Study On The Variable Valve Control Strategy Of Low Temperature Combustion In Internal Combustion Engines At Low Loads

As an advanced combustion concept,low temperature combustion(LTC)can achieve extremely low NOx and particulate matter(PM)while maintaining high thermal efficiency,however,it suffers from high HC and CO emissions as well as low combustion efficiency at low loads,especially for high octane fuel with a misfire.The variable valve technology is an important technical method for LTC to improve combustion stability,increase combustion efficiency and reduce harmful emissions at low loads.In this paper,the study on the variable valve control strategy of diesel and gasoline LTC at low loads has been carried out.The camless electro-hydraulic valve train system was developed and the control scheme of increasing system response and reliability was studied.The results indicate that,the multiple pulse control strategy can achieve low valve seating velocity,but the corresponding engine speed is also declined significantly;the one-way throttle valve connected in parallel with a switch electromagnetic valve can effectively shorten the buffer distance and reduce the valve seating velocity simultaneously.Then an intake valve closing mechanism based on the cam driver was designed,which further increases the system operating frequency and meets the requirement of higher engine speed.Based on the development of the variable valve train,diesel LTC at low loads has been investigated first.The results show that,extremely low NOx can be obtained with high exhaust gas recirculation(EGR)rates;the external cooled EGR especially with the late intake valve closing effectively reduces soot,but results in sharply increased HC and CO emissions at lower loads;the internal EGR caused by a secondary intake or exhaust opening can achieve low emissions and high exhaust temperature,however,the ignition delay period is shortened with the increase of load,resulting in sharply increased soot emissions;the homogeneous degree and temperature distribution characteristic of the secondary intake opening mixture is between the secondary exhaust opening and the only external cooled EGR,so its harmful emission also falls in between;the soot for the internal EGR can be reduced with a high injection pressure,while HC and CO emissions for the external EGR can be reduced with a low injection pressure;for the control strategy of internal EGR coupling with external EGR,increasing the internal EGR ratio can reduce HC and CO emissions but the improvement is weakened,and at the same time a higher injection pressure is required to suppress soot emissions,while increasing the external EGR ratio can achieve higher thermal efficiency.Given the problem of low combustion stability for gasoline LTC at low loads,the study of the secondary exhaust opening strategy has been explored.The result shows that,the double-injection strategy can obtain more ideal mixture concentration and thermal stratification,which is beneficial to improve the combustion process,however,leaner mixture is adverse to the realization of stable combustion with the decrease of load;the single-injection near top dead center combined with high internal EGR rate can achieve low NOx,high thermal efficiency and improved combustion stability,however,lower oxygen under the low load condition has negative effects on combustion reaction;the intake boosting is an effective method to extend the lower load limit.The lowest load limit of gasoline LTC can be extended to idle operation with intake boosting,higher internal EGR rate and single-injection near top dead center.The study of control strategies for gasoline LTC at medium loads shows that,the late injection strategy is difficult to give consideration to NOx and soot emissions with the rapid increase of load;the effects of increasing injection pressure and intake pressure for the late injection strategy on performance and emissions are limited;the early injection can achieve low NOx and soot emissions at the same time,but CO and HC emissions increase sharply with obviously decreased thermal efficiency;increasing injection pressure reduces the requirement of internal EGR rate and injection timing for the early injection,and achieves higher thermal efficiency;the early injection combined with intake boosting can further reduce soot and improve thermal efficiency,however,the NOx emission increases rapidly due to the increased combustion rate;a higher operating region for high-efficient and clean gasoline LTC can be obtained when the internal EGR is gradually replaced by the cooled EGR.