From what I understand, a turbo-compound engine is a standard reciprocating engine that uses excess exhaust gasses to spin a turbine, kinda like a turbochagree. The difference is the turbine is much larger and is used to directly add power, which can also greatly increase fuel efficiency at high-throttle. The early versions of this were pioneered at the end of the piston-engine airliner era and met their demise cause of the turbojet. From what I know, applications of this engine have been limited to a few truck and tank engines. Locomotives seem like the perfect application for these kinds of engines, which would allow greater fuel efficiency and could also benefit from having the turbine power a generator/alternator like the reciprocating engine it's attatched to. From what I've known they haven't been tried on trains. Why did they never catch on and were there ever at least plans to put one in a locomotive?
It should be noted that the DC7's and Super Constellations equipped with turbo compound radials were retired and cut up well before many DC6's, Constellations and other models without such engines, mostly because of the complexity of the engines.
The Wright Turbo-Compound (and its contemporaries) worked because the combined output was mechanical. Turboelectric drive has not been practical for aircraft for a number of reasons.
Likewise, most of the properly so-called turbocompounding efforts for trucks have involved either mechanical or hydrokinetic transmissions. The much more flexible solution was to take the additional 'turbine power' and use it to generate ancillary electricity, rather than couple it to the crankshaft; this is much more significant in stop-and-go operation of a heavy diesel truck with a multispeed transmission, where the turbo follows the variation in exhaust mass flow.
Packaging issues on larger and lower-speed diesel engines can make plumbing and installing a turbine directly connected to the engine crankshaft 'involved'. Newer high-speed engines have common staged compression and relatively responsive wheels, so lag is less of an issue.
There are so many practical advantages to a reversible electric turbocharger 'drive' on locomotives that there is no reason to debate a mechanical alternative; even a comparatively small battery handles the full requirement of, for example, the EMD mechanical supplemental system that allows the centrifugal blower of the turbo to do the scavenging-air job of Roots blowers at low engine rpm. And, of course, if you have hybrid assist of any type, the battery and management systems are essentially free both to run the turbo spooling and harvest excess power.
The DC6's were powered by Double Wasps which had a much better reputation for reliability than the Wrights in either standard or TC form. With over 100,000 made, parts were also easier to come by. The Wasp Major was another engine with a bad rep for maintennance issues, note the freighter versions of the DC7s were still being flown after the last B377 was retired.
A couple of tidbits: Wright's reason for going with the turbo-compound was as in part driven maximizing horsepower per pound, as simple turbocharging would add more weight than TC. Allison had done some work on a turbo-compound V1710 and the sfc was even better than the Wright TC.
One reason that airplanes got turbo compounds and not locomotives is that airplane engines are usually run at a relatively high constant power, which makes it easy to tune for best efficiency. Locomotive engines are run a mix of power settings, so a simple machanical connection won't work very Having said that, the processing of "pinning" the turbochargers on EMD engines to derate them sounds like unintentional turbo-compounding. Another way to turbo-cmpound could be done with the exhaust turbine driving a generator and the power from the generator split between traction and the compressor (supercharger).
An Explanation of Turbo Compound as opposed to Turbo Charging.
https://www.youtube.com/watch?v=gbMgwDIdScY
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