| With the development of the automotive industry,the pursuit of energy conservation and emission reduction has become the top priority of the automotive industry.Especially with the gradual exhaustion of petroleum resources,the development direction of new automotive engines is to find alternative fuels,reduce fuel consumption,reduce emissions and improve the brake thermal efficiency.Combining with the National Natural Science Foundation program,based on the transformation of traditional gasoline engines,this thesis used the injection strategy of gasoline intake port injection and hydrogen direct injection with the technology of the split hydrogen injection,together with three-dimensional simulation software CONVERGE,to research the mechanism of combustion and emission performance of hydrogen/gasoline dual-fuel engine.In this thesis,a four-cylinder four-stroke SI engine was modified.The direct injection system of the engine was connected to the high-pressure hydrogen supply system,so that the engine can realize the injection strategy of gasoline intake port injection and hydrogen direct injection.By using the d SPACE,a control system was built to control the injection,ignition and other parameters of the engine.In the test bench,a large number of sensors were built to control and monitor the operating parameters of the engine,so that the engine can run on the expected conditions.And the sensors can display the combustion and exhaust performances of the engine.At the same time,an engine simulation model was built.By the simulation study and the experimental study,the mechanism of the effects of the hydrogen mixture distribution on the combustion and emission performance of the engine was studied.The research was mainly divided into four kinds of injection mode,pure gasoline model,single hydrogen injection model(gasoline plus single hydrogen direct injection),the homogeneous hydrogen model(gasoline plus homogeneous hydrogen mixture distribution)and split hydrogen injection model(gasoline plus split hydrogen direct injection).Different injection modes can form different kinds of hydrogen mixture distribution in the engine cylinder,affecting the combustion and emission performance of the engine.On the basis of experimental research,this thesis used simulation research to explore the influence of different injection strategies on the hydrogen mixture distribution in the engine cylinder,so as to study the internal mechanism of the influence of the hydrogen mixture distribution on the combustion and emission performance of the engine.The main conclusions of the study are as follows:(1)Through three-dimensional simulation research,it was found that the hydrogen mixture distribution in the engine cylinder formed by the single hydrogen injection has great limitations.With the earlier hydrogen injection timing,the hydrogen in the engine cylinder is homogeneous without the hydrogen concentration area around the spark plug,which can not accelerate the ignition and combustion process of the engine well.With the later hydrogen injection timing,it can make hydrogen concentration around the spark plug in the engine,so that the engine combustion characteristic can be improved.But due to hydrogen is concentrated in a small area,it can not be in the effective use of the characteristics of hydrogen to reduce the quenching distance to reduce HC emissions,and partial combustion temperature of engine will become too high due to the hydrogen concentration,which produces a large number of NOX emissions.Therefore,under the single hydrogen direct injection,the combustion performance and emission performance of the engine cannot reach the best at the same time due to the limitation of hydrogen injection timing.(2)In order to solve the disadvantage of the hydrogen mixture distribution in engine cylinder with single hydrogen injection,this thesis presented the injection mode of split hydrogen direct injection.Split hydrogen direct injection can use two hydrogen injection,with the help of the adjustment of the split injection proportion and the injection timing,to effectively organize the hydrogen mixture distribution in the engine cylinder and optimize the combustion and emission performance of the engine as much as possible.With the split hydrogen direct injection,the first hydrogen injection can form a relatively homogeneous hydrogen distribution in the whole cylinder to reduce emissions,while the second hydrogen injection can form a hydrogen concentration zone around the spark plug to enhance the ignition and combustion speed of the engine,so as to meanwhile do well in the combustion and emission performance of the engine.(3)With the split hydrogen injection model,the brake thermal efficiency of the engine is slightly higher than that of the single hydrogen injection model and the homogeneous hydrogen mode,and the emission performance is between the single stratified hydrogen and the homogeneous hydrogen mode.This is because the split hydrogen injection mode can make full use of the hydrogen to accelerate the entire combustion process and further improve the brake thermal efficiency by reasonably organizing the hydrogen mixture distribution in the engine cylinder.At the same time,as the hydrogen concentration around the cylinder wall increases,compared with the single hydrogen injection mode,the HC emission of the engine decreases.Compared with the single hydrogen injection model,the hydrogen mixture distribution with split hydrogen injection mode is homogeneous.Therefore it is no longer a local of high temperature area,and the NOX emission of the engine reduces.(4)The addition of hydrogen can greatly expand the lean burn limit of the engine.In this condition,the excess air ratio of the the lean burn limit increases from 1.5 to about 2.8 to 3 after the addition of hydrogen.The lean burn limits of different hydrogen injection mode is slightly different,and the lean burn limit of single hydrogen injection mode is the highest.With the increase of excess air ratio,the hydrogen injection mode corresponding to the best brake thermal efficiency is gradually changed from the split hydrogen injection mode to the single hydrogen injection mode.This is because as the excess air ratio increases,the engine needs more hydrogen to stabilize and accelerate the ignition process.At the same time,with the increase of excess air ratio,the emission performance of the engine is also improved.(5)With the change of engine operation parameters,the hydrogen mixture distribution of split hydrogen injection mode can be adjucted to meet different requirements by changing the split injection proportion and the injection timing.Under the lean burn condition,the split hydrogen injection mode can continuously increase the proportion of the second hydrogen injection to ensure the stability of engine ignition.With the increase of hydrogen injection pressure,the split hydrogen injection mode can reduce the injection proportion of the second hydrogen injection,using the more concentrated hydrogen to ensure the ignition performance of the engine,and use more hydrogen to accelerate the engine combustion process.(6)The improvement of engine combustion and emission by hydrogen decreases with the increase of engine speed and load.The addition of hydrogen can make the combustion speed of gasoline engines faster,the combustion temperature higher and the combustion stability enhanced.However,when the condition of the engine changes from small speed and small load to high speed and large load,the combustion speed,combustion temperature and combustion stability of the gasoline engine will be improved accordingly.Therefore,the improvement of hydrogen is more obvious under the small engine speed and small load condition.(7)In this thesis,the research aims to optimize the combustion and emission of the engine by changing the engine injection strategy under different conditions.Various hydrogen injection modes of the engine have their own advantages and disadvantages with the change of different conditions.Under the lean burn condition,the brake thermal efficiency of the single hydrogen injection model is the highest.Under normal conditions,the brake thermal efficiency of the split hydrogen injection model is the highest.Under the whole conditions,the emission performance of homogeneous hydrogen mode is the best.Through proper calibration and control,the engine can optimize the combustion and emission under the whole conditions by adjusting the injection strategy. |
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