Tornatore, C., Siano, D., Marchitto, L., Iacobacci, A. et al., "Water Injection: a Technology to Improve Performance and Emissions of Downsized Turbocharged Spark Ignited Engines," SAE Int. J. Engines 10(5):2319-2329, 2017.
Knock occurrence and fuel enrichment, which is required at high engine speed and load to limit the turbine inlet temperature, are the major obstacles to further increase performance and efficiency of down-sized turbocharged spark ignited engines. A technique that has the potential to overcome these restrictions is based on the injection of a precise amount of water within the mixture charge that can allow to achieve important benefits on knock mitigation, engine efficiency, gaseous and noise emissions.One of the main objectives of this investigation is to demonstrate that water injection (WI) could be a reliable solution to advance the spark timing and make the engine run at leaner mixture ratios with strong benefits on knock tendency and important improvement on fuel efficiency. Experiments were carried out on a downsized port fuel injection (PFI) twin-cylinder turbocharged spark ignition engine equipped with a variable valve actuation (VVA) system over the speed range from 2500 to 4500rpm (step of 500 rpm) under a medium-high load condition. The engine was equipped with a prototype low-pressure injection system including two solenoid injectors installed in the runners, upstream of the gasoline ones, able to inject, at phased timing, a controlled amount of water within the intake ports.Experiments were carried out at wide open throttle (WOT) conditions choosing a water to gasoline mass ratio of 0.2, as a result of a previous investigation carried out to optimize the water amount in terms of engine performance. At the different engine speeds and at full gasoline operations, the relative air-fuel ratio (λ - measured by a lambda meter located in the exhaust, upstream of the three-way catalyst) was that set by the standard electronic control unit (ECU) map corresponding to the knock limited spark advance. As the WI was activated the amount of fuel was steadily reduced up to reach the stoichiometric condition and a sweep was performed up to the most advanced spark timing without knock occurrence.In-cylinder pressure data, acquired by pressure sensors flush-mounted within the combustion chamber of both cylinders, allowed estimating the main combustion parameters such as the rate of heat release and the combustion phasing. The effect of water injection on turbine inlet temperature and combustion efficiency, estimated from the exhaust gaseous emissions (HC, CO, CO2), are also presented. Further, combustion noise analysis carried out post-processing the in-cylinder pressure signal, decomposed into three sub-signals corresponding to the relevant physical phenomena: pseudo-motored operation (compression-expansion), combustion, and combustion chamber resonance. Such pressure signal decomposition was applied to find cause-effect relationships between the source signal generated by the engine during combustion process and both the objective and subjective parameters of the noise. The results coming from such analysis are discussed to determine the capability of water injection to reduce combustion noise level and improve sound quality index.