piston guy wrote: ↑Mon Feb 21, 2022 12:05 pmIt's interesting Jack that when I created the spherical design everyone scoffed at it. I tried to get the company I was working for at the time to patent the design but they chose not to. BME , CP and others began doing them too. The torque increase was more than the HP by almost double and doing that with two degrees less timing told me we were more efficient. I haven't seen a Nascar piston for about ten years so I have no clue what they are doing now. The spherical design is still good and being used by some piston companies. It does favor a small combustion chamber for sure.
I've been thinking about that. I see Wiseco uses it on their standard bore YZ250F piston, but not the big bore versions. I don't know which came first.
One direction that makes sense is that a "chamber mirror" piston is emphasizing getting the squish charge toward the flame, but may compromise in getting the flame back out to the edge of the chamber as the piston moves away from TDC and opens the space. It makes sense to me that the spherical bowl could be a good compromise between effective squish and providing enough space for the turbulent combustion to grow back into the gap.
Also, I've read that when piston velocity goes to zero at TDC, the air and fuel left in the squish zone slow way down, and the fuel can stick to the piston or flat surface of the head, making it much more difficult to vaporize and burn. The wedge of the spherical bowl promotes movement more than a flat-round dish, but still leaves a little space so the movement can continue through TDC. Air and fuel keep moving, so the fuel doesn't stick to the metal as much.
Anyway, just some brain dribblings. I don't have simulation software to do a halfway decent job of modeling it, just thinking about it.