|July 10, 2000
The new Audi A8 3.3 TDI quattro: Top TDI for the luxury class
Audi is now offering its A8 saloon with a new high-performance TDI engine, the 3.3 litre V8. With a torque of 480 Nm from 1800 to 3000 rpm, a power output of 165 kW (225 bhp) at 4000 rpm and outstanding acoustics and refinement, this engine underscores Audi's TDI expertise once more.
Audi perfects these qualities with the V8 TDI. From a capacity of just 3.3 litres the engine develops a peak torque of 480 Nm that is already available at 1800 rpm and remains at this level up to 3000 rpm. Its best specific consumption is just 205 grams per kilowatt hour and thus extremely low for an eight-cylinder engine. With a high specific output of almost 50 kilowatts per litre displacement, it is the absolute frontrunner compared with the competition.
Equipped with quattro permanent four-wheel drive as standard, the most powerful A8 with diesel engine majestically converts this potential into maximum dynamism, while remaining easy for the driver to control. It will come as no surprise that the performance figures of the A8 3.3 TDI quattro qualify it for the sports car league. This luxury-class saloon, which has a 5-speed tiptronic transmission as standard, reaches 100 km/h in 8.2 seconds. Its top speed is even more impressive at 242 km/h. However, one thing that may come as a surprise in view of this high performance level is the car's fuel consumption: the A8 3.3 TDI quattro requires just 9.7 litres of diesel per 100 kilometres (total fuel consumption according to 93/116/EC).
At the same time, four valves per cylinder, common rail injection, two electronically controlled turbochargers with variable turbine geometry (VTG) and cooled charge-air and exhaust-gas recirculation also help the Audi A8 3.3 TDI quattro to comply with the strict emission limits as specified in EU III.
Since 1989 Audi has revolutionised the diesel world with its TDI engines. This is an engine concept that can also compete with the spark-ignition engine in terms of dynamics and driving pleasure, while consuming around 30 percent less fuel and producing outstanding torque even at the lowest engine speeds. The TDI thus strikes a balance between seeming opposites, combining the pleasure derived from sporty driving with environmentally conscious mobility and the ability to cover a considerable distance on a single tank of fuel.
High tech helps to save weight
The long-stroke engine, with its 86.4 mm stroke and 78.3 mm bore, has a 90° angle between its cylinder banks. The cylinder block is of vermicular-graphite cast iron (VGC), a high-strength material, and is therefore over ten percent lighter than a grey cast iron construction.
In contrast to conventional designs with individual bearing covers, Audi has for the first time used a particularly sturdy, laterally supported main bearing frame, which is bolted several times from below - beneficial both in terms of strength and acoustics.
The oil sump is raised level with the centre of the bearings. This helps to reduce sound radiation thanks to the acoustic decoupling of the crankshaft bearings and the oil sump.
The two four-valve cylinder heads are made from aluminium. The geometry of the two intake ports, with swirl and charging ports, together with the offset star-pattern valve layout and the central position of the injectors, which are perfectly vertical in the cylinder axis, ensures optimum combustion conditions.
For the 3.3 TDI Audi has used six-jet injection nozzles and a suitably designed piston with a broader and flatter combustion chamber recess. Pre-injection has improved the combustion quality, especially when cold starting, to such an extent that it has been possible to reduce the compression ratio to 18.5:1. This is low for a direct-injection engine and results in a favourable balance between power output and emissions together with good cold-starting behaviour.
The broader piston bowl and the six-hole injection nozzle, together with the two intake ports, produce a virtually ideal swirl action in the bowl, which means that it was not necessary to use controlled swirl-type flaps. The piston is oil-cooled by cast-in cooling ducts.
Low fuel consumption, low emissions
As a twin turbo engine, each cylinder bank of the V8 TDI has its own VTG turbocharger with adjustable guide vanes. Maximum absolute pressure is approx. 2.2 bar. Directly behind the turbocharger, the exhaust gas flows through two primary catalytic converters, with the two main catalytic converters situated under the vehicle floor. In order to prevent the exhaust gases from cooling down, which would reduce catalytic efficiency and increase emissions, the manifolds and the pipes between primary and main catalytic converters are designed as a double-shell construction with air-gap insulation.
Special attention has been devoted to the intake system. The intake path has been designed so that the same volume of air is supplied to both turbochargers. Two hot-film air-mass meters with precision sensors coordinate the turbochargers.
In addition, the charge air is cooled in an air-to-water cooler which causes a much lower pressure loss than an air-to-air cooler. Thanks to this efficient Audi-specific solution, it was possible to save space by accommodating the intercooler in the "vee" of the two cylinder banks. The cooling water for the intercooler has its own low-temperature cooling circuit with electric water pump and auxiliary cooler.
This very compact construction with its short flow paths makes optimum use of the available space while keeping pressure loss to a minimum. In this system the charge air is cooled to around 80 degrees Celsius. Moving the intercooler inside the "vee" has helped to keep the engine extremely short at only 717 mm. It is also just 842 mm wide and 688 mm high.
The recirculated exhaust gas is also cooled by water. This reduces NOx emissions by a further 20 percent or so, and also has a positive effect on particle formation. Although HC and CO are increased slightly, this is compensated for by the catalytic converters installed close to the engine. Thanks to the air-gap-insulated sheet-metal manifolds, the catalytic converters reach their operating temperature and thus their maximum efficiency very quickly, and the response of the turbochargers is also improved further.
Common rail piles on the pressure
In contrast to the V6 TDI, which operates with the radial-piston pump VP 44, a common rail injection system is used on the 3.3 TDI. The high-pressure pump builds up a pressure of 1350 bar. This principle ensures that the amount of fuel delivered and compressed is only slightly greater than the amount actually injected. An intake restrictor is used to reduce both the pump's power consumption and fuel heating. A fuel cooler in the return line lowers the temperature still further.
Each cylinder bank is supplied by its own common rail, with very short connecting lines to the injectors. The pilot and main injection stages are controlled by an electric solenoid valve in the injector head.
The same toothed belt that actuates the two intake camshafts also drives the high-pressure pump. This is located together with the two combined coolers for charge air and exhaust-gas recirculation within the "vee" between the cylinder banks.
The high-pressure pump, which operates continually unlike cam-actuated injection systems, clearly reduces stress on the toothed belt, thus ensuring that the drive belt has a long service life.
The strengths of the common rail system are the high pressure it supplies even at low speeds and during part-load operation, the flexible start of injection with pilot and main injection and the pump's constant power consumption.
Audi has already produced just under one million TDI vehicles. The new Audi A8 3.3 TDI quattro is the current highlight in a development that has revolutionised diesel drive in the car sector and made a major contribution towards Audi's image boost in accordance with its motto "Vorsprung durch Technik". There is no other system today that offers the same high efficiency and can thus also meet the demands of passenger-car traffic in future. This new high-tech eight-cylinder TDI will surely make a great many new friends - especially in the prestigious luxury market segment where the diesel sound, which can now hardly be distinguished from a petrol engine, has never been particularly popular.
Audi A8: lightweight design and luxury
By virtue of its revolutionary general concept with Audi Space Frame ASF, quattro permanent four-wheel drive and TDI technology, the Audi A8 has stood for pioneering technical progress and responsible mobility since its market launch back in 1994. The latest developments in the top segment of the vehicle market demonstrate how much the A8 has since become a role model in its class.
The A8 has reversed the upward weight spiral in the luxury class and introduced a new trend in car manufacturing. Despite the generous amount of space available inside and luxurious standard equipment, its body weighs 140 kilograms less than functionally identical steel bodies. This adds up to outstanding performance coupled with extremely economical fuel consumption.
This special lightness is made possible by a body concept which can still be described as trendsetting. Its central component is the Audi Space Frame ASF. This high-strength frame construction consisting of aluminium sections and cast nodes also functions as a safety cage. The passenger cell is considerably more rigid than is the case with comparable steel bodies, and offers the occupants outstanding protection.
This extraordinarily rigid ASF construction on the Audi A8 also provides for excellent ride comfort and exceptional handling. The suspension, which is also manufactured exclusively from aluminium, significantly reduces unsprung masses thus guaranteeing perfect running characteristics and initial suspension response.
The active safety of the Audi A8 is also of the highest calibre. The standard Electronic Stability Program ESP helps the driver to overcome potentially critical driving situations supremely well.
The Audi A8 also sets standards in the field of passive safety with its standard head airbag system SIDEGUARD: an airbag on each side of the car for the head and shoulder region protects front and rear vehicle occupants simultaneously.
Three-point belts for all seats with pyrotechnic belt tensioners and side airbags, integrated into the outboard seat backrests, and full-size front airbags for driver and front passenger round off the safety equipment.
Even the standard equipment specification for the Audi A8 satisfies practically every requirement - from the luxurious materials in the interior to deluxe automatic climate control and the Driver Information System DIS, the A8 offers every attribute that is in keeping with the ambience of a luxury saloon. There are in addition numerous other possibilities for A8 drivers to configure their exclusive dream car. Navigation systems with or without TV reception, an extensive range of wheels, audio systems of concert hall quality and natural leather equipment and trim - just a few of the many optional extras available for the Audi A8.