It is widely recognized that air-fuel mixing, combustion and pollutant formation inside internal combustion engines are strongly influenced by the spatial and temporal evolution of both marco- and micro- turbulent scales. Particularly, in spark ignited\ud engines, the generation of a proper level of turbulence intensity for the correct development of the flame front is traditionally based on the onset, during the intake and compression strokes, of a tumbling macro-structure.\ud Recently, in order to both reduce pumping losses due to throttling and develop advanced and flexible engine control strategies,\ud fully variable valve actuation systems have been introduced, capable of simultaneously governing both valve phasing and lift.\ud Despite the relevant advantages in terms of intake system efficiency, this technology introduces uncertainties on the capability of the intake port/valve assembly to generate, at low loads, sufficiently coherent and stable structures, able therefore to promote\ud adequate turbulence levels towards the end of the compression, with relevant effects on the flame front development.\ud It is a common knowledge that 3D-CFD codes are able to describe the evolution of the in-cylinder flow field and of the\ud subsequent combustion process with good accuracy; however, they require too high computational time to analyze the engine\ud performance for the whole operating domain. On the contrary, this task is easily accomplished by 1D codes, where, however, the\ud combustion process is usually derived from experimental measurements of the in-cylinder pressure trace (Wiebe correlation).\ud This approach is poorly predictive for the simulation of operating conditions relevantly different from the experimental ones. To\ud overcome the above described issues, enhanced physical models for the description of in-cylinder turbulence evolution and combustion to be included in a 1D modeling environment are mandatory.\ud In the present paper (part I), a 0D (i.e. homogeneous and isotropic) phenomenological (i.e. sensitive to the variation of operative\ud parameters such as valve phasing, valve lift, intake and exhaust pressure levels, etc.) turbulence model belonging to the K-k\ud model family is presented in detail. The model is validated against in-cylinder results provided by 3D-CFD analyses carried out with the Star-CD code for motored engine operations. In particular, a currently produced small turbocharged VVA engine is analyzed at different speeds, with valve actuations typical of full load and partial load (EIVC) operations, as well.\ud The proposed turbulence model shows the capability, once tuned, to accurately estimate the temporal evolution of the in-cylinder turbulence according to the engine operating conditions. In the subsequent part II of the same paper, the developed turbulence model will be employed within a quasi-dimensional fractal combustion model
As discussed in the part I of this paper, 3D models represent a useful tool for a detailed description of the mean and turbulent flow fields inside the engine cylinder. 3D results are utilized to develop and validate a 0D phenomenological turbulence model, sensitive to the variation of operative parameters such as valve phasing, valve lift, engine speed, etc. In part II of this paper, a 0D phenomenological combustion model is presented, as well. It is based on a fractal description of the flame front and is able to sense each of the fuel properties, the operating conditions (air-to-fuel ratio, spark advance, boost level) and the combustion chamber geometry. In addition, it is capable to properly handle different turbulence levels predicted by means of the turbulence model presented in the part I. The turbulence and combustion models are included, through user routines, in the commercial software GT-Power". With reference to a small twin-cylinder VVA turbocharged engine, the turbulence/combustion model, once properly tuned, is finally used to calculate in-cylinder pressure traces, rate of heat release and overall engine performance at full load operations and brake specific fuel consumption at part load, as well. An excellent agreement between numerical forecasts and experimental evidence is obtained
The paper investigates the low-temperature cranking operation of a current production automotive Gasoline Direct Injected (GDI) by means of 3D-CFD simulations. Particular care is devoted to the analysis of the hollow cone spray evolution within the combustion chamber and to the formation of fuel film deposits on the combustion chamber walls. Due to the high injected fuel amount and the strongly reduced fuel vaporization, wall wetting is a critical issue and plays a fundamental role on both the early combustion stages and the amount of unburnt hydrocarbons formation. In fact, it is commonly recognized that most of the unburnt hydrocarbon emissions from 4-stroke gasoline engines occur during cold start operations, when fuel film in the cylinder vaporize slowly and fuel can persist until the exhaust stroke.In view of the non-conventional engine operating conditions (in terms of injected fuel amount, engine speed, ambient and wall temperature and almost null fuel atomization and breakup), an understanding of the many involved phenomena by means of an optically accessible engine would be of crucial importance. Nevertheless, the application of such technique appears to be almost unfeasible even in research laboratories, mainly because of the relevant wall wetting.CFD analyses prove then to be a very useful tool to gain a full insight of the overall process as well as to correlate fuel deposits to both the combustion chamber design and the injection strategy. In order to better understand where, and how thick, these wall films are formed during the intake and compression, a detailed description of the spray interaction with both the piston wall and the intake valves was performed by the authors in a previous paper [ 1 ]. Subsequently, a wide set of injection strategies was simulated in order to better understand the physics of spray/wall interaction and to minimize the formation of deposits in the combustion chamber most critical locations [ 2 ].In order to limit the overall number of modeling uncertainties (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) the spray model was at first validated against experimental data under low injection pressure, and results from the comparison were reported in [ 1 ].In the present paper, cold start operations at decreasing ambient temperatures are modeled and results are analyzed in terms of both fuel film distribution on the combustion chamber walls and resulting fuel/air mixture distribution within the combustion chamber. The use of CFD simulations prove to be useful to investigate and understand the influence of both combustion chamber design and injection profile on the amount and distribution of fuel deposits, showing a high potential to address future engine optimizatio
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