| In the current situation of increasingly serious energy and environmental problems,the study of new energy vehicles has been paid more and more attention by the majority of scholars.However,in the short term,internal combustion engine still occupies the largest share in the passenger car market,so the field of internal combustion engine is one of the main areas of energy conservation and emission reduction.Variable valve is a kind of technology which can effectively improve the economy and emission performance of engine.At present,there is still a large exploration space on the configuration and application of variable valve mechanism.In this study,a mechanical-electro-hydraulic variable intake and exhaust valve lift mechanism was developed based on the test prototype.The motion characteristics and reliability of valve were studied by using a motored platform.After the mechanism was transplanted to the test prototype,the thermodynamic engine test platform combined with GT-Power one-dimensional simulation software was used to study the effects of different valve lift strategies on engine combustion,economic and emission parameters,and the optimal economic valve control strategy was obtained.Based on the intake and exhaust valve strategy,the influence of valve response on combustion and emission performance of engine in transient process was analyzed.In view of the deteriorative transient condition,the influence of different valve response on the in-cylinder energy conversion process under the deterioration cycle was explored.Then,Taguchi method was used to clarify the influence degree of inlet and exhaust valve lift changes on the boundary conditions and the boundary conditions on the combustion and emission parameters of the engine.On this basis,the sensitivity analysis of combustion and emission parameters to different control parameters was carried out,and the transient condition was optimized by using the control parameters that play an obvious role during the combustion process,so as to achieve a stable and fast load transient process.The main research contents and conclusions are as follows:1.The self-developed mechanical-electro-hydraulic variable inlet and exhaust valve lift mechanism can realize the valve lift in 0mm-9mm flexible adjustable,with the reduction of valve lift valve opening timing slightly back and closing timing advance,each valve lift corresponds to a valve opening duration.The cushion section of cam on the original mechanical camshaft has lost its function,but the valve soft seat is an important premise for the valve and mechanism life.The control effect of a check valve with a thin wall hole on the seating speed of the valve was studied on the motored platform.The suitable diameter of the small hole and the moving distance of the check valve were determined through the test.The structure can ensure that the valve seating speed can be controlled below 0.5m/s.The oil inlet check valve spring mainly affects the valve opening process,the softer spring makes valve opening process without buffer and the impact is strong,the harder spring will make the valve opening time delay.The hydraulic oil temperature has a great influence on valve lift.The higher the temperature of hydraulic oil is,the more sensitive the valve lift is to the change of throttle opening angle.Valve lift is not sensitive to the change of oil supply pressure,but the oil supply pressure has a great impact on the valve opening duration,low pressure makes valve opening time lag,higher pressure will make the valve close difficult and even rebound,0.5MPa is the suitable oil supply pressure for this system.2.The self-developed variable valve mechanism was transplanted to the engine and different valve lift control strategies were studied on the thermodynamic engine test bench.Compared with the traditional throttle engine,intake valve lift load control strategy can not only reduce the pumping loss,inhibit the attenuation of in-cylinder turbulent kinetic energy,but also can make a part of the exhaust gas trapped in the cylinder based on the characteristics of the mechanism,so the economy and emission are improved.But low valve lift led to a fast airflow speed make combustion unstabe.Based on the intake valve lift load control strategy,the exhaust valve lift was coupled to adjust,and the internal EGR can be further utilized.The SICI combustion mode was realized and the BSFC and NOx emissions were further reduced.However,the higher internal EGR rate further worsens the combustion stability,so the application range of this valve strategy is limited.Increasing intake valve lift and reducing air flow velocity under small load is the key method to improve engine stability without throttle.Combined adjustment of inlet and exhaust valve lift with throttle,or shorten valve duration,both can improve engine combustion stability.In contrast,the inlet and exhaust valve lift control strategy with short valve duration can maximize the advantage of variable valve,and improve engine economy and emissions more significantly.Compared with the original throttle control strategy,BSFC can be improved by up to 19.3%.At the same time,it was found that on the basis of this strategy,the internal EGR coupled lean burn can further improve the economy and emission performance.3.The transient processes between different engine loads were studied by using the inlet and exhaust valve lift strategy which can obtain the optimal BSFC under different valve response.The test shows that when the engine transit from high load to low load,the engine can complete in 2-4 cycles with no obvious distortion of combustion parameters under 50 ms valve motor execution time.When the valve response speed decreased,the number of cycles required for engine stability increased.When the engine transit from low load to high load,the engine will appear instability cycle and combustion parameters will significantly deviate from the steady-state value under the high valve response speed.This engine instability phenomenon can be improved after reducing the valve response speed,but the number of cycles required to achieve stability increased,so the engine transient process from low load to high load needs to be optimized.According to Taguchi’s analysis,the intake valve lift plays a leading role in the change of excess air coefficient and gas flow rate at the intake valve under small load.The influence of the exhaust valve lift on excess air coefficient and gas flow rate increases after the load increased,and the exhaust valve lift always has the greatest influence on the initial combustion temperature and the internal EGR rate.Under small load,APmax is mainly affected by internal EGR rate and intake flow rate,and the effect of excess air coefficient on APmax is enhanced after load increase.ITE and heat transfer loss are the most sensitive to the change of excess air coefficient.NOx emission is greatly affected by internal EGR rate and excess air coefficient.It can be seen that different combustion and emission parameters have different sensitivities to boundary conditions,and inhibiting the fluctuation of boundary conditions under transient conditions will play a positive role in stable transient process from low load to small load.4.According to Taguchi analysis,among the four control factors of ignition timing,injection pulse width,excess air coefficient and intake temperature.Ignition timing and injection pulse width have the largest influence on the combustion process,especially for ignition timing,but NOx,HC and CO emissions are more sensitive to the change of excess air coefficient.By adjusting ignition timing and injection pulse width which are most convenient to control,the transition process from low load to high load was more stable.The results show that the downward distortion of Pmax and IMEP can be eliminated by advanced ignite within two cycles after the trigger signal was sent,but the excess air coefficient still fluctuates and the NOx emission still deteriorates briefly.After the trigger signal was sent out,the injection pulse width was adjusted,which can eliminate the fluctuation phenomenon of excessive air coefficient under medium and small load.When the small load transited to the large load,the excessive air coefficient will still float upward,but the recovery speed accelerated,and the NOx deterioration phenomenon was improved.This study provides a new idea for new configuration of variable valve mechanism and reference for the optimization of engine load control strategy.Besides that,a theoretical basis for the combustion and emissions deterioration during the transient process of the engine without throttle was proposed,and showed reference for solving the key engineering problem of stable operation under transient conditions for variable valve engines. |
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