Monday, 7 March 2016
Audi adopts 48V electrical compressor for V8 TDI
Audi claims its ‘new’ 4-litre V8 diesel engine makes the new SQ7 TDI “the most powerful diesel SUV on the market”.
Audi adds that the engine makes the car the world’s first series production car to combine turbocharging with an electrically-powered compressor (EPC).
Engineers also claim the electrically-powered turbo “consigns ‘turbo-lag’ to history”, although it does require a 48V sub-system.
The new Audi SQ7 TDI with its “redesigned” V8 BiTDI engine now offers drivers 435 PS and 900 Nm (663.8 lbft) of torque. The combined power and torque offer 0-62 mile/h in 4.8 s. Top speed is limited to 155 mile/h, and in the New European Driving Cycle (NEDC), the SQ7 TDI returns up to 38.2 mile/gal combined. This corresponds to CO2 emissions of 194 g/km.
The 3.956-litre TDI, with its compacted graphite iron (CGI) vee cylinder block, has two exhaust-gas turbochargers activated selectively using sequential charging. Thus exhaust gases flow through one turbocharger at low and intermediate load while the second turbine is activated only at higher loads.
The EPC augments the work of the two turbochargers, particularly in the lower engine speed range, providing responsive off-the-line performance.
Forced induction concept and electrical system: Electric powered compressor (EPC) and 48 volt subsystem
Audi claims its electrically powered compressor is a world first in a production vehicle. The EPC is placed in the air path downstream of the intercooler, close to the engine.
As the EPC does not require any exhaust-gas energy to develop boost, it can be used at any time, thus making it the solution for the traditional weaknesses of the classic exhaust-gas turbocharger. With this technology, turbo lag is history.
Audi launches valve-lift
The EPC provides the engine with the boost energy in under 250 milliseconds. Driven by a compact electric motor, its compressor wheel spins up to 70,000 rev/min.
The Audi valve-lift system (AVS) also makes its debut in a diesel model from Audi. The inlet and exhaust camshafts 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 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.
Power for the EPC, which reaches a maximum of 7kW, is provided by the 48-volt electrical subsystem.
The SQ7 TDI requires a 48V electrical system to facilitate the higher-power systems supporting the electromechanical active roll stabilisation (EAWS) and EPC. In order to meet their high power and energy requirements, the electrical subsystem requires its own 48V lithium-ion battery which is mounted beneath the luggage compartment. It has a nominal energy content of 470Wh and peak output of up to 13kW. A DC/DC converter connects the 48V and 12V electrical systems.
The required energy for the system is provided by a more powerful generator of 3 kW output. This MOSFET (Metal Oxide Semiconductor Field Effect Transistor) generator is said to reduce electric losses and increase efficiency. MOFSET replaces diodes used previously.
In addition, the 48V storage unit supports the 12V electrical system when required, reducing the load on the 12V lead acid battery