Search Results

An innovative quasi-dimensional multizone combustion model for the spray formation, combustion and emission formation analysis in DI diesel engines was assessed and applied to an optical single cylinder engine. The model, which has been recently presented by the authors, integrates a predictive non stationary 1D spray model developed by the Sandia National Laboratory, with a diagnostic multizone thermodynamic model. The 1D spray model is capable of predicting the equivalence ratio of the fuel during the mixing process, as well as the spray penetration. The multizone approach is based on the application of the mass and energy conservation laws to several homogeneous zones identified in the combustion chamber. A specific submodel is also implemented to simulate the dilution of the burned gases. Soot formation is modeled by an expression which derives from Kitamura et al.'s results, in which an explicit dependence on the local equivalence ratio is considered.

Among the others, natural gas (NG) is regarded as a potential solution to enhance the environmental performance of internal combustion engines. Low carbon-to-hydrogen ratio, worldwide relatively homogeneous distribution and reduced price are the reason as, lately, many researchers efforts have been put in this area. In particular, this work focuses on the characterization of the injection process inside a constant volume chamber (CVC), which could provide a contribution to the development of direct injection technologies for a gaseous fuel. Direct injection of a gaseous fuel involves the presence of under-expanded jets whose knowledge is fundamental to achieve the proper mixture formation prior to the combustion ignition. For this reason, a density based solver was developed within the OpenFOAM library in order to simulate the jet issued from an injector suitable for direct injection of methane.

Compressed Natural Gas (CNG) is regarded as a promising fuel for spark-ignited (SI) internal combustion engines (ICE) to improve engine thermal efficiency and reduce both carbon dioxide and pollutant emissions. Significant advantages of CNG are higher-octane number, higher hydrogen to carbon ratio, and lower energy-specific CO2 emissions compared with gasoline. More, it can be produced in renewable ways, and is more widespread and cheaper than conventional liquid fossil fuels. In this regard, the direct injection of CNG engines can be considered a promising technology for highly efficient and low-emission future engines. This work reports an experimental and numerical characterization of high-pressure methane jets from a multi-hole injector for direct injection engines.

In recent years, the increase of gasoline fuel injection pressure is a way to improve thermal efficiency and lower engine-out emissions in GDI homogenous combustion concept. The challenge of controlling particulate formation as well in mass and number concentrations imposed by emissions regulations can be pursued improving the mixture preparation process and avoiding mixture inhomogeneity with ultra-high injection pressure values up to 100 MPa. The increase of the fuel injection pressure in GDI homogeneous systems meets the demand for increased injector static flow, while simultaneously improves the spray atomization and mixing characteristics with consequent better combustion performance. Few studies quantify the effects of high injection pressure on transient gasoline spray evolution. The aim of this work was to simulate with OpenFOAM the spray morphology of a commercial gasoline injected in a constant volume vessel by a prototypal GDI injector.

The common realization of the necessity to reduce the use of mineral sources is promoting the use of alternative fuels. Big efforts are being made to replace petroleum derivatives in the internal combustion engines (ICEs). For this purpose it is mandatory to evaluate the behavior of non-conventional fuels in the ICEs. The optical diagnostics have proven to be a powerful tool to analyze the processes that take place inside the engine. In particular, 2d imaging in the infrared range can reveal new details about the effect of the fuel properties since this technique is still not very common. In this work, a comparison between commercial diesel fuel and two non-conventional fuels has been made in an optically accessible diesel engine. The non-conventional fuels are: the first generation biofuel Rapeseed Methyl Ester (RME) and an experimental blend of diesel and a fuel with high glycerol content (HG).

An investigation has been carried out on the spray penetration and soot formation processes in a research diesel engine by means of a quasi-dimensional multizone combustion model. The model integrates a predictive non stationary 1D spray model developed by the Sandia National Laboratory, with a diagnostic multizone thermodynamic model, and is capable of predicting the spray formation, combustion and soot formation processes in the combustion chamber. The multizone model was used to analyze three operating conditions, i.e., a zero load point (BMEP = 0 bar at 1000 rpm), a medium load point (BMEP = 5 bar at 2000 rpm) and a medium-high load point (BMEP = 10 bar at 2000 rpm). These conditions were experimentally tested in an optical single cylinder engine with the combustion system configuration of a 2.0L Euro4 GM diesel engine for passenger car applications.

During gasoline direct injection (GDI) in spark ignition engines, droplets may hit piston or liner surfaces and be rebounded or deposit in the liquid phase as wallfilm. This may determine slower secondary atomization and local enrichments of the mixture, hence be the reason of increased unburned hydrocarbons and particulate matter emissions at the exhaust. Complex phenomena indeed characterize the in-cylinder turbulent multi-phase system, where heat transfer involves the gaseous mixture (made of air and gasoline vapor), the liquid phase (droplets not yet evaporated and wallfilm) and the solid walls. A reliable 3D CFD modelling of the in-cylinder processes, therefore, necessarily requires also the correct simulation of the cooling effect due to the subtraction of the latent heat of vaporization of gasoline needed for secondary evaporation in the zone where droplets hit the wall. The related conductive heat transfer within the solid is to be taken into account.

In the last years, increasing concerns about environmental pollution and fossil sources depletion led transport sectors research and development towards the study of new technologies capable to reduce vehicles emissions and fuel consumption. Direct-injection systems (DI) for internal combustion engines propose as an effective way to achieve these goals. This technology has already been adopted in Gasoline Direct Injection (GDI) engines and, lately, a great interest is growing for its use in natural gas fueling, so increasing efficiency with respect to port-fuel injection ones. Alone or in combination with other fuels, compressed natural gas (CNG) represents an attractive way to reduce exhaust emission (high H/C ratio), can be produced in renewable ways, and is more widespread and cheaper than gasoline or diesel fuels. Gas direct-injection process involves the occurrence of under-expanded jets in the combustion chamber.

Very high pressures for injecting gasoline in internal combustion (i.c.) engines are recently explored for improving the air/fuel mixing process in order to control unburned hydrocarbons (UBHC) and particulate matter emissions such as for investigating new combustion concepts. The challenge remains the improvement of the spray parameters in terms of atomization, smaller droplets and their spread in the combustion chamber in order to enhance the combustion efficiency. In this framework, the raise of the injection pressure plays a key role in GDI engines for the trade-off of CO2 vs other pollutant emissions. This study aims contributing to the knowledge of the physical phenomena and mechanisms occurring when fuel is injected at ultra-high pressures for mapping and controlling the mixture formation.

Today, Direct-Injection systems are widely used on Spark-Ignition engines in combination with turbo-charging to reduce the fuel-consumption and the knock risks. In particular, the spread of Gasoline Direct Injection (GDI) systems is mainly related to the use of new generations of multi-hole, high-pressure injectors whose characteristics are quite different with respect to the hollow-cone, low-pressure injectors adopted in the last decade. This paper presents the results of an experimental campaign conducted on the spray produced by a GDI six-holes injector into a constant volume vessel with optical access. The vessel was filled with air at atmospheric pressure. Different operating conditions were considered for an injection pressure ranging from 3 to 20 MPa. For each operating condition, spray images were acquired by a CCD camera and then post processed to evaluate the spray penetration and cone angles.

The numerical reconstruction of the liquid jet generated by a multi-hole injector, operating in flash-boiling conditions, has been developed by means of a Eulerian- Lagrangian CFD code and validated thanks to experimental data collected with schlieren and Mie scattering imaging techniques. The model has been tested with different injection parameters in order to recreate various possible engine thermodynamic conditions. The work carried out is framed in the growing interest present around the gasoline direct-injection systems (GDI). Such technology has been recognized as an effective way to achieve better engine performance and reduced pollutant emissions. High-pressure injectors operating in flashing conditions are demonstrating many advantages in the applications for GDI engines providing a better fuel atomization, a better mixing with the air, a consequent more efficient combustion and, finally, reduced tailpipe emissions.

This paper is focused on the development and application of a CFD methodology that can be applied to predict the fuel-air mixing process in stratified charge, sparkignition engines. The Eulerian-Lagrangian approach was used to model the spray evolution together with a liquid film model that properly takes into account its effects on the fuel-air mixing process into account. However, numerical simulation of stratified combustion in SI engines is a very challenging task for CFD modeling, due to the complex interaction of different physical phenomena involving turbulent, reacting and multiphase flows evolving inside a moving geometry. Hence, for a proper assessment of the different sub-models involved a detailed set of experimental optical data is required. To this end, a large experimental database was built by the authors.

The interaction between impacting and splashed droplets on the combustion chamber walls and inlet air motion plays a fundamental role in the mixture formation process. It covers a crucial aspect for the correct operation of both DI diesel and gasoline engines as it greatly influences the combustion process and the exhaust emissions. A complete understanding of spray impingement is quite complex. In this paper, a numerical-experimental approach is proposed. On the experimental side, the behaviour of a Diesel impinging spray emerging from a common rail injection system (injection pressures of 80 MPa and 120 MPa) and of a gasoline hollow cone spray generated by a swirled injector (injection pressures up to 10.0 MPa) have been analysed. The impinging spray has been lightened by a pulsed laser sheet generated from the second harmonic of a Nd - YAG laser. The images have been acquired by a CCD camera at different times from the Start of Injection.

Nowadays HCCI combustion process is revealing the most useful technique for reducing pollutant emission from internal combustion engines. In the present paper, HCCI combustion was realized by means of single late injection at high pressure and heavy EGR, up to 50%. A transparent Direct Injection (DI) diesel engine equipped with high pressure Common Rail (CR) injection system was used. The engine was fed with commercial diesel fuel and ran in continuous mode. Digital imaging and spectroscopic techniques, with high temporal and spatial resolution, were applied to study the low temperature combustion process. Injection and combustion phases were analysed by digital imaging. Mixing process, autoignition and pollutants formation were investigated by Broadband Ultraviolet - Visible Extinction Spectroscopy (BUVES) and flame emission measurements. Radicals and species such as OH, CH and CO were detected in the combustion chamber.

In this paper we report how optical techniques were applied in the cylinder of an optically accessible engine equipped with latest-generation EURO V diesel engine head. The injection strategy with high percentage of EGR, characteristic of real engine operating point, was adopted. In particular, the combustion behavior at 1500 rpm\2 bar BMEP was investigated. Alternative diesel fuels were used. In particular, rapeseed methyl ester (RME) and gas to liquid (GTL) were selected as representative of 1st and 2nd generation alternative diesel fuel, respectively. Combustion analysis was carried out in the engine combustion chamber by means of visible digital imaging. These measurements helped to analyze the chemical and physical events occurring during the mixture preparation and the combustion development. Ultraviolet (UV) digital imaging was also performed and the presence of characteristic radical, like OH, in the various phases of combustion was detected as well.

A numerical investigation is performed with the aim of understanding the potential benefits of multiple injections in the mixed mode boosting operation of a Gasoline Direct Injection (GDI) engine. The study is carried out by firstly characterizing a high pressure multi-hole injector from the experimental point of view in the split injection operation. Measurements of the fuel injection rate are made through an AVL Meter operating on the Bosch principle. The injector is tested using gasoline in a double pulse strategy. The injection pressure is varied between 5.0 and 25.0 MPa with the pulse durations calibrated for delivering a total mass up to 50 mg/str. The choice of the dwell time between two successive injection events is achieved by firstly defining the minimum time compatible with the mechanical characteristics of both the injector and the injector driver.

A numerical methodology to simulate the high pressure spray evolution and the fuel-air mixing in diesel engines is presented. Attention is focused on the employed atomization model, a modified version of the Huh and Gosman, on the definition of a turbulence length scale limiter and of an adaptive local mesh refinement technique to minimize the result grid dependency. All the discussed models were implemented into Lib-ICE, which is a set of libraries and solvers, specifically tailored for engine simulations, which runs under the open-source CFD technology OpenFOAM®. To provide a comprehensive assessment of the proposed methodology, the validation procedure consisted into simulating, with a unique and coherent setup of all models, two different sets of experiments: a non-evaporating diesel fuel spray in a constant-volume vessel with optical access and an evaporating non-reacting diesel fuel spray in an optical engine.

Diesel combustion process was studied and characterized by digital imaging and ultraviolet-visible flame emission, extinction and scattering spectroscopy. Optical measurements were applied to a transparent diesel engine, realized by modifying a single cylinder, air-cooled, 4-stroke diesel engine by means of an external combustion chamber on the top of the engine, connected to the main chamber by a tangential passage. Diesel engine was equipped with a fully flexible electronic controlled ‘Common Rail’ injection system. Measurements were performed at 1000 rpm engine speed for two typical injection strategies. The first one consisted of a main injection in order to compare the results with those ones obtained by conventional injection system operating at low pressure. The other one was based on a pilot and main injection that is typical of current direct injection diesel engines.

Mixture formation is fundamental for the development of the combustion process in internal combustion engines, for the energy release, the consumption, and the pollutant formation. Concerning the spark ignition engines, the direct injection technology is being considered as an effective mean to achieve the optimal air-to-fuel ratio distribution at each operating condition, either through charge stratification around the spark plug and stoichiometric mixture under the high power requirements. Due to the highest injection pressures, the impact of a spray on the piston or on the cylinder walls causes the formation of liquid film (wall-film) and secondary atomization of the droplets. The wall-film could have no negligible size, especially where the mixture formation is realized under a wall-guided mode. The present work aims to report the effects of the ambient pressure and wall temperature on the macroscopic parameters of the spray impact on a wall.

In spark ignition engines, the nozzle design, fuel pressure, injection timing, and interaction with the cylinder/piston walls govern the evolution of the fuel spray inside the cylinder before the start of combustion. The fuel droplets, hitting the surface, may rebound or stick forming a film on the wall, or evaporate under the heat exchange effect. The face wetting results in a strong impact on the mixture formation and emission, in particular, on particulate and unburned hydrocarbons. This paper aims to report the effects of the injection pressure and wall temperature on the macroscopic behavior, atomization, and vaporization of impinging sprays on the metal surface. A mono-component fuel, iso-octane, was adopted in the spray-wall studies inside an optically-accessible quiescent vessel by imaging procedures using a Z-shaped schlieren-Mie scattering set-up in combination with a high-speed C-Mos camera.