Spray ignition experiments for biodiesel surrogates

Abstract

This report documents the experimental and numerical work conducted in determining the combustion characteristics of C9 and C10 esters as surrogates for modern biodiesel fuels (BDFs). A novel instrumented constant volume combustion chamber (CVCC) is used to collect data for spray combustion of the fuels. Four distinct straight chain esters are tested: methyl decanoate, ethyl nonanoate, methyl 9-decenoate, and methyl 5-decenoate. Fuel is injected into the CVCC containing air at ambient pressures of 21.4 bar and 40 bar and temperatures ranging from 623 K to 818 K. The injector fuel pressure and chamber pressure are measured during injection, ignition, and combustion. These signals are used to determine the time of injector opening and closing, as well as completion of atomization, onset of combustion (ignition), and energy release history during combustion.

Chamber pressure traces are compared for each fuel across the temperature spectrum showing two-stage combustion processes with differences in the duration and energy release in each stage with fuel and condition variation. Examination of ignition delays shows that the unsaturated esters are less reactive than the saturated esters, with methyl 5-decenoate showing the lowest reactivity. Ethyl nonanoate exhibits a faster onset of oxidation when compared to methyl decanoate, but the oxidation process is slower. Existing literature kinetic mechanisms are compared to experimental results showing that while the models exhibit two-stage ignition processes, they may not accurately describe the order of reactivities for methyl decanoate, methyl 5-decenoate, and methyl 9-decenoate. Further experimental work should be completed in both the CVCC and the Ignition Quality Tester (IQT) for correlation of results from the two instruments for determination of derived cetane number (DCN) from the CVCC. Revisions to existing kinetic models should also be made to reflect the reactivity trends seen in experimental studies.