Exhaust Emission System
Exhaust Emission System
Exhaust emission system
The engine satisfies the EURO 5 exhaust emission standards, or LEVII with the US version (LEV = Low Emission Vehicle). The engine has a catalytic converter close to the engine. The US version also has a catalytic converter on the vehicle underbody. 2 oxygen sensors are used for the oxygen sensor emissions control. A broadband oxygen sensor (Bosch: LSU advanced type) is used as the control sensor upstream of the catalytic converter close to the engine. A bistable sensor (NTK: FLO type) serves as the monitoring sensor downstream of the catalytic converter.
Brief component description
The following components for the exhaust emission system are described:
Engine-proximate catalytic converter
The catalytic converter reduces the pollutant emissions:
- The carbon monoxide (CO) reacts with oxygen (O2) to produce carbon dioxide (CO2).
- The hydrocarbons (HC) react with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O).
- The nitrogen oxide (NOx) is converted into nitrogen (N) and oxygen (O2).
At all times, the Digital Engine Electronics (DME) regulate the fuel-air mixture with regard to the following criteria:
- Exhaust emissions
- Consumption
- Power development
- Catalytic converter protection
Here, the DME picks up the residual oxygen content in the exhaust gas via the oxygen sensors and corrects the fuel injection rate on the basis of this data.
A model for the exhaust-gas temperature integrated in the DME meets (among others) the following specifications:
- The catalytic converter heater ensures that the catalytic converter quickly reaches operating temperature and conversion capability following engine start.
- 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.
Broadband oxygen sensor (control sensor)
The sensor system of the broadband oxygen sensor consists of ceramic layers of zirconium dioxide (laminate). The heating element inserted in the laminate rapidly ensures the required operating temperature of at least 750 ° C. The broadband oxygen sensor has 2 cells, a so-called measurement cell and a reference cell. The two cells are coated with electrode made of platinum.
The oxygen sensor is characterized by an extended measuring range: The broadband oxygen sensor can be used to measure an air/fuel ratio steplessly between 0.6 and 2.5 (constant characteristic curve). Additional benefits of the oxygen sensor are its higher temperature stability, short response times of less than 30 milliseconds and high degree of signalling precision. It reaches a state of operating readiness in less than 5 seconds which allows emission levels to be reduced in the engine warm-up phase. The high measuring dynamics of the sensor allow the air/fuel ratio in each engine cylinder to be determined and adjusted separately.
Bistable sensor (monitoring sensor)
The monitoring sensor with a steep characteristic curve is a bistable sensor. This bistable sensor is used downstream of the catalytic converter for diagnostic purposes. The bistable sensor reliably detects deviations from Lambda = 1. The oxygen sensor cannot deliver precise information on the extent of the mixture deviation.
System functions
The following system function for the exhaust emission system is described:
Oxygen sensor emissions control
For complete and perfect combustion, a air/fuel ratio of 1 kilogram of fuel and approx. 14.7 kilograms of air is necessary. The air mass corresponds to around 11 cubic meters. Lambda refers to the ratio between the amount of air actually supplied and the stoichiometric amount of air. During normal operation of the vehicle, the Lambda value fluctuates. The engine has its best performance with a lack of air (Lambda approx. 0.9 = rich mixture). The engine has its lowest consumption with excess air (Lambda approx. 1.1 = lean mixture). The catalytic converter can most effectively reduce pollutant emissions if the fuel-air mixture is in the region of Lambda = 1. The conversion rate, i.e. the proportion of converted pollutants, is 98 % to virtually 100 % in the case of modern catalytic converters. The Digital Engine Electronics (DME) control the optimized composition of the fuel-air mixture. The oxygen sensors deliver essential information on the composition of the exhaust gas.
The front oxygen sensor measures residual oxygen in the exhaust gas. The fluctuation values of the residual oxygen are forwarded to the DME control unit as a voltage signal. The DME corrects the mixture composition via the fuel injection. A second oxygen sensor (monitoring sensor) is built in behind the catalytic converter. The catalytic converter has a high oxygen storage capacity. This means there is only a little oxygen behind the catalytic converter. The monitoring sensor supplies a virtually constant (attenuated) voltage. With increasing age, the oxygen storage capacity of the catalytic converter declines. The monitoring sensor then reacts increasingly to oxygen sensor deviation with voltage fluctuations. These characteristics are used by a special diagnostic function for catalytic converter monitoring. A malfunction of the catalytic converter is indicated by the emissions warning light.
Notes for Service department
General notes
IMPORTANT: Protect plug connection of monitoring sensor against dirt contamination.
Ambient air is required in the interior of the monitoring sensor. The ambient air enters the interior via the plug connection through the cable. This is why the plug connection must be protected against soiling, e.g. by wax or sealant. If oxygen sensor emissions control faults develop, check the plug connection on the monitoring sensor for dirt contamination. If necessary, the plug connection must be cleaned.
Diagnosis instructions
The following monitoring functions test the condition of the exhaust system:
CO adjustment
In vehicles without oxygen sensor emissions control, the carbon monoxide emissions at idle are set via the diagnosis system. The adjustment values are preset in this case.
Oxygen sensor adaptation
The purpose of oxygen sensor adaptation (mixture adaptation) is to compensate for component tolerances and ageing effects affecting the fuel-air mixture. Factors such as excess air and fuel pressure also have an impact on the oxygen sensor adaptation (partial compensation) which is why precise oxygen sensor control limits for a fault cannot be specified. The following distinctions are made for the purpose of oxygen sensor adaptation:
- additive fuel mixture adaptation
- multiplicative mixture adaptation
Additive fuel mixture adaptation takes effect at or around idle speed. This effect steadily decreases as the engine speed increases. Multiplicative mixture adaptation is effective across the entire characteristic map. An important factor is, for example, the fuel pressure.
The delivery status of the adaptation values and equipment specifications can be reset using the "Reset adaptation values" service function. The adaptation values must subsequently be relearned. In order to learn the values for mixture adaptation, the engine must be operated for an extended period between idle and partial load.
Catalytic converter diagnosis
The catalytic converter diagnosis uses continuous oxygen sensors upstream and bistable sensors downstream of the catalytic converter. The diagnosis tests the oxygen storage capacity of the catalytic converter. The oxygen storage capacity provides an indication of the catalytic converter's conversion capability. A rich fuel-air mixture is specified during the initial phase of the catalytic converter diagnosis (approx. 3 seconds) until the oxygen sensor voltage reaches a defined value. As the oxygen content of rich exhaust gas is low, the amount of oxygen stored in the catalytic converter falls. A lean mixture with a high oxygen exhaust gas content is specified in the second phase. The longer it takes to reach the maximum oxygen storage capacity, the higher the conversion capability of the catalytic converter will be.
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