In the search for more environmentally friendly energy sources, biofuels have emerged as an attractive alternative to traditional petroleum fuels. In addition to being close to carbon-neutral and renewable, biofuels have similar properties to traditional hydrocarbon fuels, allowing them to be implemented in existing combustion engines with few changes to either the engines or supporting infrastructure. The fundamental ignition properties of petroleum fuels are reasonably understood. Although the engine properties such as ignition delay and pollutant emissions have been studied for biofuels, their fundamental ignition properties of these biofuels are still unknown. Studies of the fundamental ignition properties of biofuels are important for the safety and handling of these fuels. The objective of this study was to compare the fundamental hot surface ignition properties of biofuels relative to petroleum fuels. Properties included in this study are: ignition energy, time interval for ignition, ignition surface temperature, and flame front velocities. The fuels studied were Jet A, as the petroleum fuel, and canola methyl ester (CME), soy methyl ester (SME), and palm methyl ester (PME), as the biofuels/biodiesels. Equivalence ratios of 0.75 through 2.00 were examined for each fuel. The fuels were studied as pre-vaporized mixtures in stagnant, constant pressure and constant volume conditions. The combustion chamber was approximately 1.56 L in size with a 5 cm by 23 cm window on one side and heated walls to prevent fuel condensation. A commercially available silicon carbide dryer ignitor was used as an ignition source and was located in the center of the combustion chamber. A high speed camera recorded the propagation of the flame following ignition, allowing for the calculation of the flame front velocity. K-type thermocouples measured the temperature of the mixture at selected points inside the combustion chamber. A current transformer, shunt resistor, and voltage meter were used to calculate the current and power supplied to the ignitor, which could then be used to calculate the ignitor temperature and ignition energy. The setup can be used to study the relative differences in ignition properties of pre-vaporized fuel/air mixtures.

Both Jet A and the biofuel flames were blue in color across all equivalence ratios. Equivalence ratios near 1.3 produced the brightest flames, while equivalence ratios near 0.5 and 2.0 produced dim flames which propagated slowly. The ignitor temperature increased linearly at a rate of 110 K/s. Ignition temperature of the fuel/air mixture was determined to be nearly constant at 630°C for all examined equivalence ratios and fuels. Ignition energy was found to be six orders of magnitude greater than that in spark ignition energies due to thermal energy diffusion, aided by natural convection effects and a larger volume of mixture to heat near the ignitor. The ignition energies and time intervals of ignition of the biodiesel fuels were comparable to that of Jet A, and decreased with increased equivalence ratios. Flame velocities peaked near an equivalence ratio of 1.3 for both Jet A and the biofuels. The flame velocities for CME, SME, and PME were only 70-83% of those of Jet A, in agreement with results found in literature.