40 CFR § 1039.230 - 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.
(b) Group engines in the same engine family if they are the same in all the following aspects:
(2) The cooling system (water-cooled vs. air-cooled).
(3) Method of air aspiration.
(4) Method of exhaust aftertreatment (for example, catalytic converter or particulate trap).
(5) Combustion chamber design.
(6) Bore and stroke.
(7) Cylinder arrangement (such as in-line vs. vee configurations). This applies for engines with aftertreatment devices only.
(8) Method of control for engine operation other than governing (i.e., mechanical or electronic).
(9) Power category.
(10) Numerical level of the emission standards that apply to the engine.
(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) If you combine engines from different power categories into a single engine family under paragraph (d) of this section, you must certify the engine family to the more stringent set of standards from the two power categories in that model year.