The effect of port water injection on knocking and emission characteristics of natural-aspirated GDI engine operated under cruising condition

The negative impacts of climate change and worsening air quality, driven by anthropogenic emissions from activities like construction, industrial production, energy generation and transportation, are well-documented. To reduce greenhouse gas emissions from road transport, automakers are downsizing engine displacement to increase brake thermal efficiency, thereby improving fuel economy and reducing CO₂ emissions. However, downsized gasoline direct injection (GDI) engines are prone to knocking and emit higher levels of NO _x and particulate matter (PM) compared to port-injected gasoline engines. This study investigates the effects of port-water injection (PWI) on the combustion and emission characteristics of a naturally aspirated GDI engine fuelled with gasohol E20 under steady-state conditions. Injecting up to 30% water (relative to fuel mass) with optimised ignition timing resulted in an 80% reduction in NO _x , decreased PM diameter and mass and complete knock suppression. PM morphology analysis showed enhanced reactivity due to the generation of smaller particles with increased surface area. Adverse effects on engine performance and emissions from water injection were mitigated by advancing spark ignition timing. Optimal ignition timing for maximum thermal efficiency did not align with the Knock Limiting Spark Angle (KLSA), but a slightly lower advance favoured NO _x reduction. Our findings suggest that automakers can produce GDI engines with minimal NO _x and PM emissions without relying on precious metal-based catalysts, promoting sustainability in the transportation sector.