An Investigation and Evaluation of VVT/VVA Strategies in a Diesel HCCI Engine using 3D CFD

A three-dimensional CFD modeling was carried out to investigate effects of VVT/VVA on gas exchange and fuel-air mixing processes in a diesel HCCI engine with early fuel injection. Four VVT/VVA strategies were conducted for this study: i) NVO strategy with fixed EVO and IVC timings but variable valve lifts – referred as NVO Strategy; ii) NVO strategy with fixed valve profiles but variable EVO and IVC timings – referred as EVO Strategy; iii) NVO strategy with fixed valve lifts and fixed EVO and IVC timings but variable EVO and IVC timings – referred as EVC Strategy; iv) VVA with just variable valve lifts – referred as VMAX Strategy. The results indicate that suitable NVO settings will enhance in-cylinder tumble and then increase turbulence intensity before compression-end, though the increased NVO has a negative contribution to swirl ratio. It was found that reducing valve lifts alone is not an efficient way to retain the residual gas, but the function of reduced valve lifts will become significantly obvious by combining it with increasing NVO. For the effect of NVO on in-cylinder temperature, longer NVO will not only increase in-cylinder temperature due to higher residual gas rate, but also improve the in-cylinder temperature homogeneity. Lowering the maximum valve lift or increasing NVO, the unmixed region of in-cylinder charge shrinks. The rich fuel region expands because of the high intake velocity and enhanced turbulence intensity. This is beneficial to the forming of global homogeneous charge. It has been noted from the current study, as the droplet distribution may be influenced more by the in-cylinder air motion caused by NVO when the average droplet size is smaller, it is recommended that future studies explore the effects of VVT/VVA on diesel HCCI mixing and combustion with various advanced fuel injection strategies.