You are right @sanman, there might be more efficient thermal engine designs out there.
The practical challenge I see here is the $$$ behind it in our market. Unfortunately we cannot drive innovation in hardware design with a low 2 digit million $ world wide spend on paramotor engines a year (just guessing based on paramotor serial numbers, I have no prove for this value). E.g. EOS is building a new 4 stroke paramotor engine (“Quattro”, some youtube video about it), similar to the Bailey V5. I flew the prototype this year and it is definitely interesting, but still had some room for improvement as well. It takes years to get from a basically well known concept to a reliable production grade product.
Quoting Elon Musk: “production hell”
That said I really hope we could just buy and try suitable motor alternatives to the classical 2 stroke engines. The liquid piston engine looks really interesting, but I doubt we can get it in the 2 - 4.000 $ price range (engine only) of our 2-strokes. Besides being able to buy it small numbers, we also need to get spare parts in small charges for many years. Many manufactures will prefer the opposite: a small number of large customers, e.g. like the few big companies building military gear.
The great advantage with OpenPPG electro related parts (motors, esc, bms, battery cells…) is that we can use great parts from manufactures that create their main revenue selling these or similar sized parts to other way larger markets. That helps driving maturity up and cost down. E.g. Mad Motors is for sure making most of their revenue from selling to drone markets. With the existing know how and supply chain it is relatively easy to produce a product that is just right for us and thus affordable.
The Whitehead brothers do an awesome job testing / optimising setups and selecting the right parts. I would have loved to get my SP140 this year (just 7h left over here), but I prefer getting it well tested rather than faster.