Simulation Of In-cylinder Flow And Composition Distribution Of A Gasoline Hcci Engine With Variable Valve Actuation

Recently HCCI has received wide concentration because of its potential of high efficiency and low emission. Dominated by chemical reaction, it is hard to control HCCI combustion directly like traditional SI combustion. Change of turbulence intensity affects combustion process with its effect on the transition of thermal and composition between burnt zones and unburnt zones. With 4VVAS, valve parameters would be flexiblely varied to change load. Exhaust valve parameters affect the residual gas fraction(RGF), while the change of intake valve parameters have great effect on gas exchange and air flow characteristic, resulting in different distribution of RGF and temperature, which leads to different process of combustion accordingly.This research focuses on the effect of inhomogeneous distribution of temperature and RGF on HCCI combustion. When the parameters of exhaust valve is fixed, there are four intake valve moving manners: fixed intake valve lift(IVL) with various intake valve timing(VVT), fixed intake valve opening time(IVO) with variable various IVL, fixed intake valve closing timing(IVC) with various IVL and fixed intake valve peak timing(IVP) with various IVL.CFD simulation is carried out to study the rules of exchange process and temperature/RGF distribution with different intake valve timing and intake valve lift under the four different control strategies .Simulation results are validated by the images from PIV experiment. Results show that, during the intake stroke, there are two parts of flow entering into the cylinder. The lift flow enters directly into the cylinder along the cylinder wall on the side of intake valves; the right flow is introduced into the cylinder along the exhaust valves and the cylinder wall on this side. These two parts of flow generate two vortices in cylinder obviously. The location of high-temperature region and high-RGF region is dominated by the two vortices under intake valve and exhaust valve. The early back-flow and late back-flow have great effect on the two vortices.Intake time and inlet mass flow affect in-cylinder inhomogeneity. It gets lower when intake time gets longer. Higher inlet mass flow leads to stronger flow in the cylinder and better mixing.Residual gas is partly premixed during early back-flow, but intake time becomes short, making RGF inhomogeneity higher. Meanwhile it makes the temperature changing time get longer and that affects the temperature inhomogeneity. In that way, variety of IVO results in different change of RGF inhomogeneity and temperature inhomogeneity. The kinetic energy of charge is minimized when late back-flow occurs, which is not good for mixing in the compression stroke. The lower efficient compression ratio leads to higher temperature inhomogeneity. In that way, various IVC can be used to make different change of RGF inhomogeneity and temperature inhomogeneityIn order to find the efferent methods to control the distribution of temperature and composition,external EGR and EGR-rebreathing is studied. External EGR can reduce the RGF inhomogeneity while keep the same condition of temperature in the cylinder. Different inhomogeneity of RGF and temperature can be achieved. EGR-rebreathing has lower in-cylinder temperature, reducing the relationship between the quantity and thermal of residual gas. In-cylinder inhomogeneity gets higher when Later Exhaust Valve Closing strategy is applied. But the distribution of RGF is still similar with that of temperature. When exhaust valve is reopened during intake stroke, in-cylinder inhomogeneity gets lower with different distribution of RGF and temperature.