Effect of late intake valve closing on the particle number emissions of a turbocharged gasoline direct injection engine

To achieve carbon-neutrality, efficient gasoline engines with minimal particle number emissions are being increasingly researched. In this study, the particle emission mechanism associated with late intake valve closing (LIVC) was investigated using borescopic method, and particle image velocimetry (PIV) in a 4-cylinder turbocharged gasoline direct injection engine, and a 2-cylinder optically accessible engine, respectively. The direct flame visualization of the borescope and flow visualization of the PIV measurement were used to analyze the blue flame propagation characteristics, mean in-cylinder flow velocity, and turbulent kinetic energy associated with the LIVC along with combustion analysis. The retardation of the LIVC timing reduced the particle number emissions, increased the in-cylinder flow velocity, and decreased the effective compression ratio. The improved flow velocity and reduced effective compression ratio mitigated the production of particle precursors at the expense of decelerated initial flame propagation. However, this decelerated flame propagation was compensated by the enhanced late-stage combustion generated from the increased turbulent kinetic energy of the retarded LIVC timings. The obtained results verify that the improved mixture formation and initial lower in-cylinder temperature conditions of the retarded LIVC timing reduced the particle production, which in turn enhanced the particle oxidation owing to the accelerated late-stage combustion.