What is so wrong with having the exhaust at one end of the cylinder and the inlet at the other? Uniflow.
It would offer a clean cylinder fill with (tuned right) no transfer charge escaping, especialy if using a small air buffer between the inlet and exhaust. No EFI required. Out of phase cranks can provide enough exhaust blow down (not needed anyway apparently), with smooth balanced operation.
Combustion chamber is compromised by having two pistons against each other (without having to have odd shapes), plugs need to be in the side of the cylinder. If same size pistons there is no squish.
If some trickery were used to make the exhaust piston cause HCCI (assumimg same size pistons) then back off and provide an exhaust path, ie open some exhaust ports, what if?
On all my Uniflow engine designs I've used the gear train as a power take off, the revs are already reduced. This reduction in revs is almost always needed in IC applictions anyway. So the perceived complication of needing a drive from one end of the engine to the other is partly null and void.
All my Uniflow engines to date used two cylinders and four pistons to get the crank case pumping charge the correct volumes, as it happens this my have been the wrong approch, perhaps enough pumping to fill half the cylinder, using the exhaust (chamber) to do the rest. In other words make a simpler single cylinder with two pistons, crank case pumping only from one end (transfer end). Exhaust end would be like a fourstroke, in oil, or perhaps a Ryger?
The trick is in the mechanisim that makes the exhaust piston "suddenly" increase compression just at the right time and then just work like a normal piston until next cycle.
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