Multiple injection strategies with high water port injection rates for enhancing performance and emissions in a boosted SI engine

Abstract

• Multiple injection strategies are applied to enable high water injection rates up to 1.0 by mass. • Detailed thermodynamic state analyses were conducted with high water injection rates. • The relative thermal efficiency improvement was 3.90% with 1.0 water to fuel mass injection ratio and triple injections. • Nitrogen oxides emissions were reduced up to 52.7% with combined multiple and water injections. • The major drawbacks of high-water injection rates (particulate matter and unburned hydrocarbon emissions) were significantly improved with multiple fuel injections. Water injection has been proven both to reduce emissions of nitrogen oxides and to improve fuel conversion efficiency via knock limit extension in turbocharged spark ignition (SI) engines. However, water injection still has several obstacles to maximize the benefits, such as an increase of particulate matter and unburned hydrocarbon emissions. In order to ameliorate the problems with a sufficient amount of water injection (water to fuel mass ratio > 0.5), this study adopted multiple fuel injection strategies, which inject fuel multiple times in a cycle. A boosted single-cylinder research engine equipped with a water port injector and a piezoelectric gasoline direct injector was utilized for this study over a range of water to fuel mass ratio (up to 1.0) and multiple fuel injection (single vs. triple) conditions. The results show that the drawbacks of water injection including combustion instability, unburned hydrocarbon, and particulate emissions were significantly improved with multiple fuel injections, and the relative thermal efficiency improvement and nitrogen oxide emission reduction were maximized up to 3.9% and 52.7%, respectively, with the combined injection strategy. In addition, the increased carbon monoxide emissions typical with multiple fuel injections were largely mitigated by the water injection strategy.