Since the NOx emitted by gasoline engines can be solved by three-way catalysts or lean combustion, the NOx catalysts in the diesel engine exhaust under oxygen-rich conditions are still under research and development, and there is no successful catalyst available at present. How to maintain the good performance of diesel engines while reducing The generation of NOx is a major technical challenge currently facing.
The CO and HC emissions of diesel engines are much lower than those of gasoline engines, but the NOx is in the same order of magnitude as gasoline engines, and the emissions of particulates and soot are dozens of times or more larger than those of gasoline engines. Therefore, the emission control of diesel engines focuses on NOx and particulates, followed by HC. Reducing particulate and soot emissions goes hand-in-hand with improving diesel combustion processes, but NOx emissions are often contradictory, creating particular difficulties for diesel emissions control. Since the NOx emitted by gasoline engines can be solved by three-way catalysts or lean combustion, the NOx catalysts in the diesel engine exhaust under oxygen-rich conditions are still under research and development, and there is no successful catalyst available at present. How to maintain the good performance of diesel engines while reducing The generation of NOx is a major technical challenge currently facing.
The root cause of pollutant emissions from diesel engines is the poor mixing of fuel and air. When the diesel engine is running, the average excess air coefficient a is generally below 1.3. If the ideal mixing is achieved, it is impossible to generate soot, and the generation of NOx will not be much. However, poor mixing leads to local hypoxia, resulting in a large amount of soot. At the same time, there are many high NOx generating areas with a=1.0~1.1. Therefore, the emission control of diesel engines should focus on the central task of improving the mixture of oil and gas, preventing local a from exceeding 0.9 (which is conducive to NOx generation) and lower than 0.6 (which is conducive to soot generation), so as to achieve oil, gas, and chamber (combustion). This is the most common and fundamental measure to reduce diesel engine emissions.
1. Purification measures in diesel engine
1. Choose a low-polluting combustion system. In the internal purification measures of diesel engines, the influence of the structure type, shape and structural parameters of the combustion chamber on pollutant emissions plays a major role. Compared with the direct-injection combustion system, the indirect-injection combustion system (swirl chamber, pre-combustion chamber) is a low-pollution combustion system.
The underground diesel power equipment produced abroad generally adopts the indirect injection combustion system, such as DeutzFL413FW series diesel engine (two-stage combustion system), Caterpillar3306PCT diesel engine (pre-combustion chamber) and Deutz-MWMD916/D932 series diesel engine (pre-combustion chamber) Wait.
The reason why the pollutants of the indirect injection diesel engine is much lower than that of the direct injection diesel engine is that the indirect injection combustion system has strong airflow movement, and it is two times of mixing and two combustion (one is in the auxiliary combustion chamber, that is, the swirl flow) The second is the mixing and combustion in the main combustion chamber after the gas flows out of the auxiliary combustion chamber), and there are compression vortex and combustion vortex. Therefore, the mixture formation and combustion of the indirect-injection combustion system are more complete and perfect, so the CO emission is much lower than that of the direct-injection type.
For HC, due to the greatly improved quality of the mixture formed by the indirect injection diesel engine, the over-rich and over-lean areas in the mixture are reduced, and the unburned, cracked and incomplete HC in the over-rich and oxygen-deficient area is reduced. Combustion reduces HC emissions when hydrocarbon fuels outside the lean limit cannot be fully burned leaving HC.
The NOx emission of the indirect injection diesel engine is also lower than that of the direct injection type, because although the temperature in the swirl chamber (or pre-combustion chamber) is high, the concentration of oxygen is very low, which is not conducive to the generation of NOx. When the incompletely burned high-temperature gas is sprayed into the main combustion chamber with the unburned fuel, it is quickly mixed with the air. At this time, although there is a surplus of oxygen, the piston has begun to descend, the temperature of the gas is lowered, and the gas stays at high temperature. The time is short, which is not conducive to NOx generation.
In terms of soot particles, the indirect injection diesel engine is also lower than the direct injection type, because the indirect injection combustion system has a strong air swirl movement, and the mixture is mixed and burned twice. The soot generated in the auxiliary combustion chamber is oxidized to a considerable extent by subsequent combustion in the main combustion chamber.
The direct-injection combustion chamber has the obvious advantages of low fuel consumption and good cold start performance, so it has been widely used in diesel engines, but its main disadvantage is serious pollution. At present, people strive to take into account the requirements of low fuel consumption and low emissions by developing new direct injection combustion systems. The representative ones are micro-vortex type semi-open combustion chambers, such as quadrangular combustion chambers, micro-hybrid combustion systems (HMMS), micro Swirl Combustor (MTCC) etc. These combustion chambers rely on the proper matching of airflow movement, fuel supply system and combustion chamber to reduce exhaust pollution while maintaining low fuel consumption.
2. The improvement of the oil supply system includes the following steps:
1) Delay fuel injection timing. Delaying fuel injection timing is an effective measure to reduce NOx emissions from diesel engines. Since the injection timing is retarded, the maximum combustion temperature in the cylinder is lowered, thereby suppressing the generation of NOx. However, delaying fuel injection timing will cause increased soot and particulate emissions as well as loss of power and economy.
2) Increase the fuel injection rate. Increasing the fuel injection rate can shorten the fuel injection duration, which can prevent more fuel from being injected into the high temperature flame after ignition. Because the latter will cause thermal binding effect, and high temperature pyrolysis of fuel to form soot and particulates. Therefore, the diesel engine should have a higher fuel injection rate. However, increasing the fuel injection rate increases NOx emissions.
3) Reduce the volume of the fuel injection nozzle pressure chamber. The injector pressure chamber volume has a great influence on HC emissions. For the injector without pressure chamber (VCO, that is, the needle valve closes the nozzle), the HC emission is significantly lower than that of the injector with pressure chamber. Therefore, to obtain low HC emissions, for the fuel injector, no pressure chamber can be used or the volume of the pressure chamber can be reduced to a minimum. However, the reduction in the volume of the pressure chamber is limited by the life of the injector.
4) Adopt high pressure injection system. Studies have shown that higher injection pressure is beneficial for improving fuel atomization, oil-air mixing, shortening combustion duration and improving emissions, which can reduce HC, smoke and particulate emissions. At present, the injection pressure of the traditional pump-pipe-nozzle system can reach 110~130MPa, and the pump-nozzle system can provide a fuel injection pressure of 150MPa or higher for high-speed diesel engines.
5) Oil supply system with pump-nozzle structure. The pump-nozzle has the advantages of high fuel injection pressure and fuel injection energy and is not easy to occur secondary fuel injection and cavitation, so it has attracted more and more attention and favor. Diesel engines using pump-nozzle not only have good fuel economy, but also have low emissions, especially HC, soot and particulate emissions. If the fuel injection is delayed appropriately, NOx can be reduced.
3. Improve the intake system. The trend of diesel engine technology development is to increase the fuel injection pressure and reduce the intake swirl intensity to reduce the intake (pressure) loss, and to cooperate with the multi-hole and small-bore injectors to obtain a good mixture.
The improvement of the intake system can be carried out in three ways:
1) Adopt resonance air intake system. The general inertial charging system can only work in a narrow range of operating conditions of the diesel engine, while the variable resonance intake system can adjust the intake air volume in the entire speed range of full load and partial load.
2) Adopt variable swirl air intake system. Since the intake swirl intensity has a great influence on NOx, the variable swirl intake system is used to control the swirl intensity according to the change of speed and load, which can reduce the amount of NOx generated without sacrificing economy.
3) Adopt multi-valve technology. The use of multi-valve (4 valves per cylinder) can expand the total flow cross-sectional area of the intake and exhaust valves, increase the charge coefficient, and the injector can be vertically arranged on the cylinder axis, which is conducive to the uniform distribution of fuel in the combustion chamber space, The mixture formation and combustion conditions are greatly improved, the heat release law is more reasonable, the NOx is reduced, the CO is also reduced, and the soot is also significantly reduced when the load is large.
4. Adopt supercharging and intercooling technology. Because supercharging can improve diesel engine performance and reduce emissions, diesel engines will more commonly use exhaust gas turbocharging technology.
After the diesel engine is supercharged, the amount of CO and HC can be reduced due to the increased air density entering the cylinder. Without an intercooler, the amount of NOx emissions may increase due to the high intake air temperature. After using the intercooler, the intake air temperature can be reduced, so the NOx emission will decrease accordingly.
5. Reduce the consumption of lubricating oil. Studies have shown that lubricating oil consumption has a significant impact on the size of diesel particulate emission limits. Therefore, the consumption of lubricating oil should be strictly controlled. The target is that the consumption of lubricating oil is less than 0.19g/kW·h under high load and high-speed operation conditions; When the oil gets particulate emission, the lubricating oil consumption is less than 0.5g/h×displacement (L). The control target of particulates generated from lubricating oil is not more than 0.0408 g/kw·h.
Strategies to reduce lubricant particulate emissions include:
1) Reduce the consumption of lubricating oil on the cylinder wall.
2) Reduce the consumption of lubricating oil entering the combustion chamber and the supercharger.
3) Develop low consumption lubricating oil.
6. Adopt electronically controlled fuel injection technology. In order to meet the increasingly stringent emission regulations of diesel engines, electronic control of diesel engines has become an indispensable technology for diesel engines. The diesel engine adopts electronically controlled fuel injection technology to optimize the control of the fuel injection law and fuel injection quantity, and to control the fuel quantity of the premixed combustion and diffusion combustion parts. Pre-injection before main injection can effectively reduce NOx and noise, because pre-injection suppresses premixed combustion, delays diffusion combustion, and reduces the average gas temperature, so NOx generation is reduced.
