The fifth-generation Audi A4 is planned to go on sale in the second half of this year.
This engineering mule, the latest in a long line of prototypes to be spotted testing, wears bodywork from the current car. Previous prototypes have worn heavy disguises, but the latest interpretation of Audi's front grille, seen on the new TT, should feature alongside that car's angular LED headlights.
Based on the second-generation MLB platform, the new A4 will be part of a push for improved economy from Audi.
A range of longitudinally mounted 1.4, 1.8 and 2.0-litre TFSI engines are expected to power the new A4, some featuring cylinder shut-down technology, alongside 1.6 and 2.0-litre diesel units. At least one model should offer CO2 emissions of less than 100g/km.
Audi is also planning to introduce 3.0-litre TFSI and TDI engines, such as the one shown in Vienna last year, but those will likely be mainly for the US market.
A plug-in hybrid model, using the same petrol-electric powertrain as the A3 e-tron and with an electric-only range of around 30 miles, is also planned. A diesel-electric system is also under development and could be introduced later on to feed a growing demand for hybrids in Europe.
Both six-speed manual and nine-speed automatic gearboxes should be offered, but today's multitronic CVT will be dropped from the line-up. All A4 models will be front-wheel drive, with four-wheel drive available as an option. Stop-start technology, brake energy recuperation and thermal management should also feature. There's also the chance to offer all-wheel drive via an electrically driven rear axle.
The saloon variant of the A4 will launch first, with an Avant model appearing soon after. Reports suggest an Allroad version will follow later in the year, while successors to the A5 coupé, cabriolet and Sportback are also planned for 2016.
The A4's length and height will remain close to the current car, but its width will increase in order to house a new chassis with wider tracks. The wheelbase of the new car should also be longer than the 2810mm of the current A4. The new A4's boot capacity should remain the same as the current A4, at 480 litres.
Inside, the new A4 should offer a minimalistic design with extras such as a head-up display available as options. The latest version of Audi's MMI infotainment system should also be available, controlled via a touchpad as seen on the new Audi TT.
Currently one of the top-ranking compact executive models, the new A4 will face stiff competition from the BMW 3-series and new Mercedes-Benz C-class. The A4 is also Audi's best-selling model, with a total of 338,449 units sold in 2013.
The new MLB platform has been in development for five years, and represents a huge investment for Audi. Dumping the traditional welded steel monocoque structure, the MLB platform uses a mixture of aluminium and steel - and, eventually, some lightweight composite materials - that are joined by rivets, screws and high-tech adhesives.
The move has caused Audi to completely retool the factories that will build all of its models from the A4 upwards. Audi engineers believe that a mixed-materials architecture is superior to one made mostly of steel or mostly of aluminium. The theory is that being able to use different materials in different parts of the structure results in a more ideally optimised structure.
It is understood that the A4 will use high-strength steels for the sills, body pillars and crash structures in the nose. The front suspension turrets and parts of the engine bay will be made from aluminium castings and extrusions, while aluminium is also expected to be used for the roof panel, bonnet and front wings.
More exotic variations of the MLB platform, such as that set to be used on the new Q7 SUV, could also get super-stiff composite structures around the bulkhead and composite floor panels.
Get the latest car news, reviews and galleries from Autocar direct to your inbox every week. Enter your email address below: