Gasoline effects on spray characteristics, mixing and auto-ignition processes in a CI engine under Partially Premixed Combustion conditions

Abstract

Recent research has shown that one of the paths to reduce pollutant emissions in diesel engines is to bring the operating conditions towards those of a gasoline engine, through homogeneous combustion and high octane fuels. Reduced soot, NOx, and fuel consumption are some of the benefits of using gasoline fuel combined with EGR in compression ignition engines in partially premixed combustion modes. Thus, a comparative study using diesel fuel and gasoline has been conducted focusing on the spray characteristics, mixing and autoignition process in these new combustion modes using a conventional diesel injection system. In this work, an experimental study has been carried out comparing the effects of penetration and cone angle under non-evaporating conditions for both fuels in a constant volume test rig. Besides, liquid length measurements and combustion process under partially premixed combustion (PPC) conditions in a single cylinder transparent engine have been done. The analysis of the results for both fuels shows no major differences on penetration and spray cone angle between them. Under vaporizing conditions, diesel spray exhibited a significantly longer liquid length than gasoline, due to the higher volatility of gasoline. In quantitative terms, liquid length was found to be 1.8 to 2.4 times longer for the diesel fuel. 1D spray model calculations confirmed the experimental results. The combustion process evaluation under PPC conditions shows differences in ignition delay and RoHR even under same thermodynamic conditions. Besides, Combustion images indicate differences in the autoignition and combustion process for diesel and gasoline fuels. Gasoline shows lower soot radiation and better combustion phasing. Differences in the mixing process evaluated with the 1D model show the mixing process is governed mainly by the fuel reactivity, the ignition delay and, to a lower extent, by the spray characteristics. The overall equivalence ratio obtained through the 1D model at the start of combustion is leaner for the gasoline case. This results agrees with a longer ignition delay for this fuel, due to lower fuel reactivity, and with the lower light intensity detected in the combustion images, where the leaner the equivalence ratio the lower the intensity.