| With foreseeable depletion of conventional crude oil, alternative oil sources, e.g. oil sands, oil shale, and coal liquids, will be increasingly used for transportation fuels production. Cyclic hydrocarbons will consequently play a more significant role in combustion devices. However, understanding of the oxidation of cyclic hydrocarbons lags behind the transition and few cyclic compounds have been studied in depth. In this work, five cyclic hydrocarbons, methylcyclopentane (MCP), cyclohexane (CH), methylcyclohexane (MCH), 1,2,3,4-tetrahydronaphthalene (tetralin), and decahydronaphthalene (decalin), are studied in a motored CFR octane rating engine with controlled extent of oxidation. The focus is on low temperature oxidation and autoignition behavior which is critical to combustion processes in internal combustion engines. Reactions are conducted at constant engine speed (600 rpm), intake pressure (∼1 bar), and equivalence ratio (0.25). Compression ratio and engine intake temperature are adjusted to control the reaction extent.;By analyzing stable reaction intermediates from engine exhaust gases, global reactivity and oxidation chemistry are revealed. Molecular structure exerts a profound effect on low temperature oxidation reactivity. Decalin is the most reactive compound whose oxidation increases monotonically with increased temperature and pressure. MCH is more reactive than CH, but both show clear negative temperature coefficient behavior in that the fuel conversion decreases with increased temperature and pressure. MCP and tetralin exhibit little low temperature reaction before critical conditions for autoignition are reached. Octane numbers of these compounds correlate with the trend of low temperature reactivity but do not correlate with the trend of critical compression ratios or engine intake temperatures. Increasing reaction temperature inhibits low temperature reactivity and promotes NTC behavior. Except for decalin, low temperature oxidation of cyclic hydrocarbons releases little chemical energy and results in negligible change in engine cylinder pressure and temperature. Decalin is the only compound showing two-stage ignition.;Qualitative conformational analysis is conducted to understand how the cyclic structure affects the reactivity in low temperature oxidation. It is revealed that the hydrogen distribution and rigidity of the chair form cyclohexane ring significantly limited (1,5) H-shift during isomerization of fuel peroxy radicals (ROO·→·QOOH), the key step in low temperature chain branching. Such H-shift can only proceed via an axial peroxy group abstracting an axial hydrogen. Also important is the degeneracy of (1,5) isomerization of fuel peroxy radicals, i.e. the number of hydrogens available to (1,5) H-shift for a given peroxy group. The number of axial hydrogens in decalin, MCH, CH, and tetralin is 10, 6, 6, and 4, respectively. Decalin has also four equatorial hydrogens available for (1,5) H-shift. Degeneracy in (1,5) H-shift for decalin, MCH, CH, and tetralin is 3∼4, 2∼3, 2, and 1, respectively. These numbers explain the low temperature reactivity of these compounds observed in the experiment. The higher reactivity of MCH than CH is due to the equatorial preference of the methyl group which forces the peroxy group to stay at an axial position and facilitates (1,5) H-shift. These qualitative conclusions are supported by ab initio quantum mechanical calculations being performed outside this work.;Detailed products analyses reveal that ring dehydrogenation to form conjugate olefins---the olefins of the same molecular structure as the fuel compound but with one C=C bond---is a common and major reaction channel among cyclic hydrocarbons in pre-ignition reactions. The amount of conjugate olefins formed correlates with reactivity in low temperature oxidation: less reactive compounds produce more olefins. This is the direct result of the cyclic steric structure. The cyclic structure limits the (1,5) isomerization channel for peroxy radicals which alternatively abstract hydrogens on adjacent carbons via (1,4) H-shift to form conjugate olefins. It is also noted that CH and decalin produce similar amounts of propene relative to ethene, while the methyl substitution in MCP and MCH promotes the production of propene relative to ethene. In these experiments, over 84% of fuel carbon is recovered in the products of MCP, CH and MCH, while carbon balance is not obtained with tetralin and decalin due to the high boiling point. Nevertheless, major oxidation products, in particular, those of similar structure to the fuel molecule, are identified for all the compounds. Primary oxidation mechanisms are constructed adopting the new understanding from conformational analysis. For MCP and MCH, yields of conjugate olefin isomers are exploited to derive the percentage of fuel peroxy radicals converted into these olefins. |
