Audi is renowned for winning races and it announced another first at its Annual General Meeting at Ingolstadt, Germany, last week: The first application of an electric-powered compressor on a series production car.
The EPC (Electrically Powered Compressor), with its compact electric motor, is to be used on the first diesel “S” version of Audi’s flagship SUV, the 7-seat Q7, complementing a pair of sequential turbochargers to boost output to 320 kW (429 hp) and peak torque to 900 N·m (664 lb·ft).
The 2017 SQ7 also gets electromechanical active body roll stabilization (EAWS) and a 48-V electrical subsystem. And Audi’s Valvelift System is used on a VW Group diesel engine for the first time.
Turbo lag “is history” according to Audi executives— at least in this model. But expect the technology to be cascaded through the brand's ever-widening range. At the AGM, Audi Chairman Dr. Rupert Stadler announced the company will invest more than €3 billion in 2016 for future mobility technologies, while pushing forward "with the electrification and digitization of our products.”
e-booster eliminates turbo lag
The new EPC technology in the SQ7 "is a world first in the competitive environment,” claimed Dr. Stefan Knirsch, Member of the Audi Board of Management for Technical Development. The Audi SQ7’s engine is described by the company as a redesigned V8 BiTDI. Dr. Knirsch claimed that the electrically powered compressor "dispenses with any sign of turbo lag from step-off acceleration.”
The EPC is placed in the air path downstream of the intercooler, close to the engine and can deliver power in less than 250 ms. Its compressor wheel spins up to 70,000 rpm.
Claimed performance figures for the SQ7 include 0-100 km/h in 4.8 s. Top speed is limited to 250 kmh (155 mph), and its NEDC fuel consumption is 7.4 L/100 km combined, with CO2 emissions of 194 g/km. Audi describes this as the fuel consumption level of a 6-cylinder diesel.
The two exhaust-gas turbochargers are activated selectively, controlled by the sequential charging system. Exhaust gas flows through one turbocharger at low and intermediate load, the other at higher load. The EPC is particularly beneficial at lower engine speeds, so markedly improving off-the-line performance.
Another first for an Audi diesel is the use of the company’s AVS (Audi Valvelift System). The inlet and exhaust camshafts each have two cam contours per valve, explains the company. On the inlet side, one cam contour supports starting off in conjunction with the EPC, while the other optimizes cylinder filling and so power at high engine speeds. The AVS system on the exhaust side enables activation of the second exhaust-gas turbocharger.
The exhaust streams from the two exhaust valves are hermetically separated, with each driving one of the two turbochargers. In the lower engine speed range, one valve per cylinder remains closed, so that the full exhaust stream flows to the active turbocharger.
When load and engine speed increase, the AVS opens the second exhaust valves. This directs flow to and activates the second exhaust-gas turbocharger. The engine achieves its maximum output in this biturbo mode. The switching by the AVS is said to enable fast and precise activation of the second exhaust-gas turbine.
48-V powers active roll stabilization
Power for the EPC (maximum 7 kW) comes from the car’s 48-V electrical subsystem. This is also used to power the electromechanical active roll stabilization (EAWS).
The SQ7’s optional EAWS uses a compact electric motor with a 3-stage planetary gearbox separating the two halves of the stabilizer. On an uneven road surface, they are actively decoupled from one another for improved ride comfort. During faster driving, the tubes are interconnected and twisted against each other to “significantly” reduce body roll, claims Audi.
Together with the transmission, the electric motors produce anything up to 1200 N·m (885 lb·ft). The vehicle's front and rear stabilizers can be adjusted independently of each other. Use of a 48-V system provides more power, enabling the system to work faster and to be activated even at low speeds. The electromechanical active roll stabilization is maintenance-free.
A 48V lithium-ion battery mounted beneath the luggage compartment has a nominal energy content of 470 W·h and peak output of up to 13 kW. A DC/DC converter connects the 48V and 12V systems.
A MOSFET generator (Metal Oxide Semiconductor Field Effect Transistor) reduces electric losses and increases efficiency, with an efficiency of over 80% at an output of up to 3 kW. MOFSETs replace the diodes used previously. The 48-V storage unit supports the 12-V electrical system when required to reduce load on the 12-V lead battery.
The car drives through an 8-speed automatic transmission.
Electromechanical power steering, the Audi drive select driving dynamics system, and adaptive air suspension with S-specific tuning, are standard. Carbon fiber-ceramic disc brakes will also become available later this year following the SQ7’s launch (dependent on market). Wheels are 20 in with 285/45 tires, with options up to 22 in.
All-wheel steering is also an option and the car’s Quattro (AWD) system can be fitted with a sport differential.