Effect of Water Injection on Fuel Efficiency and Gaseous and PN Emissions in a Downsized Turbocharged SI Engine

Water injection represents a promising technology to overcome limitations of knock occurrence and fuel enrichment that reduce the efficiency of downsized spark-ignition (SI) engines. This paper presents the results of an experimental investigation performed on a portfuel injection turbocharged spark-ignition engine within the speed range 3,000-4,500 revolutions per minute (rpm) under medium-high load conditions. Engine tests were carried out by setting the gasoline baseline conditions at 0.9 as the relative air-fuel ratio according to the standard electronic control unit engine map. As the water injection was activated, the fuel amount was steadily reduced to reach the established relative air-fuel ratios (1 and 1.05). A spark timing sweep up to the most advanced one without knock was carried out, and results of engine performance, exhaust gaseous emissions, particle number (PN), and particle-size distribution were measured and compared with the baseline gasoline case. The main findings showed that the water injection was able to stop mixture overfueling and improve fuel efficiency without engine load penalties. Regarding particulate emissions, the higher knock tolerance and better combustion phasing allowed by water injection produced significant reductions in particle number emissions, which were primarily marked at higher engine speeds