Marine engine means a nonroad engine that is installed or intended to be installed on a marine vessel. This includes a portable auxiliary marine engine only if its fueling, cooling, or exhaust system is an integral part of the vessel. There are two kinds of marine engines:
(1) Propulsion marine engine means a marine engine that moves a vessel through the water or directs the vessel's movement.
(2) Auxiliary marine engine means a marine engine not used for propulsion.
40 CFR § 91.3
The following definitions apply to this part 91. All terms not defined herein have the meaning given them in the Act.