- Published in News.
Audi is the latest OEM to apply dual-stage turbocharging to a six cylinder diesel. The series-sequential system increases the 3.0-litre V6's output to 230kW/ 650Nm – benchmarks in the class – and appears first in the A7 Sportback and A6 sedan.
OEMs looking for high specific ratings are reaching the limits of single stage boosting. Dual-stage systems enable higher torque figures, which increase performance. This engine cuts the A7 Quattro's 0-100km/h time by 1s to just 5.3s.
The basis for development is the 2,967cc V6 diesel which, with a single turbo, develops either 150kW/ 400Nm or 180kW/ 500Nm. Changes to the boosting and fuel injection system deliver the higher ratings.
Audi's head of diesel engine development, Richard Bauder, said: “With the new V6 TDI Biturbo we now have an optimum engine family with efficiency-and performance-oriented variants. The Biturbo has very high torque – the prerequisite was an innovative concept for stepped turbocharging.”
A Honeywell system featuring variable turbine geometry on the smaller, high-pressure turbo is used. The larger turbo is regulated with an electronically-controlled wastegate.
Below 2,300rpm the smaller turbo does all the work, reducing pumping losses and improving transient response. The two turbos work together is series until 3,400rpm when the larger turbo supplies all the boost. Maximum engine speed is 5,200rpm – relatively high for a big diesel.
Packaging sequential turbos in vee engines is challenging – Audi located the system at the back of the engine. “The low-pressure turbo is positioned transversally; the high-pressure turbo is mounted longitudinally,” said Bauder.
The Bosch commonrail system now operates at 2,000bar and uses eight-hole piezo injectors. Changes to the piston crown geometry reduce the compression ratio from16.8 to 16.0:1, reducing NOx emissions.
Peak combustion pressure remains unchanged at 185bar but is maintained over a wider range of the engine's operating map. To cope with the extra thermal loads the cylinder heads get two-part water jackets for improved cooling.
The high torque rating enabled Audi to increase the overall ratio of the eight-speed automatic transmission by 7%, helping to reduce fuel consumption. The increased performance still comes at a price – the A7 Quattro's emissions go from the 180kW variant's 158g/km CO2 to 169g/km.
The Audi V6's output is the highest in the class. BMW's 2,993cc straight six uses a BorgWarner series-sequential system with a variable geometry turbo on the high-pressure turbo and develops 225kW/ 600Nm.
Jaguar's 2,993cc V6 has a Honeywell parallel-sequential system, featuring variable geometry on the low pressure turbo, and develops 202kW/ 600Nm.