For the compressor, Volkswagen engineers chose a Roots-type supercharger (also known as a “blower”). Unlike some other types of supercharger, a Roots supercharger doesn’t actually compress air within the device. With two counter-rotating lobes, it moves a fixed volume of air per rotation (“fixed displacement”). Compression occurs in the intake manifold.
Roots superchargers can deliver a large amount of boost even at low engine speed. The main disadvantage is that they create a lot of heat.
Tsi_airflow
Air flow through the VW Twincharged TSI. Click to enlarge.
The compressor and the turbocharger are connected in series. A control valve ensures that the fresh air required for a given operating state can get through either to the exhaust turbocharger or the compressor.
The control valve is open when the exhaust turbocharger is operating alone. In this case, the air follows the normal path as in conventional turbo engines, via the front charge-air cooler and the throttle valve into the induction manifold.
The compressor is operated by a magnetic clutch integrated in a module inside the water pump. Under turbocharging conditions, the clutch disengages the compressor.
The maximum boost pressure of the Twincharger is approximately 2.5 bar at 1,500 rpm, with the exhaust turbocharger and the mechanical supercharger being operated with about the same pressure ratio (approx. 1.53). The compressor alone delivers a boost pressure of 1.8 bar even just above idling speed.
A conventional exhaust turbocharged engine without compressor assistance would achieve only a pressure ratio of about 1.3 bar.
The more rapid response of the turbocharger enables the compressor to be depressurized earlier by continuous opening of the bypass valve. Compressor operation is restricted to a narrow engine map area with predominantly low pressure ratios and, therefore, low power consumption.
In practice, this means the compressor is only required for generating the required boost pressure in the engine speed range up to 2,400 rpm. The exhaust turbocharger is designed for optimum efficiency in the upper power range and provides adequate boost pressure even in the medium speed range.
For acceleration, an automatic boost pressure control decides if the compressor needs to be switched on to deliver the tractive power required, or if the turbocharger alone can handle the situation.
The compressor is switched on again if the speed drops to the lower range and then power is demanded again. The turbocharger alone delivers adequate boost pressure above 3,500 rpm.