With its advanced turbocharged downsized gasoline engine, its seven-speed dual clutch transmission, its integrated electric motor-generator and its plug-in hybrid recharging capability, the Audi A1 concept shown at the Paris Mondial de l’Automobile in October is in many ways the template for the energy-efficient small car of the near future.

Solid evidence is provided in the shape of a CO2 emissions figure of 92 grams per kilometer in the European drive cycle. This represents gasoline consumption of 3.9 liters per 100 km, or 60.3 US mpg – an impressive achievement for a compact car with a top speed of 200 km/h (124 mph) and 0-100 km/h (62 mph) acceleration in 7.9 seconds.

Only a handful of current production cars, most of them diesels, can undercut the psychologically important 100 g/km CO2 barrier. Among them are the Ford Fiesta Econetic and the diminutive Smart Fortwo CDI. The current Toyota Prius gasoline hybrid emits 104 g/km.

The A1 Sportback concept follows on from the A1  project quattro hybrid version originally revealed in Tokyo in October 2007. Unlike the Tokyo car, which featured quattro four wheel drive through the use of an electric rear axle, the Sportback has its hybrid elements sandwiched between the engine and the transmission.

Adding its 20 kW to the 1.4 liter gasoline engine’s 150 horsepower, the hybrid module boosts the total available power to 130 kW; the already good combustion engine torque spread of 240 Nm between 1600 and 4000 rpm is supplemented by 150 Nm from the electric motor.

Transmitting these substantial outputs to the front wheels are Audi’s seven-speed S tronic dual dry clutch transmission and a new form of electrically controlled differential which takes its inputs from the electronic stability control system.

The choice of a dry-clutch transmission is self-evident in a compact model range where efficiency and CO2 emissions will be highly visible elements in the appeal of individual versions to consumers.

The move away from the oil-based clutch system of the VW group’s larger DQ250 transmission offers benefits on several fronts. In a wet-clutch system a constant flow of high-pressure oil is required to keep the clutch plates cool: the pump providing this flow of cooling oil is necessary configured to cope with the most extreme vehicle-use requirements, for instance a series of full-power starts from rest.

It is inevitable, therefore, that under normal running much less cooling performance is required and that the energy absorbed by the oil pump is being wasted in unnecessary cooling oil flow. In a dry-clutch arrangement no such pump is required: the clutches are cooled by the ambient air.

In a presentation to engineers at the 2007 Vienna Motor Symposium Volkswagen’s team of DCT specialists estimated the average energy draw of the mechanical oil pump in the wet-clutch DCT to be 500 W over the standardized European fuel consumption drive cycle (NEDC). On the dry-clutch DCT, where the only power draw required is that for the self-
contained mechatronic shifting system, the energy required during the NEDC cycle is a “practically constant” 30 W.

Where VW’s six-speed manual transmission has a mechanical efficiency of 94 percent and the wet-clutch DQ250 DCT is 85 percent, the dry-clutch DQ200 is notably better at 91 percent. By comparison, VW’s six-speed torque converter planetary automatic scores 83 percent.

Translated into fuel consumption and CO2 emissions, the benefits of the dry-clutch unit and its seven speeds are even greater: its values are just 88 percent of those of the six-speed manual, while the wet-clutch six-speed values are 95 percent.

A further simple but practical benefit of the dry clutch principle is that much less oil is required: just 1.7 liters, compared with 7.0 for the wet-clutch unit. This helps allow a weight reduction from 80 to 73 kg, and the further bonus that the lubricant can be of much simpler specification as well as filled for life.

The hybrid system, including the starter-generator integrated in the flywheel, the control electronics and the compact lithium ion  battery pack situated at the rear of the vehicle add some 40 kg (88 lbs) to the overall weight, says Audi. Yet intelligent management of the two power sources and the dual clutch transmission – including stop-start in traffic and energy recuperation under deceleration – mean energy consumption is 30 percent less than if the vehicle were running on gasoline power alone.

On battery power alone Audi claims the A1 can drive for up to 50 km (31 miles) and hit a top speed of 100 km/h (62 mph) before requiring recharging. The A1’s concept is not that of a true plug-in hybrid, where large (and costly) batteries always drive the wheels and the combustion engine is simply there as a range extender; instead, Audi is proposing a more conventional route with the battery assisting the engine. This allows the luxury of the choice of three drive modes – gasoline only, battery only, or mixed operation.

In the A1 this would appear to give the best of both worlds - the advantages of compelling performance and the fun to drive qualities of a turbo engine linked to a dual clutch gearbox – and a decent battery range to allow the daily commute or a foray into a future zero-emission city to be carried out without having to use the engine at all. And, as such, this concept must be seen as a convincing template for the sophisticated small cars we will be driving in years to come.

Story Filed: 1/16/2009
By Tony Lewin, managing editor DCTfacts.com