This is me trying to find a way around (some of) the complexity that doomed the Napier Nomad of 1947. The Nomad, now a museum piece, is still the most fuel-efficient aircraft engine ever built (in terms of simple mechanical output per fuel input), but it didn’t work very well and came out right before big dumb lightweight turbojets and cheap post-war oil:
(below is a video, not a picture)
Like ye ole Nomad, it’s basically a gas turbine with a very-compressed diesel engine for a combustor. Since a diesel can handle significantly higher instantaneous maximum temperatures than a turbine can, better fuel efficiency results.
Plus, the gas turbine also acts as a bitchin’-powerful turbocharger for the diesel and keeps its size=weight down when compared to a regular diesel running by itself.
And finally, it’s apparently a better deal to let both the diesel and turbine produce power, as opposed to a diesel with a simple turbocharger that only drives itself.
But, instead of using scary gears to join the gas turbine and diesel together onto a single output shaft, the Parallel Compound Cycle engine cheats. No gears. Each side, the diesel and turbine, drives its own propulsion fan.
So the diesel and turbine sides are connected only by tubes (compressed air and exhaust to/from the diesel), not gearing. They’re both propelling the same airplane, so their power outputs are combined that way.
Further, since there are two “actuator disks” per engine (one for the diesel’s fan, and another for the turbine’s) instead of one, propulsive efficiency (thrust per horsepower) is upped a little bit to boot.
This was probably written about and shot down decades ago but dangit, I just haven’t found anything about it. Just in case this is a new idea, I offer it up here. Thank you.
Longshot City 
October 21, 2008 at 3:56 am
This looks to me like you could model it (for some purposes) as a standard diesel with an extra fancy turbocharger.
That suggests that you could start it with a conventional starter motor on the diesel.
Also, the way that conventional turbocharged engines deal with too high an intake pressure is with a either a wastegate that routes the exhaust gas around the exhaust turbine, or with a variable-geometry exhaust turbine (http://en.wikipedia.org/wiki/Variable_geometry_turbocharger). I’m not sure if that’s applicable to an axial turbine like those found in turbojets, though.
October 22, 2008 at 6:50 pm
Sad to say, but “New Ideas” == “Insolence” in just about every field more than 10 years old, not just aerospace. And the older the field, the more jaded.
January 26, 2009 at 7:36 am
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