Since 1996 more than 37,000 MTU 4000s have been sold for surface ship, train, mining, industrial and energy-pump uses.
Lower range 600 kW MTU 12V 4000s may already serve as emergency backup/return-to-port diesels on nuclear powered Astute class SSNs.
The MTU 4000s for submarine are reportedly more fuel efficient and have higher peak power than MTU 396s.
The TKMS future Type 212CD may use 2 x MTU 4000s.
Two MTU 4000s may be used in Singapore's new Invincible class Type 218SG. Anonymous in February 21, 2019 noticed 2 x MTU 4000s on a Type 218SG youtube diagram 42 seconds in, here https://youtu.be/I_poBnLJMew?t=42s.
As MTU 4000s are mass produced in Yulin, China they may be used about China's latest Yuan class conventional submarines.
On the MTU 4000 for submarine, see Arndt von Drathen’s, Applications Battery Charging Technologies for Advanced Submarine Requirements, MTU, 2011 https://mtu-online-shop.com/print/3100721_MTU_General_WhitePaper_SubmarineChargingUnit_2011.pdf see page 2:
"Since the introduction of the Series 396 in the early 1980s into the submarine application...Conventional submarines have grown in size over the last decades to meet increased transit distance requirements to the operational areas and to accommodate various Air Independent Propulsion technologies into the hull.
Despite hydrodynamic improvements of the hull shapes, the larger displacements have led to higher power demands. This trend for more power will be even further accelerated by new Li-Ion battery technologies. Li-Ion batteries are going to increase underwater endurance and performance of submarines significantly. However, the diesel engine driven charging unit technology needs to adapt to the new requirements: firstly, more electrical power and secondly, provide rated power almost the entire operational time to fully utilize the Li-Ion advantages."
Looking at the red "submarine-specific" portion on the page 2 diagram of the MTU 4000 is the portion on top, showing a large cylinder. Is this a muffler for quieter operation of the diesel engine?
The available photo below of an MTU 4000 for submarine shows muffler? cylinder in unpainted metal color.