Irkut chose Pratt and Whitney geared turbofans as an engine option on MS-21:
http://www.flightglobal.com/articles/20 ... tform.html
MS-21-400 shall require engines of 15 600 kgf each. The biggest PW GTF previously launched was the engine for CS300, at 10 500 kgf thrust. And Cseries needs fan diametre of 185 cm.
How big fans shall MS-21 have?