I forgot to make one more point clear about turbo compounding.
The exhaust valve is the current Achilles heel of the aircraft
piston engine. If you fail to lean your air cooled piston
engine properly your exhaust valves will fail sooner rather than later.
Exhaust valves run cherry red and slam open and shut 1200 times
a minute or more in the case of geared engines. It's a wonder they 
last 500 hours. Current exhaust valves are made from the finest materials 
available. In the case of Lycoming they are sodium cooled. They are at
the limit of their development and have been for many years. No significant 
improvement can be made in the foreseeable future.  This was the downfall 
of the CW R3350 turbo compound engine. Exhaust valves would break up and 
take out the exhaust turbine as the parts exited the engine.

The Wankel rotary has no exhaust valves of course.
What this means is the rotary is the ONLY viable intermittent 
combustion engine that is suitable for turbo compounding. A 20% 
improvement in BSFC due to turbo compounding will make
the Wankel rotary the best in every respect light aircraft engine bar none by
a wide margin. Assuming someone develops aluminum end housings,
not only will the Mazda Wankel TC rotary have a much better power to weight 
ratio than a piston engine, it will have lower fuel burn AND a much longer life.

Furthermore if Diamond Engines develops a reliable turbo compound side exhaust
port engine and they install it as standard equipment in all their aircraft 
models I predict Diamond Aircraft will soon become the largest light 
plane manufacturer in the world. They have a very good head start on all other
aircraft engine builders Lycoming and Continental included. They will also
get the lions share of the UAV market as well.

Here are some scans from Taylor's classic books on ICE's.

Paul Lamar
 
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