• Energy Research

Technologies for direct injection of fuel in compression ignition engines are in continuous development. One of the most investigated components of this system is the injector; in particular, main attention is given to the nozzle characteristics as hole diameter, number, internal shape, and opening angle. The reduction of nozzle hole diameter seems the simplest way to increase the average fuel velocity and to promote the atomization process. On the other hand, the number of holes must increase to keep the desired mass flow rate. On this basis, a new logic has been applied for the development of the next generation of injectors. The tendency to increase the nozzle number and to reduce the diameter has led to the replacement of the nozzle with a circular plate that moves vertically. The plate motion allows to obtain an annulus area for the delivery of the fuel on 360 degrees; while the plate lift permits to vary the atomization level of the spray. The experimental activities have been performed on a single-cylinder metal engine in order to evaluate the new injector concept functionality in typical engine working conditions. Then a deeper investigation of injector the characteristics has been performed in an optical single-cylinder diesel engine via high speed digital imaging in order to catch information on its operation. The results have shown a good response of the injector fuel delivery control but penalties in terms of emissions and efficiency compared to multihole nozzles. Images of the injection process showed that the fuel assumed an asymmetric shape at the exit of the injector affecting the mixing quality and, then, the combustion efficiency.

This paper investigates the energy distribution and the waste heat energy characteristics of a compression ignition engine for micro-cogeneration applications, at different engine speeds and loads. The experimental activity was carried out on a three-cylinder, 1028 cc, common-rail engine. Tests were performed with diesel fuel and a 20% v/v biodiesel blend (B20). The quantity and the quality of the waste heat energy were studied through energy and exergy analyses, respectively. Combustion characteristics were investigated by means of indicating data. Gaseous emissions were measured and particles were characterized in terms of number and size at exhaust. It was found out that the addition of 20% v/v of RME to diesel fuel does not affect significantly the brake fuel conversion efficiency and the energetic flows. On the other hand, biodiesel blend allows to reduce the combustion noise and the pollutants emissions in most of the operating conditions. A proper phasing of the injection strategy for the biodiesel blend could further reduce the exhaust emissions, mainly at high engine speeds. The results presented in this paper could be useful for the development of diesel engine based micro-cogeneration systems working at different engine speeds and loads.

Present work investigates both experimentally and numerically the benefits deriving from the use of split injections in increasing the engine power output and reducing the tendency to knock of a gasoline direct injection (GDI) engine. The here considered system is characterized by an optical access to the combustion chamber. Imaging in the UV-visible range is carried out by means of a high spatial and temporal resolution camera through an endoscopic system and a transparent window placed in the piston head. This last is modified to allow the view of the whole combustion chamber almost until the cylinder walls, to include the so-called eng-gas zones of the mixture, where undesired self-ignition may occur under some circumstances. Optical data are correlated to in-cylinder pressure oscillations on a cycle resolved basis. The numerical investigation is performed through a properly developed 3D CFD model of the engine under study, which employs a flamelet model for the combustion initiated by the spark plug, and a low-temperature self-ignition model in the zones not yet reached by the flame front. The difference in the engine behavior if powered under single or double injection strategies and their influence about knocking are discussed. Split injection reduces engine cycle-by-cycle variability with respect to the single injection case, all the others relevant parameters remaining unchanged. Benefits are also obtained as regards the resistance to knocking. This is a consequence of the different flow fields arising under the two powering modes, which obviously affect the formation of chemical intermediate species in the low temperature regimes preceding self-ignition.

The effective compression ratio of spark ignition engines is influenced by several factors, and can feature a relatively wide range even for production power units. This is given mainly by the tolerances used for different components, but also due to the actual operating parameters that can result in different thermal regimes and therefore in dimensional variations. Given their design, optical engines are characterized by high crevice volumes and increased blow-by losses. Within this context, this work was aimed at developing a procedure for estimating the effective compression ratio and charge losses, based on the analysis of in-cylinder pressure traces. Compared to existing methods that require heat release rate modeling for ensuring good accuracy, the proposed route follows a different approach, that uses sub-models only for simulating heat transfer and blow-by losses. In this way, acceptable error levels were ensured, with minimum computational demand. The new procedure was applied on two different engine configurations, as well as operation with different fuels. Estimation of combustion efficiency was found to be an important issue for correct blow-by losses determination, while compression ratio evaluation depended mostly on the thermal regime of the piston.

<div class="section abstract"><div class="htmlview paragraph">It is well known that ethanol can be used in spark-ignition (SI) engines as a pure fuel or blended with gasoline. High enthalpy of vaporization of alcohols can affect air-fuel mixture formation prior to ignition and may form thicker liquid films around the intake valves, on the cylinder wall and piston crown. These liquid films can result in mixture non-homogeneities inside the combustion chamber and hence strongly influence the cyclic variability of early combustion stages. Starting from these considerations, the paper reports an experimental study of the initial phases of the combustion process in a single cylinder SI engine fueled with commercial gasoline and anhydrous ethanol, as well as their blend (50%_vol alcohol). The engine was optically accessible and equipped with the cylinder head of a commercial power unit for two-wheel applications, with the same geometrical specifications (bore, stroke, compression ratio). Ultra-violet (UV) natural emission spectroscopy measurements ranging from 250nm to 470nm wavelength and simultaneous thermodynamic analysis were used to better understand the effect of ethanol content on flame kernel inception and development. All experiments were conducted at wide open throttle (WOT), with stoichiometric air-fuel mixtures, fixing the engine speed at 2000rpm. Optical investigations allowed to follow the evolution of chemical species that marked the spark discharge (cyano CN and hydroxyl OH radicals) as well as flame front initial growth (OH and carbyne CH radicals). Vibrational and rotational temperatures were calculated during the arc and glow phase by the ratio between the emission intensity of CN and OH radicals. Results were compared with adiabatic flame temperature traces obtained by applying a two zone model.</div></div>

Digital imaging and spectroscopic techniques, with high temporal and spatial resolution, were applied in order 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 and flame emission measurements. Moreover, fuel distribution and oxidation were studied as well. Liquid fuel and vapour phase, injected around the top dead centre, were analysed. The liquid diesel fuel was observed by extinction measurements when the liquid jet reached the bowl rim and aromatic compounds due to fuel decomposition were identified. On the other side, the vapour fuel was detected about 2° after the injection start and liquid fuel disappeared. Then, radicals and species were detected in the combustion chamber. They are interesting in order to study the chemical kinetics of low temperature combustion process. The chemiluminescence spectra of HCCI combustion appeared as well as several distinct peaks corresponding to the emission from HCO, HCHO, CH, and OH. In particular, this latter was clearly evident during the whole premixed combustion and dominated the process also after the end of the premixed phase of the heat release. Advancing the combustion, bright spots due to not homogeneous charge were detected. They were the source of the very little soot amount detected at the exhaust pipe. Finally, the injection pressure effect on the development of low temperature combustion was analysed.

Particle emissions from engines have long been topic of research studies because of their detrimental impact on human health and environment. Light duty vehicles represent the main contributor of particles in the congested urban areas and in the next future thought there will be a drastic reduction of fossil fuels, an increasing of the electric vehicles and an improvement of battery technology, a good control of exhaust particle emissions must be yet guaranteed using accurate and effective measurements for their characterization. This paper is an excursus of commercial and innovative techniques used for the evaluation of the particle emissions due to the engine exhaust. The effect of the main parameters on the soot formation and particle emissions were analyzed and described such as the influence of the injection strategies, the fuel type including commercial and not commercial fuels, and the lubricant oil. The particulate matter was characterized at the exhaust by means of the measure of number and size. Moreover, non-conventional diagnostics based on the optical detection of the natural flame chemiluminescence was applied to follow the combustion evolution and to evaluate the in-cylinder soot concentration. The correlation between the formation mechanisms of soot in the combustion chamber and the particle emissions at exhaust allows a comprehensive analysis. Particular attention was payed to the particles smaller than 23 nm that will be object of the next European emission regulation. The difficulties correlated to their measurement caused by the large fraction of volatiles in the 10 - 23 nm range that can nucleate or condense on existing particles was highlighted. In this regard a methodological approach consisting in the sampling and conditioning of the particles at two different temperature settings was defined within the project Sureal-23 to evaluate the volatile organic fraction of these particles. The knowledge developed within the exhaust particle emissions allowed to develop an experimental layout and a methodology for the sampling and measurement of non-exhaust particles.

The increment of battery temperature during the operation caused by internal heat generation is one of the main issues to face in the management of storage systems for automotive and power generation applications. The temperature strongly affects the battery efficiency, granting the best performance in a limited range. The investigation and testing of materials for the improvement of heat dissipation are crucial for modern battery systems that must provide high power and energy density. This study presents an analysis of the thermal behavior of a lithium-polymer cell, which can be stacked in a battery pack for electric vehicles. The cell is sheltered with layers of two different materials: carbon and graphene, used in turn, to dissipate the heat generated during the operation in natural convection. Optical diagnostics in the infrared band is used to evaluate the battery surface temperature and the effect of the coatings. Experiments are performed in two operating conditions varying the current demand. Moreover, two theoretical correlations are used to estimate the thermal parameters of the battery with a reverse-logic approach. The convective heat transfer coefficient h and the specific heat capacity cp of the battery are evaluated and provided for the Li-ion battery under investigation for different coatings’ conductivity. The results highlight the advantage of using a coating and the effect of the coating properties to reduce the battery temperature under operation. In particular, graphene is preferable because it provides the lowest battery temperature in the most intense operating condition.