Shock tubes have been applied to ignition research for a long time, because they can heat a combustible fuel mixture instantaneously to high temperatures and pressures. In recent years, the shock tube studies have gained attention as they could observe not only high-temperature ignition but also low-temperature ignition, due to the improvement of the equipment and novel experimental techniques. As a result, it became an effective tool for solving various problems of internal combustion engines such as knock phenomenon of gasoline engines and HCCI combustion. In the present paper, I introduce a high-pressure shock tube capable of observing low-temperature ignition, and have reported some results of our ignition research obtained since 2007. In order to aid future shock tube studies, the issues of the high-pressure shock tube and their latest solutions are also mentioned.
Soot formation process has been extensively studied over the last decade and is still a crucial issue for reduction in exhaust emission of internal combustion engines. In this article, researches on soot formation using shock tubes are introduced in terms of measurement techniques of soot particles such as laser scattering/extinction method and laser-induced incandescence. Furthermore, ambient temperature rise during soot formation process is mentioned, with reference to measurement of soot particle temperature. Besides research progress in the soot formation, an operation principle of a standard shock tube is described together with several cautions when using a shock tube as a chemical reactor to realize homogeneous high temperature fields.
Shock tube / laser absorption (ST/LA) spectroscopy is a common method to monitor a time dependent concentration profile of single chemical species. The rates of reactions are estimated fitting the concentration profiles to the model derived from simple reaction rate equation. Ignition delays are defined as the maximum intensity of emission signal at 431 nm, which is emitted from excited CH* radicals produced via the reaction C2H+O=CO+CH*. Reaction mechanism is necessary to estimate the ignition delays as C2H and O are produced via complex reactions. The concentration profiles obtained in a single-pulse shock tube are very useful to validate reaction mechanisms constructed for the pyrolysis and oxidation at high temperatures. The techniques of single-pulse experiments including gas sampling and the time histories of shock heating are discussed. The rate constant of 1,1,1-trifluoroethane, which is known as chemical thermometer, was estimated from ST/LA spectroscopy, and applied to the validation of single-pulse method. The cases of diethyl ether and butanol isomers are introduced in the last of this article. The production of acetaldehyde resulted from the pyrolysis of isobutanol shows importance of keto-enol tautomerization in the gas phase reaction.
This paper describes fundamentals of coherent anti-Stokes Raman spectroscopy/scattering (CARS). A brief review of the theoretical background of CARS as one of the third-order nonlinear optical processes. The third-order nonlinear optical susceptibility and the calculation of theoretical spectrum are explained including pressure narrowing phenomenon. The effects of the slit function, the spectral line shape of pump and Stokes lasers are discussed, and the examples of the observed CARS spectra are shown. Temperature measurement and improvement of accuracy are explained. The experimental results of unburned gas temperature in a spark ignition engine are shown as an example of the high accuracy CARS thermometry. The chirped-probe-pulse femtosecond CARS thermometry is introduced as a latest research activity.
A spark plug with flanges attached to a conventional spark plug has been proposed as a method for reliable ignition in a lean mixture with strong turbulence. When the spark plug is applied to a cogeneration engine, the maintenance cost is expected to be reduced by the reduction of the replacement frequency of the spark plug. Ignition probability tests in mixture were conducted using conventional and newly devised dome flanged spark plugs. The shock energy recovery effect of the dome-shaped flange confirmed that that reliable ignition can be performed with small ignition energy. Furthermore, as a result of the durability test in the high temperature and high pressure atmosphere, it was confirmed that the consumption of the electrode can be suppressed by reducing the spark energy. A spark plug having a concentric ground electrode has a disadvantage that the ignition probability is low even when large spark energy is given due to the small spark gap. However, it was clarified that the ignition characteristics are improved even with small energy and the durability is also improved by the reduction of the ignition energy by the increase of the spark gap and the installation of the dome-shaped flange.
In this study, ignition delay times (IDTs) of C9H20 (Nonane) isomers were investigated with a newly developed shock tube facility and numerical simulation with KUCRS. It was found that ignition delay time (IDT) of 2,2,4,4-tetramethylpentane (2244mC5) is considerably longer than n-nonane (n-C9) especially in the negative temperature coefficient (NTC) region. The maximum difference is about one order of magnitude at 1000/T = 1.3 for 1.0 MPa and Φ = 1.0, where IDT of n-C9 is 3.6 ms while 29.9 ms for 2244mC5. Despite the significant difference in IDT, 0-D simulation with the reaction model generated by KUCRS could successfully reproduce the experimental results. According to the reaction path analysis and sensitivity analysis, it was considered that 2244mC5 tends to form cyclic ether due to its highly branched structure, which inhibits the increase of OH, and thus, chain branching reactions.