The Simulation Study Of The Knocking Mechanism In An Opposed-Piston Hydraulic Free Piston Engine

Hydraulic free piston engine(HFPE)integrates an internal combustion engine and a hydraulic pump into a compact structure.In an HFPE,the energy released during combustion will be stored and delivered by fluid.Compared to conventional engine,the piston motion of HFPE is linear and not restricted by a crank mechanism,which enables variable piston trajectories and compression ratios.Besides,HFPE also has a good potential for multi-fuel.HCCI combustion has a high burn rate because its ignition occurs simultaneously in a spatially large region without a flame front,so it can solve the problem of shorter residence time at the top dead center(TDC)in HFPE.Meanwhile,HFPE has low ignition timing control requirements owing to their variable piston trajectories and compression ratios;this can relieve the control problem of ignition timing during HCCI combustion.Combining HFPE and HCCI combustion is a promising prospect for developing a new alternative fuel engine with high efficiency and low emissions.Methanol has a good anti-detonating quality and it is suited to high compression ratio engine.So methanol is used in the simulation of HCCI combustion operating on HFPE.HCCI combustion combines the advantages of spark ignition(SI)engines and compression ignition(CI)engines.It can reduce both NOx and PM simultaneously and still maintain high efficiency.However,the operating range of HCCI engines is limited by the knock phenomenon which is caused by the high burn rate of HCCI in high load conditions.In this paper,simulation study on the knock phenomenon during the HCCI combustion process was conducted and the effect of different operating parameters was analyzed.A three-dimensional computational fluid dynamics(CFD)model was established with AVL FIRE on the basis of the opposed-piston hydraulic free piston engine.Knock in HCCI combustion was studied using LES(large eddy simulation)coupled with detailed methanol chemical kinetics.The effects of mixture equivalence ratio,intake temperature,compression ratio and EGR on knock in the engine were analyzed.The results show that when the equivalence ratio increases,the ignition time delays and the combustion duration decrease.Therefore,the peak heat release rate increases,and the pressure oscillation in the cylinder becomes drastic leading to the increase of the knock intensity.When the intake temperature increase,the ignition time advances.But the peak heat release rate and the knock intensity don’t monotone increasing.When the intake temperature gets a certain temperature,the ignition time is too early resulting in the decrease of the peak heat release rate and the knock intensity.Meanwhile,the effects of the EGR temperature are similar to the intake temperature.When the compression ratio increases,the ignition time advances,the combustion duration decreases and the peak heat release rate increases.As a result,the knock intensity increases significantly.The dilution effect of EGR was studied by changing EGR rate.The results show that with the increase of EGR rate,the ignition time delays,the combustion duration increases,and the peak heat release rate decreases.In consequence,the maximum rate of pressure rise is lower and the maximum rate of pressure rise decreases significantly when EGR rate is 60%.The effects of the above parameters during HCCI combustion were analyzed and the results can provide theoretic reference for the load expanding of HCCI combustion.The analysis shows that the HCCI combustion of methanol need intake heating and the minimum intake temperature decreases with the increase compression ratio.In a high compression ratio,EGR can decrease the knock intensity.However,the EGR rate should be large because the effect of reduce knock is not obvious with a low EGR rate.