Exploring the potentials of water injection to improve fuel consumption and torque in a small displacement PFI spark-ignition engine

This work deals with the effects of port water injection (WI) on torque and fuel consumption improvements of a port gasoline injection (PFI) boosted Spark Ignition engine. First, an experimental investigation is carried out at high loads and various speeds to acquire performance and combustion data under WI operation. The engine is fully schematized in a 1D commercial code and enhanced by advanced sub-models for the in-cylinder processes description, including refinements for WI impact on flame development and knock onset. The model is validated against experiments at different speed/load points and water amounts. The validated model is utilized in a predictive way for virtual engine re-calibration and re-design aiming to explore WI potentials on the performance increase at full load. A first calibration strategy is proposed with the aim to minimize the brake specific fuel consumption (BSFC) through water injection, with a fixed value of water to fuel mass ratio (W/F = 0.5). The numerical results highlight the potential of water in reducing fuel consumption, reaching a BSFC gain up to 14% at low speed, while greater advantages up to 32% were obtained in the medium/high-speed region, mainly due to suppression of mixture enrichment. Then, a second calibration strategy is conceived to increase the full load torque curve complying with compressor surge, knock and turbine inlet temperature limits, while minimizing the water consumption. The numerical outcomes highlight a maximum increase in torque up to 13.8%, combined with minimal BSFC penalizations compared to the first calibration, and a minimum evaluated W/F?0.1. The proposed methodology and automatic control strategies prove to be a valid numerical support for an engine recalibration aimed at improving its performance.