Water Injection Applicability to Gasoline Engines: Thermodynamic Analysis

Abstract

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 incylinder 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.

Experimental Analysis of Water Pressure and Temperature Influence on Atomization and Evolution of a Port Water Injection Spray

“…In this frame, water injection technology can be used as an alternative way to control the charge temperature at the end of the intake process and during the combustion phase, thus reducing the risk of abnormal combustion in downsized and highly boosted GDI engines. Water injection can potentially enable λ = 1 operation in high-load and highspeed operation, even adopting high compression ratios [7][8][9][10][11][12][13][14][15][16][17], thereby gaining significant benefits in terms of engine efficiency while preserving the after-treatment system efficacy in controlling exhaust emissions.…”

“…In this analysis, CFD simulations of the water spray evolution in the intake duct were carried out to investigate the spray-air interaction, but the effect of the spray characteristics with different operating conditions was not investigated. In other numerical analyses of water injection systems, the water spray characteristics, or even its evaporation rate, are assumed to be constant or similar to fuel sprays generated at the same injection pressure level [13,15,16].…”

Advanced Combustion Modelling of High BMEP Engines under Water Injection Conditions with Chemical Correlations Generated with Detailed Kinetics and Machine Learning Algorithms

“…The relative injection pressure was set to 10 bar for using the current PFI and UREA technologies, thus reducing costs. An injection pressure lower than 10 bar reduces the flexibility in the injection timing selection and sweep, as already discussed by the authors in [2]. Injection pressures greater than 20 bar are beyond the practical interest because of the associated costs.…”

“…A high-bmep and high-power S.I. GDI turbo-charged engine has been virtually designed at the University of Bologna and slightly modified compared to the previous one [2]. In Figure 1 the engine is visible, in the DWI version in Figure 1a and in the PWI configuration in Figure 1b, respectively, with the same compression ratio for both.…”

“…If the thermodynamic effects can be currently well captured with standard RANS CFD simulations, the effects of the water dilution on combustion duration, and indirectly also on the TiT and the engine knock limit, ask for the accurate prediction of the flame laminar speed, the flame laminar thickness and the auto-ignition time. In fact, if the main positive aspect of the water injection is its strong mixture cooling capability, besides it, it is well known that the water vapor acts as a mixture diluter, thus diminishing the reactants burning rate and changing the auto-ignition delay time [1][2][3][4]. Before entering the actual topic of scientific work, a brief analysis of the literature is necessary.…”