Pilla, G., Kumar, R., Laget, O., De Francqueville, L. et al., "Simulation and Optical Diagnostics to Characterize Low Octane Number Dual Fuel Strategies: a Step Towards the Octane on Demand Engine," SAE Int. J. Fuels Lubr. 9(3):443-459, 2016, doi:10.4271/2016-01-2164.
Reduction of CO2 emissions is becoming one of the great challenges for future gasoline engines. Downsizing is one of the most promising strategies to achieve this reduction, though it facilitates occurrence of knocking. Therefore, downsizing has to be associated with knock limiting technologies. The aim of the current research program is to adapt the fuel Research-Octane-Number (RON) injected in the combustion chamber to prevent knock occurrence and keep combustion phasing at optimum. This is achieved by a dual fuel injection strategy, involving a low-RON naphtha-based fuel (Naphtha, RON 71) and a high-RON octane booster (Ethanol, RON107). The ratio of fuel quantity on each injector is adapted to fit the RON requirement as a function of engine operating conditions. Hence, it becomes crucial to understand and predict the mixture preparation, to quantify its spatial and cycle-to-cycle variations and to apprehend the consequences on combustion behavior - knock especially. Therefore, experimental and numerical work were conducted on different dual-fuel strategies involving Port Fuel Injection, Lateral Direct Injection and Central Direct Injection. Tests were conducted on an optical single-cylinder engine to characterize mixture preparation and combustion behavior. A semi-quantified 2-color Laser Induced Fluorescence technique was used to evaluate each fuel concentration in the chamber during the mixing process, and high-speed color imaging was performed for visualizing ignition and soot phenomena. 3D RANS simulations with ECFM combustion model were carried out showing good agreement with experimental results. Aside this validation aspect, the computations serve as numerical diagnostics to study spray to aerodynamics interactions regarding the different injection strategies and the subsequent effects on mixture formation and on the combustion. Thus, little interaction among sprays is observed, leading to a fuel distribution in the chamber driven by injectors independently. Also, regarding the operating and the inherent octane requirement, it is shown how the dual injection strategy can be adapted to maximize the anti-knock capacity of the overall fuel mixture.