Abstract
Keywords- Oxygen content, diglyme, diesel, smokeless combustion, nitric oxides
Abstract - Reduction of exhaust emissions is a major research task in diesel engine development in view of increasing concern regarding environmental protection and stringent exhaust gas regulations. Simultaneous reduction of nitrogen oxide emissions and particulate matter is quite difficult due to the soot/NOx trade off and is often accompanied by fuel consumption penalties. Thus, improvement of fuel properties is also essential for the suppression of diesel pollutant emissions along with the optimisation of combustion-related design factors and exhaust after-treatment equipment. The use of Diglyme as an oxygenated additive appears to be promising approach to further reduce particulate emissions. For this reason, it is decided to use a simulation model to study the effect of oxygen percentage on DI diesel engine performance and emissions. The motive for this is the extremely high experimental time and cost required to prepare and examine fuels with various oxygen content percentages. The objective is to provide information concerning the required oxygen percentage in the fuel to attain a desired soot reduction and at the same to examine the penalty, if any, on NO emissions. Before using the simulation it is required to examine its predictive ability as far as the effect of oxygen percentage is concerned on engine performance and emissions. This is achieved by comparing computational results with experimental data obtained from engine tests conducted on a Ricardo Hydra diesel engine using three test fuels with an oxygen content ranging from 0% to 9%. The comparison reveals that the simulation captures the effect of the fuel oxygen fraction on engine performance and pollutants emissions at least qualitatively. The computational investigation is then extended to fuels having as oxygen content up to 36%. The increase of fuel oxygen content resulted to an earlier initiation of combustion and slightly higher peak combustion pressures. This was attributed to the increase of cetane number in the case of addition of Glycol Ethers. The high oxygenation of diesel fuel resulted to a transposition of high local temperature areas from the periphery inside the fuel jet core revealing thus, an enhancement of fuel oxidation rate due to higher local oxygen availability. This fact had a positive effect on local Soot concentrations and a negative one on local NO emissions. An increase of NO emissions with fuel oxygen content was observed for values up to 25%. For higher oxygen contents, a decrease of NO emissions is observed confining thus, the overall deterioration of NO emissions with diesel fuel oxygenation. An abrupt decrease of Soot emissions with fuel oxygen percentage was acknowledged at all engine conditions examined leading to smokeless diesel combustion for high oxygen contents (pure Diglyme). Both NO and Soot emissions correlated well with fuel oxygen content. Thus, the results obtained herein can be used as guidelines for future experimental investigation aiming to the estimation of an O2 content threshold as far as Soot and NO emissions are concerned.