How about the Napier Nomad. The Nomad was one the most advanced turbo compound engines. A liquid cooled opposed diesel engine coupled to an axial flow turbine. Thermal efficiency as high as 42%.
Type: Horizontal opposed compression ignition 2-stroke engine
Cylinders: flat 12 arrangement
Bore and stroke: 6 x 7.375 in
Swept volume: 2502 cu in ( 41.1 litre)
Compressions ratio: 3.5:1
Boost pressure: 89 psi
Revolution: 2050 crusing
HP range: 3030 cruising, 4100 take-off
Specific fuel consumption (lb/hp/h): 0.33
Weight, lbs: 3580
Not the heaviest but quite bulky and probably the most efficient aero-engine built to date. Unfortunately it was hideously complex as only a Napier engine could be
There's one in East Fortune Museum if your up that way