The Ford Sigma is a small straight-4 automobile engine introduced in 1995 by Ford Motor Company. Its first evolution was sold as the Zetec-S (not to be confused with the trim level), then Zetec-SE and finally, in later years, renamed Duratec. The last upgrade of the engine is named Duratec Ti-VCT. Conceived for Ford's smaller models, the motor was intended to replace the older HCS (a derivative of the even older Kent unit) and smaller capacity CVH units.