Back in the late 1940's, 1950's and 1960's over 9000 turbo compound
  aircraft engines were built. They reached a TBO of 3000 hours in
airline service.

  As far as  turbo compound piston engines are concerned they  are very
hard on
  exhaust valves and much energy is therefore lost in the exhaust valves.
Needless
  to say the rotary has no exhaust valves so in theory it would be
significantly
  more efficient.

  Paul Lamar


When my father was a Flight Engineer at United flying the DC-7C, the
average
time between in-flight shutdowns (on each airplane) was 400hr.

I also had a long conversation with a professor of mine who was a USMC
C-119
squadron commander, he had buckets of trouble with the 3350TC, had to get
a CW
engineer in to consult and teach everybody how to operate the engines,
and
reliability got to an acceptable level.

It is probably more of a function of that particular design then that of
the
whole idea of compounding.

The heat efficiencies are just too good to overlook. It's a shame that
the
commercial GA manufacturers are completely hamstrung by 1) certification
costs,
and 2) a complete lack of want to change the product.

Jim Tiemann

As I said most of the trouble was with the exhaust valves.
When a valve broke it would take out the turbine blades.
The rotary has no exhaust valves.

Paul Lamar


  I read somewhere that the way to tell the difference between the DC 6
and DC 7 was the 6 was a four engined  (P&W) airplane with 3 bladed props
and the 7 was a three engined  (CW) airplane with 4 bladed props.

   Murry

Is true. The Connie was called the best tri motor airplane in the world.
All due to the failed exhaust valves. Usually when TWA got to London from
SFO in 19 hours non stop they had to change out at least one engine.

Paul Lamar