The engine is the "main part" of the drilling rig. The engine does not add much raw power, instead improving the entire drilling rig; an engine adds a tiny raw digging value but also introduces a multiplier, which is added as the final operation in the Watt calculation.
A level 4 engine, for instance, brings 10W to the table, the same as the previous level 3 engine. However, it increases the multiplier from x1.5 to x4, improving the overall value of the digging machine far beyond the level it had with a level 3 engine. For more information about the Watt calculations, please see Drill.