Ignition Characteristics of Diesel and Canola Biodiesel Sprays in the Low-Temperature Combustion Regime

The objective of this work is to investigate the spray ignition characteristics of diesel and canola-derived biodiesel in a rapid compression machine at the low temperatures (676 816 K) and reduced oxygen concentrations (12 and 18%) that are used in low-temperature combustion strategies of advanced diesel engines. A method for testing ignition delay times and apparent heat release rates at a series of temperatures is developed and characterized, whereby a given temperature is attained prior to the fuel spray by altering the charge cooling time after the end of compression. A single-zone heat release model is formulated and used with the experimental pressure data to calculate two unique ignition delay periods. For diesel, the total ignition delays measured with this approach are found to correlate very well with diesel ignition delay data published in the literature. When the first ignition delay period is compared, which approximates the initial time of heat release after the start of injection, it is noted that biodiesel ignites marginally faster (<1 ms) than diesel. However, the total ignition delay period, which measures the time to reach the maximum heat release rate after the start of injection, shows that canola-derived biodiesel ignites 23% faster than diesel under the tested conditions. The total ignition delay data also indicates decreased temperature sensitivity at increasing test temperature, which may be evidence of negative temperature coefficient behavior. The decrease in temperature sensitivity occurs at a lower temperature for diesel (∼740 K) than for canola-derived biodiesel (∼770 K). The sensitivities of the ignition delay times and the apparent heat release rates to changes in oxygen and fuel concentrations are also reported. It is noted that, when the maximum apparent heat release rates are normalized by the total ignition delay and on an input energy basis, the biodiesel and diesel fuels are remarkably similar for the tests using 12% oxygen but are distinguishable for tests using 18% oxygen at reaction zone temperatures in excess of 700 K.