Del. Code tit. 7, § 6706

Such codes, rules and regulations shall establish standards and requirements for the control of air contaminants which can reasonably be attained by properly functioning motor vehicles without the addition of any pollution control devices, systems, or engine modifications; provided such vehicles were not manufactured with pollution control devices, systems, or engine modifications in accordance with the “National Emission Standards Act” [42 U.S.C. § 7521 et seq.].