Reduction of Cold-Start Emissions through Valve Timing in a GDI Engine

Abstract

This work examines the effect of valve timing during cold crank-start and cold fast-idle (1200 rpm, 2 bar NIMEP) on the emissions of hydrocarbons (HC) and particulate mass and number (PM/PN). Four different cam-phaser configurations are studied in detail: 1. Baseline stock valve timing. 2. Late intake opening/closing. 3. Early exhaust opening/closing. 4. Late intake phasing combined with early exhaust phasing. Delaying the intake valve opening improves the mixture formation process and results in more than 25% reduction of the HC and of the PM/PN emissions during cold crank-start. Early exhaust valve phasing results in a deterioration of the HC and PM/PN emissions performance during cold crank-start. Nevertheless, early exhaust valve phasing slightly improves the HC emissions and substantially reduces the particulate emissions at cold fast-idle. The combined strategy consisting of late intake and early exhaust phasing shows a considerable reduction in both the cold crank-start HC and PM/PN emissions of 30%. In fast idle, the HC and PM emissions respond differently to the different valve timing strategy. The combined late IVO and early EVC configuration with moderate combustion phasing retard gives the best HC and PM emissions and reasonable engine stability.