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
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