There are always fundemental problems when arriving at a
combustion chamber shape that is NOT Round or Cyndrical.
Basically there are two problems:
1. Maximum thermal efficiency is had with a sperical combustion chamber.
(Minimum surface area to volume ratio) This is not possible
without what I call a hydraullic 'Bubble Combustion' process.
Hydraullic engines have their own set of inherent problems I will not go into here.
The next-best thing to a pulsing sphere is of course, a conventional cylinder.
This is WHY cyndrical piston engines are hard to beat, efficiency-wise.
Just because cyndrical piston engines have been around for more than
a century does not mean they can be improved upon all that easily.
2. Losses of combustion gases past the seals are most usually worse in
a combustion chamber that is not round (additional linear inches of seal
for the same conbustion volume as a conventional cyndrical combustion chamber).
Not to mention higher emissions that result from both items 1 & 2.
These issues above killed the NSU/Wankel Rotary for most applications.
Nearly every design I've seen that varies from cyndrical combustion chambers
would tend to suffer from the inherent inefficiency of adding combustion surface area
and seal area vs the near-perfect cylinder with a 'square' (equal) bore/stroke ratio.
The main attraction of an opposed-piston engine (aka Deltic, Fairbanks-Morse, etc.)
is to SPLIT the stroke to two crankshafts, which cuts the piston speed IN HALF
for the same operating frequency. Allowing a large cylinder to operate at
twice the RPM of the same cylinder with one crankshaft.
Unfortunately most of the Opposed-Piston engines have been diesels with
very undersquare Bore/Stroke ratios (not taking advantage of the RPM potential)
their undersquare ratios also elongates the combustion chamber, lowering
thermal efficiency once again.
So WHY NOT an Opposed-Piston Engine with a combined SQUARE bore/stroke ratio
Example: 100mm bore, 50mm stroke per crank, giving an overall 100mm x 100mm
near-vibrationless single cylinder engine capable of TWICE the RPM of a
conventional engine of same displacement and same bearing/piston speeds.
In this variation on the piston engine we have the potential to DOUBLE
the frequency (hp-per-liter) while all bearing velocities remain unchanged.
I'd like to see that done before we move on towards wierd-shaped chambers
because it preserves the inherent thermal efficiancy of a cylinder.
Yes, you now have two sets of rings but at twice the RPM those rings
would be negated on a linear inches of seal-per-CFM basis.
If your engine has a reliable high-rpm potential with good breathing
the additional linear seal lengths and increased combustion surface areas
could likewise be somewhat mitigated by the higher HP/liter ratio.
OK, starting to ramble on now....
Jeff Krause
EVA CASES, EVA CASE DESIGN, Custom Protective Cases, Ever Ready Cases by Designer Jeff Krause