Onetrack, I can see your concern with the high pressure rail on a common rail engine bursting, and the pipe within a pipe would appear to be the answer. You certainly wouldn't want fuel at the pressures involved being sprayed around under the cowling.
Having dealt with the burst, the next problem is to stop the engine stopping, and the opposed piston engine allows one to have a separate injection pump and injector on each side of the cylinder. Extra complexity, yes, but so is dual ignition on a petrol engine.
I do wonder if we are all hoping for too much by wanting a sudden huge step in efficiency, given the extra hoops a aero engine has to jump through before it can go into use? As with a lot of things in the real world, a small percentage improvement, across lots of components is probably more realistic. It will also only be by getting out and doing it (flying the engines), that engine and airframe manufactures will get the answers. Engines on paper and on dynamometers are not operating in the real world.
Either engines stay in the dark ages for eternity, or one must accept that electronic monitoring and control is the way forward. You can't cut the safety margins in the interests of efficiency without knowing to what you are cutting them. As an example, how far would you be prepared to lean out a Lycoming if you didn't have any indication of cylinder head temperature or exhaust gas temperature?
Your example of common rail being around in the 1920s, is a good example. It has taken until now with modern solenoid injectors to make it a viable proposition for cars. The basic technology was there for common rail, but the control for the injector was not.
If you want efficiency, then precise engine mapping and control is the way to go, if you want simplicity, then compromises must be made and fuel burn goes up. You can't have your cake and eat it.
Last edited by Mechta; 16th Jan 2011 at 13:35.