| Braking safety of heavy-duty engines has always been the focus of the research,and the after-treatment thermal management and fuel economy during low-load operation of heavy-duty engines have also received much attention in recent years.The previous studies show that two-stroke compression release braking is an effective strategy to enhance the braking safety of heavy-duty engines and cylinder deactivation has the potential of improving the fuel economy and after-treatment thermal management of heavy-duty engines during the low-load operation.Therefore,it is very necessary to realize two-stroke compression release braking and cylinder deactivation modes on a traditional four-stroke engine.In this article,a new variable mode valve actuation(VMVA)system which can realize flexible switching among four-stroke driving,two-stroke compression release braking and cylinder deactivation modes valve events on a traditional four-stroke engine is proposed,which can realize four-stroke hierarchical driving mode and two-stroke hierarchical braking mode on multi-cylinder engines.Firstly,two-stroke compression release braking can achieve excellent braking performance.However,the bigger valve load and the complex valve actuation are the major obstacles to its wide application.In order to obtain the ideal valve parameters that take into account both the brake power and the valve load,the influence of the valve parameters and their interaction on the two-stroke compression release braking performance was studied by the single factor variable method and orthogonal design variance analysis method in this paper.According to the significant order of the influence of each factor on brake power and maximum cylinder pressure,the optimal valve parameters which can result in high braking power and maintain the maximum cylinder pressure at a lower level are obtained in turn.On the basis,the full-cycle numerical model with the hydraulic valve actuation of the 6-cylinder turbocharged engine was established and the two-stroke braking performance with the dynamic valve lift of the VMVA system was validated by the numerical model.The results indicated that the two-stroke compression release braking power of the VMVA system achieved 137.6 kW at 1300 r/min,341.5 kW at 1900 r/min and 393.5 kW at 2200 r/min,44.9%,60.4%and 48.4%higher than the four-stroke compression release braking power,respectively.Besides,the maximum cylinder pressure of the two-stroke compression release braking only 1.80 MPa at 1300 r/min,4.35 MPa at 1900 r/min and 5.4 MPa at 2200 r/min,39.6%,13.0%and 0.5%lower than the four-stroke compression release braking mode,respectively.It is verified that the VMVA system has excellent braking performance and the load of valve actuation is greatly reduced.In addition,the performance of the two-stroke compression release brake under different cylinder deactivation strategies was analyzed.The results indicated that the combination of 3-cylinder deactivation mode and the non-cylinder deactivation mode can meet the demand of the brake power in the full braking range of the vehicle.Secondly,the VMVA system can improve the efficiency of the after-treatment system and fuel economy during the low engine load through four-stroke hierarchical driving mode.In this paper,the numerical simulation method was used to analyze the effects of 2-cylinder deactivation,3-cylinder deactivation and 4-cylinder deactivation on the exhaust temperature,average exhaust flow and fuel economy under the low load conditions.The results indicated that all the cylinder deactivation modes can effectively improve the exhaust temperature and greatly reduce exhaust flow rate after the turbine during the idle,low load at the rated speed and the low load of the WHSC test cycle.When the engine operates in lightly-load idle,due to the air-fuel rates is big,all the cylinder deactivation can improve the fuel economy and the more cylinders deactivated,the greater the improvement of fuel economy obtained;when the engine load is smaller than 50%at the rated speed,all the cylinder deactivation can improve the fuel economy.When the engine load is higher than 50%at the rated speed,cylinder deactivation has little improvement on fuel economy or begins to deteriorate.Besides,the mass flow rate of the turbine is greatly reduced after cylinder deactivation and the engine speed is relatively low during the low load condition of the WHSC cycle,the intake and exhaust flow rate were greatly reduced after cylinder deactivation,so the efficiency of the turbine is very low.Therefore,the improvement of BSFC is very small under the low load condition of the WHSC cycle.Comprehensive analysis of the impact of different cylinder deactivation modes on the after-treatment system efficiency and fuel economy of the engine under different working conditions.The results show that the after-treatment system efficiency and fuel economy can be improved most significantly by 3-cylinder deactivation hierarchical driving mode.Finally,based on the two-stroke hierarchical braking performance,four-stroke hierarchical driving performance and system cost,the 3-cylinder deactivation VMVA system layout scheme was determined.In this paper,a hydraulic-mechanical multi-body dynamics model coupled with Amesim and ADAMS was established to investigate the switching response and the optimum switching angle of the VMVA system.When the engine is operating at 1900 r/min,it takes 35?A for the four-stroke driving module switching from the effective state to failure state and it takes 90?A for the four-stroke driving module switching from the failure state to effective state.Meanwhile,when the two-stroke braking module is switching from the failure state to the effective state,it requires 40?A for the displacement of the spool valve is greater than 3 mm and 108?A for accomplishing the switching.Meanwhile,when the two-stroke braking module is switching from the effective state to the failure state,it requires 77?A for the displacement of the spool valve is less than 3 mm and 95?A for accomplishing the switching.Therefore,the four-stroke driving module and two-stroke braking module of the VMVA system have excellent switching response,and the switching between different modes of the engine can be accomplished in on four-stroke cycle.In conclusion,the VMVA system can effectively enhance braking safety,improve the efficiency of the after-treatment system and fuel economy under the low load of heavy-duty engines.Therefore,it has an excellent application prospect for heavy-duty engines. |
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