The Small doors on the Top are for the APGS or Auxiliary Power Generation System. Equivalent of an APU or Auxiliary power unit larger Jet airliners use to start engines. One door is Inlet and the other door is the Exhaust duct. Smoke seen during start is the APGS bursting air into the main engines to rotate for a start. Smoke is residual condensation and smoke from the APGS or possibly the APGS using one of its Cartridge starts.