Evaluation of the Ignition of Diesel Fuels on Hot Surfaces

Hot surfaces are known ignition sources for flammable and combustible liquid fuels released from motor vehicles. In automobile fires, this has been termed hot surface ignition (HSI). It can occur on any heated vehicle component including the exhaust manifold and catalytic converter. In this study, infrared (IR) thermography was used to measure surface temperatures and record the evaporation and ignition events for gasoline, diesel and bio-diesel fuels applied as single droplets to two heated surfaces: 409 stainless steel and a stainless steel heat shield. The data were used to determine the temperature range and ignition probability, from no-ignition to ignition, for the various fuel/surface combinations. The critical temperatures and ignition probability were different for the different fuels on the same hot surface. Similarly, the ignition probability curves and temperature ranges for a given fuel were not the same for both material surfaces, indicating a material effect on ignition. The minimum HSI temperature of gasoline on stainless steel was 520°C, with 100% ignition probability at 660°C. The HSI temperatures for the biodiesels, for the most part, were found to be in the same range as those for traditional diesel, with minimum values for ignition near 450°C and 100% ignition probability at 500°C. HSI temperatures for aged 5% soy-based diesel and a 5% tallow-containing diesel did not fall in this range, while a 100% soy-based bio-diesel exhibited a unique and more violent vapor ignition on the heated heat shield surface. On surfaces exposed to repeated ignition tests, the ignition probability curve also changed indicating the importance of the local nature of the surface to the critical ignition temperature. It is evident that characteristics of the hot surfaces, such as composition, geometry, aging, contamination and catalytic effects play a significant role in determining critical temperatures and ignition probabilities for HSI events. The minimum surface temperature and the temperature range resulting in an ignition event are summarized for the different fuels and compared with available auto-ignition temperature data.