J. Engines 5(2):2012, doi:10.4271/2012-01-0386.
Yes very enligthening comments there. You said it and actually it can be viewed another way: The mere fact that these testst were OPTIMIZED for igntion timing yet still all of the 50% burn off occuring at almost the same crank angle tells us Precisely that the best power is when your middle of your combustion occurs at this angle in this specific engine... No matter how advanced your timing or fast your combustion or state of fuel it all squares up at the same spot.
We can see how quick the TJI is here if you compare the gaps between the "spark" and burn angle plots we can see how small the gap is for TJI.. And yet it still gives best output when the middle of the combustion reaction occurs the same as standard spark. Not to be confused with peak pressure mind other readers..