Modelling and diagnostic study of flow in an optical engine with negative valve overlapping for homogeneous charge compression ignition

The three-dimensional bulk gas motion in an engine cylinder with different flow scales generates a varied range of shear stresses and deformations in the fluid, which leads to the gas turbulence. These flow irregularities combined with periodic engine operation induce gas fluctuations over time and cylinder volume. This affects the thermal inhomogeneity of the in-cylinder gas mixture composition. A novel approach to the formation of in-cylinder flow that promote Homogeneous Charge Compression Ignition HCCI engine by generating various patterns of the incylinder flow has been proposed. Two types of inlet manifold diffusers were experimentally applied to a 4-valve pentroof optical engine and the spatial flow inhomogeneities were carefully analysed. Additional special effects in the in-cylinder flow fields were induced by using the Negative Valve Overlap (NVO) technique. Case studies for internal nonreactive ‘COLD’ flow structures were investigated using a motored single cylinder optical engine and Particle Image Velocimetry (PIV) technique to validate a 3-D CFD engine model. The optical diagnostics and CFD modelling of the in-cylinder HCCI ‘COLD’ flow show that two different porting strategies with fixed Negative Valve Overlap (NVO) significantly affect the in-cylinder flow structures. A comparison of in-cylinder flow effects generated by different intake deflectors indicate that a low-swirl port configuration can potentially improve the fuel and thermal homogeneity by producing higher values of Root Mean Square (RMS) of velocity fluctuation and turbulence intensity. The high-swirl port configuration generates high stratification of gas flow field at Top Dead Centre (TDC). The final part of the study investigates the stratification of gas temperatures. The 3 studies show that both of the intake shrouds generate differences in the rate of change of the large scale rotating flow affecting the gas transport and mixing process within the engine cylinder