Abstract
•Coconut-derived biodiesel is more reactive than canola-derived biodiesel.•The trends in biodiesel reactivity depend on the relative proportion of saturated and unsaturated components.•Biodiesel unsaturated content has a limited effect on the timing of heat release.•The location of the unsaturated bonds in the carbon chain could improve agreement with the computed results for biodiesel heat release.
This work reports results on the low-temperature oxidation of canola and coconut-derived biodiesel in a motored engine. A lean premixed fuel–air mixture of equivalence ratio 0.25 at an intake temperature of 125 °C was subject to reaction with pure compression. The extent of the reaction was controlled by changing the compression ratio in the range of 5.5–9.0. In-cylinder pressure measurements were used for heat release analysis, and detailed speciation analysis of the reaction products was done using a gas chromatography-mass spectrometry system. The canola-derived biodiesel shows minor low-temperature heat release, while coconut biodiesel exhibits a relatively higher heat release. It was found that the low-temperature oxidation chemistry for these biodiesels is similar to saturated single-component methyl esters. However, the heat release was somewhat suppressed due to unsaturated compounds in the two biodiesel fuels. The heat release suppression correlates well with the extent of unsaturation. The oxidation end products under the test conditions were similar for both fuels. The relatively higher reactivity of the coconut over the canola biodiesel was also seen in the speciation results, and its stable intermediate concentrations were much higher than the canola. The experimental results were compared to computations using an existing kinetic model. The onset of heat release was predicted with reasonable accuracy. However, the peak and accumulated heat release were significantly overpredicted.