No other modern engine has smoke trails quite like the RD 93/33 engines. Like I said, they're interim.
J31 is flying with interim Russian RD33 engine. Russian RD33/93 family of engines had known problem with incomplete combustion as seen in mig29 smoky exhaust. Russia claims to have fixed the problem of smoky exhaust in the latest generation of RD33/93 engines. China (or Russia) has also tweaked the RD93 currently used in JF17/FC1 to correct smokiness and improve performance, Both J31 and JF17 are going to be present in Zhuhai, and you'll be able to judge for yourself.
China is working on indigenous WS13 engine to replace RD33/93. No news of their progress is heard lately. I am sure reporters will be anxious to find out the status of that an other engines.