Twin-scroll turbocharger
The N55 is equipped with a single twin-scroll turbocharger instead of 2 smaller, separate turbochargers of the kind employed with the N54 powerplant.
The following illustration shows the components of the twin-scroll turbocharger.
| Index | Explanation |
|---|---|
| 1 | Wastegate valve |
| 2 | Wastegate valve lever |
| 3 | Exhaust tract 2 |
| 4 | Exhaust tract 1 |
| 5 | Coolant return |
| 6 | Outlet to intercooler |
| 7 | Vacuum unit, wastegate valve |
| 8 | Inlet to intake silencer |
| 9 | Ring tract |
| 10 | Blow-off valve |
| 11 | Oil return |
| 12 | Coolant duct |
| 13 | Turbine |
| 14 | Connection to catalytic converter |
The turbine is only rarely exposed to constant exhaust-gas pressure. At low engine speeds the exhaust gases arrive as pulsating pressure waves. This pulsation produces brief spikes in the pressure ratio at the turbine. Because higher pressures are accompanied by a simultaneously rise in operating efficiency, the pulsation enhances the effective supercharging pressure to increase the N55 engine's torque generation. The benefits are particularly pronounced at low engine speeds. To prevent interaction between individual cylinders during gas- exchange processes, the exhaust gases from cylinders 1-3 (bank 1) and cylinders 4-6 (bank 2) are conducted to separate, individual downpipes. The respective exhaust-gas streams remain separated within the turbocharger as they proceed through the exhaust tubes and onto the turbine in a spiral flow path (scrolls). This design ensures optimal exploitation of the pulsation to generate boost pressure.
A wastegate valve is employed to limit the boost pressure.