Chemical Kinetics of Hydrocarbon Ignition in Practical Combustion Systems

Chemical kinetic factors of hydrocarbon oxidation are examined in a variety of ignition problems. Ignition is related to the presence of a dominant chain branching reaction mechanism that can drive a chemical system to completion in a very short period of time. Ignitio n in laboratory environments is studied for problems including shock tubes and rapid compression machines. Modeling of the laboratory systems are used to develop kinetic models that can be Used to analyze ignition in practical systems. Two major chain branching regimes are identified, one consisting of high temperature ignition with a chain branching reaction mechanism based on the reaction between atomic hydrogen with molecular oxygen, and the second based on an intermediate temperature thermal decomposition of hydrogen peroxide. Kinetic models are then used to describe ignition in practical cOmbustion environments, including detonations and pulse combustors for high temperature ignition, and engine knock and diesel ignition for intermediate temperature ignition. The final example of ignition in a practical environment is homogeneous charge, compression ignition (HCCI) which is shown to be a problem dominated by the kinetics intermediate temperature hydrocarbon ignition. Model results show why high hydrocarbon and CO emissions are inevitable in HCCI combustion. The conclusion of this study is that the kinetics of hydrocarbon ignition are actually quite simple, since only one or two elementary reactions are dominant. However, there are many combustion factors that can influence these two major reactions, and these are the features that vary from one practical system to another. ........................... z__ " .... 2_ 2_ Introduction In many practical steady combustion systems, ignition is simply a means of starting the system on its way to steady state: Performance and emissions are essentially independent of ignition in such systems as boilers, furnaces and burners. However, in other practical problems, ignition has a great influence on performance, emissions and other characteristics, and ignition can explain the performance of the entire system. Ignition can depend on physical, chemical, and mixing and transport features of a problem, and in some cases on heterogeneous phenomena. Excellent reviews of ignition can be found in current sources [1-4], describing thermal feedback, chemical kinetic chain branching reactions, and other elements. However, ignitionin general is an enormous subject, and the present work cannot provide a thorough treatment. This paper focuses on chemical kinetic factors in practical systems, with special attention on ignition in automotive engines. Recent advances in experimental and kinetic modeling capabilities have provided new insights into ignition, offering new possibilities for control strategies and new classes of combustors. General Features of Ignition Many interesting systems involve chain reactions, such as nuclear reactors, with neutrons as chain Carriers. Chain termination is provided by neutron absorbers, and chain branching is associated with the term "supercritical’, normally a condition to be avoided. Populations of living organisms obey the same reactivity laws. Organisms can grow exponentially via