The serious development of alternative vehicle concepts can be traced back to the 1970s. At the time, the oil crisis of 1973/74 initially promoted the electric vehicle as a possible means of transportation that was not dependent on oil. The concept of the Audi 100 experimental electric car from 1976 provides a typical example of this. However, the technical limits of battery technology at the time (energy density, power density, lifespan) very quickly led to a situation where, alongside the development of electric vehicles, there was also an increasing focus on the development of hybrid vehicles. The further oil crises of 1979/80 and 1990/91 had a similar effect to the first oil crisis in terms of the efforts made to develop electric and hybrid vehicles.
Based on the Audi 100 Avant (C3), the first generation of the Audi Duo was created in autumn 1989 and unveiled for the first time at the Geneva Motor Show in March 1990. An Audi 100 Avant quattro with a five-cylinder fuel-injection engine and 100 kW/136 hp was provided with a 12.6 hp DC electric motor on the rear axle, and thus demonstrated the feasibility of hybrid vehicles that can travel over short journeys without producing any emissions. A nickel-cadmium battery provided power for the electric drive.
Audi presented the second generation of the Audi Duo at the Geneva Motor Show in 1992. Under the body of the Audi 100 Avant quattro of the C4 series, the Audi Duo had a combustion engine which drove both axles and an electric motor which acted on the rear axle when required. The two-litre, four-cylinder engine delivered 85 kW/115 hp, and in electric mode a water-cooled, permanently excited three-phase synchronous motor delivering 21 kW/28.5 hp drove the rear wheels. Switching took place automatically at a speed of approx. 25 km/h and in electric mode the Audi Duo achieved a top speed of 65 km/h; using a sodium/sulphur battery with a voltage of 252 volts, the range was around 80 kilometres.
Audi unveiled the third generation of the Audi Duo at the International Motor Show in Berlin in mid-September 1996. The basic engineering came from the Audi A4 Avant (B5), equipped with the well-known 66 kW/90 hp four-cylinder TDI and a water-cooled three-phase synchronous motor that delivered 21 kW/29 hp. In contrast to the arrangement on the previous models, here the electric motor was flanged onto the five-speed gearbox, and it also drove the front wheels. The drive shaft of the electric motor extended over a reduction gear directly into the five-speed gearbox. In electric mode, a small electric pump supplied the hydraulics. The steering support, brake servo and ABS worked in the usual way; a fuel-powered auxiliary heater performed the role of the heating system. While travelling on the diesel engine or during braking, the electric motor acted as a generator and charged the battery cells. The capacity of the lead batteries was designed for travelling a distance of approx. 50 urban kilometres.