| The continuously variable valve lift(CVVL)technology controls the intake air volume and load of the gasoline engine by changing the intake valve lift and opening duration,which can eliminate the throttle valve and greatly reduc e the partial load pump loss,and this is one of the most promising technologies to improve engine performance at partial-load.At present,the mechanical CVVL technical solution for engineering applications needs to be used with hydraulic variable valve timing(VVT)technology.The system has some defects,such as poor ability to follow-up,insensitive response,large mechanism,high cost,complicated control strategy and long development cycle.Therefore,this paper proposes a new type of mechanically continuously variable valve lift system with self-adjusting valve timing(CVVLT).The system achieves the variable valve actuation by adjusting the center of the camshaft pivot,with using an adjusting motor to control the valve lift and timing at the same time,which eliminating the intake hydraulic VVT.It also has the advantages including simple and compact,stable and reliable,low cost and sensitive response.This paper has carries out related research on CVVLT system,the specific content and conclusion are as follows:(1)The GT-power model of original engine is built.Through the method of performance simulation,the main parameters about the valve lift curve are preliminarily determined.Based on the proposed line design method combined with performance simulation,the key components of the CVVLT system are forward developed.Based on the calculation results of the multi-body dynamic model and GT-Power model of engine performance simulation in GT-suite,the design profile is optimized and verified.The results of the profile optimization design are in good agreement with the design target values,the intake valve dynamics parameters are equivalent to the original engine,and the CVVLT engine performance prediction results are in line with expectations.The CVVLT system is successfully manufactured and equipped.The design method and the function of the CVVLT syst em are verified through the cylinder head cooler test platform.The verification results are in line with expectations.(2)The formation mechanism analysis of the pumping loss of the gasoline engine is carried out,and the common characteristics of the pu mping loss of the CVVLT engine and the original engine are compared and analyzed.The theoretical limits of the pumping loss of the original and CVVLT engine are discussed.Based on the bench test results of two engines,around the influencing factors after economic decomposition of gasoline engine,a quantitative analysis about it is carried out.This analysis shows that compared with the original engine,the CVVLT engine uses valve lift to control the load.In the case,the partial load does not have the throttle loss of the throttle,so the intake pressure is higher,the work consumed during the gas exchange process is less,and the pump loss is reduced.Obviously,the fuel economy is improved,and the pumping losses at 1500r/min,2000r/min and 3000r/min are reduced by up to 23%,25% and 22% respectively compared with the original engine.In addition,at 2000r/min brake mean effective pressure of 2bar and 3000r/min brake mean effective pressure of 2bar,the effective thermal efficiency is increased by 6.9% and 8.1% respectively compared with the original engine.(3)Based on the one-dimensional simulation model of original engine,design and test results,a one-dimensional simulation model of the CVVLT engine is built.Based on this model,the performance simulation analysis about influence of the intake valve timing of the CVVLT engine on the gas exchange process and the combustion process is performed.It can be seen from the simulation results,the intake valve of the CVVLT engine at partial load,delaying its opening time,the pressure difference between the inside and outside of the valve is greater during the intake process,what's more,the intake flow rate,the cylinder volume,and the air amount increases.Meanwhile,using a larger valve lift,the local throttle loss of the valve can be decreased,and the closing time of it can also be delayed.Therefore,at low load conditions,further optimization of matching intake timing and lift can achieve a 1% to 2% reduction in fuel consumption.(4)The Converge three-dimensional simulation model of the CVVLT engine is built,and the influence,the intake valve timing on in-cylinder flow,is studied.The simulation results show that,delayed the opening time of the intake valve when CVVLT engine at partial load,both the in-cylinder tumble ratio and turbulent kinetic energy are increased during the intake process.After the intake valve closed,the continued downward movement of the piston will dissipate turbulent flow energy.When the intake valve's closing timing is advanced,the piston will continue to descend in the late intake period,and the turbulent kinetic energy will be dissipated for a longer time.In addition,the sooner the intake valve of the CVVLT engine is closed,the smaller the in-cylinder tumble ratio is,the in-cylinder flow tends to be disordered,more gas molecules collide on the cylinder wall,and the viscosity between the gas molecules dissipates more,the turbulent energy consumption in the cylinder is dispersed faster.Therefore,the earlier the valve is closed,the weaker the turbulent kinetic energy is at ignition moment,and the area turbulent kinetic energy concentrated is also further away from the spark plug.As a result,the earlier the valve is closed,the slower the flame propagation speed after ignition,the longer the combustion duration.Due to adoption of the early intake valve closing strategy,combustion duration of the CVVLT engine is longer than that of the original engine.The research in this paper expands the implementation form of CVVL technology,reveals the improvement effect of CVVLT system on the economy of gasoline engine,clarifies the optimization direction of CVVLT principle prototype,and provides theoretical guidance for the engineering development of CVVLT system. |
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