Pic. 4.98. Schematic operation of the catalytic converter: 1 – ceramic carrier of honeycomb structure; 2 - metal fabric; 3 - casing made of heat-resistant alloy steel; 4 - mesh funnel
E-class diesel engines are equipped with an oxidative catalytic converter as standard. The catalyst casing and tubes are made of alloy steel. Noble metal coating on catalyst (pic. 4.98) activates additional oxidation in the exhaust gases, while hydrocarbons (NS) and carbon monoxide (SO) turn into carbon dioxide (CO2) and water (H2O). This process also reduces the emission of solids. Soot particles cannot be deposited on hydrocarbon compounds and are brought out.
Exhaust gases
carbon monoxide (SO). Measured during exhaust gas monitoring. A prerequisite for low CO content is precise control of injection quantity and timing, as well as uniform swirling of the mixture. It is poisonous in enclosed spaces, in the air it combines with atmospheric oxygen to form non-hazardous carbon dioxide, which, however, can cause a greenhouse effect.
hydrocarbons (CH). Partially may not burn in cold areas and narrow corners of the combustion chamber. Their share depends on the design of the engine (immutable size); too rich or too lean mixture increases their release. Together with nitrogen oxides, they form smog.
Nitrogen oxides (NOx). Their share increases with increasing combustion temperature. They are emitted, for example, on engines adjusted for low CO and HC content in the exhaust gases (reduced fuel consumption). At high concentrations, they cause burning in the respiratory tract. When combined with water, they form nitric acid (acid rain).
Sulfur dioxide (SO2). In small quantities, it is formed only in diesel engines. The reason is the content of sulfur in diesel fuel. In the light forms sulfuric or sulphurous acid, both lead to acid rain. However, sulphurous acids from automobile exhaust make up only 3% of all sulphurous acids produced.
Improving exhaust gas parameters when using an electronic diesel system (EDS)
In order to reduce nitrogen oxides, hydrocarbons and soot in the exhaust, diesel engines are equipped with an exhaust gas recirculation system in addition to an oxidizing catalytic converter (pic. 4.98) and adjustable suction pressure.
Exhaust Gas Recirculation (AGR)
Pic. 4.99. Nodes of the exhaust gas recirculation system: 1 - intake air temperature sensor; 2 - pressure control valve lever; 3 - cell of depression of the pressure control valve; 4 - cell of rarefaction of the exhaust gas system (ARF); 5 - control valve of the exhaust gas return system; 6 – the soaking-up pipeline of the fulfilled gases; 7 - outlet pipe; 8 - front part of the muffler
At high fuel combustion temperatures in diesel engines, the proportion of nitrogen oxides in the exhaust gases increases sharply. The possibility of reducing the temperature in the combustion chamber - the supply of exhaust gases. A part of the total exhaust gas flow of the engine is removed using the exhaust gas recirculation system (pic. 4.99). The dose is determined based on engine load and is directed back to the air supply system piping behind the pressure control valve. Since the exhaust gas contains practically no combustible substances, it cannot be burned again. However, it limits the supply of fresh air for combustion and thus helps to reduce the temperature in the combustion chamber and the proportion of nitrogen oxides.
In addition to electronically controlled exhaust gas recirculation, which helps reduce nitrogen oxide emissions, CDI engines are equipped with converters, as well as a second exhaust gas cleaner located under the bottom.
When repairing the exhaust system, it is impossible to weld metal that has undergone corrosion well. This measure is only for a short time. Rubber fasteners and clamps last longer than metal. On systems with two or more muffler sections, after replacing one, the others have to be replaced after a few months. Therefore, at service stations, as a rule, the entire system is changed completely. However, this is not always necessary. Check the condition of the system and first decide whether you will change sections individually or the entire system.
Sequence of work
1. Place the car on a jack.
2. If during dismantling it is impossible to unscrew the bolts, then cut or drill them. When mounting, use new bolts and nuts.
3. For safety, also replace rubber bands, gaskets, etc.
4. If at least one part of the exhaust system has already been replaced, then it is better to disconnect the inserted ends of the pipes when heated. For this, a welding torch is used in the workshops. You can also use a propane torch. But be sure to have a fire extinguisher handy. If these products are not available, try using a rust remover.
5. Joints of sections are disconnected by strong rotational movements or by means of blows with a hammer.
6. If this does not help, saw off the pipe 10 cm from the junction with the unusable section. Saw the rest of the section to length and knock it down with a chisel or a large screwdriver.
7. All bolted connections and nuts will be loosened the next time easier if you lubricate the threads with heat-resistant grease during installation. It is desirable to do this at the junctions of the joints.