William R. Leppard

The autoignition chemistry of paraffinic fuels and pro-knock and anti-knock additives : a detailed chemical kinetic study

A numerical model is used to examine the chemical kinetic processes which lead to knocking in spark-ignition internal combustion engines. The construction and validation of the model is described in detail, including the low temperature reaction paths involving alkylperoxy radical isomerization. The numerical model is then applied to C{sub 1} to C{sub 7} paraffinic hydrocarbon fuels, and a correlation is developed between the Research Octane Number (RON) and the computed time of ignition for each fuel. Octane number is shown to depend on the rates of OH radical production through isomerization reactions, and factors influencing the rate of isomerization such as fuel molecule size and structure are interpreted in terms of the kinetic model. The knock behavior of fuel mixtures is examined, and the manner in which pro-knock and anti-knock additives influence ignition is studied numerically. The kinetics of methyl tert-butyl ether (MTBE) is discussed in particular detail. 28 refs., 5 figs., 5 tabs.

A Detailed Chemical Kinetics Simulation of Engine Knock

Abstract Abstract–Engine knock was simulated for a stoichiometric ethane-air mixture in which the ethane oxidized according to the detailed chemical kinetics mechanism of Westbrook, Dryer, an co-workers. The simulation, using experimental temperature and pressure histories to drive the reaction kinetics, predicted times of knock occurrence that agreed to within two crank-angle degrees of available experimental engine data. In add ition, general comparisons of the species involved in the preknock reactions with limited experimental data available in the literature for other fuels showed consistent trends between prediction and experiment. The important species, including radicals, and reactions that lead to knock with ethane as the fuel are discussed.

A Comparison of Olefin and Paraffin Autoignition chemistries: A Motored-Engine Study

The autoignition chemistries of the olefins 1-butene, 2-butene, isobutene, 2-methyl-2-butene, and 1-hexene and their corresponding paraffins were examined in a motored, single-cylinder engine by measuring stable intermediate species and performing heat-release analyses. The same engine conditions were used for each olefin-paraffin pair, and compression ratio was varied to affect different levels of chemical activity. Experimental measurements for each olefin-paraffin pair are compared with each other and with literature values

Autoignition chemistry in a motored engine: An experimental and kinetic modeling study

Autoignition of isomers of pentane, hexane, and primary reference fuel mixture of n-heptane and iso-octane has been studied experimentally under motored engine conditions and computationally using a detailed chemical kinetic reaction mechanism. Computed and experimental results are compared and used to help understand the chemical factors leading to engine knock in spark-ignited engines. The kinetic model reproduces observed variations in critical compression ratio with fuel molecular size and structure, provides intermediate product species concentrations in good agreement with observations, and gives insights into the kinetic origins of fuel octane sensitivity. Sequential computed engine cycles were found to lead to stable, non-igniting behavior for conditions below a critical compression ratio; to unstable, oscillating but nonigniting behavior in a transition region; and eventually to ignition as the compression ratio is steadily increased. This transition is related to conditions where a negative temperature coefficient of reaction exists, which has a significant influence on octane number and fuel octane sensitivity.

Autoignition Chemistry of C4 Olefins Under Motored Engine Conditions: A Comparison of Experimental and Modeling Results

A detailed chemical kinetic mechanism was used to simulate the oxidation of 1-butene, 2-butene, and isobutene under motored engine conditions. Predicted species concentrations were compared to measured species concentrations obtained from a motored, single-cylinder engine. The chemical kinetic model reproduced correctly the trends in the measured species concentrations. The computational and experimental results showed the main features of olefin chemistry: radical addition to the double bond leads to the production of the observed carbonyls and epoxides. For isobutene oxidation, the production of unreactive, 2-methyl allyl radicals leads to higher molecular-weight species and chain termination. 44 refs., 10 figs.

Autoignition chemistry of the hexane isomers: An experimental and kinetic modeling study

Autoignition of the five distinct isomers of hexane is studied experimentally under motored engine conditions and computationally using a detailed chemical kinetic reaction mechanism. Computed and experimental results are compared and used to help understand the chemical factors leading to engine knock in spark-ignited engines and the molecular structure factors contributing to octane rating for hydrocarbon fuels. The kinetic model reproduces observed variations in critical compression ratio with fuel structure, and it also provides intermediate and final product species concentrations in very close agreement with observed results. In addition, the computed results provide insights into the kinetic origins of fuel octane sensitivity.

The autoignition chemistry of isobutane: a motored engine study

Isobutane autoignition chemistry was examined in a motored, single-cylinder engine by measuring stable intermediate species, performing heat release analyses, and measuring visible emissions. Experimental measurements are compared with isobutane literature values, with previous n-butane results, and specific isobutane autoignition chemistry is discussed in light of the measurements

Autoignition Chemistry of N-Butane in a Motored Engine:A Comparison of Experimental and Modeling Results

A detailed chemical kinetic mechanism was used to simulate the oxidation of n-butane/air mixtures in a motored engine. The modeling results were compared to species measurements obtained from the exhaust of a CFR engine and to measured critical compression ratios