Ethanol effect as premixed fuel in dual-fuel CI engines: experimental and numerical investigations

Dual fuel configurations in compression ignition engines are considered as a promising solution to rely on alternative fuels that allows to respect at the same time the strict regulations on pollutants emissions. Looking at ethanol as one of the future transport biofuels, the authors investigate on the use of ethanol as the premixed fraction in a dual fuel engine. The activity is specifically focused on a light duty engine, since the dual fuel concept application, already implemented in heavy duty vehicles, was recently taken into account for smaller engines in automotive applications too. In the present paper the results of two research activities are reported and compared: tests on an optical single cylinder research engine and numerical simulations, so achieving a combined approach to the study of dual-fuel configuration. Optical diagnostics has been applied in a transparent direct injection diesel engine equipped with the head of an Euro5 commercial engine and the latest generation common rail injection system. Pure ethanol is injected in the intake manifold, while n-heptane is directly injected in the cylinder. UV digital imaging is applied to the optical engine allowing to detect crucial chemical intermediates, like OH, HCO, and CO, in terms of their spatial distribution and temporal evolution. This analysis allows to identify in-cylinder zones with high reactivity of mixed fuels and to investigate the effect of the presence of ethanol on the burning dynamics of the directly injected fuel. Numerical simulations are carried out by means of the OpenFOAM library, that represents the closed valve phase of the engine cycle. Thanks to the adoption of detailed reaction kinetics, the predicted spatial distributions of the selected reaction intermediates are compared with the images provided by optical diagnostics. Moreover, the integrated emission intensity from the chosen chemical species is compared to the corresponding total concentration calculated histories, in order to verify the coherence of the numerical results also in terms of temporal evolution