| Nowadays with the growing energy crisis and environmental pollution problemsworldwide, people are becoming more stringent requirements for the engine in energy savingand control of pollutant emissions. The diesel engine has many advantages: such as economypowerful and reliability, as well as higher thermal efficiency and lower CO, HCemissions.Therefore it was widely used gradually, and the dieselization on cars will be thedevelopment trend.But there are some shortcomings,high particulate matter (PM) and NOxrestrict its development,the two emissions had caused a certain damage on the environment,Soall the countries have formulated a large number of diesel engine emission regulations in theincreasingly application,especially limiting NOxemissions without Changing the particulatematter limitation,forcing the diesel engine to take more advanced control technology to controlthe generation of pollutants.For the diesel engine, its specific exothermic ways determined by the Special combustionstyle is the expression of fuel combustion in the cylinder which has a direct impact on theengine power, economy and emissions characteristics. So if we want to achieve high efficiencyand low emissions, the key point is to control heat release rate actively.While the heat releaserate was effected by inner flow characteristics and the spray characteristics. so the main goal isto optimal match the two main objectives.According to this,the paper proposes a method ofactive control combustion process by controling the target exothermic rate, so combined thefuel injection system with the inner air movement, to control the heat release rate andconcentration field distribution, and achieve high efficiency and low emissions combustionprocess.Different shapes of combustion chamber and other parameters determine the special flowcharacteristics and the combustion rule. The way that we analyze the flow characteristics incombustion chamber have important significances on grasping the micro flow characteristics, the formation of mixture and the combustion process. When we talk about shrinkage mouthcombustion, we can’t descript the match of spray and the combustion shape intuitively with thesole concept of injection advance angle because of the difference of factors such as spray angel,injection press, the shape at shrinkage mouth and the convex platform shape. As a result, weintroduce the spray wall parameter which reflates the relation between spray and combustionshape. Firstly we analyze the impacts of different combustion shapes on flow characteristic onthe basis of the evaluation method of combustion structure. Secondly we analyze how thechange which is aroused by different impact factors of spray wall angle influence thecombustion heat release rule and the emissions. All these have universal significance on thematch of spray and the combustion shape. The result shows that, though reducing thecompression ratio is beneficial for reducing the NOxemission, it can’t apply to high low speedperformance and energy conservation; though high compression ratio can improve high lowspeed performance and realize energy conservation, it is not beneficial for NOxemission. Thatis to say adjusting the compression ratio only can’t apply to energy conservation and reducingemissions perfectly. Under the certain compression ratio conditions, we can organize andcontrol the flow characteristics in combustion chamber effectively through designing the properconvex platform shape. This is beneficial for controlling the heat release rule. The combustionchamber with flat top and concave shape of slope not only can inhibition the pre-mix processeffectively and promote diffusive combustion, but also can adjust to high speed perfectly andapply to high low speed performance. The change of spray matching angle has little impact onthe center of heat release rate. The impact of heat release duration presents difference parabolicrule which is related to engine speed. And there is a maximum of heat release duration indifference speed zone. The change of spray matching angle has great impact on initial heatrelease and little impact on diffusive combustion and is affected by engine speed. The change ofspray axis has little impact on initial heat release but great impact on diffusive combustion. Onthe contrary, the change of the spray axis’s relative position at the axis of cylinder direction hasgreat impact on initial heat release and little impact on diffusive combustion. We can control therate of pre-mixture combustion and diffusive combustion, to realize lower specific fuelconsumption and reduce NOxemission by matching best spray matching angle. The effect onheat release rule of airflow movement strength in cylinder and the distribution of airflow movement exist a weight balance on the basis of engine speed and present a trading off andtaking turns rule with the change of engine speed.The target value of the target release rate is limited by power, economy and emissions atthe same time, basing on the analysis of the gas mixture on the impact of the Combustion Law,especially on the detailed analysis of the law of NO emissions,is the to consider the importantrole of prompt NO on the NO generation, not only providing a new control method to furtherreduce the emissions of NO, but a new requirement on the analysis of the target heat releaserate. The results show that the temperature before the start of combustion period in the dieselengine is substantially all below2000K, the fast No are mainly generated this time; the gastemperature of the rapid combustion period is more than2000K, due to the composite effect ofthe temperature and mixing conditions this time, the prompt NO and Zeldovich NO willgenerate; in the late diffusion combustion period, the cylinder temperature is very high, themixed gas doctrinal area does not exist basically, so only the Zeldovich NO generates this time,not the prompt NO. In the rarefied zone relative to theoretical air-fuel mixture in the wholecombustion process, the generation of NO and temperature are closely related; while in therelative theoretical mixed gas concentration area, the generation of NO has little dependence ontemperature. Despite the small amount of the formation of prompt NO, but the generatedintermediate product of the chemical reaction of the process would greatly increase thelikehood of Zeldovich NO formation. Control of NO from the mixed gas concentration control,that is, the mixing rate of the gas mixture and temperature control, that is, the reaction rate ofthe gas mixture, the absence of any party will result in the one-sidedness of NO control. It canbe seen in the equivalent ratio of within the range of1.5to3, the prompt NO would generate atthe temperature of1800K, with the rising of temperature, the NO generation region theequivalent ratio would decreased gradually, until when the ratio decreased in the vicinity of1.2,when the temperature reached in the vicinity of the highest2400K~2500K, the NO ismainly Zeldovich NO, the proportion in the generation amount of the total NO largest. In theequivalence ratio of3and its vicinity region both prompt NO generation, another of sootgenerated, so in order to achieve reduced NOx and soot while, to avoid the superimposition ofthe high concentration region and the high-temperature region is generated.On the basis of analysis of the effect of dynamic characteristics of the mixture formation on heat release law and NO generation, proposed the concept of a target heat release rate. asbased on that analyzed the heat release rate on various operating conditions. Found the targetvalue of the target heat release rate on various operating conditions according the requirements,and researched the measures to achieve the target heat release rate from the perspective ofcontrolling the mixture formation. The results show that, the engine can achieve performancelimitation under the target heat release rate, so achieving high efficiency and low emissions.The realization of target heat release rate need a good match between the current hardware andcontrol strategies. The target heat release rate can be divided for premixed combustion stageand diffusion combustion stage. The main control parameters of pre-mixing combustion stageinclude the rate of cylinder pressure rise and the maximum combustion temperature. Thechange of in-cylinder pressure in pre-mixed combustion process not only affects the level ofcombustion noise, but also affects the emission of NOx. So controlling the pre-mixingcombustion stage with the rate of pressure rise and the in-cylinder maximum combustiontemperature, in fact, not only limits the level of combustion noise, also restrains the emission ofNOx. The combustion duration in the diffusion combustion stage affected the duration of thecylinder high temperature, and plays an important role in the generation of NOx emissions, alsodirectly affects the engine’s fuel consumption rate. So both NOx emissions and fuelconsumption rate determines the target value of the target heat release rate. If the NOxemissions is permitted, the limitations of the cylinder combustion temperature can beappropriate reduced, so that the rate of diffusion combustion is accelerate and to achieve themore fuel economy. Due to the limitation of the hardware technical level, the engine isimpossible to play the maximum performance, but from the perspective of the target heatrelease rate, optimizational match can be achieved between the various characteristics of theengine completely in support of the new technology so as to achieve high efficiency and lowemissions limit, which put forward new demands to development of equipment. |
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