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The diesel engine has the characteristics of high thermal efficiency, high power, etc. It has good economy and reliability, and has been widely used in the field of engineering machinery. For example, road rollers, loaders, excavators, bulldozers, etc. are powered by diesel engines. Although diesel engines have many advantages, they emit more exhaustive components, mainly HC, CO, SO, NO, and PM (microparticles). Especially at the construction site, due to the frequent exchanges between construction machinery and transportation vehicles, and the limitation of ventilation conditions, the harmful gases emitted by these construction machinery are seriously exceeded and will spread throughout the surface, which greatly jeopardizes the health of construction workers. The construction is carried out normally. Therefore, it is very important to control and purify the exhaust gas discharged from the diesel engine.
1 Fuel control measures
1.1 alternative fuel
The use of alternative fuels will be one of the important methods to control diesel and gasoline engine emissions, and because of the limited fossil fuels, the search for alternative fuels has become a hot topic in current internal combustion engine research. Currently, alternative fuels are mainly natural gas, compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), hydrogen, methanol, ethanol, dimethyl ether (DEM), dimethyl carbonate (DMC) and biodiesel. Etc., among which methanol, natural gas, and liquefied petroleum gas are considered to be the most promising alternative fuels for clean energy. Among them, CNG, LPG, and methanol-gasoline vehicles have received strong government support and rapid development in China.
Methanol can be extracted from raw materials such as natural gas, coal and biomass; ethanol is mainly produced by fermentation of sugar and starch-containing crops. The use of alcohol fuel engines can be close to or exceed the diesel and gasoline engines, and the exhaust gas has less harmful components, which is a promising fuel. However, when methanol and ethanol are burned, harmful components such as formaldehyde and acetaldehyde are released, which is limited in use.
Dimethyl ether (DEM) is a diesel alternative fuel that has received much attention in recent years and can be produced from coal, natural gas and biomass waste. DEM's self-ignitability is very good, it can be used as a single fuel to directly replace diesel; it can achieve efficient and gentle compression combustion of the engine, and has the same or slightly higher power and economic performance as the diesel engine; the most outstanding advantage is that DEM can completely eliminate smoke and To achieve ultra-low emissions, NOx emissions are 30% lower than diesel engines. If exhaust gas recirculation is used at the same time, NO emissions can be further reduced to 50% of general diesel engines to achieve both PM and NO.
Dimethyl carbonate (DMC) contains 53.3% of oxygen, and the soot and granules produced by combustion are lower than pure diesel. Because of the floating æ„• 谌 谌 ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ æ—Œ æ—Œ æ—Œ æ—Œ æ—Œ æ—ŒTechnical love, æ¦é±¿ ​​苡 è‹¡ 裼鸵 ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt; Produce biodiesel by processing used cooking oil, with an annual output of 300 tons. It is actually a methyl ester rapeseed oil or a methyl ester vegetable oil, and the raw materials for production are sunflower, soybean, and the like. This biodiesel, which sells for $0.50 per liter, can be used instead of diesel or mixed with diesel. Moreover, it is completely refined from renewable raw materials, and the amount of carbon dioxide produced during combustion is much lower than that of ordinary diesel, and the environmental pollution is much smaller than ordinary diesel. At present, the United States, Germany, Brazil, Argentina and other production have been put into production.
There is also a kind of biodiesel, which is a liquid fuel made from waste cooking oil and the like. The basic production process is as follows: after the waste oil is added to the reaction tank, the alcoholysis and esterification can be simultaneously carried out through a slightly acidic catalyst technology, and the reaction speed is also obviously accelerated. In addition, through a metal salt treatment agent, the key problem of using the waste animal and vegetable oil to produce high residual acid value of diesel oil is solved. Both of these key technologies have reduced the cost of biodiesel production, allowing biodiesel to enter the production plant from the laboratory.
1.2 Fuel modification
1.2.1 Increase the cetane number. The higher the cetane number of diesel, the shorter the ignition delay period and the better the ignition quality. The emission of various pollutants generally decreases with the increase of diesel cetane, the cetane number is insufficient, the ignition delay period is shortened, and the ignition quality is improved. Poor, premixed combustion is excessive, rough operation, increased noise, and increased black smoke and NO emissions. Increasing the cetane number of diesel fuel can effectively reduce PM, CO and NO emissions from engine exhaust particles.
1.2.2 Reduce the content of aromatic hydrocarbons in fuel oil. The density of aromatic hydrocarbons is relatively large, the ignition is relatively poor, and more carbon black is produced during the combustion process, so that the CO, HC, NO and PM in the exhaust gas are increased. Therefore, reducing the content of aromatic hydrocarbons can effectively control the emission of harmful pollutants.
1.2.3 Reduce the amount of S in the fuel. During the combustion process, 1% to 3% of the S in the diesel oil is converted to sulfate. The rest is mainly converted to SO. Van Beckhoven found that in the direct injection diesel engine, the S in the fuel is reduced by 0.05wt from 0.30. %, particulate emissions will be reduced by 10% to 30%. Bartlett reports that in all light diesel engines, sulfur in the fuel is reduced by 0.05% by weight from 0.30 and particulate emissions are reduced by about 7%.
1.2.4 Reduce the density. Diesel density and viscosity are important physical and chemical indicators of diesel, which will affect the quality of diesel spray, which in turn affects diesel engine emissions. Reducing the density of diesel fuel can reduce particulate matter in HC and emissions. The fuel density is reduced from 840kg/m3 to 800kg/m3 and particulate emissions will be reduced by 13%.
1.2.5 Use fuel additives. Commonly used cetane number improving additives, eliminating carbon deposit additives and smoke suppressants, but these additives are not ideal after use, and some have negative effects. For example, the application of more anti-smoke additives, the use of metal bismuth, magnesium, zinc and other soluble alkalized salts or neutral salts as smoke-eliminating additives, after adding a small amount can significantly reduce the exhaust smoke of diesel engines, but the particulate emissions increase And these metals are harmful to humans and are not recommended now.
Through the above analysis, it can be seen that the control measures for diesel fuel mainly include the following: using alternative fuels; increasing the cetane number of fuel; selecting appropriate diesel viscosity, reducing the surface tension of diesel; reducing density; reducing diesel Sulfur content and aromatic content. These technical measures have helped reduce the harmful emissions of diesel exhaust.
2 Diesel engine internal purification technology
Diesel engine internal purification technology is mainly to improve the oil and gas mixture, to prevent local excess air coefficient of more than 0.9 (which is conducive to the production of NO) and less than 0.6 (which is conducive to the production of soot). Reducing particulate and soot emissions is consistent with improving the combustion process, allowing the diesel engine to achieve uniform mixing, full combustion, soft work, reliable start-up, and low emissions. However, these reduction measures tend to increase NO emissions, which poses special difficulties for diesel engine emissions control. Therefore, in the determination of exhaust gas purification measures, it is necessary to proceed from the current advanced purification technology, according to the performance of the machine, take a variety of measures to comprehensively use, in order to achieve the purpose of purification.
2.1 Adopt new combustion method
Traditional diesel combustion is divided into two parts: premixed combustion and diffusion combustion. The main combustion is diffusion combustion at λ≈l, the flame temperature is high, and NOx is easily generated. This problem can be solved by using a thin uniform mixture. This idea was adopted by the Uniform Charge Combustion Combustion System (HCCI) proposed by the Southwest Research Institute of the United States and the Premixed Lean Burn Combustion Process (PREDIC) of the Japan ACE Research Institute. The premixed lean burn combustion method reduces or eliminates the diffusion combustion. The lean mixture can reduce the combustion temperature and greatly reduce the NOx, which is 98% lower than that of the general diesel engine. Because the mixture in the cylinder is uniform, there is no local excessive rich mixture. The PM emission is reduced by 27% compared with the general diesel engine. The premixed lean combustion method is still in the research stage, and there is still a certain distance from the practical, but the prospect is very considerable.
2.2 Improvement of fuel injection system
2.2.1 Improvement of fuel injection law
The reasonable fuel injection law should be: the initial fuel injection rate should not be too high to suppress NOx formation; medium-term emergency fuel injection, increase fuel injection rate and fuel injection pressure to accelerate diffusion combustion speed, prevent PM increase and thermal efficiency deterioration The end of the spray should be completed quickly to avoid incomplete combustion and increase in PM.
In the absence of electronically controlled fuel injection, the fuel injection law is improved by changing the shape of the oil pump cam. The conventional cam is a tangent cam, and the modified cam is a concave arc cam. The oil supply law has an initial low, medium and medium speed and a supplemental combustion period. Features that are not prolonged. After verifying the test on a 6105 diesel engine, it was found that after improvement, NOx decreased by 6% to 13%, PM decreased by 80/15%, but fuel economy deteriorated slightly.
In order to achieve the first and last urgent fuel injection law, it is also possible to use a double spring injector to double the pressure injector. When the oil pressure rises, first overcome the soft pressure of the first stage, so that the needle valve slightly rises. Since the flow area is small, the rate of fuel injection is low; when the oil pressure rises to overcome the second stage spring pressure, the main injection is started.
2.2.2 Increase injection pressure and reduce nozzle diameter
Increasing the injection pressure and reducing the diameter of the orifice can further refine the spray particles of the fuel to increase the surface area of ​​the fuel and air contact, accelerate the mixing of the fuel and the air, and significantly reduce the carbon emissions from the PM. High-pressure injection can lead to an increase in NOx, such as the use of delayed injection time and EGR, to achieve the purpose of controlling particulate PM and NOx emissions. High-pressure injection systems need to be well matched to the combustion chamber to avoid excessive fuel injection onto the cold surface of the cylinder, reducing SOF emissions from organic solubles in HC and particulate PM; high-pressure injection technology places stringent requirements on fuel injection systems. . The entire system is required to have extremely high strength, stiffness and tightness. Such measures must also be combined with other improvements to achieve better results.
Increasing the injection pressure can effectively reduce the particulate emissions of the diesel engine; reduce the average droplet diameter of the fuel, promote the formation of the mixture; reduce the maximum pressure rise rate of the engine and reduce the combustion noise.
2.2.3 Matching of injection timing and injection rate
Controlling the injection timing of diesel engines is an important means of controlling diesel engine emissions. Delaying injection timing is one of the simple and effective measures to reduce NOx concentration in diesel engine emissions. NOx is very sensitive to the effect of injection timing. When the injection timing differs from the set value by 1 °CA, NOx will increase by about 15%. In order to reduce NOx emissions, the injection timing is gradually delayed, approaching the up-stop direction. At present, the injection timing of the electronically controlled injection has been reduced to about 5 °CA before the top dead center. The fuel injection rate has a great influence on the emission of harmful gases. In practice, it is often used to delay the injection timing and increase the injection rate, so that the CO rise caused by delaying the injection timing alone is suppressed. Both CO and NOx emissions are reduced.
2.2.4 pilot injection and multiple injection
A small amount of fuel is injected before the main injection to reduce NOx and noise. The main injection requires fast injection rate and high injection pressure to promote the formation of the mixture to shorten the slow-burning period and ensure good economy and power. Pilot injection (pre-injection) + main injection mode. In order to simultaneously reduce NOx and PM emissions, multiple injection methods may be employed, that is, after the pilot injection and the main injection are completed, a small amount of fuel is injected to form a post-injection, and the post-injection may promote the oxidation of the soot and reduce the PM emission. The use of post-injection will increase HC emissions and fuel consumption. In order to obtain good results under different working conditions, the pilot injection oil quantity, the post-injection oil quantity, and the interval angle of each injection and the control precision of the time have strict requirements, which is difficult to imagine for the mechanical fuel injection system. Only an electronically controlled high pressure common rail fuel injection system can be competent.
2.3 Improvement of the intake system
At present, the development trend of diesel engines is to increase the injection pressure and reduce the eddy current strength to reduce the pressure loss of the intake air, and cooperate with the multi-valve small-aperture injector to obtain a good mixture.
2.3.1 Using supercharged intercooling technology
Exhaust gas turbocharging can increase the intake pressure, increase the supply of air, increase the average effective pressure in the cylinder, the excess air ratio and the average temperature of the whole cycle, so that the fuel can be completely burned, and the discharge of particulate matter of the diesel engine can be reduced. About 50%, and reduce CO and HC emissions. After boosting, the fuel consumption rate decreases, and CO and HC are further reduced. At the same time, the intake air temperature is increased to increase the combustion temperature, so that the NOx after pressurization is higher than the non-pressurization. In this regard, a supercharging and intercooling method can be employed to lower the intake air temperature to control the deterioration of NOx. According to the data, the intake air temperature is reduced by 0.5, the maximum combustion temperature and exhaust temperature can be reduced by 1~3 °C. With the intercooling technology, the NOx emissions can be reduced by 60%/-70%. Therefore, the use of supercharged intercooling is one of the effective measures to reduce the exhaust emissions of diesel engines for vehicles.
2.3.2 Multi-valve design
The multi-valve design is mainly used to expand the total flow area of ​​the intake and exhaust valves, increase the intake charge, and make the combustion of the diesel more thorough; the intake vortex ratio is variable. In achieving these goals, it also has a large impact on diesel emissions. The influence comes from two aspects. First, when the 4-valve technology is adopted, the injector is arranged near the cylinder axis to make the oil and gas mix evenly, and the combustion ends early, which is beneficial to reduce NOx. In addition, the four valves cause the combustion chamber to be concave. Large eddy currents reduce the eddy current dead zone and help to reduce PM. Second, the partial passage can be closed to form the intake vortex intensity that is compatible with the rotational speed of the diesel engine. According to the influence of variable eddy current ratio, the researchers conducted a test on the influence of eddy current ratio change on NOx and PM on a 6108 diesel engine. The machine has a 4-valve structure, and the double inlet with the double intake valve is a long spiral air passage and chopped To the airway, the tangential airway vortex is nearly zero and can be throttled to achieve a variable eddy current ratio. At low speeds, the tangential air passage is closed to obtain a high eddy current ratio, thereby improving the quality of the mixture at low speeds and improving the economy, power and emissions of the diesel engine.
2.4 Optimized combustion system
Optimizing the combustion system refers to the best match of the shape of the fuel supply system, the intake flow, and the combustion chamber. From the standpoint of view, there is no optimal solution for the strength of the eddy current, the degree of high pressure fuel injection, and the type of combustion chamber. But the best combination of the three is the optimal combination. For example, when the injection pressure is low, high-intensity airflow movement is required to accelerate the oil-air mixing. On a heavy-duty vehicle diesel engine, the higher injection pressure and the larger number of orifices can reduce the intake vortex. The centrally-cranked combustion chamber has a strong airflow and can be maintained for a longer period of time, which is advantageous for accelerated diffusion combustion. The central combustion chamber with a convex combustion chamber has a rapid combustion process, and the main combustion period is short. The proper delay of the fuel supply timing can greatly reduce the NOx emissions under the premise that the fuel consumption rate and the smoke change are not large.
At present, the development trend of direct injection diesel engine is to increase the injection pressure; increase the number of injection holes of the injector and reduce the aperture; optimize the injection timing according to the diesel engine condition, so that the injection timing not only follows the rotation speed but also with the load. The change is automatically adjusted. The use of a split combustion chamber is not conducive to the formation of NOx due to the low peak temperature of the flame; most of the soot is produced in the secondary combustion chamber, and most of it is oxidized after entering the main combustion chamber, effectively reducing particulate and HC emissions, separating The combustion chamber is 1/3-1/2 lower than the NOx emissions of the direct injection combustor of the same specification. The newly developed combustion system uses a strong continuous late disturbance to effectively reduce soot and particulate emissions, which is similar to smokeless. This in turn creates conditions for further exhaust gas recirculation or delayed injection advancement to reduce NOx emissions.
2.5 Application of diesel electrical control technology
The use of electronic control not only improves the control accuracy of injection timing and fuel injection, but also allows for flexible or precise control of air control components such as EGR, bleed valve or variable geometry turbocharger with electronic control technology. The ideal solution for the control system should be to optimize fuel economy and emissions.
The application of electronically controlled diesel high-pressure injection technology (such as electronically controlled high-pressure common rail injection) enables the diesel engine to continuously adjust the relationship between engine speed and load through optimal injection timing, optimum fuel injection rate and pre-injection. The pilot injection and multiple injection technologies are used. The pilot injection oil quantity, the post injection fuel quantity, and the interval angle and timing control precision of each injection have strict requirements. These are obviously only good in the electronically controlled high pressure common rail system. achieve. The particulate emissions are greatly reduced and the emissions performance of the engine transition conditions can be significantly improved. The electronically controlled high-pressure injection control controls the fuel injection law, and can achieve the optimal fuel injection according to the engine operating conditions. At the same time, by controlling the ratio of premixed combustion and diffusion combustion, it can simultaneously reduce harmful emissions and control the air-fuel ratio of the engine, which is beneficial to Achieve effective off-board purification measures. The common rail electronically controlled injection technology is the most advanced diesel electronically controlled injection technology. The development, application and research work of the common rail system have been reported abroad. However, in China, this research is still in its infancy.
2.6 Exhaust gas recirculation (EGR)
Exhaust gas recirculation (EGR) is to ensure that the power of the internal combustion engine is not reduced. A part of the exhaust gas is introduced into the intake system, mixed with the fresh mixed gas, and then enters the cylinder to participate in combustion, which is reduced by lowering the maximum temperature of combustion in the combustion chamber. NOx emissions. The use of exhaust gas recirculation (EGR) to reduce NOx emissions, combined with electronic control, according to diesel engine load, speed, cooling water temperature sensor and start switch signal, the ECU EGR rate and EGR timing control, to ensure that the diesel engine Reduce NOx emissions in the exhaust gas under conditions of little performance impact.
At present, the application of EGR on gasoline engines is relatively successful, but it is not satisfactory on diesel engines. The main reason is that the large amount of PM and other harmful emissions from the diesel engine directly into the cylinder will increase the wear of the piston ring and cylinder liner, and will also dilute the lubricant and accelerate its deterioration. The use of EGR in a diesel engine is equivalent to adding a certain amount of CO and water vapor to the intake air to become a diluent. The increase in the EGR rate also reduces the density and O concentration of the intake medium, resulting in a combustible mixture in the cylinder. Both the burning rate and the combustion temperature are reduced, eventually leading to a decrease in the power and economy of the engine, and an increase in HC, CO and PM emissions. When the engine is in medium and small load conditions, the effect of using EGR is very significant. When the EGR rate is about 30%, the engine's emissions and overall performance are better: the use of a larger EGR rate to reduce NOx emissions is more obvious, and the engine economy is not prominent. When the engine is under heavy load conditions, if EGR is used, the economy and power of the engine will be significantly reduced. In addition, the wear of the piston ring and the cylinder liner will be increased and the deterioration of the lubricating oil will be accelerated. Therefore, EGR should not be used under heavy load conditions.
Turbocharged diesel engine under 30%~50% load. The average exhaust pressure is lower than the average intake pressure. Therefore, exhaust gas recirculation is difficult to achieve. To this end, scholars from various countries have proposed a variety of solutions for achieving exhaust gas recirculation on supercharged diesel engines. Mainly: the internal EGR is realized by adjusting the timing; the throttle valve is installed in the intake pipe or the exhaust pipe, the intake pressure is reduced or the exhaust pressure is increased by throttling; the exhaust gas is pressed into the auxiliary device or the pressure of the piston itself. The air pipe; by adding a Venturi Pipe to the intake pipe, reducing the intake pressure at the EGR joint; utilizing pressure fluctuations and the like. Among them, the Wenquli tube EGR system can easily realize exhaust gas recirculation under high working conditions. Moreover, the additional pumping loss is small, the cost is not high, and there is great superiority.
2.7 Prevent oil leakage
In addition to fuel combustion, PM in diesel exhaust emissions accounts for a considerable portion of PM produced by engine oil. PM can be divided into two parts: soluble organic matter (SOF) and insoluble organic matter (IOF), which account for about 39% and 61%. In SOF, PM produced by motor oil accounts for the majority, accounting for 29% of total PM; engine oil produces IOF in addition to SOF, and PM from engine oil accounts for 34% of total PM. At the same time, the oil that is incompletely combusted into the combustion chamber is discharged with the exhaust gas, which is an important part of the blue smoke emitted by the diesel engine. Therefore, it is necessary to prevent and reduce the oil from entering the combustion chamber. This should improve the design of the lubricating oil system, reduce the skirt clearance, optimize the design of the piston, piston ring and cylinder surface, improve the roundness of the cylinder liner and improve the intake valve lifter. The measures such as sealing and reducing the leakage of oil from the valve stem are implemented.
2.8 adding water to burn
Adding a small amount of water to the diesel fuel to form an emulsion fuel in the form of "water-in-oil". When the liquid water is heated to a vapor state during combustion, the latent heat of vaporization is absorbed, so that the combustion temperature and pressure are lowered, so that the fuel evaporation rate is increased. The thermal cracking reaction is reduced, thereby suppressing the formation of NOx. At the same time, since the liquid water is heated to become a vapor state, an "explosion" is formed, which has a further atomization effect, further mixing the fuel and the air, and reducing PM emission.
The experiment proves that when the amount of water spray is equal to the amount of fuel, the NOx+HC emissions will be reduced by about 50%, while the power is only reduced by 4%, and the emission reduction effect is better. However, water injection from the intake pipe will increase the corrosion of the intake pipe and cylinder, and the oil sump will easily accumulate water, and the required amount of water spray can be adjusted with the change of engine load. This has certain difficulties in design and structure.
3 diesel engine exhaust aftertreatment technology
The diesel engine exhaust gas has a high oxygen content, and the HC and CO content is much lower than that of the gasoline engine. The main harmful substances are NOx and soot. Therefore, the focus of diesel exhaust purification is to reduce NOx and reduce soot. The measures are: reducing NOx under oxygen-rich conditions by a selective reduction catalytic converter, reducing HC and CO emissions and organic components in particulate PM-like substances by an oxidation catalytic converter; collecting particulates in diesel exhaust by a particulate filtering device Shaped matter, etc.
3.1 NOx post-treatment measures
The most ideal way to remove NOx is to decompose NO into N and O, but the catalyst is quickly deactivated under the action of O and SO, so the practical prospects of this method are slim. At present, the research on NOX purification of diesel exhaust is mainly from the two technical routes of selective catalytic reduction and adsorption-catalytic reduction, carbon fiber reduction technology.
The most important thing for selective catalytic reduction is to determine the reducing agent and catalyst. The researchers also tried NHX selective catalytic reduction of diesel engine exhaust NOX, and the results showed that the purification rate can reach more than 65%. However, this method is only suitable for the purification treatment of the stationary diesel exhaust, and it is not suitable for the diesel engine with variable operating conditions due to the problem of NH3 leakage. In 1990, 1wamoto and Held reported that HC can selectively catalyze the reduction of NOx on Cu/ZSM-5 catalyst. So far, the studied catalysts can be divided into supported noble metals, supported metal oxides and ion-exchanged zeolites. Three major categories. Broadening the active temperature range of the catalyst or catalyst-reducing agent combination and improving the stability of the catalyst in S02 and hydrothermal environment are the future efforts.
Adsorption-catalytic reduction of NO is the storage of NOx during the lean-burn phase, while in the short-lived rich phase, NOx is released and reduced by HC in the exhaust. The active components of the adsorption-reduction type three-way catalyst are precious metals and alkaline earth metals (or rare earth metals). The main factor affecting the performance of the adsorption-reduction catalyst is the capacity of the adsorbent to adsorb NO at the exhaust gas temperature of the diesel engine and its resistance to SO2 and CO2 poisoning. Improving the performance of these two aspects is the goal of future efforts. Adsorption-catalytic reduction has been proved by Japanese automakers to be applied to some new models of NOX purification, but this method sacrifices fuel economy to a certain extent, and requires a very low S content in fuel. For ultra-low S fuels, existing adsorption-catalytic reduction technology can reduce NOX by 90%. Adsorption-catalytic technology for S high-content fuels is currently under development.
For the simultaneous purification of PM and NO, some studies have shown that carbon particles can reduce NOx on calcium-titanium and precious metal catalysts. Recently, Toyota Corporation of Japan developed a continuous simultaneous catalytic purification of PM and NOX and also has a good purification effect on NO and HC, so it is expected to simultaneously purify PM, NO, CO and HC on the same catalyst, that is, developed The so-called "four-effect catalyst", its successful development will undoubtedly greatly promote the advancement of diesel exhaust control technology.
The use of carbon fiber loading low voltage technology can effectively reduce NOx emissions. Carbon fiber has catalytic activity and can promote the redox reaction of NO and C or HC in the exhaust gas. With the increase of voltage, the NOX emission can be significantly reduced. At present, the technology is in the research stage, and no breakthrough progress has been made. At the same time, the purification effect of the technology must be based on the effective elimination of particles. The activated carbon fibers of different substrates are impregnated with nitric acid and used as a direct reducing agent to achieve the purpose of removing NO. For example, activated carbon fibers made of nitric acid-impregnated pitch-based, viscose-based, polypropylene-based, and the like are used as raw materials. However, these studies are still in the laboratory phase.
3.2 PM post-processing technology
3.2.1 Adding an Oxidation Catalytic Converter
Diesel PM post-treatment techniques include catalytic oxidation and filtration. The addition of an oxidation catalytic converter to a diesel engine is an effective measure for the effective purification of flammable gases and soluble SOF organic components in the exhaust gas. Taking this measure (using platinum Pt and palladium Pd precious metals as catalysts) can reduce HC and CO by 50% and particulate PM by 50% to 70%, and polycyclic aromatic hydrocarbons and nitropolycyclic aromatic hydrocarbons are also significantly reduced.
However, the oxidation catalyst has the disadvantage of oxidizing SO2 in the exhaust gas to SO, generating sulfuric acid mist or solid sulfate particles, and additionally increasing particulate matter emissions. A recent demonstration study of new diesel engines in the United States showed that when using a mass fraction of S of 368×10-6, catalytic oxidation can reduce PM emissions under transient conditions by 23% to 29%, HC Decrease by 52%~58%. If you use S with a mass fraction of 54×10-6, PM can be reduced by 13%. Therefore, diesel oxidation catalysts are generally suitable for diesel fuels with low sulfur content; and to ensure catalyst and carrier, engine operating conditions, engine characteristics, flow rate of exhaust gas and size of catalytic converter, and inlet temperature of exhaust gas into the converter. Wait for normal, to achieve the best purification effect.
3.2.2 Using a particulate trap
The particulate trap consists of a particulate filter and a regeneration device. The particulate trap captures solid carbon particles in the diesel exhaust and soot adsorbing soluble organic components through a filter medium (filter element) having extremely small pores therein.
The main body of the particle trap is the filter element. Currently, the commonly used filter materials are: wire mesh, ceramic fiber, ceramic foam and wall-flow honeycomb ceramic. The filter determines the filtration efficiency, operational reliability, service life of the filter and the use and regeneration of the regeneration technology. The filter element should meet the high performance index: it has high filtration efficiency, has large filtration area, good thermal shock resistance, strong mechanical performance index, good thermal stability, and can withstand high heat load. The smaller thermal expansion coefficient is better, the flow resistance is small, and the back pressure is small, the back pressure growth rate is low, and the regeneration capacity is strong and the quality is light. Currently, the most commonly used filter materials are cordierite (which is mainly composed of MgO.AlO.SiO) and silicon carbide crystal SiC.
The particulate trap has a high efficiency of carbon filtration and can reach 60%. During the filtration process, the exhaust pressure of the diesel engine will increase. When the exhaust back pressure reaches 16~20 kPa, the performance of the diesel engine begins to deteriorate. Therefore, the particles must be periodically removed to restore the filter to its original working state, ie, filtration. Regeneration. The regeneration mode of the particle trap can be divided into "passive" regeneration and "active" regeneration: the "passive" mode is catalytic regeneration, which is to impregnate the filter carrier or add additives to the fuel to reduce the oxidation reaction of the particles. The activation energy reduces the light-off temperature of the carbon particles to realize the regeneration of the particulate filter; the "active" regeneration mode is also called "thermal regeneration", that is, the regeneration mode of the applied energy (thermal energy), and the external heat source is used to accumulate in the filter body. The particles are heated and self-ignited to reduce particulate PM in the filter. According to the form of applied energy, it can be divided into: full load regeneration, fuel injection combustion regeneration, electric heating regeneration, electric self heating regeneration and microwave regeneration. Subsequently, several non-heating regeneration modes such as CRT (continuous regenerative trap) system, throttling regeneration, reverse jet regeneration, and vibration regeneration were developed.
For the time being, further problems to be solved in the research of regenerative filters are: lowering the regeneration temperature and further reducing the energy required for regeneration. It can effectively regenerate at the exhaust temperature of diesel engines, achieving the purpose of reducing energy loss and simplifying the mechanism: for vehicles using pneumatic brakes, reverse jet regeneration technology is a future development direction. However, the source and reliability of its structure and energy are subject to further study. Continuous regeneration will be an important development direction in the future, but as far as China is concerned, due to the high sulfur content in diesel oil (required to be 50×10), the domestic industry cannot be used for a long time due to the influence of chemical technology. .
In various diesel particulate trap regeneration technologies, in addition to continuous regeneration, the timing of regeneration is judged, that is, regeneration control is performed, and the regeneration control system is an indispensable part of the particulate trap. Modern automatic trap systems already have electronic monitoring in the form of an online diagnostic system that simultaneously controls the regeneration process, in addition to simply monitoring back pressure, and using complex calculations to determine soot loading. The newly developed soot sensor (such as conductivity) continuously monitors the cleanliness of the exhaust to ensure that the trap is regenerated at the right time.
3.2.3 Electrostatic particle collector
70%~80% of the diesel exhaust particles are in a charged state, and each charged particle has about 1~5 basic positive or negative charges, and the whole is electrically neutral. At present, the adsorption of soot particles with charging characteristics is carried out by using an additional strong electric field, and certain experimental results have been obtained. However, the current problem is that the equipment is too large and the cost is too high. The most difficult to use on the vehicle is the prevention of secondary dispersion and back corona in the supply and collection of high voltage electricity. But with the development of technology is also very promising.
3.2.4 Voltage capture technology
A metal mesh is installed on the upstream and downstream of the exhaust pipe of the diesel engine, and a DC voltage of about 50 V is applied between the nets. Generally, the upstream metal mesh has a large mesh and a negative voltage; the downstream metal mesh has a dense mesh and a positive voltage. When the particles pass through the upstream metal mesh, they are negatively charged, and are adsorbed when passing through the positively charged metal mesh, thereby achieving the purpose of particle purification. The method is simple in device and high in filtration efficiency.
3.2.5 pulse corona plasma chemical processing technology
This technique utilizes high energy electrons of 5-20 eV to bombard the gas molecules NOx, SO, O, HO, etc. in the reactor. After activation, decomposition, ionization and other processes produce strong free radicals such as COH, HO, atomic oxygen (O) and ozone, strong oxides rapidly oxidize carbon particles, NOx and SO under the action of water to form nitric acid and sulfuric acid, added Appropriate additives (NH, etc.) produce the corresponding ammonium salts, which can be collected by filters and electrostatic dust removal to reduce pollution. However, as this process produces new salts and other chemical components, it is possible to form secondary pollution, which is still in theoretical research and laboratory applications.
3.2.6 Electrostatic cyclone technology
The researchers conducted an exploratory study on the effect of electrostatic cyclone technology on the capture and removal of diesel PM. The results show that the high-pressure pulse electrostatic action can not only capture the diesel exhaust PM well, but also the HC and NOx in the exhaust gas have a certain removal effect. The electrostatic cyclone trap has the advantages of low exhaust resistance and simple cleaning.
4 Conclusion
4.1 Fuel aspects
The current development direction is to use alternative fuels, improve petroleum smelting technology, develop new diesel additives, eliminate sulfur in diesel, reduce aromatics in fuel, reduce the content of unsaturated hydrocarbons in diesel, improve the quality of diesel, and solve the problem from the source. The problem of exhaust emissions.
4.2 In-machine purification technology
The development direction of diesel exhaust emission control will be comprehensively applied by various measures. Using electronic control technology, through optimized design of diesel engine, supercharged intercooling and EGR are used to achieve optimal cooperation.
4.3 exhaust gas aftertreatment technology
Catalysts, oxidants and reducing agents are the development direction. In addition, the regeneration technology of particulate traps and the non-filtration technology for removing particulates have yet to be developed.
1 Fuel control measures
1.1 alternative fuel
The use of alternative fuels will be one of the important methods to control diesel and gasoline engine emissions, and because of the limited fossil fuels, the search for alternative fuels has become a hot topic in current internal combustion engine research. Currently, alternative fuels are mainly natural gas, compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), hydrogen, methanol, ethanol, dimethyl ether (DEM), dimethyl carbonate (DMC) and biodiesel. Etc., among which methanol, natural gas, and liquefied petroleum gas are considered to be the most promising alternative fuels for clean energy. Among them, CNG, LPG, and methanol-gasoline vehicles have received strong government support and rapid development in China.
Methanol can be extracted from raw materials such as natural gas, coal and biomass; ethanol is mainly produced by fermentation of sugar and starch-containing crops. The use of alcohol fuel engines can be close to or exceed the diesel and gasoline engines, and the exhaust gas has less harmful components, which is a promising fuel. However, when methanol and ethanol are burned, harmful components such as formaldehyde and acetaldehyde are released, which is limited in use.
Dimethyl ether (DEM) is a diesel alternative fuel that has received much attention in recent years and can be produced from coal, natural gas and biomass waste. DEM's self-ignitability is very good, it can be used as a single fuel to directly replace diesel; it can achieve efficient and gentle compression combustion of the engine, and has the same or slightly higher power and economic performance as the diesel engine; the most outstanding advantage is that DEM can completely eliminate smoke and To achieve ultra-low emissions, NOx emissions are 30% lower than diesel engines. If exhaust gas recirculation is used at the same time, NO emissions can be further reduced to 50% of general diesel engines to achieve both PM and NO.
Dimethyl carbonate (DMC) contains 53.3% of oxygen, and the soot and granules produced by combustion are lower than pure diesel. Because of the floating æ„• 谌 谌 ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ ç ˜î‡¯ æ—Œ æ—Œ æ—Œ æ—Œ æ—Œ æ—ŒTechnical love, æ¦é±¿ ​​苡 è‹¡ 裼鸵 ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— ç— lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt lt; Produce biodiesel by processing used cooking oil, with an annual output of 300 tons. It is actually a methyl ester rapeseed oil or a methyl ester vegetable oil, and the raw materials for production are sunflower, soybean, and the like. This biodiesel, which sells for $0.50 per liter, can be used instead of diesel or mixed with diesel. Moreover, it is completely refined from renewable raw materials, and the amount of carbon dioxide produced during combustion is much lower than that of ordinary diesel, and the environmental pollution is much smaller than ordinary diesel. At present, the United States, Germany, Brazil, Argentina and other production have been put into production.
There is also a kind of biodiesel, which is a liquid fuel made from waste cooking oil and the like. The basic production process is as follows: after the waste oil is added to the reaction tank, the alcoholysis and esterification can be simultaneously carried out through a slightly acidic catalyst technology, and the reaction speed is also obviously accelerated. In addition, through a metal salt treatment agent, the key problem of using the waste animal and vegetable oil to produce high residual acid value of diesel oil is solved. Both of these key technologies have reduced the cost of biodiesel production, allowing biodiesel to enter the production plant from the laboratory.
1.2 Fuel modification
1.2.1 Increase the cetane number. The higher the cetane number of diesel, the shorter the ignition delay period and the better the ignition quality. The emission of various pollutants generally decreases with the increase of diesel cetane, the cetane number is insufficient, the ignition delay period is shortened, and the ignition quality is improved. Poor, premixed combustion is excessive, rough operation, increased noise, and increased black smoke and NO emissions. Increasing the cetane number of diesel fuel can effectively reduce PM, CO and NO emissions from engine exhaust particles.
1.2.2 Reduce the content of aromatic hydrocarbons in fuel oil. The density of aromatic hydrocarbons is relatively large, the ignition is relatively poor, and more carbon black is produced during the combustion process, so that the CO, HC, NO and PM in the exhaust gas are increased. Therefore, reducing the content of aromatic hydrocarbons can effectively control the emission of harmful pollutants.
1.2.3 Reduce the amount of S in the fuel. During the combustion process, 1% to 3% of the S in the diesel oil is converted to sulfate. The rest is mainly converted to SO. Van Beckhoven found that in the direct injection diesel engine, the S in the fuel is reduced by 0.05wt from 0.30. %, particulate emissions will be reduced by 10% to 30%. Bartlett reports that in all light diesel engines, sulfur in the fuel is reduced by 0.05% by weight from 0.30 and particulate emissions are reduced by about 7%.
1.2.4 Reduce the density. Diesel density and viscosity are important physical and chemical indicators of diesel, which will affect the quality of diesel spray, which in turn affects diesel engine emissions. Reducing the density of diesel fuel can reduce particulate matter in HC and emissions. The fuel density is reduced from 840kg/m3 to 800kg/m3 and particulate emissions will be reduced by 13%.
1.2.5 Use fuel additives. Commonly used cetane number improving additives, eliminating carbon deposit additives and smoke suppressants, but these additives are not ideal after use, and some have negative effects. For example, the application of more anti-smoke additives, the use of metal bismuth, magnesium, zinc and other soluble alkalized salts or neutral salts as smoke-eliminating additives, after adding a small amount can significantly reduce the exhaust smoke of diesel engines, but the particulate emissions increase And these metals are harmful to humans and are not recommended now.
Through the above analysis, it can be seen that the control measures for diesel fuel mainly include the following: using alternative fuels; increasing the cetane number of fuel; selecting appropriate diesel viscosity, reducing the surface tension of diesel; reducing density; reducing diesel Sulfur content and aromatic content. These technical measures have helped reduce the harmful emissions of diesel exhaust.
2 Diesel engine internal purification technology
Diesel engine internal purification technology is mainly to improve the oil and gas mixture, to prevent local excess air coefficient of more than 0.9 (which is conducive to the production of NO) and less than 0.6 (which is conducive to the production of soot). Reducing particulate and soot emissions is consistent with improving the combustion process, allowing the diesel engine to achieve uniform mixing, full combustion, soft work, reliable start-up, and low emissions. However, these reduction measures tend to increase NO emissions, which poses special difficulties for diesel engine emissions control. Therefore, in the determination of exhaust gas purification measures, it is necessary to proceed from the current advanced purification technology, according to the performance of the machine, take a variety of measures to comprehensively use, in order to achieve the purpose of purification.
2.1 Adopt new combustion method
Traditional diesel combustion is divided into two parts: premixed combustion and diffusion combustion. The main combustion is diffusion combustion at λ≈l, the flame temperature is high, and NOx is easily generated. This problem can be solved by using a thin uniform mixture. This idea was adopted by the Uniform Charge Combustion Combustion System (HCCI) proposed by the Southwest Research Institute of the United States and the Premixed Lean Burn Combustion Process (PREDIC) of the Japan ACE Research Institute. The premixed lean burn combustion method reduces or eliminates the diffusion combustion. The lean mixture can reduce the combustion temperature and greatly reduce the NOx, which is 98% lower than that of the general diesel engine. Because the mixture in the cylinder is uniform, there is no local excessive rich mixture. The PM emission is reduced by 27% compared with the general diesel engine. The premixed lean combustion method is still in the research stage, and there is still a certain distance from the practical, but the prospect is very considerable.
2.2 Improvement of fuel injection system
2.2.1 Improvement of fuel injection law
The reasonable fuel injection law should be: the initial fuel injection rate should not be too high to suppress NOx formation; medium-term emergency fuel injection, increase fuel injection rate and fuel injection pressure to accelerate diffusion combustion speed, prevent PM increase and thermal efficiency deterioration The end of the spray should be completed quickly to avoid incomplete combustion and increase in PM.
In the absence of electronically controlled fuel injection, the fuel injection law is improved by changing the shape of the oil pump cam. The conventional cam is a tangent cam, and the modified cam is a concave arc cam. The oil supply law has an initial low, medium and medium speed and a supplemental combustion period. Features that are not prolonged. After verifying the test on a 6105 diesel engine, it was found that after improvement, NOx decreased by 6% to 13%, PM decreased by 80/15%, but fuel economy deteriorated slightly.
In order to achieve the first and last urgent fuel injection law, it is also possible to use a double spring injector to double the pressure injector. When the oil pressure rises, first overcome the soft pressure of the first stage, so that the needle valve slightly rises. Since the flow area is small, the rate of fuel injection is low; when the oil pressure rises to overcome the second stage spring pressure, the main injection is started.
2.2.2 Increase injection pressure and reduce nozzle diameter
Increasing the injection pressure and reducing the diameter of the orifice can further refine the spray particles of the fuel to increase the surface area of ​​the fuel and air contact, accelerate the mixing of the fuel and the air, and significantly reduce the carbon emissions from the PM. High-pressure injection can lead to an increase in NOx, such as the use of delayed injection time and EGR, to achieve the purpose of controlling particulate PM and NOx emissions. High-pressure injection systems need to be well matched to the combustion chamber to avoid excessive fuel injection onto the cold surface of the cylinder, reducing SOF emissions from organic solubles in HC and particulate PM; high-pressure injection technology places stringent requirements on fuel injection systems. . The entire system is required to have extremely high strength, stiffness and tightness. Such measures must also be combined with other improvements to achieve better results.
Increasing the injection pressure can effectively reduce the particulate emissions of the diesel engine; reduce the average droplet diameter of the fuel, promote the formation of the mixture; reduce the maximum pressure rise rate of the engine and reduce the combustion noise.
2.2.3 Matching of injection timing and injection rate
Controlling the injection timing of diesel engines is an important means of controlling diesel engine emissions. Delaying injection timing is one of the simple and effective measures to reduce NOx concentration in diesel engine emissions. NOx is very sensitive to the effect of injection timing. When the injection timing differs from the set value by 1 °CA, NOx will increase by about 15%. In order to reduce NOx emissions, the injection timing is gradually delayed, approaching the up-stop direction. At present, the injection timing of the electronically controlled injection has been reduced to about 5 °CA before the top dead center. The fuel injection rate has a great influence on the emission of harmful gases. In practice, it is often used to delay the injection timing and increase the injection rate, so that the CO rise caused by delaying the injection timing alone is suppressed. Both CO and NOx emissions are reduced.
2.2.4 pilot injection and multiple injection
A small amount of fuel is injected before the main injection to reduce NOx and noise. The main injection requires fast injection rate and high injection pressure to promote the formation of the mixture to shorten the slow-burning period and ensure good economy and power. Pilot injection (pre-injection) + main injection mode. In order to simultaneously reduce NOx and PM emissions, multiple injection methods may be employed, that is, after the pilot injection and the main injection are completed, a small amount of fuel is injected to form a post-injection, and the post-injection may promote the oxidation of the soot and reduce the PM emission. The use of post-injection will increase HC emissions and fuel consumption. In order to obtain good results under different working conditions, the pilot injection oil quantity, the post-injection oil quantity, and the interval angle of each injection and the control precision of the time have strict requirements, which is difficult to imagine for the mechanical fuel injection system. Only an electronically controlled high pressure common rail fuel injection system can be competent.
2.3 Improvement of the intake system
At present, the development trend of diesel engines is to increase the injection pressure and reduce the eddy current strength to reduce the pressure loss of the intake air, and cooperate with the multi-valve small-aperture injector to obtain a good mixture.
2.3.1 Using supercharged intercooling technology
Exhaust gas turbocharging can increase the intake pressure, increase the supply of air, increase the average effective pressure in the cylinder, the excess air ratio and the average temperature of the whole cycle, so that the fuel can be completely burned, and the discharge of particulate matter of the diesel engine can be reduced. About 50%, and reduce CO and HC emissions. After boosting, the fuel consumption rate decreases, and CO and HC are further reduced. At the same time, the intake air temperature is increased to increase the combustion temperature, so that the NOx after pressurization is higher than the non-pressurization. In this regard, a supercharging and intercooling method can be employed to lower the intake air temperature to control the deterioration of NOx. According to the data, the intake air temperature is reduced by 0.5, the maximum combustion temperature and exhaust temperature can be reduced by 1~3 °C. With the intercooling technology, the NOx emissions can be reduced by 60%/-70%. Therefore, the use of supercharged intercooling is one of the effective measures to reduce the exhaust emissions of diesel engines for vehicles.
2.3.2 Multi-valve design
The multi-valve design is mainly used to expand the total flow area of ​​the intake and exhaust valves, increase the intake charge, and make the combustion of the diesel more thorough; the intake vortex ratio is variable. In achieving these goals, it also has a large impact on diesel emissions. The influence comes from two aspects. First, when the 4-valve technology is adopted, the injector is arranged near the cylinder axis to make the oil and gas mix evenly, and the combustion ends early, which is beneficial to reduce NOx. In addition, the four valves cause the combustion chamber to be concave. Large eddy currents reduce the eddy current dead zone and help to reduce PM. Second, the partial passage can be closed to form the intake vortex intensity that is compatible with the rotational speed of the diesel engine. According to the influence of variable eddy current ratio, the researchers conducted a test on the influence of eddy current ratio change on NOx and PM on a 6108 diesel engine. The machine has a 4-valve structure, and the double inlet with the double intake valve is a long spiral air passage and chopped To the airway, the tangential airway vortex is nearly zero and can be throttled to achieve a variable eddy current ratio. At low speeds, the tangential air passage is closed to obtain a high eddy current ratio, thereby improving the quality of the mixture at low speeds and improving the economy, power and emissions of the diesel engine.
2.4 Optimized combustion system
Optimizing the combustion system refers to the best match of the shape of the fuel supply system, the intake flow, and the combustion chamber. From the standpoint of view, there is no optimal solution for the strength of the eddy current, the degree of high pressure fuel injection, and the type of combustion chamber. But the best combination of the three is the optimal combination. For example, when the injection pressure is low, high-intensity airflow movement is required to accelerate the oil-air mixing. On a heavy-duty vehicle diesel engine, the higher injection pressure and the larger number of orifices can reduce the intake vortex. The centrally-cranked combustion chamber has a strong airflow and can be maintained for a longer period of time, which is advantageous for accelerated diffusion combustion. The central combustion chamber with a convex combustion chamber has a rapid combustion process, and the main combustion period is short. The proper delay of the fuel supply timing can greatly reduce the NOx emissions under the premise that the fuel consumption rate and the smoke change are not large.
At present, the development trend of direct injection diesel engine is to increase the injection pressure; increase the number of injection holes of the injector and reduce the aperture; optimize the injection timing according to the diesel engine condition, so that the injection timing not only follows the rotation speed but also with the load. The change is automatically adjusted. The use of a split combustion chamber is not conducive to the formation of NOx due to the low peak temperature of the flame; most of the soot is produced in the secondary combustion chamber, and most of it is oxidized after entering the main combustion chamber, effectively reducing particulate and HC emissions, separating The combustion chamber is 1/3-1/2 lower than the NOx emissions of the direct injection combustor of the same specification. The newly developed combustion system uses a strong continuous late disturbance to effectively reduce soot and particulate emissions, which is similar to smokeless. This in turn creates conditions for further exhaust gas recirculation or delayed injection advancement to reduce NOx emissions.
2.5 Application of diesel electrical control technology
The use of electronic control not only improves the control accuracy of injection timing and fuel injection, but also allows for flexible or precise control of air control components such as EGR, bleed valve or variable geometry turbocharger with electronic control technology. The ideal solution for the control system should be to optimize fuel economy and emissions.
The application of electronically controlled diesel high-pressure injection technology (such as electronically controlled high-pressure common rail injection) enables the diesel engine to continuously adjust the relationship between engine speed and load through optimal injection timing, optimum fuel injection rate and pre-injection. The pilot injection and multiple injection technologies are used. The pilot injection oil quantity, the post injection fuel quantity, and the interval angle and timing control precision of each injection have strict requirements. These are obviously only good in the electronically controlled high pressure common rail system. achieve. The particulate emissions are greatly reduced and the emissions performance of the engine transition conditions can be significantly improved. The electronically controlled high-pressure injection control controls the fuel injection law, and can achieve the optimal fuel injection according to the engine operating conditions. At the same time, by controlling the ratio of premixed combustion and diffusion combustion, it can simultaneously reduce harmful emissions and control the air-fuel ratio of the engine, which is beneficial to Achieve effective off-board purification measures. The common rail electronically controlled injection technology is the most advanced diesel electronically controlled injection technology. The development, application and research work of the common rail system have been reported abroad. However, in China, this research is still in its infancy.
2.6 Exhaust gas recirculation (EGR)
Exhaust gas recirculation (EGR) is to ensure that the power of the internal combustion engine is not reduced. A part of the exhaust gas is introduced into the intake system, mixed with the fresh mixed gas, and then enters the cylinder to participate in combustion, which is reduced by lowering the maximum temperature of combustion in the combustion chamber. NOx emissions. The use of exhaust gas recirculation (EGR) to reduce NOx emissions, combined with electronic control, according to diesel engine load, speed, cooling water temperature sensor and start switch signal, the ECU EGR rate and EGR timing control, to ensure that the diesel engine Reduce NOx emissions in the exhaust gas under conditions of little performance impact.
At present, the application of EGR on gasoline engines is relatively successful, but it is not satisfactory on diesel engines. The main reason is that the large amount of PM and other harmful emissions from the diesel engine directly into the cylinder will increase the wear of the piston ring and cylinder liner, and will also dilute the lubricant and accelerate its deterioration. The use of EGR in a diesel engine is equivalent to adding a certain amount of CO and water vapor to the intake air to become a diluent. The increase in the EGR rate also reduces the density and O concentration of the intake medium, resulting in a combustible mixture in the cylinder. Both the burning rate and the combustion temperature are reduced, eventually leading to a decrease in the power and economy of the engine, and an increase in HC, CO and PM emissions. When the engine is in medium and small load conditions, the effect of using EGR is very significant. When the EGR rate is about 30%, the engine's emissions and overall performance are better: the use of a larger EGR rate to reduce NOx emissions is more obvious, and the engine economy is not prominent. When the engine is under heavy load conditions, if EGR is used, the economy and power of the engine will be significantly reduced. In addition, the wear of the piston ring and the cylinder liner will be increased and the deterioration of the lubricating oil will be accelerated. Therefore, EGR should not be used under heavy load conditions.
Turbocharged diesel engine under 30%~50% load. The average exhaust pressure is lower than the average intake pressure. Therefore, exhaust gas recirculation is difficult to achieve. To this end, scholars from various countries have proposed a variety of solutions for achieving exhaust gas recirculation on supercharged diesel engines. Mainly: the internal EGR is realized by adjusting the timing; the throttle valve is installed in the intake pipe or the exhaust pipe, the intake pressure is reduced or the exhaust pressure is increased by throttling; the exhaust gas is pressed into the auxiliary device or the pressure of the piston itself. The air pipe; by adding a Venturi Pipe to the intake pipe, reducing the intake pressure at the EGR joint; utilizing pressure fluctuations and the like. Among them, the Wenquli tube EGR system can easily realize exhaust gas recirculation under high working conditions. Moreover, the additional pumping loss is small, the cost is not high, and there is great superiority.
2.7 Prevent oil leakage
In addition to fuel combustion, PM in diesel exhaust emissions accounts for a considerable portion of PM produced by engine oil. PM can be divided into two parts: soluble organic matter (SOF) and insoluble organic matter (IOF), which account for about 39% and 61%. In SOF, PM produced by motor oil accounts for the majority, accounting for 29% of total PM; engine oil produces IOF in addition to SOF, and PM from engine oil accounts for 34% of total PM. At the same time, the oil that is incompletely combusted into the combustion chamber is discharged with the exhaust gas, which is an important part of the blue smoke emitted by the diesel engine. Therefore, it is necessary to prevent and reduce the oil from entering the combustion chamber. This should improve the design of the lubricating oil system, reduce the skirt clearance, optimize the design of the piston, piston ring and cylinder surface, improve the roundness of the cylinder liner and improve the intake valve lifter. The measures such as sealing and reducing the leakage of oil from the valve stem are implemented.
2.8 adding water to burn
Adding a small amount of water to the diesel fuel to form an emulsion fuel in the form of "water-in-oil". When the liquid water is heated to a vapor state during combustion, the latent heat of vaporization is absorbed, so that the combustion temperature and pressure are lowered, so that the fuel evaporation rate is increased. The thermal cracking reaction is reduced, thereby suppressing the formation of NOx. At the same time, since the liquid water is heated to become a vapor state, an "explosion" is formed, which has a further atomization effect, further mixing the fuel and the air, and reducing PM emission.
The experiment proves that when the amount of water spray is equal to the amount of fuel, the NOx+HC emissions will be reduced by about 50%, while the power is only reduced by 4%, and the emission reduction effect is better. However, water injection from the intake pipe will increase the corrosion of the intake pipe and cylinder, and the oil sump will easily accumulate water, and the required amount of water spray can be adjusted with the change of engine load. This has certain difficulties in design and structure.
3 diesel engine exhaust aftertreatment technology
The diesel engine exhaust gas has a high oxygen content, and the HC and CO content is much lower than that of the gasoline engine. The main harmful substances are NOx and soot. Therefore, the focus of diesel exhaust purification is to reduce NOx and reduce soot. The measures are: reducing NOx under oxygen-rich conditions by a selective reduction catalytic converter, reducing HC and CO emissions and organic components in particulate PM-like substances by an oxidation catalytic converter; collecting particulates in diesel exhaust by a particulate filtering device Shaped matter, etc.
3.1 NOx post-treatment measures
The most ideal way to remove NOx is to decompose NO into N and O, but the catalyst is quickly deactivated under the action of O and SO, so the practical prospects of this method are slim. At present, the research on NOX purification of diesel exhaust is mainly from the two technical routes of selective catalytic reduction and adsorption-catalytic reduction, carbon fiber reduction technology.
The most important thing for selective catalytic reduction is to determine the reducing agent and catalyst. The researchers also tried NHX selective catalytic reduction of diesel engine exhaust NOX, and the results showed that the purification rate can reach more than 65%. However, this method is only suitable for the purification treatment of the stationary diesel exhaust, and it is not suitable for the diesel engine with variable operating conditions due to the problem of NH3 leakage. In 1990, 1wamoto and Held reported that HC can selectively catalyze the reduction of NOx on Cu/ZSM-5 catalyst. So far, the studied catalysts can be divided into supported noble metals, supported metal oxides and ion-exchanged zeolites. Three major categories. Broadening the active temperature range of the catalyst or catalyst-reducing agent combination and improving the stability of the catalyst in S02 and hydrothermal environment are the future efforts.
Adsorption-catalytic reduction of NO is the storage of NOx during the lean-burn phase, while in the short-lived rich phase, NOx is released and reduced by HC in the exhaust. The active components of the adsorption-reduction type three-way catalyst are precious metals and alkaline earth metals (or rare earth metals). The main factor affecting the performance of the adsorption-reduction catalyst is the capacity of the adsorbent to adsorb NO at the exhaust gas temperature of the diesel engine and its resistance to SO2 and CO2 poisoning. Improving the performance of these two aspects is the goal of future efforts. Adsorption-catalytic reduction has been proved by Japanese automakers to be applied to some new models of NOX purification, but this method sacrifices fuel economy to a certain extent, and requires a very low S content in fuel. For ultra-low S fuels, existing adsorption-catalytic reduction technology can reduce NOX by 90%. Adsorption-catalytic technology for S high-content fuels is currently under development.
For the simultaneous purification of PM and NO, some studies have shown that carbon particles can reduce NOx on calcium-titanium and precious metal catalysts. Recently, Toyota Corporation of Japan developed a continuous simultaneous catalytic purification of PM and NOX and also has a good purification effect on NO and HC, so it is expected to simultaneously purify PM, NO, CO and HC on the same catalyst, that is, developed The so-called "four-effect catalyst", its successful development will undoubtedly greatly promote the advancement of diesel exhaust control technology.
The use of carbon fiber loading low voltage technology can effectively reduce NOx emissions. Carbon fiber has catalytic activity and can promote the redox reaction of NO and C or HC in the exhaust gas. With the increase of voltage, the NOX emission can be significantly reduced. At present, the technology is in the research stage, and no breakthrough progress has been made. At the same time, the purification effect of the technology must be based on the effective elimination of particles. The activated carbon fibers of different substrates are impregnated with nitric acid and used as a direct reducing agent to achieve the purpose of removing NO. For example, activated carbon fibers made of nitric acid-impregnated pitch-based, viscose-based, polypropylene-based, and the like are used as raw materials. However, these studies are still in the laboratory phase.
3.2 PM post-processing technology
3.2.1 Adding an Oxidation Catalytic Converter
Diesel PM post-treatment techniques include catalytic oxidation and filtration. The addition of an oxidation catalytic converter to a diesel engine is an effective measure for the effective purification of flammable gases and soluble SOF organic components in the exhaust gas. Taking this measure (using platinum Pt and palladium Pd precious metals as catalysts) can reduce HC and CO by 50% and particulate PM by 50% to 70%, and polycyclic aromatic hydrocarbons and nitropolycyclic aromatic hydrocarbons are also significantly reduced.
However, the oxidation catalyst has the disadvantage of oxidizing SO2 in the exhaust gas to SO, generating sulfuric acid mist or solid sulfate particles, and additionally increasing particulate matter emissions. A recent demonstration study of new diesel engines in the United States showed that when using a mass fraction of S of 368×10-6, catalytic oxidation can reduce PM emissions under transient conditions by 23% to 29%, HC Decrease by 52%~58%. If you use S with a mass fraction of 54×10-6, PM can be reduced by 13%. Therefore, diesel oxidation catalysts are generally suitable for diesel fuels with low sulfur content; and to ensure catalyst and carrier, engine operating conditions, engine characteristics, flow rate of exhaust gas and size of catalytic converter, and inlet temperature of exhaust gas into the converter. Wait for normal, to achieve the best purification effect.
3.2.2 Using a particulate trap
The particulate trap consists of a particulate filter and a regeneration device. The particulate trap captures solid carbon particles in the diesel exhaust and soot adsorbing soluble organic components through a filter medium (filter element) having extremely small pores therein.
The main body of the particle trap is the filter element. Currently, the commonly used filter materials are: wire mesh, ceramic fiber, ceramic foam and wall-flow honeycomb ceramic. The filter determines the filtration efficiency, operational reliability, service life of the filter and the use and regeneration of the regeneration technology. The filter element should meet the high performance index: it has high filtration efficiency, has large filtration area, good thermal shock resistance, strong mechanical performance index, good thermal stability, and can withstand high heat load. The smaller thermal expansion coefficient is better, the flow resistance is small, and the back pressure is small, the back pressure growth rate is low, and the regeneration capacity is strong and the quality is light. Currently, the most commonly used filter materials are cordierite (which is mainly composed of MgO.AlO.SiO) and silicon carbide crystal SiC.
The particulate trap has a high efficiency of carbon filtration and can reach 60%. During the filtration process, the exhaust pressure of the diesel engine will increase. When the exhaust back pressure reaches 16~20 kPa, the performance of the diesel engine begins to deteriorate. Therefore, the particles must be periodically removed to restore the filter to its original working state, ie, filtration. Regeneration. The regeneration mode of the particle trap can be divided into "passive" regeneration and "active" regeneration: the "passive" mode is catalytic regeneration, which is to impregnate the filter carrier or add additives to the fuel to reduce the oxidation reaction of the particles. The activation energy reduces the light-off temperature of the carbon particles to realize the regeneration of the particulate filter; the "active" regeneration mode is also called "thermal regeneration", that is, the regeneration mode of the applied energy (thermal energy), and the external heat source is used to accumulate in the filter body. The particles are heated and self-ignited to reduce particulate PM in the filter. According to the form of applied energy, it can be divided into: full load regeneration, fuel injection combustion regeneration, electric heating regeneration, electric self heating regeneration and microwave regeneration. Subsequently, several non-heating regeneration modes such as CRT (continuous regenerative trap) system, throttling regeneration, reverse jet regeneration, and vibration regeneration were developed.
For the time being, further problems to be solved in the research of regenerative filters are: lowering the regeneration temperature and further reducing the energy required for regeneration. It can effectively regenerate at the exhaust temperature of diesel engines, achieving the purpose of reducing energy loss and simplifying the mechanism: for vehicles using pneumatic brakes, reverse jet regeneration technology is a future development direction. However, the source and reliability of its structure and energy are subject to further study. Continuous regeneration will be an important development direction in the future, but as far as China is concerned, due to the high sulfur content in diesel oil (required to be 50×10), the domestic industry cannot be used for a long time due to the influence of chemical technology. .
In various diesel particulate trap regeneration technologies, in addition to continuous regeneration, the timing of regeneration is judged, that is, regeneration control is performed, and the regeneration control system is an indispensable part of the particulate trap. Modern automatic trap systems already have electronic monitoring in the form of an online diagnostic system that simultaneously controls the regeneration process, in addition to simply monitoring back pressure, and using complex calculations to determine soot loading. The newly developed soot sensor (such as conductivity) continuously monitors the cleanliness of the exhaust to ensure that the trap is regenerated at the right time.
3.2.3 Electrostatic particle collector
70%~80% of the diesel exhaust particles are in a charged state, and each charged particle has about 1~5 basic positive or negative charges, and the whole is electrically neutral. At present, the adsorption of soot particles with charging characteristics is carried out by using an additional strong electric field, and certain experimental results have been obtained. However, the current problem is that the equipment is too large and the cost is too high. The most difficult to use on the vehicle is the prevention of secondary dispersion and back corona in the supply and collection of high voltage electricity. But with the development of technology is also very promising.
3.2.4 Voltage capture technology
A metal mesh is installed on the upstream and downstream of the exhaust pipe of the diesel engine, and a DC voltage of about 50 V is applied between the nets. Generally, the upstream metal mesh has a large mesh and a negative voltage; the downstream metal mesh has a dense mesh and a positive voltage. When the particles pass through the upstream metal mesh, they are negatively charged, and are adsorbed when passing through the positively charged metal mesh, thereby achieving the purpose of particle purification. The method is simple in device and high in filtration efficiency.
3.2.5 pulse corona plasma chemical processing technology
This technique utilizes high energy electrons of 5-20 eV to bombard the gas molecules NOx, SO, O, HO, etc. in the reactor. After activation, decomposition, ionization and other processes produce strong free radicals such as COH, HO, atomic oxygen (O) and ozone, strong oxides rapidly oxidize carbon particles, NOx and SO under the action of water to form nitric acid and sulfuric acid, added Appropriate additives (NH, etc.) produce the corresponding ammonium salts, which can be collected by filters and electrostatic dust removal to reduce pollution. However, as this process produces new salts and other chemical components, it is possible to form secondary pollution, which is still in theoretical research and laboratory applications.
3.2.6 Electrostatic cyclone technology
The researchers conducted an exploratory study on the effect of electrostatic cyclone technology on the capture and removal of diesel PM. The results show that the high-pressure pulse electrostatic action can not only capture the diesel exhaust PM well, but also the HC and NOx in the exhaust gas have a certain removal effect. The electrostatic cyclone trap has the advantages of low exhaust resistance and simple cleaning.
4 Conclusion
4.1 Fuel aspects
The current development direction is to use alternative fuels, improve petroleum smelting technology, develop new diesel additives, eliminate sulfur in diesel, reduce aromatics in fuel, reduce the content of unsaturated hydrocarbons in diesel, improve the quality of diesel, and solve the problem from the source. The problem of exhaust emissions.
4.2 In-machine purification technology
The development direction of diesel exhaust emission control will be comprehensively applied by various measures. Using electronic control technology, through optimized design of diesel engine, supercharged intercooling and EGR are used to achieve optimal cooperation.
4.3 exhaust gas aftertreatment technology
Catalysts, oxidants and reducing agents are the development direction. In addition, the regeneration technology of particulate traps and the non-filtration technology for removing particulates have yet to be developed.
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