Water Injection Applicability to Gasoline Engines: Thermodynamic Analysis

The vehicle WLTP and RDE homologation test cycles are pushing the engine technology toward the implementation of different solutions aimed to the exhaust gases emission reduction. The tightening of the policy on the Auxiliary Emission Strategy (A.E.S.), including those for the engine component protection, faces the Spark Ignited (S.I.) engines with the need to replace the fuel enrichment as a means to cool down both unburnt mixture and exhaust gases to accomplish with the inlet temperature turbine (TiT) limit. Among the whole technology solutions conceived to make SI engine operating at lambda 1.0 on the whole operation map, the water injection is one of the valuable candidates. Despite the fact that the water injection has been exploited in the past, the renewed interest in it requires a deep investigation in order to outcome its potential as well as its limits. Many experimental campaigns have been performed while only few researches have deeply investigated the effect of the water injection on the air-fuel mixture under engine operating conditions. Since the experiments perform like a black box and they might hide some important phenomena, CFD numerical investigation steps are mandatory in order to provide a wider and deeper overview on the thermo-fluid dynamics characteristics involved in engine operations with water injection.

This paper aims to provide an insight into those processes by using a CFD numerical approach to investigate the effect of the water injection on a non-reacting air-fuel mixture. The investigation has been carried out with the aid of CFD simulations by using AVL FIRE v.2017 solver. The influence of the physical, chemical and in-cylinder thermodynamic conditions on the effectiveness of the water cooling effect of the mixture is discussed and the application of the water injection to a spark ignition engine case at full load and high BMEP is examined. Port Water Injection (PWI) and Direct Water Injection (DWI) solutions are compared showing the high potential involved in DWI.