Page - 46 - in Advanced Chemical Kinetics

In the initiation step, a hydrocarbon (RH) reacts with oxygen (O2) to make a hydrocarbon radical (R•), which reacts with oxygen to make a peroxy radical (RO2•). (Radical species are denoted by the ‘•’ symbol next to the character.) Next, an internal hydrogen-atom abstrac- tion takes place (i.e. the abstraction of a hydrogen atom from the molecule itself). Following the internal abstraction, the radical •R′OOH reacts internally to eliminate (eject) OH and forms a compound without free valences (unpaired electrons) such as an aldehyde or ketone (Reaction (4)). The mechanism continues with a second O2 addition to the peroxy radical initially formed (Reaction (5); [23]). After a few steps, keto-hydroperoxide (HO2R”O) is formed (Reaction (10)). Keto-hydroperoxide decomposes at around 800 K, producing further hydroxyl (OH•) that consumes the fuel (Reaction (11)). In the hydrogen abstraction reac- tion (Reaction (3)), the molecule isomerizes by ‘reaching around’ and abstracting a hydrogen Figure 4. An example of HCCI combustion for (a) overall heat-release rate, (b) magnified view of heat-release rate, (c) in-cylinder temperature and (d) mole fraction. Advanced Chemical Kinetics46