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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 (Ï„) 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
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book Advanced Chemical Kinetics"
Advanced Chemical Kinetics
- Title
- Advanced Chemical Kinetics
- Author
- Muhammad Akhyar Farrukh
- Editor
- InTech
- Location
- Rijeka
- Date
- 2018
- Language
- English
- License
- CC BY 4.0
- ISBN
- 978-953-51-3816-7
- Size
- 18.0 x 26.0 cm
- Pages
- 226
- Keywords
- Engineering and Technology, Chemistry, Physical Chemistry, Chemical Kinetics
- Categories
- Naturwissenschaften Chemie