40 CFR § 1048.230 - How do I select engine families?
(a) For purposes of certification, divide your product line into families of engines that are expected to have similar emission characteristics throughout the useful life as described in this section. Your engine family is limited to a single model year.
(1) The combustion cycle.
(2) The cooling system (water-cooled vs. air-cooled).
(3) Configuration of the fuel system (for example, fuel-injected vs. carbureted gasoline engines).
(4) Method of air aspiration.
(5) The number, location, volume, and composition of catalytic converters.
(6) The number, arrangement (such as in-line or vee configuration), and approximate bore diameter of cylinders.
(c) You may subdivide a group of engines that is identical under paragraph (b) of this section into different engine families if you show the expected emission characteristics are different during the useful life.
(d) In unusual circumstances, you may group engines that are not identical with respect to the things listed in paragraph (b) of this section in the same engine family if you show that their emission characteristics during the useful life will be similar.
(e) You may create separate families for exhaust emissions and evaporative emissions. If we do this, list both families on the emission control information label.
(f) Where necessary, you may divide an engine family into sub-families to meet different emission standards, as specified in § 1048.101(a)(2). For issues related to compliance and prohibited actions, we will generally apply decisions to the whole engine family. For engine labels and other administrative provisions, we may approve your request for separate treatment of sub-families.