Naval News, has republished an excellent article written by H I Sutton of Covert Shores:
"The NewMystery Submarine Seen In China: What We Know" of June 25, 2021 concerning the PLA-N's Yuan Type-039C (or D). That is the Yuan variant with the vaguely A-26 (aka Blekinge-class) like sail and other exterior differences from the preceding Yuans - also many Yuans are suspected of having Swedish srtyle Stirling-engione AIP.
The only statement I would alter siightly is:
"This [the Yuan] likely uses a closed-cycle Stirling engine, similar to that on the A-26 class, to power the submarine when submerged. This means that it doesn’t have to snorkel to run its diesel generators [while its using AIP] to recharge the batteries." [but doers need to use the snorkel to provide air for the diesels (to recharge the batteries) when the AIP is not being used].
H I Sutton's combination of commentary, artwork and photos at Covert Shores and via Naval Newsis uniformily excellent.
Here is another interesting H I Sutton article, via Naval News"First Sighting Of New Stealth Fighter For Chinese Navy’s Aircraft Carriers" of June 8, 2021.