Investigation of VVT and spark timing on combustion and particle emission from a GDI Engine during transient operation

Transient operation is frequently used by vehicle engines and the exhaust emissions from the engine are mostly higher than those under the steady station. An experimental study has been conducted to investigate the effect of various valve timings and spark timings on combustion characteristics and particle emissions from a modern 3.0-liter Gasoline Direct Injection (GDI) passenger car engine. The transient condition was simulated by load increase from 5% to 15% at a constant engine speed with different settings of valve timings and spark timings. The transient particle emission measurement was carried out by a Cambustion DMS500 particulate analyser. The combustion characteristics of the engine during transient operation including cycle-by-cycle combustion variations were analyzed. The time-resolved particle number, particulate mass and particle size distribution were compared and analyzed between different engine settings. The existing transient lambda control cannot maintain stoichiometric combustion in the transition. Valve timings affect the intake manifold pressure and the amount of actual air intake during transition cannot be calculated only using the intake manifold pressure. During the load increase, the particle numbers were slightly lower when IVO or EVC remained unchanged compare to the other cases. The particulate masses were the same for all cases. The valve timing affected the fuel vaporization; as a result the nucleation mode particles were changed. Lowest particle number and particulate mass was observed when the spark timing was changed to 30° BTDC. Spark timings had a strong influence on the exhaust temperature even during the transient operation which indicated the oxidization of some unburned fuel.