Dual-fuel combustion fundamentals: Experimental-numerical analysis into a constant-volume vessel

Dual-fuel combustion has shown high potential for the reduction of emissions (especially nitric oxides and particulate matter) keeping almost unchanged fuel conversion efficiency compared with conventional Diesel engines. However, a deep understanding of the phenomena controlling dual-fuel ignition and combustion processes is still needed to further improving engine behavior especially at low load. To this aim, a combined experimental/numerical approach is proposed in this paper, consisting in a detailed experimental test campaign along with a numerical model to represent and then verify the similarities between engine and chamber local thermodynamics conditions. The design and operation of a tailored experimental setup to study the fundamentals of the dual-fuel combustion process at engine-like operating conditions in optically accessible constant volume combustion chamber is a challenging task. In this paper, similar conditions characterizing the engine operation are represented with a first combustion of a lean air-methane mixture. Then, methane is injected into the chamber to mimic low load engine operation condition in terms of overall equivalence ratio. The oxygen left from the first combustion supports the oxidation of the post-injected methane whose ignition is triggered by a diesel pilot injection. Special care is addressed in characterizing heat and mass losses as well as the mass of methane introduced. During experiments, chamber pressure is measured and thus the evolution of the combustion process is characterized. Numerical simulations, carried out by means of the CONVERGE CFD code, are used to check the charge distribution inside the chamber, and evaluate the local thermodynamic conditions after the gas exchange process. A comparison between the experimental and numerical pressure trace profiles has been performed to validate the numerical model. Results obtained confirm the validity of the proposed approach highlighting the need for a careful calibration of the injection parameters to achieve the target conditions close to the spray injection location.