Eye Of The Needle

With its all-new 3L DOHC V-6 turbo-diesel used for the range-topping A8 sedan, Audi AG merges several advanced-design concepts that not only enhance performance but also reduce ecological impact and physical bulk.

The new turbodiesel develops 233 hp at 4,000 rpm — and maximum torque is a mighty 332 lb.-ft. (450 Nm), which is maintained from 1,400 to 3,250 rpm.

Audi says the new V-6 is the first production diesel to combine common-rail fuel injection with piezoelectronic fuel injectors that situate the piezo elements inline with the injector needle.

Provided by Robert Bosch GmbH, the third-generation fuel system continues the previous maximum 23,200-psi (1,600-bar) injection pressure, but now channeled through the new-design piezo injectors with 75% fewer moving parts and subsequently reduced mass.

Bosch's low-mass and ultra-fast piezo design allows needle switching at twice the speed of earlier solenoid and piezo injectors that do not have the piezo elements inline with the injector needle.

The Bosch injectors can deliver up to five injections per operating stroke into the combustion chamber through 7-hole nozzles. Improved mixture formation has enabled the turbodiesel's compression ratio to be reduced to 17:1, with claimed overall benefits in power, emissions, fuel economy and combustion noise.

A DOHC/4-valve-per-cylinder layout has each camshaft of the two cylinder banks operating two inlet and two exhaust valves. Variable-swirl flaps are integrated in the inlet tracts to adjust airflow and turbulence according to engine speed and load. The flaps are electronically controlled to match engine load and speed.

Valve operation is via roller cam followers with hydraulic lash adjusters. Camshafts are driven by a complex array of timing chains; an exceptionally long primary chain on the crankshaft drives the injection pump, balance shaft and a pair of dual-diameter idler wheels. The inner coaxial sprockets of the two wheels each engage a short chain, which drives the exhaust camshaft of each cylinder bank. The inlet camshafts are driven from the exhaust cams by zero-backlash gears to reduce noise.

A refinement in the variable-geometry turbocharger is electronically controlled angling of the turbine blades. Optimizing the angle of the turbine blades exposed to the exhaust flow generates faster buildup of turbo boost pressure, particularly at low engine speeds, enhancing throttle response.

The turbocharger's individually pivoted guide blades all are linked by radial arms to an encircling ring supported on rollers. Rotation of the ring by just a few degrees tilts the blades as required. This limited movement is handled by a servo motor through a gear train with final crank action.

There are two charge-air intercoolers — uniquely mounted under the car's headlights in order to maximize the ram-air effect. They are connected in parallel to minimize pressure losses and feed-air temperature.

Other notable engine features are air-gap insulated exhaust manifolds to conserve engine heat, and water-cooled exhaust gas recirculation (EGR).

For engine bulk reduction, the entire timing chain system is at the transmission end of the block, cutting overall length of the V-6 to a compact 18 ins. (44 cm). Weight of the complete engine is kept to 485 lbs. (220 kg) partly by casting the block in vermicular graphite iron, saving about 15 lbs. (7 kg) over the standard cast iron used for the previous V-6. Also contributing to weight reduction is the large aluminum casting combining the main bearing deck and oil sump.