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The phenomenon of self-ignition of fuels mixtures is well known, yet its underlying physics is not fully understood even today. Most known fuels exhibit three distinct ignition limits (also known as explosion limits), which can be described on a pressure-temperature diagram. While the different limits were first observed from experiments, many attempts have been made over the years to understand and characterize them. The upper (or third) explosion limit has been qualitatively explained by Semonov thermal ignition theory, with a more quantitatively expressions found in Frank-Kamenestkii ignition limits. Both thermal theories treat the ignition problem from a macroscopic point of view, using heat transfer considerations with a simplified, single step chemical reaction. The intermediate and lower (or second and third) ignition limits have been treated to date by the chemical kinetics approach. Numerous models, incorporating detailed chemical kinetics mechanisms, coupled with the laws of conservation of mass and energy, have been proposed and developed over the years. While both the thermal ignition theories and some of the chemical kinetics models are able to predict the ignition limits with varying degrees of accuracy, the fundamental understanding of the physical mechanisms that govern the phenomenon is still lacking. The development of a universal ignition theory, coupled with better understanding of its physics, has many applications, such as in the design of engines or in the safety of fuel storage and transportation.
The purpose of our research is to study and understand the phenomenon of self-ignition, focusing mainly on the different ignition limits and the reason for their existence. Contrary to both the thermal ignition theories and the chemical kinetics models, our research is aimed at investigating the self-ignition phenomenon at the molecular level. Due to the statistical nature of self-ignition, as observed from the wide scatter of the experimental results, we propose the use of statistical thermodynamics and fluctuation theory in order to develop a universal ignition theory. Our plan is to evaluate and develop a new set of criteria, based on fluctuations in thermodynamic properties of the fuel mixture. We postulate that near the ignition limits, fluctuations in certain properties may be the keystone in the initiation of the self-ignition process. In parallel, we plan to employ the use of the Le Chatelier rule on the three distinct explosion limits, in order to develop a single analytical expression that can correlate the different explosion limits. Based on an extensive literature review and observations made from comparison of experimental results and the current models, we hope that by using our novel approach we will be able to gain deeper understanding of the fundamental physics of self-ignition.