Diesel engine internal purification method dialysisDiesel generator reached the maximum combustion temperature during a slow-burning, directly affect the nitrogen oxide (NOx) generation slow-burning period at the same time, if the engine is still in injection, and spray to the high temperature exhaust gas area, or a rich mixture, can all result in a lack of oxygen and generated particles. Therefore, from the Angle of the generator in purification, can by adjusting the highest temperature and fuel concentration, and reducing the generation of nitrogen oxides (NOx) can be used more than valve technology, turbocharged inter-cooled technology, injection rate and exhaust gas recirculation control by adjusting the air flow, fuel injection or organization make fuel rapidly and completely burn, reducing the generation of particles can adopt exhaust gas turbo technology, improve the injection pressure, combustion chamber structure, reduce oil consumption, the use of low sulfur fuel, control and adjust the volume of the fuel injection process.
1. Exhaust gas turbocharging technology
According to the pressure in different ways, the engine of pressurization can be divided into: mechanical pressurization, air wave pressure, exhaust gas turbo, composite pressure of exhaust gas turbo is to use engine exhaust has certain energy and waste gas into the turbine expansion work, all the power used to drive and the turbine exhaust turbine coaxial rotary compressor, the fresh air in the compressor compressed and then send one cylinder.
1.1 development status
Exhaust gas turbocharger can improve the engine air intake density, improve engine vehicle engines are more commonly used radial flow, charge quantity to meet the requirements of high speed and high response performance of turbocharger compressor parts generally adopts single-stage centrifugal structure, and turbocharging system, can be divided into constant pressure turbocharging system and pulse turbocharging system, constant pressure supercharging system of exhaust utilization rate is low, low speed torque characteristics and acceleration performance is poorer, suitable for low pressure pulse turbocharging system at low pressure for waste gas utilization rate is relatively high, scavenging effect is obvious, the exhaust volume is small, is sensitive to changes in load, good dynamic response, The diesel engine with complex structure requires high acceleration performance and torque characteristics, so the pulsed turbocharging system is often adopted.
Compared with mechanical pressure, exhaust gas turbo not consumed by the engine crankshaft output power, does not affect the engine power, does not increase fuel consumption compared with air wave pressure, the supercharging pressure is high, can reach 0.4 MPa, single machine power is greater than 35 kw when used in diesel engines, and relatively mature technology, has realized the transition compared with compound supercharging, its structure is simple, easy to control, more suitable for automotive diesel engine using exhaust gas turbocharger, diesel engine after the necessary modification, can make the power increased by 30% ~ 50%, about 5% lower fuel consumption, to improve the engine performance, fuel economy and emission performance.
But exhaust gas turbo technology also has certain defects in the first place, when low speed performance is bad when diesel engine at low speed, the power generated by turbines will also reduce, reduce compressor supercharging pressure corresponding and supercharging effect is not good second, acceleration response slow due to the use of waste gas to drive the supercharger, engine after at least one discharge waste gas quantity will increase, will reflect to the supercharger, so transient response is bad again, high sensitivity to the inlet and exhaust pressure displacement when the cylinder is too small, turbocharger surge will happen when the cylinder displacement is too large, Both of these abnormal conditions will affect the pressure and working efficiency of the supercharger.
1.2 effects of exhaust gas turbocharging on emissions
1.2.1 influence on CO emissions
CO is a product of incomplete combustion of fuel in diesel engines, mainly in the local hypoxia or low temperature of diesel engine is usually work under the condition of lean combustion, turbo technology makes the excess air coefficient, fuel atomization and mix improved, make the fuel burn more fully, further reduce CO emissions.
1.2.2 impact on HC emissions
The HC in diesel engine is mainly by the original fuel molecules, decomposition of fuel molecules and the combustion reaction of the intermediate compound, a small group of oil cylinder by channeling generation pressurization junior increased air tightness, excess air coefficient is bigger, can improve the quality of fuel atomization, reduce the fuel oil deposition on the combustion chamber wall, HC emissions.
1.2.3 influence on NOx emission
NOx formation mainly depends on the concentration, temperature and reaction time in the process of diesel engine burning pure booster, due to the excess air coefficient increases, and combustion temperature increase NOx emissions so often in pressurization at the same time reduce the compression ratio, delayed injection, exhaust gas recirculation, reduce NOx emissions of the inlet inter-cooled technology can greatly reduce the pressurization last in temperature, effective control of combustion temperature, reduce NOx.
1.2.4 effects on particle emission
Influence particles generated reason is complex, mainly by excess air coefficient, fuel atomization quality, injection rate, combustion process and fuel quality effect usually can lower the emission of NOx measures of them are good for particle generator network (taizhou) after pressurization, inlet density increase, the filling quantity increase, cooperate inter-cooled technology, high-pressure fuel injection, electronic control common rail injection and valve technology, etc., can more effectively control the release of the particles.
1.2.5 impacts on CO2 emissions
CO2 is an important greenhouse gas, which can lead to a global temperature rise (taizhou generator network) at the same time, CO2 emissions is one measure of engine fuel economy supercharged diesel engine make full use of the exhaust gas energy, high efficiency, the machine of mean effective pressure increase, the CO2 emission is better than that of gasoline engine.
1.3 future development trend of turbocharging technology
Variable cross-section turbo is a supercharged technology with development potential future as a result of the traditional turbocharger cannot adjust the nozzle, along with the change of rotating speed, load can satisfy the high speed of good work, but not for good work of low speed, low speed when the charging efficiency of low variable cross-section turbine can be reduced during low speed turbine nozzle area, to improve the effect of boost pressure, ensure good working on low speed.
2 waste gas recirculation (EGR)
In order to solve the NOx emission, exhaust gas recirculation (EGR) system, its principle is to put the part of waste gas into the combustion chamber, increasing the combustion chamber of gas heat capacity, reduce the highest temperature of the combustion gas, thus inhibiting NOx emissions.
2.1 development status of EGR
Starting in the 1970 s, began the study of exhaust gas recirculation system abroad, now some diesel car an EGR system has been installed, has paved the way for the diesel meet European Ⅳ standards.
For turbocharged medium-cooled diesel engines, there are usually two ways: take the air from the front of the turbine and return it to the EGR system behind the compressor. From gas turbine after return to the front of the compressor of the EGR system of turbocharged diesel engine cooling recirculation structure design is suitable for use of the former way, can avoid recycling waste gas pollution compressor and cold, reduce silting and corrosion problem, at the same time to avoid an EGR change with working condition of response lag.
Because of the diesel engine combustion, peroxide of di diesel engine EGR rate more than 40%, non di could reach 25% to prevent particles produced, middle and low load larger EGR rate, is often used to full capacity without EGR, to ensure that the engine power performance and fuel economy when speed improve EGR rate, guarantee more fresh air to enter, the calibration experiment measured the best EGR pulse spectrum.
On the precise control of EGR rate is used more electronic signals according to engine speed, oil pump rack signals (oil) and water temperature, etc., according to the predefined pulse spectrum change EGR rate for diesel oil pressure difference between inlet and exhaust pipe is lesser, diesel E - GR reflux pipe diameter is larger, and the diesel engine EGR rate is higher, but on the intake pipe and throttle, low load, through the inlet throttling to increase the pressure difference between inlet and exhaust pipe at the same time, using cold EGR, can further reduce NOx emissions of diesel engine exhaust of SO2 produces sulfuric acid, High quality lubricants and low sulphur diesel should be used for corrosion of EGR system pipes and valves and cylinder walls.
2.2 effects of waste gas recycling on emissions
2.2.1 influence on NOx emission
Exhaust gas recirculation technology to reduce the combustion chamber can achieve the maximum combustion temperature, reduce the intake charge quantity, thus inhibiting the NOx emission experiments show that when the engine speed is constant, the proportion of NOx in the exhaust gas, will decrease with the increase of the exhaust gas recirculation rate when the engine at different loads, NOx emissions decline rate and EGR rate are the approximate linear relationship between the larger exhaust gas recirculation rate to cause a decline in diesel engine power, in high load, an EGR rate is low, in a small load, EGR rate is higher, according to the different operating mode, select the appropriate EGR rate.
2.2.2 impacts on particle emission
When the engine speed must, particulate emissions will changes over the EGR rate in general, the introduction of the waste gas will cause into the fresh air in the cylinder is reduced, easy to cause local oxygen and fuel combustion is incomplete, cause the increase of the particles, with the increase of EGR rate, the engine exhaust particles also will increase, high load, but in fact, the injection is more, the combustion time is shorter, E - GR rate on the excess air coefficient is larger, the influence of the particle was increased more than in a small load, less injection, EGR rate on the influence of excess air coefficient is relatively weakened, The trend of particle increase is also relatively small, and the linear relationship with NOx is different. The relationship between particle emission increase rate and EGR rate is a secondary response, so the percentage of particle increase is relatively larger.
With the introduction of exhaust gas, less NOx emission, particle value rises, load working condition of larger particles increased obviously, should limit the EGR rate under the condition of high load at the same time, the HC components in the engine exhaust particles with an EGR system needs to two aspects of NOx and particles less choose proper EGR rate.
2.2.3 influence on HC and CO emissions
With the increase of EGR rate, change of engine emissions of HC and CO in the tail gas is relatively consistent, rise in engine speed of a certain situation, with the increase of EGR rate, HC and CO are the emissions produced by combustion of fuel is not sufficient when filling the of exhaust gas in the cylinder increases, bound to lead to participate in relative reduce the amount of oxygen the combustion, fuel combustion conditions evil HC emissions at high load showed a trend of increase, in the small load during ignition delay period showed a trend of decline the HC emissions come mainly from the formation of lean mixture, so the HC emissions associated with the length of ignition delay period, the lower the load. Ignition delay period, the more extremely lean mixture formation, the higher the concentration of the HC in engine exhaust at low load, the same gas reflux ratio, the greater the heating air intake, the more obvious, the effect of ignition delay period will shorten, is good for improving HC emissions.
2.2.4 impacts on CO2 and fuel consumption rate
The test results show that the excess air coefficient decreases when the engine exhaust gas recycling rate increases, but the CO2 emission and fuel consumption rate only fluctuate slightly and remain basically unchanged.
2.3 future development trend of EGR
Developed countries in Europe and the us in the field of gasoline engine and light diesel engine EGR has been is a mature industrial technology, the development direction is to perfect it: how the EGR and particle capture, electronically controlled high pressure fuel injection technology, such as inlet oxygen enrichment technology closely together, reduce a variety of harmful emissions full; How to realize the precise control of the EGR rate variable condition, and the improvement of dynamic response are the key to achieve the Ⅳ standard, for the later research still need to further improve the EGR rate, EGR when applied to turbo engine, corrosive HeJinPai pressure deficit problems need to study, to get a comparatively ideal solution in the field of heavy-duty diesel engine, the application of EGR problem more and more complex, in heavy duty diesel engine high load situation, with the increase of EGR rate particulate emissions increase speed is accelerated, the durability and reliability of the engine by shadow the EGR in the application of heavy-duty diesel engine is a key research direction of abroad, It can be predicted that EGR will be widely used in heavy diesel engines in the near future.
3 mixed combustion of diesel engine
The above two methods is by changing the diesel engine combustion chamber combustion mode to achieve the purpose of reducing NOx or particles but, because of NOx and particulate emissions laws are often at odds, usually is to reduce a while another increase, therefore, can be considered through the way of improving fuel to reduce NOx and reduce the particles
3.1 diesel engine mixed combustion LPG (liquefied petroleum gas)
Experiments show that the diesel engine with LPG after burning, because of LPG in mixture with air inlet, is relatively uniform, the combustion process of local hypoxia condition improved, particulate emissions, be suppressed NOx, HC, CO will change along with the change of the amount of LPG and the working condition of NOx emissions in small and medium load decreases with increasing amount of LPG, full load condition, the LPG amount less, NOx and decreased with increment of LPG amount when LPG volume continues to increase, NOx emission is slightly higher HC and CO emission increased with increment of LPG amount, can reduce emissions by reducing fuel delivery advance Angle.
3.2 diesel engine mixed combustion CNG (natural gas)
Natural gas is a kind of common fuel, diesel/CNG dual fuel engine has the transition with a small amount of diesel ignition natural gas engine, for example, the mixture premixed combustion, the combustion process produces a lot of ignition and combustion of local hypoxia condition improved, reduced emissions of particulates burn faster at the same time, select the appropriate proportion of natural gas, diesel, can reduce NOx emissions.
3.3 diesel engines are mixed with hydrogen
Material is a kind of high calorific value of hydrogen, hydrogen in the process of combustion, the flame propagation speed soon, won't produce HC, CO and CO2, is a very clean fuel, and extremely rich resources when mixing hydrogen diesel engine, can greatly improve the combustion situation hydrogen plasma technology has application prospect in the car, can provide rich hydrogen gas engine, to improve the thermal efficiency, at the same time hydrogen combustion speed, can shorten the ignition delay period, can inhibit the NOx emission current, hydrogen plasma technology in the application of improved gasoline engines, also can have good effect on the diesel engine.
3.4 other alternative fuels
As energy shortage, there will be a new type of fuel more involved in the optimization of diesel engine combustion method by improving the quality and composition of fuel, improve combustion process in cylinder, in order to improve the engine efficiency and emissions performance, there must be a very broad prospects.
(1) using the exhaust gas turbo technology, can improve fuel economy, reduce HC, CO and particulate emissions, but worsen the NOx emission generator network (taizhou) so need to add the post-processing equipment, can reduce the various emissions across the board.
(2) the exhaust gas recirculation technology mainly for lean combustion produces a large number of NOx emissions of diesel engine, can significantly reduce the emissions of NOx, will increase the generation of other emissions compared, especially particulate emissions with EGR rate increases, the rapid increase, with particulate trap emissions will improve effect.
(3) by blending other fuel, such as: natural gas, liquefied petroleum gas, hydrogen etc can partly improve the NOx in the exhaust and the number of particles, need to adjust the proportion, according to working condition of further studies are needed