Direct Water Injection Strategies for Performance Improvement of a Turbocharged Spark-Ignition Engine at High Load Operation

Water injection (WI) could be a viable tool for the reduction of CO2 emissions of spark-ignition engines. At high loads, the performances of this kind of engines are constrained by knock phenomena, thermal limits of engine components and maximum tolerable in-cylinder pressure. Water injection, mainly due to its cooling effect, helps mitigating knock and reducing the exhaust gas temperature. Furthermore, it allows to obtain greater spark advances, better combustion phasing and leaner mixtures with a consequent improvement in terms of engine efficiency. In this work, the authors investigated the effects of a particular direct water injection (DWI) strategy on the performance of a turbocharged PFI spark-ignition engine at high load operation. The analysis has been carried out using a validated 1D model that reproduces the entire engine layout. A knock model allows to identify the knock-limited parameters in the various operating points analyzed. Threshold values for both exhaust turbine inlet temperature and maximum in-cylinder pressure have been imposed. The results of this analysis show how a particular DWI strategy allows optimizing engine parameters in order to improve the fuel economy of the analyzed engine at full load operation. In particular, a double water injection strategy (pilot and post WI) has been performed in almost all the operating conditions analyzed. The adopted DWI configuration allows an increase of indicated mean effective pressure up to 19% and a reduction of indicated specific fuel consumption up to 36% with respect to engine operation without WI.