johnny comelately wrote:re accumulation grooves, does anyone know the facts on how they work in regard to sealing?
as an example the model aero pistons that have no rings and just an accumulation groove.
there does not seem to be any leakdown, which is surprising
The basic principle of how a piston with multiple accumulation grooves spaced along its upper circumference functions to limit combustion gas blow-by is not difficult to understand. You can analyze it as a system of flow orifices and volumes in series. The first flow orifice is the clearance area between the piston crown and cylinder bore, the volume of the space being filled is that of the accumulation groove, and the pressure delta is that between the combustion chamber and the first accumulator groove. Then the same gas flow situation occurs between the first and second accumulator grooves. And so forth with the remaining accumulator grooves.
There are flow losses and a pressure drop from one accumulator groove to the next. So eventually the gas flow will stagnate if there are enough accumulator grooves in series and the spaces between the piston and cylinder bore are small enough. But this does not mean this "sealing" arrangement would be more efficient than a conventional system using piston rings.
