Like many other manufacturers, Audi has chosen the theme of fuel efficiency for its Paris Motor Show exhibitions, and the latest addition to the group is the Audi A4 TDI Concept e.

Built around a highly efficient diesel engine that's good for 58.95mpg (3.99L/100km) in mixed driving, the car is an attempt at using a great deal of technology to build a car that is conventional in nature but exceptional in fuel performance. The idea isn't new, but Audi's implementation appears to be remarkably successful.

Rated at 120hp (88kW) and 213lb-ft (290Nm) of torque, the A4 TDI Concept e's engine is a standard direct-injection TDI diesel unit. Though 120hp may not be the sort of power you'd find in a sport sedan, it's enough to roughly match Toyota's Prius in acceleration, clocking in at 10.7 seconds to get to 62mph (100km/h) and a top speed of 128mph (206km/h). With better combined fuel mileage than the petrol-based hybrid Prius, the A4's efficiency and luxury specification make a strong case for production.

Though the A4 TDI Concept e isn't a hybrid, it does use regenerative braking to store energy in a battery. That energy is then used for auxiliary systems and to power the car's stop-start functionality. Other advanced technology features include a magnesium transmission housing to cut weight, insulation on the gearbox to improve warm-up times, and gear ratios designed to optimize efficiency. The six-speed manual unit also makes for efficient and inexpensive operation in comparison to the more costly dual-clutch solutions becoming increasingly popular.

The optimization of the A4 took into account just about every aspect of the vehicle. Electromechanical power steering is speed-sensitive and only takes power from the engine when the wheel is being turned. Rolling resistance is minimized with advanced tires, and parasitic losses on the engine are mitigated with an advanced computer control system that cuts out the air conditioning system's compressor whenever it's not required. Even the coefficient of drag has been optimized, down to 0.25cd from the production A4's 0.27.

The A4 TDI Concept e fits in with the existing line of 'e' models from Audi, including the A3 1.9 TDI e and its Sportback variant, the new A6 2.0 TDI e and Avant, and the A8 2.8 FSI e. The A4 is expected to be the next expansion of this line, filling in the mid-size sedan option. Further expansion of the 'e' brand is planned through 2012, though which models will be added remains unclear.