Exhaust aftertreatment in an FSI engine leads to a further reduction in emissions
Adjustable three-way catalytic converter
All models sold in Germany are equipped with an adjustable three-way catalytic converter. The converter during its life on average reduces the content of carbon monoxide by 85 percent, hydrocarbons by 80, and nitrogen oxides by 70 percent. However, over time, the effectiveness of the neutralizer decreases. Name «adjustable» means that the reduction in the content of harmful substances is regulated depending on the composition of the air-fuel mixture.
Oxidation process in catalytic converter (scheme): The catalytic converter consists of a honeycomb structure of a ceramic substrate 1, the ceramic substrate is embedded in an elastic metal fabric 2, which dampens vibrations. This package is enclosed in a heat-resistant case 3, which is made of special steel. In order for the incoming exhaust gases to pass by the cells evenly, a funnel-shaped grid 4 is placed in front of the converter.
Catalytic converter operating temperature
In order for the catalytic converter and oxygen sensor to function properly, they must be very hot (neutralizer: up to approx. 300°C). As a rule, this takes 20-80 seconds. However, both the converter and the sensor are very sensitive to overheating. For example, if the unburned mixture ignites in a hot catalytic converter, the temperature rises to dangerous levels. If the temperature in the catalytic converter reaches 900°C, then there is a rapid aging of the converter, and at a temperature of more than 1200°C it collapses. At temperatures above 1400'C, the ceramic body of the oxygen sensor melts. The exhaust system then becomes clogged and the engine loses power.
Handling the catalytic converter
In the event that the engine does not start due to a dead battery, avoid towing or pushing to start the engine, as this may introduce unburned fuel into the catalytic converter and permanently damage the converter.
Misfires and misfiring indicate a malfunction of the ignition system, have the system checked immediately by a workshop.
When applying the anti-corrosion protective coating of the underbody, the anti-corrosion agent must not come into contact with the catalytic converter.
From time to time, with the vehicle on jack stands, check the heat shield above the catalytic converter for damage.
Due to a crack in the exhaust pipe in front of the oxygen sensor, the sensor detects an increased oxygen content in the exhaust gas, as a result, the air-fuel mixture is enriched and fuel consumption increases.
Exhaust system for FSI engines
In order to achieve and even exceed the requirements of the stringent Euro 4 exhaust emission standard with an efficient and clean engine with petrol direct injection, the vehicle concept must include an expensive exhaust gas aftertreatment system. This is due to the fact that during layer-by-layer mixing (see chapters «Engine management and petrol injection system») a large amount of nitrogen oxides are produced, which must be converted into harmless nitrogen on their way to the exhaust pipe.
Therefore, in the 2.0l FSI engine, in addition to a small three-way catalytic converter, which is located directly behind the engine and therefore starts immediately after a cold start, there is an additional NO storage converter, and (for the first time in the world) NOx sensor. In the mode of layered mixing, nitrogen oxides are retained in the accumulative neutralizer by barium molecules. The sensor determines the moment when the capacity of the neutralizer is exhausted, and it threatens «overflow». This happens approximately every 60 seconds. After that, the sensor generates a pulse that enters the central control unit of the engine. The control unit switches the engine to regeneration mode for approximately two seconds with a richer mixture and a higher exhaust gas temperature.
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