| In order to further improve the thermal efficiency of the high compression ratio lean-burn engine,experiments are carried out from the fuel stratification strategy and the water direct injection strategy in this paper.The lean-burn limit of the engine is expanded by stratified lean combustion,and ultra-lean stratified combustion is realized;the optimized direct water injection strategy is used to realize the load expansion of the engine under lean-burn conditions;The simulation analysis of lean burn and direct water injection explains the reasons why fuel stratification and direct water injection improve engine performance from the in-cylinder combustion mechanism.Firstly,a strategy of fuel stratification is proposed for the homogeneous lean limit limited by the stability of combustion instability.The effects of fuel stratification strategy on ultra-lean combustion characteristics were studied,including the effects of secondary injection ratio and secondary injection timing on engine combustion characteristics,power performance and economy.After that,the optimized fuel stratification strategy is adopted,that is,the main injection time is-300°CA ATDC,the main injection ratio is 0.6,the secondary injection time is-180°CA ATDC,the secondary injection ratio is 0.4,and the working speed is 2000 rpm and the load is 8.5bar.In this case,the lean limit is extended to 2.0,and the indicated thermal efficiency of the engine is increased to 46.53%.Then,an experimental study on the effect of direct water injection strategy on engine knock,combustion characteristics,power performance and economy under high load is carried out.And based on the research on the water injection strategy,the optimized water injection strategy was used to carry out the large-load expansion test of the engine,and the thermal efficiency performance of 47.07% of the indicated thermal efficiency was achieved under the working conditions of 2000 rpm and 10 bar load.Although direct water injection has the potential to further expand the load,the thermal efficiency cannot continue to be improved due to the instability of lean combustion at large loads.Finally,a numerical model of a high compression ratio lean-burn engine is built,and the comparative analysis of in-cylinder mixture formation and combustion process shows that,compared with homogeneous lean combustion,the in-cylinder mixture structure under stratified lean combustion is better than that of homogeneous lean combustion.The ideal mixture concentration gradient distribution realizes the fast and efficient combustion process under the influence of various factors such as in-cylinder turbulence and mixture equivalence ratio distribution.The calculation results under high load show that the direct water injection pair significantly reduces the temperature of the in-cylinder mixture and the combustion temperature,and suppresses the occurrence of knocking.Based on the research in this paper,on the basis of the original engine load limit of 8.5 bar,the lean limit of 1.8,and the indicated thermal efficiency of 45.35%,the fuel stratification strategy is used to realize the expansion of the engine lean limit to 2.0 and the indicated thermal efficiency to 46.53%.Layer combustion is adopted,and the direct water injection strategy is used to realize the load expansion to 10 bar under the condition that the lean burn limit is basically unchanged,and the indicated thermal efficiency is increased to 47.09%. |
