1. Could you explain the significance of your article to the non-specialist?

Non-traditional hydrocarbon fuels, such as those derived from tar sands or oil shale, are rich in cyclic alkanes. Advanced engine designs that promise high efficiency and low emissions increasingly rely on autoignition chemistry to time the ignition event. Autoignition behavior is strongly dependent on radical plus oxygen kinetics, and these reactions remain poorly understood, especially for radicals derived from cyclic alkanes. This research combines new first-principles kinetic theory and experimental measurements for the reaction of a prototypical cyclic alkyl radical, cyclohexyl, with molecular oxygen. The optimized kinetic model should allow more accurate prediction of the ignition of these fuels. 

 

2. What has motivated you to conduct this work?  

The reactions of hydrocarbon radicals with oxygen have been a subject of study in our groups for years, both because of their technological importance and because of the rich fundamental physical chemistry they exhibit. The reaction of cyclohexyl radical with O₂ was of particular interest because earlier work by Prof. Ray Walker's group in Hull^1,2 showed that the OH-producing channels were unusually important and that ring-opening products were prominent. The present theoretical investigation highlights the complexity of this reaction. Furthermore, the low-temperature chemistry of cyclohexyl radicals with molecular oxygen may initiate the dehydrogenation of cyclohexane to benzene³ that has been observed in low-pressure cyclohexane-oxygen flames.⁴ Finally, the increasing presence of cyclic alkanes in transportation fuels, especially in areas of North America where tar-sands are a significant part of the fuel stream, makes investigation of the ignition chemistry of fuels like cyclohexane especially timely.

 

3. Where do you see this work developing in the future? 

One of the long-standing aims of this work has been to improve the knowledge of the relationship between fuel molecular structure and low-temperature oxidation chemistry, especially for the chemical activation pathways. The fate of the adduct formed in the initial reaction of alkyl radicals with O₂, the RO₂ species, depends in part on internal isomerization pathways to form reactive and less-stable hydroperoxyalkyl radicals, often denoted by QOOH. These ephemeral QOOH radicals are crucial to low-temperature chain branching and future work will increasingly try to focus on them and their reactions with O₂.

 

4. Are there any particular challenges facing future research in this area? 

There are several key challenges ahead for ignition chemistry. The pressure dependences of the critical low-temperature oxidation pathways are not understood in sufficient detail to permit reliable predictive control of ignition timing in advanced engines. The changes in the fuel stream as non-traditional hydrocarbon sources and renewable alternative fuels enter the market will require understanding the fundamental chemistry of a broader range of molecules. These problems are being attacked by many groups worldwide. Finally, the ignition chemistry of pure fuels is already very difficult to investigate and predict over the large range of temperature and pressure required for advanced engine designs. Real fuels are mixtures of hundreds of compounds; the effects of fuel blending on ignition behavior are a formidable challenge for combustion chemistry to model, and for detailed experiments to unravel.