They function the same as normal engineers. They only appear couple of times in arabian campaigns.

Engineers can be assigned to construct and operate siege and tower engines, or to use the oil smelter to carry oil pots.

Siege engines can be constructed on the field for gold in the form of a tent and engineers will try to man them if enough manpower is supplied. When unmanning a tower engine, engineers automatically go back to their assembly points.

A smelter will supply an engineer with oil if one is already carrying and loading the cauldron with pitch. When they unload their pots, they run back to the smelter to refill it.