The two exhaust gas driven turbochargers, mounted within the 90-degree angle of each cylinder bank to provide them with short flow paths, run in a sequential process, with the smaller turbocharger engaged at low and intermediate throttle loads and the second larger turbocharger activated at higher loadings. The electrically powered compressor, or EPC has commonly referred to, augments the efforts of the turbochargers by increasing the throughput of air within the induction system at lower low throttle loads in a bid to improve overall response.
Modern day diesel engines tend to drop their revs quite markedly when you back away from the throttle as result of the induction pressure being dramatically reduced as the boosting effect of the turbochargers is halted.
The idea behind the new induction process applied by Audi to its new V8 is to use the additional air forced into the induction system by the EPC, whose compressor wheel spins at up to 70,000 rpm, to keep the smaller turbocharger primed and ready to spin back up to its maximum boost pressure as fast as possible so that full torque is available virtually the moment you get back on the throttle.
“The electrically driven compressor in the SQ7 TDI is a world first, with which Audi underscores its claim Vorsprung durch Technik,” said Stefan Knirsch, member of the AUDI board responsible for technical development.
In the SQ7 TDI the new engine produces 429bhp and 663lb ft of torque on a band of revs between 1000 and 3250rpm. This is 94bhp and 74lb ft more than Audi’s old twin-turbocharged 4.2-litre V8 diesel used in the superseded first-generation Q7 4.2 TDI. However, it is not the most powerful diesel to feature in the top-of-the-line Audi SUV. That honour rests with the discontinued twin-turbocharged 6.0-litre V12 diesel engine, which produced 493bhp and 737lb ft in the memorable Q7 6.0 TDI.
Predictably, the new V8 receives a selective catalytic converter, which is integrated into a particulate filter and uses urea injection to reduce NOx levels. In keeping with the sporting pretensions behind the latest Q7 model, it also uses a sound actuator that allows the driver to vary the acoustic qualities, with the synthetic exhaust note played back through the speakers.
The stout reserves of the SQ7 TDI are fed through an eight-speed automatic gearbox and Audi’s Torsen torque sensing four-wheel drive system with a self locking centre differential to all four wheels. Buyers can also specify an optional sport differential, which provides a torque vectoring effect with a variable amount of drive between each of the individual rear wheels dependant on traction levels and wheel speed, making it the only Q7 model to offer it.
Audi quotes an official 0-62mph time of 4.8sec – some 1.3sec faster than the most powerful of existing Q7 models, the 328bhp supercharged 3.0-litre V6 petrol engine powered 3.0 TFSI. Top speed is limited to 155mph. Combined cycle consumption on the European test cycle is put at 38.2mpg, endowing the SQ7 TDI with an average CO2 emission figure of 194g/km.
Although not yet made official, Autocar can reveal the new V8 forms the basis for smaller V6 diesel featuring a similar electric powered compression (EPC) enhanced induction system.
The chassis of the SQ7 TDI is described as the most advanced applied to a road going Audi model yet. Together with the electro-mechanical four wheel steering, air suspension and adaptive damping functions brought over from standard second-generation Q7 models, it also uses a new electro-mechanical roll stabilization system similar to that available on the latest BMW 7-series. Fitted with an electric motor boasting a three-stage planetary gearbox that separate the two halves of the stabilizer bar, it is claimed to significantly reduce lean. In off-road driving, the stabilizers are decoupled.