Theoretical Study Concerning the Effect of Oxygenated Fuels on DI Diesel Engine Performance and Emissions 2004-01-1838
Diesel engine manufacturers have succeeded in developing engines with high power concentration and thermal efficiency without disregarding to comply with the continuous stringent emission regulations. Nowadays, several techniques such as injection control strategies, EGR and exhaust after treatment devices have been used to reduce diesel emissions. However, emission control alternatives are often accompanied by fuel consumption or cost penalties and also, the request for improving the pollutant emissions behavior of the existing diesel vehicle fleet has become mandatory. Thus, research scientists and engineers have focused also on the area of fuel composition for the reduction of pollutant emissions. Of major importance seems to be the use of oxygenated additives to reduce particulate emissions. According to recent studies, soot emissions are decreased following the increase of oxygen percentage. But the effect of oxygen content and type of oxygenate on soot formation and oxidation process and NO formation mechanism have not been fully elucidated yet. A question is also raised as whether the presence of fuel-bounded oxygen affects the soot formation or oxidation mechanism. A further question concerns the NO emissions and specifically, up to what extent the presence of oxygen inside the fuel affects its local formation rate and thus, tailpipe emission values. For this reason, in the present study a multi-zone combustion model is used after modifications to examine the effect of oxygenated additive percentage and type on combustion and pollutant formation processes of DI Diesel engines. One conventional and two oxygenated fuels having oxygen content between 0% and 9% are examined. Rapeseed methyl ester, diglyme and butyl-diglyme are mixed at different proportions with a commercial diesel fuel. Representative chemical formulas are derived for the two oxygenates and used to examine the effect of fuel-bounded oxygen on combustion characteristics in the simulation. The effect of oxygenates on soot formation and oxidation process and on NO formation mechanism is also investigated. The predictive ability of the multi-zone model with regard to cylinder pressure history and pollutant emissions is verified against corresponding results obtained from tests conducted in a single cylinder DI Ricardo Hydra diesel engine. Higher peak combustion pressures are experienced for the oxygenated fuels compared to conventional one. An earlier combustion commencement is observed for the oxygenated fuels as compared to the conventional one due to increase of cetane number and earlier shift of dynamic injection timing. The results reveal for the in-cylinder soot history that the fuel-bound oxygen enhances soot oxidation during expansion stroke. High fuel equivalence ratio and soot concentration areas within the fuel jet are confined with increasing oxygen content. It is shown also that the oxygen enrichment of the conventional fuel is not accompanied by a sharp increase of the in-cylinder NO concentration as expected due to decrease of the local temperature as a result of the lower fuel heating value.