Auto-ignition behavior of methyl decanoate

The auto-ignition behaviors of n-heptane, a diesel-relevant compound, and methyl decanoate (capric acid methyl ester), a biodiesel-relevant compound, were studied. A premixed charge of fuel and air at an equivalence ratio of 0.25 was delivered to the engine, and the extent of auto-ignition was increased by increasing the compression ratio. The cool-flame heat release accounted for 16% of the total for n-heptane, and 13% for methyl decanoate. For both fuels, the cool-flame produced high concentrations of aldehydes and ketones, as well as CO. No CO₂ was detected for n-heptane until the onset of the main combustion event, but methyl decanoate produced significant amounts of CO₂ before the onset of second stage ignition. Formation of CO₂ early in the ignition process was due to unimolecular decomposition of the ester molecules. This direct CO₂ was inefficient from a soot suppression standpoint relative to oxygenated fuels that only form CO directly from the parent fuel. Because of its structure, biodiesel might not be as effective as reducing particulate emissions as other oxygenated fuels, such as ether molecules, where only one oxygen atom is bonded to any given carbon atom. This is an abstract of a paper presented at the 230th ACS National Meeting (Washington, DC 8/28/2005-9/1/2005).