Investigating Effects of Water Injection on SI Engines

Licentiate thesis, 2019

In recent years, there has been a major shift towards the partial or complete electrification of vehicles that have traditionally been powered by conventional internal combustion (IC) engines; almost all major automotive manufacturers have rightly stated that they will make such a shift. This has been motivated in large part by pressure from governments and policymakers to minimize vehicular emissions, especially those of the greenhouse gas CO2, in order to control climate change.

A widely recognized way of facilitating this shift is to introduce vehicles having both an electric motor and a downsized turbocharged spark-ignited engine. Downsized SI engines are designed to have lower fuel consumption and tailpipe emissions than conventional engines while maintaining a comparable power output by increasing thermal efficiency. Unfortunately, this generally necessitates higher cylinder pressures and temperatures, both of which increase the engine's knock propensity. At present, engine knock is mitigated by retarding the ignition timing or fuel enrichment, both of which reduce thermal efficiency. During the last decade, research building on trials conducted with aircraft engines has shown that water injection may be a viable alternative knock mitigation strategy that mainly suppresses knock by reducing local in-cylinder mixture temperatures. Adding sufficient water to the cylinder can enable knock-free engine operation under stoichiometric conditions, reducing fuel consumption and enabling full utilization of a three-way catalytic converter (TWC).

This licentiate thesis presents studies on the performance of a water injection system that were conducted within the framework of a broader project seeking to optimize SI engines for use in high efficiency hybrid powertrains. The results presented originate from two experimental campaigns. During the first campaign, a 3-cylinder 1.5L turbocharged engine was operated using 91, 95, and 98 RON gasoline fuel to assess the effects of water injection on knock mitigation, thermal efficiency, and emissions. Full- and part-load curves obtained with different fuels and water injection strategies are presented and discussed. In the second campaign, the effect of varying the moisture content of the ambient air (i.e. the relative humidity) was investigated using both experimental and theoretical methods to clarify the mechanism responsible for the knock suppression and performance enhancement caused by water injection. The engine was operated at three humidity levels that were maintained using a humidity control system developed in-house. Particulate emissions were also measured at each studied operating point and their dependence on relative humidity is discussed.