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behavior where the autoignition delay times increase with increasing initial temperature. Similar to that of DME, the autoignition delay time for n-heptane also shows NTC behavior with relatively high-fuel autoignition reactivity due to very low octane number (=0). For iso- octane, surprisingly, NTC behavior is observed despite of high octane number (=100) even for relatively longer than autoignition time delay of DME and n-heptane. As shown in Ref. [21], which systematically investigates autoignition properties of iso-octane at conditions relevant to practical combustion devices using RCM, iso-octane can exhibit NTC region under condi- tions of elevated initial pressure. 3. Chemistry of HCCI combustion As discussed above, reactants in HCCI combustion begin at room temperature and are steadily heated during the compression stroke by piston. As the reactant temperature increases, the specific elementary reactions that contribute to fuel consumption in general and chain branch- ing and autoignition in particular also change. The reactants pass through three distinct tem- perature ranges, each with its own unique chain branching reaction pathways that contribute to the eventual autoignition. With reference to Figure 4 as an example of HCCI combustion, this section explains the chemical reactions that play a role in the process, which are classi- fied as low-temperature reactions (LTR), intermediate-temperature reactions (ITR) and high- temperature reactions (HTR). 3.1. Low-temperature reactions Virtually, no significant reaction takes place until the reactant temperature reach about 550 K. As the reactant heats up during the compression stroke, chemistry becomes increas- ingly active at temperatures above 600 K. At these conditions, fuel dissociation is described by the following low-temperature mechanism [22]. RH + O2 ⇒ R• + HO2 initiation Re.(1) R• + O2 ⇔ RO2• first O2 addition Re.(2) RO2• ⇒ •R’OOH internal H-atom abstraction Re.(3) •R’OOH ⇒ R’O + OH• chain propagation Re.(4) •R’OOH + O2 ⇔ HO2R’O2• second O2 addition Re.(5) HO2R’O2• + RH ⇒ HO2R’O2H + R• external H-atom abstraction Re.(6) HO2R’O2H ⇒ HO2R’O• + OH• chain branching Re.(7) HO2R’O• ⇒ OR’O + OH• chain propagation Re.(8) HO2R’O2• ⇒ HO2R”O2H internal H-atom abstraction Re.(9) HO2R”O2H• ⇒ HO2R”O + OH• chain propagation Re.(10) HO2R”O ⇒ OR”O• + OH• chain branching Re.(11) Autoignition and Chemical-Kinetic Mechanisms of Homogeneous Charge Compression Ignition... http://dx.doi.org/10.5772/intechopen.70541 45