luigi allocca

The combustion of fossil-based fuels in ICEs, resulting in a huge amount of greenhouse gases (GHG) and leading to an immense global temperature rise, are the root causes of the more stringent emission legislations to safeguard health and that encourage further investigations on alternative carbon-neutral fuels. In this respect, the hydrogen has been considered as one of the potential clean fuels because of its zero-carbon nature. The current development of hydrogen-based ICEs focuses on the direct injection (DI) strategy as it allows better engine efficiency than the port fuel injection one. The behavior of the fuel jet is a fundamental aspect of the in-cylinder air-fuel mixing ratio, affecting the combustion process, the engine performances, and the pollutants emissions.In the present study, comprehensive investigations on the hydrogen jet behavior, generated by a Compressed Hydrogen Gas (CHG) injector under different operative conditions, were performed. A measuring system, suitable for the gaseous fuels, was used for measuring the flow rate. The fuel jet morphology was studied in a constant volume vessel filled with nitrogen as function of the injection pressure (up to 4.0 MPa) and different backpressure in the vessel, through the measurements of the jet penetrations, total areas, and cone angles showing a strong dependence from the set parameters. The cycle-resolved schlieren imaging technique by a high-speed camera was used to follow the jet spreads while the images were processed by a home-made procedure allowed to identify the contours of the hydrogen jet in the nitrogen gas and hence to measure the main parameters characterizing the jet structure.

The direct injection of gaseous fuels involves the presence of under-expanded jets due to the high pressure-ratios and the strong gas compressibility. Understanding the physical development of such processes is essential for developing Direct Injection (DI) devices suitable for application in internal combustion engines fueled by methane or hydrogen. In this work a coupled experimental-numerical characterization of a spray, issued by a multi-hole injector, was performed. The experimental characterization of the jet evolution was recorded by means of schlieren imaging technique and then a numerical simulation procedure was assessed using the measurements for validating. A density-based solver, capable of simulating highly compressible jets and developed within OpenFOAM environment, was used to study the effects of thermodynamic conditions on the development of the injection process. The obtained results shed more light on the characteristics of the gaseous spray demonstrating how these features really affects the development of the injection process and the quality of the air/fuel mixture.

Nobody escapes the importance of the air/fuel mixture preparation in internal combustion engines, whatever is the thermodynamic cycle they work on and the adopted fuel. The short time between the fueling and the start of the combustion is crucial for the best mixture formation and its burning for energy optimization and the pollutant production. The always stringent vehicle emissions rules led engine research toward the use of high-pressures and direct-injection systems for allowing a best atomization/vaporization of the fuel while, as drawback, the downsizing of the combustion system results in increasing of fuel impact on the combustion chamber and cylinder line with HC and particulate matter increase at the exhaust. A complete description of the injected fuel, its spread in the chamber with primary and secondary breakups, the vaporization, the mixture with the air and the combustion are of basic importance to govern the process and help new design architectures. Optical characterization has shown it’s powerful in describing these processes without interfering with their evolution both in stationary and reciprocating devices. Imaging and spectroscopic techniques allow us to follow the physic-chemical transformations of the initial bulk of fuel in the mixture, production of primary and secondary species and individuate the potential sites of pollutant production. Furthermore, the results constitute a data set for initializing and calibrating numerical codes for provisional, results and behavior of diverse injecting systems and at different ambient conditions, and last but not the least inside the engine. In this paper, a tracking shot of optical techniques adopted to depict the fuel spread in a quiescent optical accessible vessel at engine-like conditions is reported for practices of fuel direct injection in engines.

In the present study the fuel spray of a gasoline direct injected engine with multi-hole injector is simulated. Simulation inputs data, injection flow rate and spray cone angle are obtained from previous experimental studies. Log-normal distribution with different standard deviation is used for initial droplet size as the primary break-up model in order to reach the agreement between experimental and calculated spray tip penetration. As the first step, only one plume of spray injected into a quiescent air environment is simulated and validated by varying break-up model and standard deviation. Then, with coefficient obtained from the single jet simulation all six spray jets are simulated based on the injector nozzles geometry. The comparison between single-jet simulation and multi-jet simulation shows that validated model coefficients for the single-jet spray cannot be used for multi-jet spray simulation without significant modifications due to adjacent jet interaction and pressure drag. A set of new coefficients for the multi-jet spray is presented.