Evaluation of Water Injection Strategies for NOx Reduction and Charge Cooling in SI Engines

Water injection in spark-ignition engines is one key technology for increasing power, improving fuel efficiency by mitigating knock and meeting future emission standards and is extensively researched today. Finding a suitable injection strategy is necessary to commercially exploit the potential of water injection. In order to evaluate water injection strategies, a better understanding of the mixture formation with water injection is required. Indirect port and direct water injection strategies were compared by highly time resolved in-cylinder water concentration and temperature measurements using an optical spark plug probe. The ignition conditions, mixture formation and performance were assessed in a single cylinder research engine equipped with water injection technology. Variations of engine speed, water/fuel ratio and water injection timing were performed and the effects on exhaust gas temperature and engine-out emissions analyzed in detail. At different operating points, a comparison of direct and indirect water injection concepts was made with regard to mixture formation. The spark plug probe enabled a local assessment of the ignition conditions near the spark. Furthermore, a water efficiency number could be obtained by measuring the in-cylinder temperature directly. In conclusion, a better understanding of water injection is reached and optimum water injection strategies in terms of NOx reduction and charge cooling are derived.