Experimental Study of Combustion and Emissions Characteristics of Methyl Oleate, as a Surrogate for Biodiesel, in a Direct Injection Diesel Engine

This study evaluates the combustion and emissions characteristics of methyl oleate (C₁₉H₃₆O₂ CAS# 112-62) produced by transesterification from oleic acid, one of the main fatty acid components of biodiesel. The ignition delay of ultra-low sulfur diesel#2 (ULSD) and its blends with methyl oleate (O20-O50), varied between 6.5-9.7 CAD, depending on speed, at constant load of 8 bar imep (100% load). The CN was 47 for ULSD and increased up to 51 for O50, which resulted in the start of combustion's premixed phase being advanced by about 2 CAD while reducing the maximum apparent heat release of about 30%. The combustion duration varied in the range of about 56-67 CAD and the maximum total heat flux rate, presented values from 4.2 to 5.5 MW/m₂, which correlate well with the increase of the convection flux because of the speed increase. The maximum cycle temperature was in the range of 2500K for the speeds from 1200 to 1800 rpm for both fuels. The mechanical efficiency decreased from 83% to 77% by increasing the speed from 1200 to 1800 rpm, while the thermal efficiency was around 33.5% for methyl oleate blends and reduced by 0.5% compared with ULSD. NOx displayed a maximum reduction of 20% for O50, and the unburned hydrocarbons (UHC) remained constant irrespective of speed or methyl oleate content. Soot emissions show low values for all fuels and maintain almost constant for all methyl oleate concentration, with a slight increase for O50 at 1200 rpm. The results suggest that methyl oleate has favorable combustion characteristics and is similar with that of ULSD, further supporting the investigations of methyl oleate as a surrogate for biodiesel.