Audi SQ8 – Mild hybrid system with electric powered compressor (EPC)
An electric powered compressor (EPC) in Audi SQ8 TDI supplements the work of the two turbochargers when starting off and when accelerating at a low engine speed, allowing the V8 TDI to always build up its power spontaneously.
The EPC operates with a constant voltage of 48 volts. It is situated in the intake air path behind the intercooler and is active in the following two operating statuses: During start-up, the bypass closes and the EPC precompresses the air flow. This increases the filling of the combustion chamber. When driving, if the load requirement from the accelerator is high while the amount of energy available in the exhaust gas is low, the bypass flap closes and the intake air flows into the EPC, where it is compressed for the second time. A compact electric motor with an output of up to 7 kW accelerates the compressor wheel of the EPC to up to 70,000 rpm in approximately 250 milliseconds.
Increasing efficiency: the mild hybrid system
The drive power for the EPC comes from the 48-volt main electrical system of the Audi SQ8 TDI, which is also the backbone of the mild hybrid system (MHEV). A compact lithium-ion battery with an energy capacity of 0.5 kWh serves as its energy storage unit. The second major element of the MHEV system is the belt alternator starter (BAS), which is connected to the crankshaft. During braking, it can recover up to 8 kW of power and feed the energy into the battery. A DC/DC converter connects the 48-volt system and 12-volt electrical system.
The MHEV technology has the potential to reduce real-world fuel consumption by up to
0.5 liters (0.1 US gal) per 100 kilometers (62.1 mi). If the driver lifts off the accelerator at a speed between 55 and 160 km/h (34.2 and 99.4 mph), the Audi SQ8 TDI can, depending on the situation, either recuperate, roll at idle or coast with the engine deactivated for up to
40 seconds. The BAS restarts the engine the next time the accelerator is depressed, and it does so faster or more gently than a conventional starter. Start-stop operation begins at a residual speed of 22 km/h (13.7 mph).