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 § 94.2
The definitions of this section apply to this subpart. They also apply to all subparts of this part, except where noted otherwise.