Introduced in 1995, the Ford Sigma is a small straight-4 automobile engine sold by Ford Motor Company. First evolution of engines were sold as the "Zetec-SE" (although frequently badged as Zetec-S, not to be confused with the trim level). After the upgrade Ford renamed Sigma to Duratec. Last upgrade of engine is named Duratec Ti-VCT. The engine was introduced in the mid-1990s and production continues through the present. Like the SHO V6 and SHO V8, the Sigma was designed with help from Yamaha. Conceived for Ford's smaller models, it was intended to replace the older HCS (a derivative of the even older Kent unit) and smaller capacity CVH units.