News; Diesel engine exhaust emission purification method

Published: Saturday 15 October, 2011

Preface
Diesel hydrocarbon mixed, uneven fuel cannot burn, causing the decomposition to dominated by carbon particles. At the same time, compared with the gasoline engine, diesel engine excess air ratio is high, and partial high temperature in the combustion, leading to nitrogen oxides (NOx) spawning, but carbon monoxide (CO) and hydrocarbons (HC) emissions are much lower than gasoline engines, the fuel economy is also very good. From here we can see, performance optimization of diesel engine emissions, mainly to reduce nitrogen oxides (NOx) and particulate emissions.
Diesel engine combustion temperature reaches its maximum in slow burning issue, directly affect the nitrogen oxide (NOx) generation. At the same time during the slow burning, engine fuel injection, and sprayed onto hot gas or mixed gas concentration, causes the particles generated from lack of oxygen.
Therefore, from the perspective of purification inside the machine, you can adjust the maximum temperature and the concentration of fuel, reducing nitrogen oxides (NOx) generation. Multiple-valve technology, the turbocharged and intercooled, control of fuel injection rate and exhaust gas recirculation. By adjusting the airflow fuel injection or organization so quickly and completely as combustion of fuels, reducing particle generation. Using exhaust gas Turbo technology, improve the blast pressure, improved combustion chamber, reducing oil consumption, the use of low sulphur fuel, fuel injection process control and adjust the quantity.
1. exhaust gas Turbo technology
Under pressure in different ways, engine pressurization can be divided into mechanical pressurization, pressure wave supercharged and turbocharged, composite booster. Turbocharged is utilizing engine exhaust have certain energy entering the turbine exhaust gas and expansion work, full power exhaust gas turbine used to drive axis with the turbine compressor, compressor will be fresh air again this cylinder compression, as shown Figure 1.
(1) development
Exhaust gas turbine supercharger increases engine intake air density, improvement of engine charge. Most vehicle engine using radial to meet requirements for high speed and high performance. Turbocharger compressor parts of general use single-pole structure of centrifugal and turbocharging system, can be divided into time-pressure turbocharger systems and impulse Turbo system. One, the pressure booster systems for exhaust low utilization rates, poor low-speed torque characteristics and Acceleration performance, intended to be used when a low pressure. Pulse-Turbo system low pressure when relatively high exhaust gas utilization, scavenging effect obvious, exhaust pipe volume is small, sensitive to load changes, dynamic response, and complex structure. Diesel, higher demands on acceleration and torque characteristics, so the use of pulse-Turbo system.
Compare with mechanical booster, turbocharged by engine crankshaft output power not consumed, without affecting engine power and no additional fuel consumption. Compared with the pressure wave supercharged, its high boost pressure, up to 0.4MPa, when used in diesel engines than 35kW single power, and relatively mature technology and existing products. Compared with the combined charging, its simple structure, easy to control, more suitable for diesel use. Use exhaust gas turbine supercharger, diesel engine after necessary modifications, you can increase the power 30%~50%, fuel consumption reduces 5% or so, will help improve the whole machine of power, economy and emission performance.
Exhaust gas Turbo technology also has some flaws. First poor low speed performance. When you lower engine speed to drive turbines generated power will be reduced, leading to compressor supercharging pressure lower, final rinse well. Secondly, accelerating response slow. Using exhaust-driven supercharger, after at least one cycle of the engine emissions will increase, will not reflect on the supercharger, so transient response is not good. Third, the intake and exhaust pressure sensitivity is high. When the cylinder exhaust volume is too small, turbocharger surge occurs. When cylinder exhaust volume too large, supercharger is blocked. Both non-normal condition will affect the turbocharger boost pressure and efficiency.
(2), exhaust gas turbo-effect on emissions
1) effect on emissions of CO
CO is a product of incomplete combustion of fuel in diesel engines, mainly formed under the local hypoxia or hypothermia. Diesel engines usually work under lean-burn condition, Turbo technology allows excess air coefficient larger, improved fuel atomization and mixing, fuel burn more fully, further reducing CO emissions.
2) impacts on HC emission
HC is primarily in the diesel engine by the original fuel molecules, and decomposition of the fuel elements and compounds in the middle of the composition in combustion reactions, generation cylinder lubricating oil fraction by the fleeing man. Pressurized air density increases, excess air ratio goes up, you can improve the quality of fuel diffusion, reducing sediment in the fuel on the combustion chamber wall, HC emission reductions.
3) influence on NOx emissions
NOx generation depends largely on the combustion process of concentration, temperature and reaction time. Pressurized diesel engine simple, resulting from excess air ratio increases the combustion temperatures and NOx emissions increased. Often reduce its turbocharging while compression ratio and retarded injection, EGR, reducing NOx emission. Adopting inlet air cooling technology can significantly reduce pressure in less advanced temperature, effective control of combustion temperatures, which will help reduce NOx.
4) impacts on particulate emissions
Causes influencing the particles generated more complex mainly affected by excess air ratio, fuel atomization quality, injection rate, combustion and fuel quality affected. Usually not conducive to measures in favour of reducing NOx emission of particulates. Pressurized, intake air density increase charge increases, combined with cooling technology and high-pressure fuel injection, electronically controlled common rail injection, multiple-valve technology to more effectively control the emission of particulates.
5) impact on CO2 emissions
(3) Turbo technology development trend
Variable geometry turbocharging is a supercharged technology development potential in the future. Traditional turbocharger cannot adjust the nozzle section speed, load variations to meet high speed good work but don't meet low speed good, low speed pressurized less efficient. Variable geometry turbine nozzle area to decrease when the low speed, to increase the boost pressure effect, guarantee the good working of low speed.
2. exhaust gas recirculation (EGR)
In order to resolve the NOx emissions, exhaust gas recirculation (EGR), architecture diagrams as shown in Figure 2. The principle is to import a portion of the exhaust gas combustion chambers, increasing the heat capacity of gases in combustion chamber, reducing the maximum temperature of the combustion gases, thus inhibiting NOx emissions.
(1) Current development of EGR
In the 1970 of the 20th century, the foreign begins study of exhaust gas recirculation, EGR system is already installed on some diesel vehicles now, laying a foundation for diesel vehicles meet Euro IV standards.
For turbocharged and intercooled diesel engine, usually have the following two ways: from taken from returning before the turbine to the compressor EGR system; taken from after the turbine before gas back into the compressor of the EGR system. Structure design of turbocharged diesel engine of cooling recycled before appropriate to adopt a way, avoiding any recycling exhaust gas pollution of compressor and intercooler, reduce the blockage and corrosion problems, while avoiding EGR response lag over working conditions.
Due to oxygen combustion for diesel engines, direct injection diesel engine EGR rate exceeds 40%, non-direct-injection of up to 25%. To prevent the particles, often using larger EGR rate, low load, full load without EGR, to ensure the engine's power performance and fuel economy. When speed increase, reducing EGR rates to ensure that more fresh air into, measured best by an experimental calibration of EGR pulse spectrum.
On EGR rate control the use of electronic signals. According to the engine speed signal, the fuel pump rack signal (that is, fuel delivery) and water temperature signal, according to the predefined pulse spectrum change of EGR rate. Diesel pressure difference between inlet and exhaust pipe small, diesel E-GR returning tube diameter is large, and diesel engines needed high EGR rates, intake manifold and throttle, low load, through the air intake restrictor to increase pressure difference between inlet and exhaust pipes. At the same time, cooled EGR, to further reduce NOx emissions. SO2 in the exhaust of diesel engine generates sulphuric acid, EGR system of pipes and valves and cylinder wall of forming corrosion, should select high-quality lubricating oils and low-sulphur diesel.
(2) effect of EGR on emissions
1) effect on NOx emissions
EGR technology to reduce the combustion temperature in the combustion chamber of the highest, reducing the intake charge, so as to suppress NOx emissions. Experiments show that when the engine speed when certain, ratio of NOx in flue gas, will reduce over EGR rate increases. When the engine is under different load, reduction in NOx emissions rate showing approximate linear relationship with the EGR rate. Higher EGR rates will lead to a decline in diesel engine, at the time of high load, EGR rate lower, small load, high EGR rates, depending on the operating mode, select appropriate EGR rate.
2) effect on particulate emissions
When the engine speed when certain, particulate emissions from EGR rates vary. In General, emissions will result in the introduction of fresh air into the cylinder less prone to local hypoxia and incomplete fuel combustion, causing particles increases. As the EGR rate increases, increasing the engine exhaust particulate. But in fact, high load, fuel injection, and a shorter burning time, E-GR rate of significant impact on excess air ratio, larger particles increases. In a small load, fuel injection and less influence of EGR rate for excess air coefficient relative weakening particles upward trend is also relatively small. And unlike linear relationship between NOx and particulate emissions from EGR rate and rate of increase as second response, consequently particles increase is relatively greater.
With the introduction of emissions, NOx emissions would be reduced, the particle will rise in the value, condition particle loads a larger increase in trend is very clear, limit of EGR rate under high load conditions. At the same time, HC with EGR system for engine exhaust particulate components less. Need proper selection of integrated both NOx and particulate EGR rate.
3) effect on HC, CO emissions
As the EGR rate increases, emissions of HC and CO in the exhaust of the engine changes more consistent, rise. In the case of the engine speed must, as the EGR rate increases, HC and CO are inadequate fuel combustion emissions. As the charge into the cylinder exhaust gas increase, will inevitably lead to participation in the relative reduction in the amount of oxygen combustion, fuel combustion conditions deteriorate. HC emission in heavy load, rendered in an upward trend, in low load-time rendering of a downward trend. HC emissions come mainly from forming within the delay period of very lean mixture, HC emissions associated with delay period length. Load the lower the delay period in the formation of very lean mixture more higher the concentration of HC in the engine exhaust. In the same low load, exhaust gas recirculation rate, heated air intake more, delay period is shortened, good for improve HC emissions.
4) on the effect of CO2 and fuel consumption
Tests showed that when the engine exhaust gas recirculation rates increased, excess air ratio has decreased, but the CO2 emissions and fuel consumption of only a very small fluctuations, has remained virtually unchanged.
(3) Future development trend of EGR
In Europe and developed, EGR in gasoline engine and light diesel engine area has is a mature of industrial technology, development direction is will its perfect: how will EGR technology and particles capture technology, and electric controlled high pressure spray oil technology, and inlet rich oxygen technology, close combination up, makes various harmful emissions property full reduced; how implementation EGR rate variable workers condition Shi of precision control and dynamic response characteristics of improve are is yihou of research focus. To meet Euro IV standards, EGR rate need to be further improved, EGR when applied to turbo engines, corrosion problems and study on exhaust pressure deficit problem needs, to achieve a more satisfactory solution. In the field of heavy duty diesel engines, EGR problems more and more complex, in the heavy duty diesel engine high load situations, as EGR rate increases particulate emission increases accelerated and engine durability and reliability is affected. Current EGR application in heavy diesel engines aboard a priority research directions, predictable in the near future EGR in the heavy diesel engines have broad application field.
3. diesel engine combustion
Both is through changes in combustion chamber of diesel engine combustion to reduce NOx or particle purposes. However, for NOx and particulate emissions often contrary to the law, are usually reduced one another grows, therefore, to consider the adoption of improved fuel means to achieve not only reduces NOx reducing particulate emissions.
(1) diesel engines burning LPG (liquid petroleum gas)
Experiment that after diesel engines burning LPG, as LPG in inlets and air more evenly, improved local oxygen during combustion, particle emissions are curbed. NOx, HC, CO with the LPG joined volume and conditions change. NOx emissions increase and decrease in small and medium-sized load with the volume of LPG, when full-load condition, less LPG, NOx decrease with the LPG increases. When LPG volume continues to increase, slightly higher NOx emissions. HC, CO emissions rise with the LPG increases and can reduce fuel delivery advance angle to reduce emission.
(2) diesel engine burning CNG (natural)
Natural gas is a relatively common form of fuel, CNG/diesel dual fuel engine has product. For example, with a small amount of natural gas ignited by pilot diesel engine, its mixture of premixed combustion, combustion ignition point arising in the course of many, local hypoxic conditions improve combustion, reducing the emission of particulates. At the same time burn faster, select the appropriate proportion of natural gas, diesel, can reduce NOx emissions.
(3) diesel engine burning hydrogen
Is a substance with high heating value of hydrogen gas, hydrogen during burning, the flames spread quickly, does not produce HC, CO and CO2, is a very clean fuel, but resource rich. When mixed with hydrogen combustion for diesel engines, will greatly improve combustion. Plasma hydrogen production technology in automobile applications, can provide engine with hydrogen-enriched gas, improving thermal efficiency, while hydrogen burning speed, reduces the delay period can suppress NOx emissions. Currently plasma hydrogen technology application in gasoline engines make progress in diesel engines also have good effect.
(4) other alternative fuel
With the shortage of energy, with participation of more fuel to the optimization of diesel engine combustion method. By improving fuel quality and composition, improve the in-cylinder combustion process, to improve engine efficiency and emissions performance, will have extremely broad prospects.
4. conclusions
(1) the use of exhaust gas Turbo technology can improve fuel economy, reduce the emissions of HC, CO and particulates, but deteriorated NOx emissions. Result needed after the installation of process equipment, can go to reduce emissions.
(2) EGR technology is mainly used for lean burn large amounts of NOx emission of diesel engines, can significantly reduce NOx emissions, compared to increases the volume of other emissions, particularly particle emission with EGR rates rise, rapid increases, combined with particulate filter effects will improve emissions.
(3) by burning other fuels, such as: natural gas, liquefied petroleum gas, hydrogen, and so can to some extent improve the amount of NOx and particulates in the exhaust, need it relative to the operating conditions, further research is required.
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