Emission Control Systems: Description and Operation
Exhaust Emission System
Exhaust emission system
The N55 is a BMW TwinPower Turbo engine featuring dual forced induction. The N55 engine utilizes a single twinscroll 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
- Centre silencer
- Rear silencer.
The following illustration portrays the N55 engine's exhaust system on the F07.
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.
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).
Brief component description
This section describes the following exhaust-system components:
- Exhaust manifold
- Twin-scroll turbocharger
- Blow-off valve
- Oxygen sensor
- Near-engine catalytic converter
- Exhaust flap (on specific models)
- Engine PVC heater.
Exhaust manifold
The exhaust manifold features air-gap insulation and a 6 into 2 runner system. Combining three exhaust runners provides ideal flow characteristics to power the twin-scroll turbocharger. The exhaust manifold and the twin-scroll manifold are welded together to form a single unit.
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.
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.
Blow-off valve
The compressor bypass valve opens to prevent intense oscillations in the impeller when the throttle valve is suddenly closed (for instance, during gearchanges). The sets up a flow around the twin-scroll turbocharger. The compressor bypass valve prevents air from pulsating against the closed throttle valve.
The compressor bypass valve serves the following purposes:
- Improved engine acoustic properties
- Protection for the twin-scroll turbocharger.
Additional effect: The twin-scroll turbocharger responds rapidly to renewed opening of the throttle valve. If the compressor bypass valve were not present the twin scroll turbocharger would be forced to overcome the backpressure from the closed throttle valve, which would reduce its speed. The twin-scroll turbocharger would respond more slowly when the throttle valve reopens.
The following illustration shows how the compressor bypass valve operates.
The compressor bypass valve is mounted on the twin-scroll turbocharger.
In contrast with the arrangement employed on the N54 engine, the bypass valve on this powerplant is not pneumatically operated. The compressor bypass valve on the N55 assumes the form of an electric actuator controlled directly by the DME digital engine electronics system. Placing the compressor bypass valve on the twin-scroll turbocharger has made it possible to substantially reduce the number of components.
The compressor bypass valve is either open or closed, with no intermediate positions.
Oxygen sensor
The oxygen sensor consists of a ceramic coating made of zirconium dioxide (laminated). The heating element inserted in the laminate rapidly ensures the required operating temperature of at least 750 °C. The oxygen sensor contains 2 cells, a measuring cell and a reference cell. The two cells are coated with electrode made of platinum.
The oxygen sensor provides seamless, continuously-variable response while monitoring fuel ratios ranging from 0.65 to 2.5 (continuous characteristic curve). This oxygen sensor works with a lower heater output than a conventional oxygen sensor. In addition, this oxygen sensor is ready for operation more quickly. Current is applied at the measuring cell. This pumps oxygen into the reference cell in a process that continues until a voltage of 450 millivolts is present between the reference cell's electrodes. The applied current at the measuring cell is the measured variable for the fuel-air ratio. This enables the oxygen sensor emissions control to set any desired air/fuel ratio in the combustion chamber.
Near-engine catalytic converter
Two monoliths are located within the catalytic converter's housing. The catalytic converter's total volume is 2.7 liters. The monoliths have various coatings.
Monolith 1 on the N55 engine has the following characteristics:
- Volume = 1.2 liters
- Diameter = 125 mm
- 600 cells.
Monolith 2 on the N55 engine has the following characteristics:
- Volume = 1.5 liters
- Diameter = 125 mm
- 400 cells.
The catalytic converter reduces emissions of the following pollutants:
- With the aid of oxygen (O2) carbon monoxide (CO) is converted into carbon dioxide (CO2).
- Hydrocarbons (HC) react with oxygen (O2) to form carbon dioxide (CO2) and water (H2O).
- Nitrous oxides (NOx) break down into nitrogen (N) and oxygen (O2).
The following illustration shows the catalytic converter on the F10.
At all times, the Digital Engine Electronics (DME) regulate the fuel-air mixture with regard to the following criteria:
- Exhaust emissions
- Consumption
- Power development
- Catalyst protection.
The DME digital engine electronics system relies on the oxygen sensors to monitor levels of residual oxygen in the exhaust gas and then dials in corrections in fuel-injection quantities using these data. A model for exhaust-gas temperature integrated in the DME digital engine electronics system discharges the following functions (among others):
- The catalytic converter heater ensures rapid heating and catalytic conversion soon after the engine starts.
- The effect of the catalytic converter protection is that the exhaust-gas temperatures, in particular at full load, are regulated in such a way that a thermal overload of the catalytic converter is prevented.
Exhaust flap (on specific models)
The exhaust flap installed in exhaust systems on vehicles with a single rear silencer varies according to specific model. The exhaust flap is controlled by the electric exhaust flap changeover valve.
The exhaust flap is integrated within the rear silencer. The exhaust flap reduces the noise level when the engine is idle and when the engine speed is close to idle. The exhaust flap provides active noise control on the order of 8 dB.
The DME digital engine electronics system controls the electric changeover valve for the exhaust flap. The partial vacuum applied opens the exhaust flap. When air impacts against the diaphragm in the aneroid capsule (vacuum system depressurized) a spring closes the exhaust flap.
An aneroid capsule is mounted on the rear silencer's endpipe. The diaphragm box is linked to the exhaust flap via an adjustment mechanism. A vacuum hose leads from the exhaust flap's electric changeover valve to the aneroid capsule.
When the engine is off no power is supplied to the exhaust flap and the exhaust flap is closed. Both conditions, open and closed, occur during normal vehicle operation.
Control of the exhaust flap is governed by:
- Load
- Gear engaged
- Engine speed.
Engine ventilation heating
The N55 engine is equipped with a PCV heater. The engine's PCV heating system is a special-equipment option (Option 842 Cold-climate version). Under steady-state operation at temperatures below -25 °C and during extremely short trips the positive crankcase ventilation system could potentially freeze and cause engine damage. PCV heating relies on an electric auxiliary heater that dials in temperatures of roughly 80 °C.
Switch-on conditions:
- Terminal 15 on
- Ambient temperature less than 2 °C
- Time since engine start is less than 360 seconds
(time can be extended based on engine-oil temperature, coolant temperature and vehicle speed).
System overview
System functions
The following section describes the following system functions:
- Lambda closed-loop mixture control.
Oxygen sensor emissions control
For complete and perfect combustion, an air/fuel ratio of 1 kilogram of fuel to approximately 14.7 kilograms of air is necessary. The air mass corresponds to around 11 cubic meters. The ratio between the air quantity that is actually being supplied and that required for a stoichiometric ratio is designated as lambda. During normal operation of the vehicle, the Lambda value fluctuates. The engine provides maximum performance operating with low air (lambda roughly 0.9 = rich mixture). The engine attains maximum fuel economy (minimal consumption) running on excess air (lambda of roughly 1.1 = rich mixture). The catalytic converter furnishes its best reduction of pollutant emissions with an air-fuel mixture in the range of lambda = 1. With modern catalytic converters the catalyst's conversion rate, meaning the proportion of pollutants that are actually converted, lies between 98 and almost 100 percent. Optimal composition of the air-fuel mixture is controlled by the DME digital engine electronics system. The oxygen sensors deliver essential information on the composition of the exhaust gas. The pre-catalyst oxygen sensor continuously monitors levels of residual oxygen in the exhaust gas. The fluctuating values for residual oxygen are relayed to the DME digital engine electronics system in the form of a voltage signal. The DME manipulates the fuel-injection system to correct the mixture composition. A second oxygen sensor is installed behind the catalytic converter (monitor sensor). The catalytic converter has a high oxygen storage capacity. This means there is only a little oxygen behind the catalytic converter. The post-catalyst oxygen sensor transmits a virtually constant (attenuated) voltage. With increasing age, the oxygen storage capacity of the catalytic converter declines. The post-catalyst sensor then responds with progressively more pronounced fluctuations in its lambda readings. This characteristic is exploited by a special diagnostic function employed to monitor the catalytic converter.
A malfunction of the catalytic converter is indicated by the emissions warning light.
Notes for Service department
General notes
NOTICE: Engine PCV heater installed according to equipment level.
The special-equipment cold-climate version (SA 842) is mandatory in various countries (standard in US and Canada).
NOTICE: Protect the oxygen sensor's plug connection to prevent contamination from entering.
The oxygen sensor requires ambient air within its interior in order to operate. The ambient air enters the interior via the plug connection through the cable. For this reason it is important to guard the plug connection against contamination from substances such as wax and sealants. When malfunctions occur in the lambda closed-loop control system the oxygen sensor's plug connection should always be checked. If necessary, the plug connection must be cleaned. Avoid all contact between the plug connection and contact spray, cleansers and solvents, as these substances could destroy the oxygen sensor.
NOTICE: Defective exhaust flap.
If the exhaust flap is defective the exhaust system will emit a hissing noise a high engine speeds when it is closed.
If the exhaust flap is defective and partially open a humming noise will be generated at idle.
Diagnosis instructions
The following monitoring functions check the condition of the exhaust system:
- CO adjustment
- Oxygen sensor adaptation
- Catalytic converter diagnosis.
CO adjustment
On vehicles without lambda closed-loop mixture control the carbon monoxide emissions at idle are adjusted using the diagnostic system. Proceed by performing the "CO adjustment" service function step-by-step using the specified adjustment data.
Oxygen sensor adaptation
The lambda adaptation (mixture adaptation) compensates for component tolerances and ageing that can affect the air-fuel ratio. Factors such as vacuum leaks and fuel pressure can also affect the lambda adaptation (partial compensation). This is why it is not possible to define precise lambda control limits as fault causes.
Multiplicative mixture adaptation is effective throughout the entire characteristic map. An example of an essential factor is fuel pressure. The "Reset adaptation values" service function can be used to reset the adaptation values and the equipment versions to their initial delivery status. Then the adaptation values must be relearned by the system. In order to initialize the system and allow it to "learn" the mixture-adaptation data it is necessary to operate the vehicle for an extended period between idle and part-load.
Catalytic converter diagnosis
The catalytic converter diagnosis function tests the ability of the catalytic converter to store oxygen. The oxygen-storage capacity serves as an index of the catalyst's ability to perform chemical conversion.
During the first phase of the catalyst diagnosis process (lasting roughly 3 seconds) the system dials in a rich mixture and maintains it until the oxygen sensor's voltage reaches a predefined value. Because rich exhaust is low in oxygen, there is a corresponding reduction in the amount of oxygen stored in the catalytic converter.
In the 2nd phase a lean mixture with oxygen-rich exhaust gas is supplied. The catalytic converter's ability to store oxygen is calculated based on the time that elapses before the post-catalyst oxygen sensor reacts to the adjustment in exhaust-gas composition. The operative assumption is that no change in the oxygen sensor's voltage occurs until the catalytic converter reaches the limit of its ability to store oxygen.
The system recognizes the catalytic converter as defective when its ability to store oxygen falls below a specified threshold value.
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