Issue: Nov 2008


Efficiency for the future



by Nick Palmen

Audi is committing major resources in order to continue improving fuel efficiency and reducing emissions. The A4 TDI concept e introduced at the Paris Auto Show offers the typical experience of driving an Audi, yet uses just 3.99 liters of fuel per 100 km (58.95 US mpg) – corresponding to a CO2 output of 105 grams per km (168.98 g/mile). 

Audi uses an electromagnetic servo assembly for steering, while electrically actuated brakes are fitted at the rear. The 225/45 R17 tires have been optimized for road resistance. The driver can select from an economy training program, or have the navigation system calculate the most fuel-efficient route to his destination.

The engine

The heart of the sedan is a direct-injection diesel engine – a 2.0 TDI with output of 88 kW (120 hp). From 1,750 to 2,500 rpm, the four-cylinder engine produces a constant torque of 290 Nm (213.89 lb-ft). The piezo injectors in the common-rail system inject fuel at a pressure of up to 1,800 bar, and the piston geometry provides ideal thermodynamics in the combustion chambers.

The 2.0 TDI features additional efficient technologies. The oil cooler, oil nozzles for cooling the piston crowns, oil and water pumps can be switched off when inactive. New piston rings produce lower tangential forces, the spring pressure on the intake valves was reduced slightly, and the brake booster vacuum pump received a new actuator. These measures reduced emissions by five grams of CO2/km. Other engine modifications include reduced internal friction, a thermal management system, and an electronic fuel pump. 

As in the production version, Audi is using a thermostatically controlled cooling circuit with an internal heat exchanger for the air conditioning system. The system uses 20% less fuel than its predecessor. A new controller disconnects the air conditioning system’s compressor from its engine drive whenever possible.

Like many Audi production models, the A4 TDI concept e uses a recuperation system that converts energy during deceleration into electrical power. Electrical energy generated by deceleration is stored in an absorbent glass mat battery. When the sedan accelerates, the battery reduces the alternator load by feeding the energy back in.

Performance has not been sacrificed. The sedan sprints from 0 to 100 km/h (62.14 mph) in 10.7 seconds, and accelerates from 80 to 120 km/h (49.71 to 74.56 mph) in fifth gear in just 10.9 seconds. Top speed is 206 km/h (128 mph).

Drivetrain and chassis

The 2.0 TDI uses a six-speed manual transmission with “considerably reduced” internal friction customized for the high-torque TDI engine, with taller ratios for gears three through six. Five kilograms have been saved through the use of a mainly magnesium housing with insulating foam-packed skin which ensures the transmission oil heats up faster. The drive shafts to the front wheels have also been redesigned. 

The transmission is coupled to a system that switches the engine off whenever the sedan stops, the shift lever is in the neutral position, and the driver’s foot is off the clutch pedal. It starts up in two tenths of a second as soon as the driver steps on the clutch. 

The start-stop system improves the fuel efficiency of the Audi A4 TDI concept e  further, meaning that five grams less of CO2 are produced per 100 kilometers – as measured on the dynamometer. And in actual terms this reduction may turn out even higher, as many trips to work are less than ten kilometers long and are frequently interrupted by traffic lights, according to Audi.

Another important feature is that an electro-mechanical part replaced the electro-hydraulic power steering. This yielded a further reduction of four grams of CO2/km. It does not have to circulate any hydraulic fluid, nor absorb any energy while the vehicle is traveling in a straight line – its electric motor which provides power assistance only kicks in if the driver turns the wheel. 

The electrical brakes are another technology module that shows how meticulous the Audi engineers were in their approach to efficiency. Thanks to active brake control, residual brake torque is almost non-existent. It only arises if the small air gap that is necessary for a stiff feel to the pedal and immediate brake response results in a slightly unevenly shaped disk rubbing on the lining. Using electrical rear-axle brakes will achieve the same effect on the front axle: here, an enlarged air gap is also possible since the power brake system only has to carry out the lining displacement for one axle.

Body and cockpit

The A4 TDI concept e glides “sleekly” through the wind, according to Audi. A trunk lid with a restrained separating edge adds a further dimension to the aerodynamics. The partially closed grid in the single-frame grille and sporty suspension tuning with optimized ride height also contribute to a drag coefficient of 0.25 (production model: 0.27). The underbody of the technology study is faired more than the production model; the rear lights feature low-consumption LED technology. New dip-polished 7.5J x 17 cast aluminum wheels improve flow of air around the wheels and wheel housings. The tires are designed for smooth running and grip due to their 225/50 R17 wide format.


The technology package from Audi’s modular efficiency system also takes the most important external factor of all into account as far as possible: the person behind the wheel. Despite the prominence of high-end technology, the driver remains primarily responsible for how fuel-efficient his vehicle is. 

The driver will discover two major aids in the A4 TDI to help him drive economically. He can call up an efficiency program that shows up on the instrument panel display and which acts like an ecology trainer – pointing out open side windows or recommending rapid upshifting. And the navigation system will always offer a fuel-saving route as an alternative whenever the driver enters a destination.
World engine

The TDI with ultra low emission system can be employed worldwide, even in California and all other US states that currently have the most stringent emission standards. Compared with the fleet average of the gasoline engines typically found in the US, the TDI achieves a fuel-efficiency advantage of up to 40%.

Integrated efficiency strategy

Audi says the TDI is just one part of its integrated technology strategy to further reduce CO2 and the emission of pollutants in all models. Audi in the meantime relies on the potential of its combustion engines and on the components of its modular efficiency system: with direct injection and turbocharging, the TFSI gasoline engines and TDI diesel engines already provide convincing performance with comparatively low consumption, while their technologies form a solid basis for further significant reductions of consumption in coming years.

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