Table of contents: Adjustable three-way catalytic… ↓ Operating temperature of the… ↓ Handling the catalytic converter ↓ Exhaust system for FSI engines ↓
Fuel is mainly composed of carbon and hydrogen. When burned in an engine, carbon combines with oxygen in the air to form carbon dioxide (CO₂), and when hydrogen (H₂) combines with oxygen (O₂) to form water (H₂ O). When one liter of fuel is burned, about 0.9 liters of water is produced, this water evaporates from the system under the influence of heat. In winter, after starting a cold engine, you often see white clouds of exhaust gases - this is a sign of condensation.
Exhaust gas aftertreatment in the FSI engine leads to further reduction of harmful emissions
Adjustable three-way catalytic converter
All models sold in Germany are equipped with a regulated three-way catalytic converter. Over the course of its service life, the converter reduces the content of carbon monoxide by 85 percent, hydrocarbons by 80 percent, and nitrogen oxides by 70 percent. However, the efficiency of the converter decreases over time. The name "regulated" means that the reduction of harmful substances is regulated depending on the composition of the fuel-air mixture.
The oxidation process in the catalytic converter of exhaust gases (diagram): The catalytic converter consists of a cellular 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 housing 3, which is made of special steel. In order for the incoming exhaust gases to pass by the cells evenly, a funnel-shaped mesh 4 is placed in front of the converter.
Operating temperature of the catalytic converter
In order for the catalytic converter and the oxygen sensor to function properly, they must be very hot (catalytic converter: up to approximately 300°C). This usually 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 converter, the temperature rises to dangerous values. If the temperature in the catalytic converter reaches 900°C, the converter ages quickly, and at temperatures above 1200°C it is destroyed. 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
If the engine does not start due to a dead battery, do not tow or push the engine to start it, as this may introduce unburned fuel into the catalytic converter and cause long-term damage to the converter.
Misfires and interruptions in ignition indicate a malfunction of the ignition system, immediately check the system in a workshop.
When applying an anti-corrosion protective coating to 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 that the heat shield over the catalytic converter is not damaged.
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 of which the fuel-air mixture becomes enriched and fuel consumption increases.
Exhaust system for FSI engines
In order to achieve and even exceed the stringent Euro 4 exhaust emission standard in the case of an efficient and clean direct petrol injection engine, the vehicle concept must include an expensive exhaust gas aftertreatment system. This is due to the fact that during stratified mixture formation (see Chapters "Engine management and petrol injection system") a large amount of nitrogen oxides is produced, which must be converted into harmless nitrogen on the way to the exhaust pipe.
Therefore, in the 2.0 l FSI engine, in addition to a small three-way catalytic converter, which is located directly behind the engine and therefore begins to work immediately after a cold start, there is an additional NO storage converter, and (for the first time in the world) nOx sensor. In the stratified mixture formation mode, nitrogen oxides are retained in the accumulating neutralizer by barium molecules. The sensor determines the moment when the capacity of the neutralizer is exhausted and it threatens to "overflow". This happens approximately every 60 seconds. After that, the sensor produces a pulse that goes to the central control unit of the engine. The control unit switches the engine to regeneration mode for approximately two seconds, forming a richer mixture and a higher exhaust gas temperature.
The original version of the article is posted on the website audimanual.ru