Behavior of a fuel oil during the combustion cycle of a direct injection diesel engine

Abstract

In-cylinder sampling and analysis of particulate (soot and condensed hydrocarbon species), light hydrocarbons, and gaseous inorganic species has been performed at two positions of a single cylinder direct injection diesel engine by means of a fast sampling valve, in order to follow the behavior of a diesel oil during the engine cycle. It has been shown that the combustion process in a diesel engine proceeds through a preignition phase of heating and vaporization of the injected fuel, which causes the transformation of the fuel components in light gaseous products and oxygenated compounds that rapidly disappear at the ignition. After ignition, the formation of soot has been found, particularly in the high-temperature position, but the formation of soot precursors, i.e., acetylene and combustion-formed polycyclic aromatic hydrocarbons (PAHs), was not in evidence. In whichever phase of the combustion cycle the PAHs are composed of unburned fuel PAHs but in the low-temperature position the PAH percentage increases, as opposed to the high-temperature position, and this can be due to the high resistance of the fuel PAHs to complete oxidation in this temperature condition. At the same time the appearance of high-molecular-weight material of oxygenated character has been found at the low-temperature position. This could derive from the oxidation, perhaps in liquid phase, of the heavy aromatic compounds of the fuel. The chemical analysis of the hydrocarbons collected in the oxidation phase and in the exhaust revealed that unburned fuel, reentraining the sampling zones from wall quenching and ring crevices, is the main source of hydrocarbon emission from the engine.