40 CFR §1036.101

1. (a)You must show that engines meet the criteria pollutant standards for NOX, HC, PM, and CO as described in § 1036.104. These pollutants are sometimes described collectively as “criteria pollutants” because they are either criteria pollutants under the Clean Air Act or precursors to the criteria pollutants ozone and PM.
2. (b)You may optionally demonstrate compliance with the emission standards of this part by testing hybrid powertrains, rather than testing the engine alone. Except as specified, provisions of this part that reference engines apply equally to hybrid powertrains.