Mechanism Study On High Efficiency And Clean Combustion Of Diesel Methanol Dual Fuel Engine For China Ⅵ Emission Regulations

In order to cope with the dual pressure of energy shortage and environmental protection,the application of methanol as low carbon fuel in internal combustion engine attracts extensive attention.Previous studies have shown that the diesel methanol compound combustion（DMCC）achieves the World Harmonized Steady-State Cycle（WHSC）emission regulations in compression ignition engine without using the selective catalytic reduction（SCR）.At the same time,the brake thermal efficiency（BTE）was improved to a certain extent compared with the prototype diesel engine which achieves the same emission regulations.DMDF mode achieves low NOx emissions and improves the BTE of the engine only by in-engine purification,which is different from the technical route of NOx emissions reduction in diesel mode by SCR urea injection.This new technical route shows great potential for the future internal combustion engine to realize low-carbon and clean combustion.Therefore,it is of great theoretical significance and engineering practice value to explore the reasons why DMDF mode achieves low emissions and high BTE and to reveal its mechanism of high efficiency and clean combustion.This subject studied the mechanism of high BTE and low NOx,and analyzed the formation conditions of regulated and unregulated emissions from the thermal conditions in the cylinder.The energy loss and influencing factors in the DMDF mode and the effect of thermal management on the further improvement of BTE were studied by thermodynamic analysis.In addition,a numerical model of the working process of DMDF was constructed,which can be used as a prediction tool to further improve the performance.The main research contents and conclusions are as follows:（1）The main reason for the higher BTE of DMDF mode compared with pure diesel mode was revealed.First,based on the analysis of in-cylinder combustion process,DMDF mode was the coordinated combustion mode of homogeneous compression combustion with a small amount of diffusion combustion.DMDF mode was more conducive to thermal power conversion due to quick combustion.Second,the quick combustion of DMDF reduced the heat transfer from the cylinder.Third,the vaporized methanol and EGR reduced the actual intake air flow in DMDF mode,resulting in the reduction of the charge involved in combustion in the cylinder,which reduced the intercooling loss and the energy loss taken away by the EGR.Fourth,the characteristic of methanol contained oxygen itself ensured that the combustion can be fully carried out at low oxygen concentration.The combination of the above factors improved the BTE of DMDF mode.（2）The mechanism of low NOx emissions in DMDF mode was clarified.It was found that the low temperature,low oxygen and nitrogen concentration in the cylinder created by DMDF mode combined with the diesel methanol quick combustion was the fundamental to achieve low NOx.The large amount of methanol and EGR used in the DMDF mode significantly reduced the temperature of mixture and the total amount of air in the cylinder,which reduced the total amount of nitrogen and oxygen involved in the combustion.The combined effect of the above factors formed the conditions of low temperature,low nitrogen and oxygen concentration and quick combustion,which inhibited the generation of NOx.However,methanol mixture contained oxygen itself ensured the formation of low PM in the combustion process.In addition,the DMDF mode with high MER and high EGR greatly weakened the trade-off relationship between NOx and PM and realized WHSC emission regulations without using SCR.（3）The main causes of high HC emissions in DMDF combustion mode were clarified.The research showed that the engine that used the port injection of methanol to form a homogeneous mixture and then was compression ignition with diesel in the cylinder had higher concentrations of HC emissions.Based on the phenomenon,camshafts with different valve overlap were designed and special research was carried out.It was found that the sources of high HC emissions were mainly from incomplete combustion in the cylinder and the leakage from valve overlap.Methanol leakage was small at valve overlap of 40°CA.The emission of methanol and formaldehyde with increasing valve overlap was studied by means of simulation and bench test.The results showed that methanol leakage increased rapidly when the valve overlap was greater than 60°CA.This was because the larger valve overlap led to the increase of effective flow area.（4）The complex combustion mode of DMDF was studied by exergy analysis.DMDF meeting the ChinaⅥemission regulations was a complex reaction including methanol,diesel,fresh air and EGR.The combustion process of DMDF also changed from diffusion combustion of diesel to premixed and partial diffused combustion of DMDF.Therefore,exergy analysis can more accurately analyze the cause of exergy loss of complex combustion in DMDF mode from the perspective of total energy.The influence of MER,high/low pressure EGR and main injection timing on the exergy balance of DMDF were found.First,the addition of high pressure EGR improved the in-cylinder temperature of DMDF mode with high MER,which reduced the incomplete combustion and irreversible combustion exergy loss and increased the power capacity of the engine.Low pressure EGR only reduced NOx generation and had little impact on BTE.Second,the main injection timing of diesel was moderately advanced,which was conducive to improving the BTE.With the advance of main injection timing,the BTE of the engine first increased and then decreased.（5）The thermal management strategies to improve the BTE of DMDF engine was proposed.Increasing intake air temperature obviously improved the combustion efficiency and reduced incomplete combustion loss under heavy load.Increasing cooling water temperature reduced cooling loss and incomplete combustion loss.The effect of intake air temperature on BTE was higher than that of cooling water temperature.The in-cylinder temperature and EGR flow rate decreased with the decrease of exhaust pressure,which greatly reduced the heat transfer in the cylinder and the cooling loss of EGR,and significantly improved the BTE.（6）A prediction model of DMDF mode was established based on GT-Power coupled genetic algorithm.On the basis of a large number of research results,the influence of MER,oxygen concentration,CO₂ concentration and main injection timing on engine performance and emissions was summarized.The prediction model suitable for DMDF engine was established by GT-Power and the multi-parameter optimization was realized by coupling genetic algorithm.The experimental verification results showed that the optimization results of the prediction model were in good agreement with the experimental results.Therefore,the prediction model can be used as a prediction tool to further improve the performance of DMDF mode.