| The China sixth-stage standards for heavy vehicles have posed new challenges to engine emissions and economy.In the present thesis,based on the LCCE(Liquids Consumption Cost Equivalent),considering the overall consumption of fuel and urea,combing with the idea of engine working condition partition design,and applying advanced methods including computational fluid dynamics simulation,virtual calibration,and automatic calibration et al.,key technologies of YCS04 new diesel engine such as combustion system,engine emission control system and the thermal management of the equipped vehicle,were systematically investigated.On the overall technical route,in view of the national conditions of our country and the factors such as fuel economy,adaptability and emission upgrading,this research adopted the cooling EGR+DOC+DPF+SCR/ASC emission control technical route.On this basis,this thesis applied LCCE throughout the whole engine development process and established a complete engine development methodology and some key technologies from the design of combustion system components such as intake ports,combustion chamber,injector,emission after-treatment system to the fine MAP calibration.In the aspect of combustion system design,in view of the precondition of increasing fuel injection pressure of China VI engine,the key structures and parameters of intake ports,combustion chamber,injector nozzle diameter,injection cone angle and camshaft height were determined preliminarily by using computational fluid dynamics simulation,and then experimentally tested for typical operating points under test cycle by utilizing LCCE and partition design method.Such design and selection finally determined the combustion system structure to be a combination of low swirl ports(swirl ratio of 1.3)with a type combustion chamber with throat height of 5.8 mm and throat diameter of 60 mm,in which the intake ports adopt“Y”-shape cross-cylinder design to ensure the induction accessibility.In the emission after-treatment system design,key components including DPF,SCR,and DOC were selected.In DPF selection,the pressure difference characteristics and regeneration control of DPF are mainly investigated.In order to ensure the consistency of carbon load in the measurement process,the DPF test method for engine is designed.Meanwhile,the regeneration ability of DPF was also tested under WHTC cycle.The optimized DPF uses cordierite as carrier,3g/cft precious metal coating amount,and Pt/Pd ratio of 12:1.On the other hand,the configuration of different SCR and DOC combinations was studied,and the NO_xconversion efficiency and conversion rate of SCR were tested.For the first time in the development of China VI engine,observing the conversion rate of the engine under transient operating conditions was conducted.Finally,the DOC with ceramic carrier,30g/cft precious metal coating amount,and Pt/Pd ratio of 2:1,and SCR with ceramic carrier and copper-based coating were selected as optimized DOC+SCR configuration.In the aspect of external EGR control,in order to ensure the accuracy of closed-loop control of EGR rate,the influence of exhaust pressure wave was reduced by optimizing the layout of EGR valve and the shape of Venturi tube,and the accuracy of EGR flow measurement was greatly improved.On this basis,by using advanced virtual calibration technology,the robustness of engine fuel consumption and emissions on the pressure difference offset of Venturi flowmeter was studied.The results show that the influence of pressure difference signal offset on NO_x at steady state is directly resulted from closed loop control of recycling exhaust gas,and its forward offset will lead to a significant increase of NO_x in the original exhaust.Transient cycle test results show that the relationship between pressure difference signal offset and cycle ratio NO_x and average specific fuel consumption is linear,and the influence on fuel consumption is less.Such results could provide important reference for EGR control.On the basis of the design and selection of the abovementioned engine sub-systems,considering the complex changes of engine operating environment such as different road conditions,variable vehicle loads and driver driving habits in China,the engine control parameters,DPF regeneration strategy and SCR operation strategy were further carried out based on vehicle thermal management to achieve efficient collaborative operation between engine combustion system and emission after-treatment system.In the DPF regeneration control strategy,the inlet temperature of DOC was increased by firstly calibrating combustion parameters,then performing the secondary injection when the DOC temperature reaches the ignition point,and further conducting dynamic compensation and closed-loop control calibration of the secondary post injection according to the transient conditions.In the SCR operation strategy,by adjusting the heat management of cooling EGR system,combined with fuel injection calibration,common rail pressure and throttle opening,and controlling the opening of EGR valve properly,the engine exhaust temperature was raised by20～50°C,and especially obvious at low loads,which helped ensuring the conversion efficiency of SCR.Finally,the YCS04 diesel engine which adopted the abovementioned key technologies and the corresponding vehicle equipped with the engine have successfully passed the tests of the China-6b standard with a considerable allowance.As such,the key technologies development of engine combustion and emission in the present study were successfully completed. |
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