• 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.

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.

<div class="section abstract"><div class="htmlview paragraph">Although in the latest years the use of compression ignition engines has been a thread of discussion in the automotive field, it is possible to affirm that it still will be a fundamental producer of mechanical power in other sectors, such as naval and off-road applications. However, the necessity of reducing emissions requires to keep on studying new solutions for this kind of engine. Dual fuel combustion concept with methane has demonstrated to be effective in preserving the performance of the original engine and reducing soot, but issues related to the low flame speed forced researcher to find an alternative fuel at low impact of CO₂. Hydrogen, thanks to its chemical and physical properties, can be a perfect candidate to ensure a good level of combustion efficiency; however, this is possible only with a proper management of the in-cylinder mixture ignition by means of a pilot injection, preventing uncontrolled autoignition events as well. Moreover, an effective injection strategy can be beneficial for a further reduction of carbonous pollutants from the diesel fuel pilot. Therefore, this work is aimed to numerically analyze the sensitivity of the combustion development in a diesel engine converted to operate in dual fuel mode, where hydrogen is injected in the intake manifold and diesel pilot is directly injected in the cylinder. Starting from a test case at a constant engine speed of 2000 rpm experimentally validated, numerical simulations are carried out with the software ANSYS Forte, using a Turbulence-Kinetics interaction model and the Autoinduced Ignition Flame Propagation model for diesel and hydrogen, respectively. Lookup tables were specifically implemented for the evaluation of the laminar flame speed through H₂/air mixtures.</div></div>

Dual fuel engines induce benefits in terms of pollutant emissions of PM and NOx together with carbon dioxide reduction and being powered by natural gas (mainly methane) characterized by a low C/H ratio. Therefore, using natural gas (NG) in diesel engines can be a viable solution to reevaluate this type of engine and to prevent its disappearance from the automotive market, as it is a well-established technology in both energy and transportation fields. It is characterized by high performance and reliability. Nevertheless, further improvements are needed in terms of the optimization of combustion development, a more efficient oxidation, and a more efficient exploitation of gaseous fuel energy. To this aim, in this work, a CFD numerical methodology is described to simulate the processes that characterize combustion in a light-duty diesel engine in dual fuel mode by analyzing the effects of the changes in engine speed on the interaction between fluid-dynamics and chemistry as well as when the diesel/natural gas ratio changes at constant injected diesel amount. With the aid of experimental data obtained at the engine test bench on an optically accessible research engine, models of a 3D code, i.e., KIVA-3V, were validated. The ability to view images of OH distribution inside the cylinder allowed us to better model the complex combustion phenomenon of two fuels with very different burning characteristics. The numerical results also defined the importance of this free radical that characterizes the areas with the greatest combustion activity.

The knowledge of piston temperature during internal combustion engine operation represents a precious information to evaluate heat losses and engine efficiency. Experimental measurements of piston temperature during engine functioning is very challenging; hence, modeling this process can be very helpful. In the present work, temperature measurements have been collected using a research compression ignition engine, both in motored and fired mode. They have been used to set-up a 1d model of heat transfer through the piston optical window. A good agreement has been obtained. Moreover, the model can provide information not available from experiments.

Homogeneous Charge Compression Ignition (HCCI) combustion was applied to a transparent diesel engine equipped with high pressure Common Rail (CR) injection system. By means of CR system the quantity of fuel was split into five injections per cycle. Combined measurements, based on digital imaging and spectroscopic techniques, were applied to follow the evolution of HCCI combustion process with high temporal and spatial resolution. Digital imaging allowed to analyse injection and combustion phases. Broadband ultraviolet - visible extinction spectroscopy (BUVES) and flame emission measurements were carried out to evaluate the presence of radicals and species such as HCO, OH, CH, and CO. In particular, BUVES measurements were performed to follow fuel oxidation, and pollutant formation and oxidation. During injection and cool combustion, bands of aromatic compounds and alkyl peroxides, indicating fuel decomposition, and hydrogen peroxides were detected. Autoignition was characterized by the presence of OH radical which was homogenously distributed in the chamber and provided for the reduction of particulate matter in the cylinder. Finally, OH could be considered as a marker of HCCI combustion and it could result a suitable tool to identify the different phases of combustion. The effect of different injection pressures was analysed.

The premixed charge compression ignition (PCCI) combustion represents a possible solution for decreasing the pollution with respect to diesel engine, while maintaining the efficiency rate at values that are comparable, and in some cases higher, than those of a diesel engine. This paper investigates the operation of an optical-access compression ignition engine (bore: 82 mm, stroke: 90 mm) running at PCCI combustion with neat bio-ethanol and European commercial diesel fuel injected in the intake manifold and into the cylinder, respectively. In its original configuration, the engine burned diesel and this case was used as reference of compression ignition combustion. Then, different amounts of bio-ethanol were injected varying the energizing time of the injector set in the intake manifold. This allowed to create PCCI combustion with high levels of pre-combustion mixing, and to ensure low equivalence ratio and low flame temperatures too. Moreover, both the amount and the start of diesel injection was varied to investigate their effects on the several combustion phases. UV-Visible imaging and spectroscopic measurements were performed in the engine and the autoignition of the charge, the combustion process and the chemical species involved were detected and analysed. In particular, optical diagnostics allowed to observe how the mixture burned: no luminous flame emission in the visible range was recorded; while in the ultraviolet wavelength range numerous species, like HCO, HCOH, OH, and CO and others were detected. Varying the in-cylinder premixed ratio the combustion retarded and the rate of heat release passed from single to three-phase premixed combustion, so revealing a phase due to intermediate temperature reactions. The same behavior was observed varying the start of injection and the quantity of diesel that affected the premixed ratio. Spectroscopic measurements revealed that during the intermediate temperature heat release large amount of OH radical governed the start of combustion of the charge. It was also observed that the preignition combustion was mainly due to the stratified mixing of the diesel fuel close to the bowl wall. Finally the presence of OH radical was monitored for the whole combustion process