Computational Fluid-Dynamics Analysis of the Effect of Ethanol-Air Mixture Supply in a Heavy-Duty Engine

As part of the Bio-FiRE-for-EVer research project aiming to propose a solution for off-grid charging stations based on the adoption of a reciprocating engine, this study investigated the combustion development and pollutant emissions of an 8.7 l six-cylinder heavy-duty PFI internal combustion engine fueled by ethanol. The reference experimental case features critical issues in the formation of the air-fuel, mainly due to the slow evaporation rate of the alcohol fuel inside the intake manifold via a single point injection, providing a non-uniform and averagely rich (λ=0.89) reactant mixture inside the cylinders. For this purpose, an in-depth analysis of the in-cylinder phenomena is performed by using a CFD solver for the reacting flow. A geometry of the cylinder system complete with intake and exhaust ducts is created for calculations with the three-dimensional Ansys FORTE code. The inclusion of the inlet duct in the computational domain allows the experiencing of several setups of the mixture. Indeed, due to the uncertainties on the complete vaporization of ethanol, experimental data allowed a preliminary validation of the CFD based predictions by considering the presence of liquid fuel fraction (30%) in the inlet duct. After the model calibration, firstly, a more favorable air-fuel ratio condition of λ=1 is examined and then, two alternative solutions are proposed to optimize the engine performance via a multipoint injection upstream of the intake valves but still considering a rich mixture. Based on the results it is demonstrated that the presence of liquid represents a more realistic condition achieving outputs closer to the experimental measurements. The adjustment of air-fuel ratio to a stoichiometric value by only enhancing the amount of air leads to significant improvements in terms of mechanical outputs and CO emissions. Besides, an optimized injection setup can overcome the maldistribution of fuel among cylinders, its incomplete oxidation and reduce the percentage of fuel that remains liquid forming a film on the duct’s wall.