For which model year did the Clean Air Act take effect for engines?

The Clean Air Act Amendments of 1990 significantly expanded the authority of the Environmental Protection Agency (EPA) to regulate air quality and emissions from various sources, including engines. The new standards set forth by these amendments began to apply to new engines starting with the model year 1994. This means that manufacturers were required to meet more stringent emissions standards for engines produced for that model year and subsequent years. Therefore, the correct answer indicates that the implementation of the Clean Air Act's new regulations for engines took effect in 1994, aligning with the significant legislative changes designed to improve air quality and reduce pollution levels.