energies Article CFD Study and Experimental Validation of a Dual Fuel Engine: Effect of Engine Speed Roberta De Robbio 1 , Maria Cristina Cameretti 2, *, Ezio Mancaruso 1 , Raffaele Tuccillo 2 and Bianca Maria Vaglieco 1   Citation: De Robbio, R.; Cameretti, M.C.; Mancaruso, E.; Tuccillo, R.; Vaglieco, B.M. CFD Study and Experimental Validation of a Dual Fuel Engine: Effect of Engine Speed. Energies 2021, 14, 4307. https:// doi.org/10.3390/en14144307 Academic Editor: Jamie W.G. Turner Received: 19 June 2021 Accepted: 15 July 2021 Published: 17 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Istituto di Scienze e Tecnologie per l’Energia e la Mobilità Sostenibili (CNR), 80125 Napoli, Italy; roberta.derobbio@stems.cnr.it (R.D.R.); ezio.mancaruso@stems.cnr.it (E.M.); biancamaria.vaglieco@stems.cnr.it (B.M.V.) 2 Department of Industrial Engineering (D.I.I.), Università di Napoli Federico II, 80125 Napoli, Italy; raffaele.tuccillo@unina.it * Correspondence: camerett@unina.it Abstract: Dual fuel engines induce benefits in terms of pollutant emissions of PM and NO x 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. Keywords: dual fuel; CFD analysis; combustion; ultraviolet visible spectroscopy 1. Introduction The diesel engine has always played a central role in the energy and transportation sectors. However, in recent years, a number of limitations related to exhaust emissions have made it the subject of debate. Indeed, diesel engine reliability and low consumption have led the academic and industrial world to offer alternatives with new fuel blends as well as innovative injection and combustion techniques to comply with current emission regulations. The dual fuel diesel engine still represents a valid alternative to the traditional engine, especially for the reduction of carbon dioxide. Nonetheless, a typical drawback of this new strategy is the employment of a fuel for which the engine setup is not optimized. Therefore, a deep investigation is needed to outline the processes that characterize the conversion of the chemical energy of a fuel source into mechanical work. In this work, the authors, who have already been involved in the study of dual fuel for several years [1–8], present a computational fluid dynamics (CFD) application to investigate the phenomenon of combustion of the two fuels used in dual fuel engines (diesel and natural gas/methane). In a dual fuel engine, diesel fuel is injected in minimal quantities to facilitate the ignition of the mixture in the cylinder consisting of air and natural gas that have been premixed in the intake duct. Energies 2021, 14, 4307. https://doi.org/10.3390/en14144307 https://www.mdpi.com/journal/energies