Superior power and exemplary efficiency – the new A8 makes its debut on the market with a choice of two powerful V8 engines, one gasoline and the other a TDI, with a six-cylinder TDI to follow later. All three engines feature direct fuel injection and consume much less fuel than their predecessors – as much as 22 percent less. A second variant of the 3.0 TDI with 150 kW (204 hp) and front-wheel drive will follow at a later date. It is amazingly efficient. In the EU driving cycle, the new engine consumes only 6.0 liters/100 km (39.20 US mpg), analogous to 156 g CO2 per km (251.06 g/mile).
Audi uses new technologies from its modular efficiency platform in all three engines. One of these is the recuperation system. Whenever the driver lifts off the gas or brakes, the intelligent management system slightly increases the voltage of the generator so that it recharges the battery, which features a particularly high cycle strength. Electricity is fed back during subsequent acceleration, and the engine has to use less power to drive the generator. The generator uses a novel water cooling system. More efficient than an electric fan, this alone brings a benefit of roughly 0.1 liters/100 km.
The innovative thermal management is another Audi advancement. It reduces fuel consumption by around three percent. Many car trips cover less than 15 kilometers (9.32 miles), which is why an intelligent cold start and warm up program is so important.
In the new A8, electronically controlled valves completely disconnect the engine from the cooling system for a brief time following a cold start. This quickly gets the motor oil up to operating temperature and shortens the phase of elevated friction losses substantially. Once enough hot cooling water is available, the transmission oil for the eight-speed tiptronic is heated using a heat exchanger. A proprietary computer controls a multiplicity of actuators that ideally distribute the flows of heat between the engine, the transmission and the cabin in any situation – in the city and on the highway, in summer and in winter.
Efficiency in stop-and-go traffic: 3.0 TDI with start-stop system
The new 3.0 TDI is paired standard with a start-stop system. When the car comes to a stop at a traffic light or an intersection and the driver keeps the brake depressed, the system’s control unit shuts down the engine. On a hill or a downgrade, the brake system maintains the brake pressure unless the slope is greater than ten percent. A powerful starter restarts the engine quickly and conveniently as soon as the driver lets off the brake.
The start-stop system works with a particularly strong battery that continues to function even at very low outdoor temperatures. Only during the initial warm up period of the engine is the battery still inactive so that the motor oil, the coolant and the interior can be brought to temperature more quickly. The driver can deactivate the start-stop system at any time by pushing a button. In the standardized driving cycle, the start-stop system reduces consumption by roughly 0.4 liters/100 km or around 8 grams of CO2/km (12.87 g/mile). The increase in efficiency is even more pronounced in everyday driving. The majority of the high percentage of short trips mentioned above take place in city traffic.
Trim and strong: the 4.2 FSI
The 4.2 FSI draws 273 kW (372 hp) of power from 4,163 cc displacement, generating its maximum of 445 Nm (328.22 lb-ft) torque at 3,500 rpm. It accelerates the A8 from zero to 100 km/h (0 to 62.14 mph) in 5.7 seconds on its way to an electronically governed top speed of 250 km/h (155.34 mph). In the EU cycle, the powerful gasoline engine is content with 9.5 liters/100 km (24.76 US mpg) – a reduction of 1.4 liters or 13 percent compared to the previous model despite a 16 kW (22 hp) increase in power. The CO2 emission rate is only 219 g/km (352.45 g/mile).
Like all of the engines in the new Audi A8, the 4.2 FSI is a long-stroke model with a 90 degree cylinder angle and a cylinder spacing of 90 millimeters. A two-stage chain drive, placed at the back of the engine to save space, drives the four camshafts, which can be continuously moved through 40 degrees of crankshaft rotation to ideally fill the combustion chambers. A fourth, particularly quiet running chain drives the ancillaries.
New triovals – minimally triangular chain wheels – allow substantially lower tensile forces. They therefore contribute every bit as much to the reduction of internal friction as the regulated oil pump that switches between two pressure stages as a function of engine speed and load. The driver of the A8 can check the motor oil level at any time in the MMI.
The cylinder case of the 4.2 FSI is made of a lightweight aluminum/silicon alloy. The cylinder barrels are finely honed by exposing the hard silicon crystals. An improved method and optimized piston rings also reduce friction here. The lands between the cylinders are only 5.5 millimeters (0.22 in) wide. The V8 is unusually trim, weighing only 197 kilograms (434.31 lb), which has a positive effect on the total weight of the A8 and on the distribution of axial loads.
Like all gasoline engines from Audi, the V8 also uses FSI direct injection. The plastic intake manifold switches between two stages; pneumatically actuated flaps control its length and the tumble. The newly developed injectors of the common rail unit inject the gasoline into this roller-shaped movement of air at a pressure of 140 bar. It vaporizes on the walls of the combustion chamber, thus cooling them. This allows a high compression of 12.5:1 for greater efficiency. All of this activity is managed by a state-of-the-art engine management unit.
The V8 has an appealing, very cultivated sound throughout the entire speed range. Among the many measures employed by Audi to further improve the acoustics is a torsion damper that decouples the drive for the air conditioning compressor. Pressure losses through the engine’s airways were minimized – from the new dual admission tract at the front of the vehicle to the exhaust system, whose backpressure has been reduced by 20 percent. The two catalytic converters sit directly behind the exhaust manifolds so that they quickly commence operation after the engine is started. The entire exhaust system weighs eight kilograms (17.64 lb) less than in the previous model, with higher-grade steels that get by with thinner walls making the difference here.
Massive pulling power: the 4.2 TDI
The 4.2 TDI is also a superior high-tech engine. Its displacement of 4,134 cc yields 258 kW (350 hp) and a hefty torque of 800 Nm (590.05 lb-ft), already available at speeds between 1,750 and 2,750 rpm. The V8 dispenses its power almost casually, moving the A8 like a sports car. The sprint from a standing start to 100 km/h (62.14 mph) takes 5.5 seconds. Top speed is electronically governed to 250 km/h (155.34 mph).
This makes the standardized consumption of only 7.6 liters/100 km (30.95 US mpg), an equivalent of 199 g/km (320.26 g/mile) CO2, all the more impressive. This is a full 1.8 liters/100 km or 19 percent less while power has increased by 18 kW (24 hp) and torque by 150 Nm (110.63 lb-ft). The Audi A8 4.2 TDI is the only V8 diesel engine in the luxury class with CO2 emissions below 200 g/km (321.87 g/mile).
The advancements with the 4.2 TDI are primarily the result of the increased injection pressure of the common rail system. The piezo injectors shoot the fuel out of the eight-hole nozzles at a pressure of 2,000 bar. Cylinder pressure increased from 160 to 180 bar. The crankshaft, the pistons and their oil spray nozzles, the cylinder head, its gasket, and the oil cooler all underwent modifications to increase durability and performance. The crankcase with its monobody main bearing frame continues to be made of vermicular graphite cast iron, a sophisticated type of cast iron. The extremely strong material enables low wall thicknesses, which reduce weight.
The reduction of internal friction was another point of emphasis during the refinement of the 4.2 TDI. It manifests in the new piston rings with lower tension, in softer valve springs, in the optimized seal rings at the crankshaft and camshaft, in the two-stage oil pump like the one used in the 4.2 FSI and in the need-based, electric, in-tank fuel pump, which eliminates the need for the mechanical prefeed pump in the high-pressure injection pump. The diesel uses the same innovative thermal management as the gasoline engine.
The weight of the TDI dropped to 254 kilograms (559.97 lb). The cooling water pipes are now made of aluminum instead of steel; aluminum add-on parts such as the oil pan, the chain box, and the intake module have become lighter. The common rail system lost 1.2 kilograms (2.65 lb); the exhaust system a full 11 kilograms (24.25 lb).
The two water-cooled turbochargers with variable geometry and charge air cooling are also members of the latest generation. Thanks to improved compressor wheels, the turbos can develop high boost pressures at low speeds. Sensors monitor the work of the turbos and ensure that they are always working in their optimal operating ranges. The exhaust gas recirculating system received a modified cooler that is supplied with cold water via a separate loop. This strongly cools the recirculated exhaust, and combustion chamber temperatures and thus nitrogen oxide emissions are reduced.
The A8 4.2 TDI can travel more than 1,100 km (683.51 miles) on a single tank of fuel. The 90 liter (23.78 US gallon) tank located between the rear wheels is another high-tech component. Rubber elements decouple the plastic body from the car body, and intermediate walls attenuate the sloshing sounds of the fuel. An expansion tank collects the additional volume arising due to thermal expansion; five valves provide for perfect venting.
A whole new level of efficiency: the new 3.0 TDI
Shortly after the market launch, the two V8 engines will be joined by a six-cylinder version – the 3.0 TDI. It is new from the ground up – all that was carried over from the previous version of the engine are the 90 degree V, the cylinder spacing of 90 millimeters (3.54 in), and the displacement of 2,967 cc. All major components are new developments, from the cylinder block to the crankshaft and the connecting rods to the cylinder heads.
Like the two eight-cylinder engines, the V6 diesel, which now also operates at an ignition pressure of 180 bar, also combines strong performance with exemplary efficiency. It produces 184 kW (250 hp) from its 2,967 cc displacement and delivers 550 Nm (405.66 lb-ft) to the crankshaft at 1,500 to 3,000 rpm – increases of 13 kW (17 hp) and 100 Nm (73.76 lb-ft).
Consumption, on the other hand, decreased by 1.9 liters, or 22 percent, to 6.6 liters/100 km (35.64 US mpg). The 3.0 TDI has CO2 emissions of only 174 g/km (280.03 g/mile) – easily best in the segment. The V6 diesel accelerates the new Audi A8 from zero to 100 km/h (62.14 mph) in 6.6 seconds and to a governed top speed of 250 km/h (155.34 mph).
An important development priority was the reduction of weight. The new V6 weighs only 194 kilograms (427.70 lb), a full 23 kilograms (50.71 lb) less than its predecessor. The development engineers saved six kilograms (13.23 lb) with the crankcase alone – which like in the V8 TDI is made of vermicular graphite cast iron – four kilograms (8.82 lb) with the cylinder heads, and 2.5 kilograms (5.51 lb) with the crankshaft. The bolts and water pipes are made of aluminum instead of steel; the oil pan is made of magnesium and aluminum.
High-tech machining: laser exposure and plate honing
The 3.0 TDI is not only lightweight, it is also low friction. Audi uses two high-tech methods during its production – the laser exposure of the cylinder barrels and a new solution called plate honing, in which a plate is bolted onto the crankcase prior to the mechanical honing of the cylinder barrels. This simulates the pretension that the cylinder head will later exert late and which results in the minimal deviations from perfect roundness. This effect is then accounted for during honing, which is why Audi can fit the pistons with more slender rings that exhibit lower preload forces.
Additional measures for reducing friction target the chain drive. Two chains are enough to drive the four camshafts, the balance shaft, the oil pump, and the common rail system’s high-pressure pump. The previous engine still used four chains and a toothed belt.
The injection system, which likewise uses piezo injectors, is particularly efficient thanks to a new pump concept. Located in the air intake module is a central swirler flap that replaces the separate swirler flaps in the intake ducts that swirl the air and thus improve torque development at low engine speeds.
The single flap keeps pressure losses during intake low. The exhaust system and the charging air path of the turbo were also designed for low-loss flow. The mounting of the compressor shaft in the water-cooled turbocharger was remounted. The low-friction grooved ball bearings provide for snappy response, particularly from low speeds.
The new 3.0 TDI also has Audi's innovative thermal management system on board – a particularly advanced version of it. The crankcase and the cylinder heads each have their own cooling water loops connected to one another via a valve. During the warm up phase, the coolant in the block is not circulated. This brings the engine oil up to temperature quickly, in part because its oil cooler is bypassed during this phase. Water can often be left standing in the crankcase even when the engine is operating at low loads.
The coolant circulating through the heads heats the cabin and also feeds the cooler of the exhaust gas recirculation system, a high-performance component. The new exhaust recirculation concept is another module of the 3.0 TDI’s efficiency package – the predecessor engine still used an electric coolant pump, which is no longer needed.
In addition to the 3.0 TDI with 184 kW (250 hp), Audi will also offer a second version of the V6 with 150 kW (204 hp). Driving exclusively the front wheels, its efficiency is exemplary. In the EU driving cycle the new engine consumes only 6.0 liters/100 km (39.20 US mpg), or 156 g CO2 per km (251.06 g/mile). A particularly low-emission, clean diesel version that will already satisfy the Euro 6 standard scheduled to take effect in 2014 is currently also in preparation.
The equipment, data and prices specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.