Dual-fuel means relating to an engine designed for operation on two different types of fuel but not on a continuous mixture of those fuels (see 1036.601). For purposes of this part, such an engine remains a dual-fuel engine even if it is designed for operation on three or more different fuels.
40 CFR § 1036.801
The following definitions apply to this part. The definitions apply to all subparts unless we note otherwise. All undefined terms have the meaning the Act gives to them. The definitions follow: