Audi S6 and S7 gain 48v mild hybrid V6 diesel engines

The Audi S6 Saloon, S6 Avant and S7 Sportback join the Audi SQ5 in gaining 48-volt mild hybrid (MHEV) 3.0-litre V6 TDI engines.

An electrically-powered compressor working with a 48-volt primary electrical system combines TDI economy with 700 Nm of torque to give improved throttle response.

This takes the 3.0-litre V6 TDI’s total power output to 350 hp, which is transmitted via the eight-speed tiptronic automatic transmission to the quattro all-wheel-drive system.

The electrically-powered compressor (EPC) delivers high starting performance. As well as vigorous acceleration, the electric supercharger provides a repeatable boost function when accelerating which means the EPC pre-empts any hint of turbo lag.

The EPC’s response time is under 250 milliseconds, its peak output is seven kilowatts and its maximum speed is 70,000 rpm. The electric turbocharger’s boost function extends all the way to an engine speed of 1,650 rpm. Thanks to dual supercharging, ie. the EPC working in tandem with the exhaust turbocharger, the S models achieve a constant torque of 700 newton-metres (516.3 lb-ft) across an engine speed range of 2,500 to 3,100 rpm. 

The EPC supports the TDI’s turbocharger whenever there is insufficient energy in the exhaust gas for a spontaneous torque build-up – when starting off or accelerating at low load, from a low engine speed.

The EPC, which visually resembles a conventional turbocharger, is mounted directly on the engine in the intake air path behind the intercooler. In most operating conditions it is circumvented by a bypass. However if the load demanded by the accelerator is high and the energy available on the turbine side is low, the bypass valve closes, guiding the intake air to the EPC. The compressed air flows directly into the combustion chamber.

In everyday driving, the technology avoids frequent downshifts, keeps the engine speed level low and suppresses the turbo lag to which a conventional exhaust turbocharger is susceptible.

The V6-TDI complete with EPC propels the new S6 Saloon from 0-62mph in 5.0 seconds, with the S6 Avant and S7 Sportback taking a tenth of a second longer over the standard sprint. The top speed is electronically limited to 155mph.

While the EPC boosts the performance, the mild-hybrid system (MHEV) helps to improve fuel efficiency. The 48-volt MHEV system comprises a belt alternator starter (BAS) and a lithium-ion battery with a capacity of 10 Ah housed in the vehicle floor beneath the luggage compartment. The BAS is connected to the crankshaft. During deceleration it can recover up to 8 kW of power, which it then stores in the lithium-ion battery. When the driver accelerates again, the BAS reacts instantly by restarting the engine.

MHEV technology allows for start/stop operation from a speed as low as 13mph. Thanks in part to the integration of the mild-hybrid system and the vehicle sensors, the S models realise a fuel saving of up to 0.4 litres in real driving conditions and can coast for up to 40 seconds with the combustion engine deactivated.

On that basis, the S TDI models combine impressive driving dynamics with high efficiency, low fuel consumption and low emissions. All S TDI models have homologation to the Euro 6d temp emission standard. WLTP fuel economy values for the S6 TDI Saloon range from 35.8 mpg to 36.2 mpg, equating to NEDC-correlated CO2 emissions of 164 g/km. The figures for the S6 Avant are 34.9 mpg to 35.3 mpg and 171 g/km, and for the S7 Sportback 35.3 mpg to 35.8 mpg and 170 g/km.

As on every S model from Audi, quattro permanent all-wheel drive comes as standard. In normal driving conditions, its self-locking centre differential distributes torque between the front and rear axle in a 40:60 ratio. In the extreme, 70 percent can flow to the front wheels and 85 percent to the rear wheels.

The new large S models will be making their debut in mainland Europe and the UK in the summer of 2019.