TSI.

On the twincharger, Volkswagen combines an exhaust turbocharger with a Roots-type mechanical compressor. This involves two pistons rotating in opposite directions inside a housing, without making contact. The pump effect is generated by the opposing rotation of the two Roots pistons.

The operating principles of the compressor and exhaust turbocharger complement each other perfectly. The compressor has map control up to an operating speed of about 3,500 rpm. Its boost pressure is increased further by the in-series exhaust turbocharger. From 3,500 rpm on, only the turbocharger boosts the pressure in the cylinder.

The high-speed compressor compensates for any turbo lag towards the lower end of the engine speed range and bridges this effect. It is driven by the crankshaft via a maintenance-free belt drive with a ratio of about 1 to 5. Depending on the engine speed and accelerator position, the mechanical supercharger is engaged or disengaged by map control by means of a solenoid coupling.

By virtue of the compressor being driven directly from the crankshaft, the required boost pressure is available even at very low engine speeds. The twincharger thus guarantees a spontaneous flow of torque in every situation, including sudden transitions from coasting to full throttle, and provides a surprising amount of pulling power.

To suit the new technology, Volkswagen’s engineers also revised the petrol direct injection: the maximum injection pressure at the 6-hole injector, a design being used for the first time, is as much as 150 bar. The piston geometry with the specially shaped recess has been modified on the TSI. This measure ensures that the mixture preparation process passes off smoothly all the way from idling to full-load operation at more than 7,000 rpm. Efficient combustion and therefore more power and torque, together with lower emissions, are the result.

From idle speed up to an engine speed of approx. 2,400 rpm, the compressor operates continuously provided the driver requires a certain amount of power. Within the speed range of about 2,400 to 3,500 rpm, the compressor is engaged on demand via the solenoid coupling, for instance for overtaking.

A control flap adjusts the route of the fresh air drawn in: via the compressor, or directly to the exhaust turbocharger. When in the turbocharger-only mode, from about 3,500 rpm, the control flap is opened and the compressor shut down. The air then passes along the conventional route for turbo engines, via the front charge-air cooler and the throttle, into the intake manifold.

At these engine speeds, the exhaust turbocharger with wastegate control – which is designed for optimum efficiency at the upper end of the engine speed range – is the sole source of supercharging.
Each stage of the twincharger is thus always operated in its optimum range. This guarantees spontaneous throttle response, outstanding torque and low fuel consumption values.