Production car features electric powered compressor

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Audi electro-mechanical roll stabilisation

Andy Pye reports on the new Audi SQ7 with electrically powered compressor for keeping performance up and emissions down.

Tapping into extra accelerative power becomes as quick and easy as flicking a light switch, as the new 435PS Audi SQ7 TDI joins the range as the world’s first series production car, combining turbocharging with an electrically powered compressor bringing breathtakingly immediate response.

At the heart of the new Audi SQ7 TDI is a redesigned V8 BiTDI engine that now develops 435 PS and 900Nm (663.8lb-ft) of torque, firmly staking this performance flagship’s claim as the most powerful diesel SUV on the market. The electrically powered compressor dispenses with any sign of turbo lag from step-off acceleration, helping to set an outstanding 0-62 mph sprint time of just 4.8s. Top speed is limited to 155mph, and in the New European Driving Cycle (NEDC), the SQ7 TDI returns up to 38.2mpg combined. This corresponds to CO2 emissions of 194g/km.

The 3,956cc 4.0 TDI has been redesigned from the ground up. It combines best-in-class performance with impressive efficiency credentials through the use of two exhaust-gas turbochargers which are activated selectively according to the concept of sequential charging, since exhaust gas only flows through one turbocharger at low and intermediate load. The second turbine is only activated at higher loads. The electrically powered compressor (EPC) augments the work of the two turbochargers, particularly in the lower engine speed range, providing for extremely responsive off-the-line performance.

The electrically powered compressor (EPC) is believed to be a world first in a production vehicle. This EPC strongly supports the 4.0 TDI engine when starting off and accelerating from low load for exceptional take-up with no turbo lag. It is placed in the air path downstream of the intercooler, close to the engine. Because the EPC does not require any exhaust-gas energy to develop boost, it can be used at any time, thus solving the traditional weaknesses of the classic exhaust-gas turbocharger and eliminating turbo lag.

“Following the launch of the successful SQ5, we are now also applying the concept of a diesel-engined S model to the Q7 model line. So equipped, the SQ7 TDI with a V8 TDI engine achieves the consumption figures of a six-cylinder. The new technology of the electric powered compressor in the SQ7 TDI is a world first in the competitive environment,” said Stefan Knirsch, Member of the Board of Management of AUDI AG for Technical Development.

The EPC provides the engine with the boost energy needed for the dynamic delivery of power in less than 250ms. Driven by a compact electric motor, its compressor wheel spins up to 70,000 revolutions per minute. The 4.0 TDI thus develops its immense power immediately, with no perceptible lag.

The Audi valvelift system (AVS) is also making its debut in a diesel model from Audi. The inlet and exhaust cam shafts each have two cam contours per valve. On the inlet side, one cam contour supports starting off in conjunction with the EPC, while the other optimises cylinder filling and thus power at high engine speeds. The AVS system on the exhaust side enables activation of the second exhaust-gas turbocharger. The sequential charging system controls the two exhaust-gas turbochargers so that only one turbocharger is used at low engine speeds. The second is activated additionally at higher loads and engine speeds. The system exhibits very good torque delivery and dynamic response across the entire engine speed range.

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 enables fast and precise activation of the second exhaust-gas turbine.

The power for the EPC, which reaches a maximum of 7kW, is provided by the 48V electrical sub-system. The SQ7 TDI uses this higher-power electrical system, which facilitates the high-power systems electromechanical active roll stabilisation (EAWS) and EPC. To meet their high power and energy requirements, the electrical sub-system includes its own 48V lithium-ion battery mounted beneath the luggage compartment with a nominal energy content of 470Wh and peak output of up to 13kW. A DC/DC converter connects the 48-volt and 12-volt electrical systems.

The required energy is provided by a more powerful and highly efficient generator with an efficiency of over 80% at an output of up to 3kW. This MOSFET generator (Metal Oxide Semiconductor Field Effect Transistor), which reduces electric losses and increases efficiency. MOFSETs replace the diodes used previously. In addition, the 48V storage unit supports the 12V electrical system when required. This also reduces the load on the 12V lead battery.

The intake side of the 4.0 TDI is on the outside, the exhaust side with the two turbochargers is on the inside of the 90-degree internal V. This layout provides for short gas paths, meaning spontaneous response and short paths for the emissions control system. The common-rail system generates up to 2500bar injection pressure. Ignition pressure reaches the 200bar mark in broad sections of the characteristic.

The innovative thermal management and sophisticated measures in the crankshaft and camshaft drive reduce friction. With the combination of a NOx oxidating catalytic converter and a downstream SCR catalytic converter, which is integrated into the diesel particulate filter and uses AdBlue injection to reduce oxides of nitrogen, the V8 TDI has a particularly efficient emissions control system. A sound actuator in the exhaust system amplifies the sonorous eight-cylinder sound and SQ7 TDI drivers can vary the amount of sound it generates using the Audi drive select system.

The Biturbo V8 combined with the EPC is a core element of the global Audi diesel strategy. With high torque even at low engine speeds and low consumption figures, it is also ideally suited for markets such as the United States. Its engine characteristic combines supreme pulling power with impressive agility, exceptional step-off performance and spontaneous response.

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About Andy Pye

Andy Pye is a graduate of Cambridge University and has had a high profile career in the technical press as well as being a pioneer in web publishing.

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