You can see the smog of war follows the FC 31. True Chinese tradition, make enough smog and hide it, that is Chinese stealth design.
To those who wonder about the black smoke. It is because two things:
1, Chinese air force uses cheaper fuel, more carbon content, so the black smoke is the carbon powder.
2, Normal engine working condition will burn up those carbon powder and form carbon dioxide, so you did not see it most of the time. However, when afterburner turn up. The fuel is burning in a oxygen starve condition, so the burning is not clean, hence the black smoke.