One thing to consider is that with so few moving parts, the engine could be made of ceramic and use the fuel and exhaust gases for lubrication directly. I saw

One thing to consider is that with so few moving parts, the engine could be made of ceramic and use the fuel and exhaust gases for lubrication directly. I saw a documentary about it in the 90s and remember a guy holding a ceramic piston and discussing it but never heard anything else about it. I believe the carbon residue acted like graphite. The hardness of the components was so much higher than metal that it was like comparing the lifetime of ceramic bearings like silicon nitride (Si3N4) to metal bearings.