CFD Analysis of the Effects of Fuel Composition and Injection Strategy on Mixture Preparation and Fuel Deposit Formation in a GDI Engine

Abstract

In spark-ignited direct-injected engines, the formation of fuel pools on the piston is one of the major promoters of unburnt hydrocarbons and soot: in order to comply with the increasingly stringent emission regulations (EU6 and forthcoming), it is therefore necessary to limit fuel deposit formation. The combined use of advanced experimental techniques and detailed 3D-CFD simulations can help to understand the mechanisms driving fuel pool formation. In the paper, a combined experimental and numerical characterization of pool formation in a GDI engine is carried out to investigate and understand the complex interplay of all the mentioned factors. In particular, a low-load low-rpm engine operation is investigated for different ignition phasing, and the impact of both fuel formulation and instantaneous piston temperature variations in the CFD analyses are evaluated. The investigated engine operation shows some interesting features which are suited to deeply investigate the interplay between fuel film formation, mixing and soot. In particular, the relatively low wall temperature and low injection pressure allow the fuel to form deposits and then slowly evaporate, with possible presence of liquid fuel at the time of ignition. The simultaneous presence of slow fuel evaporation, reduced turbulence and presence of liquid fuel leads to the formation of extremely rich mixture pockets (with equivalence ratios well above 5) which are the major promoters for soot inception. Four different start of injection (hereafter SOI) values are analyzed, for which tailpipe Soot concentration measurements are available. For one SOI value, two different injection profiles are also evaluated. In particular, the analyses focus on the formation of fuel pads on the combustion chamber walls and on the mixture stratification, and a correlation between these two factors and the tailpipe soot level is found. The proposed methodology proves to be able to capture the Soot trend for the different SOI values without simulating the combustion process; it is therefore promising since it avoids the need for a dedicated calibration of the combustion model parameters and provides reasonable results (at least in terms of trends) with limited computational resources

A Combined Experimental and Computational Fluid Dynamics Investigation of Particulate Matter Emissions from a Wall-Guided Gasoline Direct Injection Engine

“…In recent years there have been significant developments in accurate 3D CFD modelling of GDI engines as this offers a useful insight into the physico-chemical mechanisms leading to particle formation. Several studies [25,38,39] demonstrated that a strong indirect correlation exists between mixture preparation and engine-out PN Den . Engine speed, injection timing and mixing time are shown to influence the preparation process, but injection pressure appears to be the most significant driver for mixture homogeneity.…”

“…Control strategies which include high injection pressure and appropriate timing to minimise unwanted jet-to-wall impact are seen to be potentially beneficial across the wider engine running envelope. Through modelling of a high-performance wall-guided GDI engine, Giovannoni et al [25] showed how retarded SOI increases the quantity of fuel film forming on the piston crown, resulting in an unwanted stratification of the equivalence ratio distribution across the entire combustion chamber. Importantly, the study demonstrates that exhaust soot concentration is intimately correlated with the summation of combustion chamber computational cells' mass, where the modelled fuel equivalence ratio at ignition timing is in excess of a set rich threshold.…”

“…In a GDI engine, poor mixture preparation is one of the major causes of PM formation [11,14,[20][21][22][23][24][25]. Mixture preparation refers normally to the process of fuel injection, vaporisation and mixing with in-cylinder air charge.…”

“…Incomplete mixing between air and fuel and the permanence of sub-stoichiometric mixture-pockets during combustion, lead to abnormal combustion and generation of PM [9,10,14,21,22,[24][25][26][27][28]. Fuel film formation is governed by injection characteristics such as injection pressure, spray angle/direction, injection timing, but is also heavily dependent on gas and especially metal wall temperature [14,25]. Higher injection pressure speeds up the process of mixing through better droplets atomisation but, at the same time, results in deeper spray tip penetration and potential spray-wall impingement.…”

“…Higher injection pressure speeds up the process of mixing through better droplets atomisation but, at the same time, results in deeper spray tip penetration and potential spray-wall impingement. If exposed to high temperature during combustion, liquid film may give rise to the mechanism of pool-fire leading to heavy PM yield [9,21,25]; on the other hand, the process of evaporation of the film may contribute to the mal-distribution of AFR along the chamber periphery [14,25]. The third mechanism, residual liquid droplets, is generally related to cold start running conditions or specific strategies such as stratified charge combustion [22,24].…”