A Numerical Investigation on the Potentials of Water Injection as a Fuel Efficiency Enhancer in Highly Downsized GDI Engines

Abstract

Engine downsizing is gaining popularity in the high performance engine market sector, where a new generation of highly downsized engines with specific power outputs around or above 150 HP/litre is emerging. High-boost and downsizing, adopted to increase power density and reduce fuel consumption, have to face the increased risks of pre-ignition, knock or mega-knock. To counterbalance autoignition of fuel/air mixture, such engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter reduces performance and induces an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A relatively simple yet effective solution to increase knock resistance is investigated by means of 3-D CFD analyses in the paper: water is port injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. Full-load engine operations of a currently made turbocharged GDI engine are investigated comparing the adopted fuel-only rich mixture with stoichiometric mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-injected engine allows the Spark Advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption

CFD Analysis of Port Water Injection in a GDI Engine under Incipient Knock Conditions

“…Numerical simulations can assist in designing water injection systems and in understanding the intertwined phenomena occurring from the water spray to the combustion. Some examples of CFD studies have been presented recently in the literature [7,11]. Two aspects combine: the realistic modeling of the water injection process and the robust knock risk prediction.…”

“…Tabulated kinetics for ignition (TKI) coupled with the extended coherent flamelet combustion model (ECFM) it is shown to provide high accuracy in predicting knock frequency and intensity. However, RANS is still the method of choice for large number of design space explorations [7,11,[13][14][15][16]. Some authors are proposing new advanced models with presumed probability distribution functions to include knock statistics upfront in RANS [14,15].…”

Effect of Spark Ignition Timing and Water Injection Temperature on the Knock Combustion of a GDI Engine

“…Water injection technology is a direct and simple method to suppress knock and reduce emissions [9][10][11]. The specific heat capacity of water is large.…”

Impact of the Primary Break-Up Strategy on the Morphology of GDI Sprays in 3D-CFD Simulations of Multi-Hole Injectors

“…For example, focusing on Gasoline Direct Injection (GDI) engines, spray directly affects air-fuel mixing [1][2][3] and, thus, combustion [4,5], knock [6,7], and emissions [8]. Moreover, knock suppressor techniques such as water (or water/methanol) injection are more and more diffused [9][10][11][12]. In this case, a detailed numerical representation of the water spray is mandatory to properly predict water evaporation or even phenomena such as liquid film formation on the intake ports, which impact on the effectiveness of water injection itself.…”