Russia and Ukraine

[The Architect Of Fate]

the T-14 is designed around an extremely compact diesel engine with cylinders arranged in a X configuration around a central crankshaft.   it is not exactly a new engine.   it was first developed in the early 1970s as candidate for the next soviet tank expected to be fielded in the 1980s.   however what engine the next soviet tank should have was the subject of a intense bureaucratic battle between two fractions within soviet military industrial complex, and the rival camp, which advocated the use of gas turbines and had as its champion the powerful soviet defence minister, won.  So the X diesel engine only reached prototype stage, and was not fully developed to an operational state. 

as it turns out, selection of gas turbine engine was recognized as a mistake by the mid 1980s, and the powerful defense minister who championed the gas turbine camp had also died.   but the soviet union fell before the X-engine development can be revived. 

fast forward 20 years, Russia is once again looking for a new tank, and it seemed like a good idea to base it on the revived X-engine which had been languishing,  because on paper its performance and compactness was still highly competitive.    Using this engine would allow the new tank to be much more compact and this better protected.

but unfortunately, either because the X-engine concept is subtlety but fatally flawed, or the capabilities of russian tank automotive research establishment is no longer the equal of what it had been under the USSR,  the teething troubles with the prototype proved insurmountable.  

So the russians are left with a tank that could only use an engine that doesn’t work.

It's essentially a dressed-up T-34 engine.

I would actually argue it's inferior to a T34 engine in many respects and that's saying something