I'd both agree and disagree with recycled flyer-a conical piston DOES offer advantages over a flat topped one-for 360 degree porting....it is however, more expensive to make and requires a matching contra piston-thereby adding two additional machining steps in the production, many manufacturers dispensed with it in the interests of production economics. I can think of several examples-(the Allbon Dart is a good example) where early models with a conical piston crown outperform later models with a flat crown-and no alteration to porting has otherwise occurred. In cross flow scavenged and loop scavenged layouts it offers no advantage (which is why the Super Tigre and Enyas didn't use it-and now that schneurle porting is virtually universal, again flat topped pistons are the norm.
Also pertinent is that diesel ignition is bordering on a detonation, not a defined burn as in a glow engine, and you have another reason for not wanting localised hotspots.
Recycled and I agree on the main purpose of a baffle.....the straight fence one though is dictated purely by production economics-no one would argue that it is an efficient design. The better performng cross flow engines (think McCoy 60, Dooling 61 etc) had quite complex piston crown shapes-and the baffles were curved and contoured quite considerably.....all in the interests of better gas flow.
For brokenenglish-the Mills and Comp Special pistons may well have used the cutouts to alter transfer timing-but with that would inevitably come imposition of directional effects on the charge as the port opens-the incoming high pressure charge sees only a narrow contricted exit which is oriented towards the upper cylinder-which is essentially what is required in any case-we don't want the cool high pressure incoming mixture mixing with the hot downgoing expanding exhaust gases if we can avoid it. By the time the transfer port fully opens, the exhaust is already fully open and gas pressure has dropped significantly. The quest of the 2-stroke designer has always been to maximise cylinder filling, maximise scavenging and minimise fresh charge loss during scavenging-all of which are mutually exclusive to a degree-though I'd argue that modern designers have done a pretty good job to date....
ChrisM
'ffkiwi'