Automotive Engineer

The more air you can force into the combustion chambers the more torque you can develop, and, with that, make vehicles accelerate more quickly. Turbochargers are the enabler, but when consumers demand ever higher outputs – not everybody wants the lowest-possible CO2 emissions – you may need more than one.

So sequential turbocharging systems are becoming popular with premium OEMs. They deliver excellent launch behaviour, transient response and high power. BMW and Jaguar Land Rover 3.0 engines use series- and parallel-sequential systems respectively.

Now Audi too has applied series-sequential boosting to its V6 diesel. It’s based on the single-turbo V6 introduced in 2010 in ratings from 150kW/450Nm to 184kW/550Nm. This latest variant, which the firm calls the TDI Biturbo, develops 230kW and 650Nm – 50Nm more than the competition. 

“The V6 Biturbo has very high torque – the prerequisite was an innovative dual-stage turbocharging system,” said Richard Bauder, Audi’s head of diesel engine development. “We now have an optimised engine family, with efficiency-oriented and high-performance versions.”

The base engine’s architecture is carried over largely unchanged but weight has increased by 16kg to 209kg, because of the extra turbocharger and associated pipes, ducts and valves.

Despite the extra power output, the compacted graphite iron cylinder block did not require any strengthening – peak firing pressure is still 185bar, although extended over a wider range of the operating map.

But the higher output meant higher thermal loads in the cylinder heads. Taking the existing castings as the starting point, simulations found hot spots in the material between the exhaust valves. These were considered excessive. The solution was the adoption of a split water jacket.

“We have much more intensive water circulation around the lands and because of this peak temperatures were reduced significantly, by up to 25°C,” said Bauder.

The pistons were uprated too. Peak temperatures around the rim of the bowl were too high so the oil splash-cooling was improved, and the salt-core cooling duct moved upwards to improve heat transfer. These changes delivered reductions of up to 11°C.

Despite the higher loads the pistons are still aluminium – Bauder said the material is good for up to 80kW/litre. Bowl geometry was changed to reduce compression ratio from the base engines’ 16.8:1 to 16.0:1. Rigidity has been increased by running the pins in a brass sleeve. And the pins now have a diamond-like carbon coating which, together with an optimised ring pack, reduces friction. 

Audi has twin-turbos on its V8 and V12 diesels, but this V6 is the OEM’s first application of a dual-stage system – so packaging is much more complex. To fit everything in, the module is located at the rear of the engine with the low-pressure turbo positioned transversly and the high-pressure turbo longitudinally.

The system is supplied by Tier One supplier Honeywell. The small high-pressure turbo has a variable geometry turbine; the larger low-pressure turbo has a wastegate. Maximum boost pressure is 3.2bar absolute and is regulated by a turbine switching valve and a compressor bypass valve.

The switching valve is integrated into the low-pressure turbine housing. Butterfly and side-mounted swing designs were evaluated, with the latter proving to have lower leakage and greater durability.

There are three modes of operation: only the smaller high-pressure turbo operates until the engine reaches 2,300rpm. Then both turbos work together until 3,400rpm, after which the larger low-pressure turbo does all the work until the engine reaches its 5,200rpm limit.

More charge air from the boosting system is only useful if it can be matched by the fuel supply. The Biturbo uses an uprated version of the Bosch 2,000bar piezo commonrail system with eight-hole nozzles.