1 Fuel economy is also a major challenge for diesel engines in the future. The development trend of interior cleaning is to use lean-burn diesel technology to provide more intake air to improve the combustion conditions, that is, to improve the fuel economy by optimizing engine combustion. Due to the full combustion, the emission of PM and CO 2 can be reduced to a lower level, while at the oxygen-enriched conditions, the NOx increases. Therefore, the above-mentioned third strategy is the development trend of the future four-way catalytic converter for diesel engines. 2 The basic principle of the four-way catalytic converter for vehicle diesel engines Many types of four-way catalytic converters for vehicle diesel engines have been developed at home and abroad, mainly based on composite technologies, namely HC, CO and particulate oxidation technologies and NOx reduction technologies. Combine. In recent years, more attention has been paid to the study of simultaneous removal of four pollutants using a single technique. 2.1 Catalytic Oxidation Technology and NOx Catalytic Reduction Technology Combining oxidizing HC and CO and SOF in particulates to reduce NOx, that is, through the combination of Catalytic Oxidation Technology (DOC) and NOx Purification Technology, to achieve diesel exhaust Simultaneous purification of CO, HC, NO x and particulates. 2.1. 1 Catalytic Oxidation Technology Catalytic oxidation is the use of catalysts to reduce the activation energy of the chemical reactions of hydrocarbons, carbon monoxide, and SOF in the exhaust gas of diesel vehicles so that these substances can react with the oxygen in the exhaust gas at a lower temperature. Oxidation reaction, conversion to carbon dioxide and water. The main factors affecting the performance of the catalytic oxidizer are the exhaust temperature and the sulfur content in the fuel. Higher exhaust temperatures will contribute to SOF oxidation and increase conversion efficiency; but if the exhaust temperature is too high (above 400 500), the amount of sulfur in SO x and fuel oil converted to sulphate will greatly increase, resulting in The particles increase, and the sulfate covers the inner surface of the catalytic oxidizer, deactivating the catalytic oxidizer. 2.1.2 Diluted NOx Catalytic Reduction Technology This technology uses HC as a reducing agent to reduce NOx with noble metals, zeolites, and Cu ZSM5 catalysts under oxygen-enriched conditions. Experiments have shown that alkanes, olefins, and diesel can themselves act as reducing agents to reduce NOx emissions. Because diesel engine itself emits less unburned hydrocarbons, the maximum NO x conversion rate is generally 15%, which is called passive reduction; if external HC reducing agent is added, it is called active reduction, and the maximum conversion rate of NOx can reach 30% and 50%. A small amount of N 2 O is formed, which is a potential greenhouse gas. In diesel engines, the process of reducing NOx is performed under oxygen-enriched conditions, and therefore an effective reducing agent is required. The efficiency of the reduction reaction depends on the type of reducing agent and the ratio of HC/NOx, and therefore, lean NO x . In the catalytic reduction technology, the key is to increase the HC/NOx ratio. The use of a NOx-absorbing NOx catalyst effectively utilizes the adsorbed HC as a NOx reduction catalyst, which can increase the reactor efficiency. It is impossible to achieve a high NOx conversion rate by using this technique alone, and if combined with plasma technology, the effect is better. Ir, Ag, Pt, etc. are also useful as catalysts, and their conversion efficiency is not high. 2.1.3 Selective Catalytic Reduction (SCR) Catalysts used for SCR reduction generally use Pt Al 2 O 3, Ag Al 2 O 3, and Cu Zeolite (zeolite). The reducing agent may be made of ammonia and various HCs, HCs may be derived from products that are burned by the engine or injected into the exhaust gas. It has been experimentally confirmed that different types of HC, NOx conversion rate is also different, the reduction characteristics of C 3 H 6 is best, and the noble metal Pt has good low temperature activity, and the NO x conversion rate is highest at 200°C. In addition, HC is consumed in large amounts due to oxidation at high temperatures, resulting in a decrease in the NOx conversion rate. SCR generally uses urea or ammonia gas, which has the advantage of improving fuel economy, and SCR is not sensitive to sulfur, even if the use of high sulfur content of diesel, can still effectively work long-term. The selective catalytic reduction technology has matured and applied to stationary diesel engines, but the reducing agent has affected its application in automobiles. The main challenge it faces is the need to establish a supply network for urea. In Europe, organizations established by automakers, SCR equipment manufacturers, and fuel suppliers have been established. The goal is to establish a European urea supply network when implementing the EU RO4 standard. In addition, due to its complex structure, ammonia is difficult to carry on automobiles, and it is prone to cause ammonia leakage pollution. The use of urea aqueous solution requires additional containers and complicated supply devices. 2.1. 4 NOx storage catalytic system NOx storage is connected to the catalytic reduction system. The storage first converts NO to NO 2 through the platinum catalyst, and then NO 2 and Ba react to form Ba(NO 3) 2. When all the storage units When they are full, the engine uses a slightly richer condition. Under the condition of rich gas mixture, unburned HC, CO, H together with Pt, Ba will replace Ba(NO 3) 2 with BaCO 3. This is to use HC as reducing agent, Rh as catalyst, releasing NO X from storage unit. It is returned to N2, H2O, CO2, but the cost is very high. It requires the control of the engine to be complicated and needs to be periodically changed to the state of rich gas mixture. It cannot use high-sulfur diesel and it has low conversion at high temperatures. The major problems faced by the composite technologies of Catalytic Oxidation (DOC) and NOx Catalytic Reduction technologies. 2. The combination of particulate traps and NOx catalytic reduction technology is often combined with SCR and DPF. Particle traps have been researched for nearly 20 years. The main problem is the regeneration of particle traps. To burn particulates requires a high temperature of 500 600, and the normal exhaust temperature of a diesel engine is far from meeting this requirement. It is necessary to increase the temperature of the exhaust gas by means of an external heat source, or by means of the action of the catalyst to reduce the temperature required for the oxidation. In addition, the high temperature of particulate combustion may cause the trap to melt or break. The common catalyst is a catalyst-loaded particulate trap. The oxidation reaction of particulates under the action of a catalyst is a reaction based on CO 2 , which can lower the reaction temperature. The reaction temperature is about 325. This is generally quite high for the exhaust temperature of a diesel engine. There is also a continuous regeneration filter, which uses the oxidation catalyst to produce NO2 to oxidize the particles continuously, but desulfurized fuel is required; and there must be sufficient NOx, and the ratio of NOx to particulates is at least 8, and only heavy diesel vehicles are generally available. Enough NOx. Others include the use of fuel additives filter, electric heating regeneration, burner heating regeneration, microwave regeneration, etc., after taking the above measures, the regeneration temperature is still higher than 300, medium and heavy diesel vehicles can meet the requirements, and light car exhaust temperature Usually less than 200, it is not suitable. For cars, there is a device that uses a combination of a NOx storage reduction catalyst system and a catalytic filter DPF. Compared to SCR, this device has a smaller volume, and a small number of automobiles are equipped with the device, and its purification effect still needs to be improved. The main problems faced by the particle trap regeneration technology. 2.3 Catalytic removal of 4 pollutants at the same time The idea of ​​catalytically removing NO x and PM simultaneously in an oxidizing atmosphere of a diesel engine exhaust is proposed by Yoshida for the first time. Cooper et al. found through experiments that NO is oxidized to NO 2 under the action of Pt catalyst, and NO 2 can further oxidize the soot of diesel engines, revealing the role of NO(NO 2 ) in reducing the emission of soot from diesel engines. In addition, Teraoka and Shangguan studied the simultaneous removal of NO x and dry soot in diesel engines. At present, these studies are all in the exploratory testing stage. The problem with using Pt catalysts is that it is extremely difficult to reduce under oxidizing atmosphere (lean conditions), and the ratio of HC and NO x in the exhaust gas is not conducive to the reaction between the two. Because most of the NO is generated at high temperatures under high loads, the HC is more completely oxidized and there is not enough HC to reduce NO. Deeba, Michel, Lui, and Yiu Kwan designed a device consisting of a two-stage zeolite catalyst. The first-stage zeolite absorbs gaseous HC at low temperatures, and at high temperatures, under the influence of second-stage zeolites and noble metal catalysts. The first stage zeolitic HC, and the soluble organic component SOF in the microparticles act as a reducing agent, reducing NOx to nitrogen and CO to CO2. At temperatures above 200, the NOx conversion can reach 80%. The technology is still in the research phase and the purification of particulates is still to be improved. In recent years, the research focus of this technology is to use rare earth materials with abundant reserves in China instead of precious metal materials, and in particular to replace the Pt catalyst with a catalyst made of perovskite-type composite oxides to achieve simultaneous removal of four pollutants. Some progress has been made, especially in the treatment of fine particles; the main problem now facing is the low conversion of NOx, which can be removed by 20% 70% under different working conditions. 2.4 Low temperature plasma technology combined with particle traps The plasma technology can simultaneously purify 4 types of contaminants. The purification mechanism is the generation of low-temperature plasma by corona or dielectric barrier discharge at atmospheric pressure. The use of plasma contains a large amount of high-energy electrons, excited particles, atomic oxygen (O), and the resulting highly oxidative freedom. Base (OH, HO 2), ozone (O 3 ), etc., initiate a series of physical and chemical reactions, so as to achieve the purpose of purifying exhaust harmful substances. The active particles in the plasma have higher energy, and some are even higher than the bond energy of certain gas molecules. When the electron energy is 10 eV, when the average energy of the electrons exceeds the chemical binding energy of the pollutant molecules, high-energy active particles and harmful substances act directly to open the molecular bonds to generate some monoatomic molecules and solid particles. This technology has almost no requirement for sulfur content and can work at lower temperatures. Diesel exhaust contains 3% oxygen at 17%. At high oxygen levels, plasma discharge is mainly an oxidation reaction. It mainly oxidizes NO to NO 2. Plasma alone has no effect on NOx, but plasma technology and catalyst Combined with the technology, the plasma enhances the selectivity of the catalyst and has a better purifying effect on NO x and particulates. Therefore, plasma technology is often used in conjunction with catalyst technology, and plasma technology is used to increase the efficiency of catalytic reduction reactions. This technology has the advantages of good low temperature activity (150), a wide operating temperature range (150 500), and no expensive metals (usually metal zeolite or alumina). Using plasma technology to oxidize NO to NO 2 and further oxidize the particles, mainly in the gas phase, it is difficult to achieve high NOx and particle purification at the same time with lower energy. Combined with particle traps may theoretically achieve better results. The main challenge of this technology is that the energy consumption is too high, followed by the limitation of the airspeed characteristics of the catalyst, unnecessary by-products, and the injection of HC. The author has conducted theoretical research on the energy consumption of plasma and proposed some specific measures to reduce energy consumption. It is believed that the combination of plasma and catalyst technology can achieve a low level of energy consumption. 3 Conclusion So far, many technologies have been developed at home and abroad for the treatment of exhaust gas from diesel engines, but each of them has faced some problems, and no good exhaust after-treatment technologies have yet been developed. Judging from the development trend, the restrictions on automobile emissions will be more stringent. Only the comprehensive application of various technologies, including the coordination of diesel engine intake air intake, internal combustion treatment, exhaust gas post-treatment, etc., and a variety of exhaust aftertreatments will be adopted. The combination of technology can meet the requirements, which will be the direction of the development of diesel engines. The combination of low-temperature plasma technology and catalytic conversion technology, because of its theoretical possibility to achieve better results, and to avoid the main problems of other four-way catalytic converters, such as the need for higher exhaust temperature, the need for complex reducing agent Supply devices, etc., have good application prospects and are expected to become broad market technologies. Bibcock, Basin Faucet,Kitchen Mixer & Faucet,Tub Faucet Water Manifold,Pipe Clamp Co., Ltd. , http://www.nbpipefittings.com