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atom from somewhere on the hydrocarbon chain. Straight chain molecules such as n-heptane
are long enough for flexible internal abstraction of hydrogen (Reaction (1)). In addition to
this, H atoms in n-heptane are bound to ‘secondary sites’ (the -CH2- backbone), which makes
them easier to abstract H atoms in primary sites, where the hydrogen is attached to the end
of a chain (the -CH3 group). Iso-octane is actually a short pentane chain with three methyl
groups attached to the chain. The short chain has difficulty ‘reaching around’ to abstract a
hydrogen atom and furthermore, most of the H atoms in iso-octane are primary, thus harder
to abstract. This flexibility and abstraction theory explains the higher reactivity and lower
octane number of n-heptane (octane number = 0) with respect to iso-octane (octane num-
ber = 100). The theory further explains the high octane number of methane (octane num-
ber = 120) where no internal abstraction is possible. The mechanism from Reaction (1) to
Reaction (11) listed above also explains the observation of so-called ‘two-stage ignition’, also
called ‘negative temperature coefficient (NTC)’ zone. At low temperature, the oxygen addi-
tion (Reactions (2) and (5)) leads to a product ‘P’ that then undergoes reactions that lead to
chain branching (Reactions (7) and (11)). These chain branching reactions lead to a rapid
increase in the temperature of the mixture. As the temperature increases, the NTC zone is
reached where the newly formed product ‘P’ can now either continue towards chain branch-
ing or decompose beak to the reactants (i.e. reverse reaction, see the bi-directional arrow
on Reactions (2) and (5)). The increase in the reverse rate results in a lower concentration of
products ‘P’ which in turn leads to a reduction of chain branching, causing a reduction in the
rate of temperature increase; the ignition delay is prolonged. As a consequence, one observes
what is called ‘two-stage ignition’. At low temperatures, the reactions are proceeding at a
slow, but observable rate. Starting at temperature below the NTC zone, the energy release
by these reactions slowly increases the temperature. With this increased temperature, the
reaction rates increase, the temperature is increasing faster and faster. This is the ‘first stage’
of ignition. The temperature increase until the NTC zone is reached. At this temperature, the
concentration of ‘P’ decreases, and thus the rate of increase in temperature slows down, but
is never zero. With time, the slowly increasing temperature reaches a point where low con-
centration of product ‘P’ is more than compensated by the increased chain branching reaction
rate and then, the system explodes: this is the ‘second stage’ of ignition. Surprisingly, if one
starts the system in the NTC zone, the concentration of ‘P’ is extremely low and the ignition
delay can be longer than if one stared the system at a temperature below the NTC zone. This
is why it is called ‘negative temperature coefficient (NTC)’ zone.
3.2. Intermediate-temperature reactions
As the temperature increases above about 850 K, where the equilibria of Reactions (2) and (5)
have effectively extinguished the low-temperature chain branching pathways, the next reac-
tion sequences involve consumption of fuel (RH), primarily by hydrogen (H) atom abstrac-
tion by OH and hydroperoxyl (HO2), and the temperature increases gradually, accompanied
by a steady increase in the level of hydrogen peroxide (H2O2), as shown in Figure 4d. This new
set of chemical reactions contributing to the increase in the level of H2O2 with the increase of
temperature is called ‘intermediate-temperature reactions (ITR)’ and is described by the fol-
lowing main intermediate-temperature mechanism [24].
Autoignition and Chemical-Kinetic Mechanisms of Homogeneous Charge Compression Ignition...
http://dx.doi.org/10.5772/intechopen.70541 47
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Buch Advanced Chemical Kinetics"
Advanced Chemical Kinetics
- Titel
- Advanced Chemical Kinetics
- Autor
- Muhammad Akhyar Farrukh
- Herausgeber
- InTech
- Ort
- Rijeka
- Datum
- 2018
- Sprache
- englisch
- Lizenz
- CC BY 4.0
- ISBN
- 978-953-51-3816-7
- Abmessungen
- 18.0 x 26.0 cm
- Seiten
- 226
- Schlagwörter
- Engineering and Technology, Chemistry, Physical Chemistry, Chemical Kinetics
- Kategorien
- Naturwissenschaften Chemie