Exhaust emission system: Notes: Overview
The N55 is a BMW TwinPower Turbo engine featuring dual forced induction. The N55 engine utilises a single twin-scroll turbocharger that is powered by two exhaust tracts (dual charging). The exhaust system on the N55 is thus less complex that that employed on the N54 engine with its two turbochargers. The exhaust system consists of the following components:
- Exhaust manifold
- Twin-scroll turbocharger
- Catalytic-converter mounted close to the engine
- Center silencer
- Rear silencer.
The following illustration portrays the N55 engine's exhaust system on the F07.
| Index | Explanation |
|---|---|
| 1 | Exhaust manifold |
| 2 | Twin-scroll turbocharger |
| 3 | Catalytic-converter mounted close to the engine |
| 4 | Center silencer |
| 5 | Rear silencer, right |
| 6 | Rear silencer, left |
Some specific models may employ only a single rear muffler behind the N55 engine; these vehicles are equipped with an exhaust flap.
The following illustration portrays the exhaust system installed with the N55 engine on vehicles with one rear silencer.
| Index | Explanation |
|---|---|
| 1 | Aneroid capsule for controlling exhaust flap |
| 2 | Exhaust flap |
| 3 | Center silencer |
| 4 | Rear silencer with exhaust flap |
Depending on the motorisation an electrical exhaust flap can also be used (see CORRESPONDING BRIEF COMPONENT DESCRIPTION ). The 6-cylinder spark ignition N55 engine complies with EURO 5 emissions standards. Two oxygen sensors are installed to support the lambda closed-loop emissions control. One oxygen sensor serves as the control sensor and is mounted between the engine and the near-engine catalytic converter. The second oxygen sensor serves as a monitoring sensor in the near-engine catalyst (between monolith 1 and monolith 2).