Page - 47 - in Advanced Chemical Kinetics

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