Seite - 43 - in Advanced Chemical Kinetics

chemical-kinetic mechanism from Lawrence Livermore National Laboratory (LLNL; 1034 species and 4236 reactions) [18] was used, which has been developed for the oxidation of primary reference fuels (PRFs), iso-octane and n-heptane, for gasoline. This mechanism was developed by combining the iso-octane [19] and n-heptane [20] mechanisms. 2.4. Comparison of autoignition delay times Autoignition, the spontaneous ignition of a fuel and oxidizer mixture in the absence of any external ignition source, occurs when slow thermal reactions initially have a large chain branching component sufficiently to maintain and accelerate oxidation. The increasing radi- cal concentration leads to the increase in reaction rate build on themselves, and eventually result in an ignition through a rapid explosive rise in radical concentration, oxidation rate and temperature. Most of these reactions typically release heat, and eventually increasing the tem- perature and pressure of the system, and at the same time, their rate is also strongly dependent on pressure, temperature and charge composition. These characteristics cause a complicated interaction of negative and positive feedback loops that determine when the ignition will hap- pen. In fact, autoignition is very sensitive to details of chain branching and chain terminating in the initial reactions, and hence depends sensitively on the chemical structure of the fuel. The autoignition reactivity of the fuel is a very important parameter, impacting the design and the potential high-load performance of HCCI engines. The accurate prediction of autoig- nition times and their dependence on pressure, temperature and composition is essential for advanced engine technologies, such as HCCI, where the ignition event is timed by chemical kinetics. An autoignition delay time (τ) of fuels is one of the crucial indicators to present the extent of fuel autoignition reactivity for the combustion optimization of internal combustion engines, especially for HCCI engines. The autoignition delay time is defined as the time inter- val required for the fuel-air mixture to spontaneously ignite at some prescribed conditions. The rapid compression machine (RCM) and shock tube are two of the most widely used facili- ties for the studies of ignition delay time. RCM gives a direct way of measuring the ignition delay time by simulating the process of adiabatic compression and ignition. While the shock tube is applied to study autoignition characteristics of gas mixtures at a higher temperature and pressure than those of RCM, RCM is used to study the autoignition characteristics of test fuels in the temperature range of low to intermediate, compared with shock tubes. To Property (unit) Methane DME n-Heptane iso-Octane Boiling point (°C) −161.5 −25.1 98.4 99.2 Liquid density (g/cm3@20°C) — 0.67 0.68 0.6878 Relative gas density (air = 1) 0.55 1.6 3.46 3.9 Vapor pressure (MPa) — 0.61@25°C 0.0046@20°C 0.0051@20°C Ignition temperature (°C) 650 235 285 417 Lower heat value (MJ/kg) 49.0 28.8 44.57 44.31 Table 1. Properties of DME [15], methane [15], n-heptane [16] and iso-octane [16]. Autoignition and Chemical-Kinetic Mechanisms of Homogeneous Charge Compression Ignition... http://dx.doi.org/10.5772/intechopen.70541 43