Research On The Combustion Characteristics And Emission Performance Optimization Of Internal Exhaust Gas Recirculation For Small-size Off-road Diesel Engine

The small off-road diesel engine has the characteristics of compact structure,reasonable price,convenient use and maintenance,which has a broad market.However,with the growing problem of energy consumption and environmental pollution,energy conservation and emission reduction have been focused on.Therefore,for the structural features and the problem of cost constraints of small off-road diesel engines,the effect of the internal exhaust gas recirculation(IEGR)on the combustion characteristics and emission performance of diesel engine were researched.The optimization of emission performance was also achieved in coordinate with electronically controlled fuel injection system parameters.The thermodynamic model of the prototype was established by AVL Boost software.Based on the model verification,the feasibility of IEGR with exhaust valve secondary opening.The influences of start timing,lift,and opening duration of exhaust valve secondary opening on the IEGR rate,power,and fuel consumption rate were studied.And the design of the IEGR schemes was completed.The results show that with the delay of the exhaust valve secondary opening timing,the IEGR rate decreases,especially after 477°CA.The increase of the exhaust valve secondary opening lift is beneficial to improve the IEGR rate.However,the dynamic and economy begin to deteriorate when the lift is greater than 1.2 mm.The secondary opening duration has a slight effect on the IEGR rate,power,and fuel consumption rate.Then,the effect rule of IEGR on the emission performance and combustion characteristics of small off-road diesel engines were investigated by the engine cylinder pressure acquisition,bench and emission performance tests.The experiment results show that under the condition of the same other combustion parameters,diesel emission performances of each IEGR scheme are significantly different.Scheme 1 has an obvious advantage on reducing NO_x emissions.Compared with the original engine,the weighted emissions of NO_x and NO_x+HC are reduced by 11.9%and 9.1%respectively.Although the Soot and CO emissions have increased,there is still a large margin from the regulatory limits.According to the analysis of combustion characteristics,the maximum combustion pressure peak in the cylinder has increased by the use of IEGR,and the corresponding crank angle has been advanced.The phase of high temperature has been delayed,while the combustion duration has increased slightly.Besides,under full load condition,the ignition delay period decreases,and the peak value of instantaneous heat release rate increases.Under low load conditions,the delay period slightly increases,and the peak value of instantaneous heat release rate decreases.Finally,the influence of fuel injection timing on engine emission performance and fuel economy was studied before and after use IEGR.The optimization of electronically controlled fuel injection strategy with IEGR was completed.The results show that after the introduction of IEGR,the total emissions of NO_x and HC can be effectively improved by the adjustment of injection timing,which can also reduces CO emissions at some operating conditions.With the optimal goal of the total NO_x and HC emissions,total emissions of NO_x and HC after optimization are reduced by 17.6%compared with the original engine.Although the Soot and CO emissions increase,there is still a large margin from the emission regulations.When maintaining the same total emissions performance of NO_x and HC with the original engine,fuel economy has been improved by 6.4%after the optimization of fuel injection strategy with the optimal goal.