Originally Posted by
JDNSW
Exhaust turbines driving the propeller shaft were a 'thing' as large aeroplane piston engines fought a last, failing, battle against turbines. The Napier Nomad, perhaps the most ambitious one, featured a 12 cylinder two stroke diesel from memory, with 70% of the power coming from the exhaust turbines. It is also considered by some to be the most complex aeroplane engine ever built (although competitors in this space were the four row radials from the other side of the Atlantic). There were, however, several versions built, with quite different design. None reached the design 6,000hp, and by the time any were ready for testing, it was clear that the future was turbines.
The major theoretical advantage of the layout was that it was effectively a turboprop engine with the combustion chamber replaced by the diesel engine, allowing both a higher combustion temperature (higher thermal efficiency) and a lower turbine inlet temperature (less demanding on blade metallurgy). But at the expense of complexity, size, and mass.