Seite - 42 - in Advanced Chemical Kinetics

• The ignition timing is much less sensitive to changes in speed and load than that of two- stage ignition fuel, which indicates that significantly less compensation will be required to maintain optimal ignition timing over the required load and speed range. • Advantages of two-stage ignition fuel for HCCI engine • Because the amount of LTHR produced by a two-stage ignition fuel increases with the local fuel/air equivalence ratio (ϕ), fuel stratification can be used for controlling combus- tion phasing. • This local ϕ-dependence of the LTHR also provides a means for reducing the peak HRR. For this work, methane and iso-octane are selected as the single-stage ignition fuel, and DME and n-heptane are selected as the two-stage ignition fuel. Methane and DME are classified as a gaseous fuel. On the other hand, iso-octane and n-heptane are classified as a liquid fuel. Figure 2 presents the chemical formula and the illustration of chemical structure, and Table 1 lists the properties for these four test fuels [15, 16]. 2.3. Chemical-kinetic mechanism for test fuels An overall reaction includes very complex and sophisticated reactions that cannot be ana- lyzed without a proposed chemical-kinetic mechanism, a series of steps that a reaction takes before reaching the final products. The chemical-kinetic mechanism is step-by-step descrip- tions of what happens on a molecular level in chemical reactions. Each step of the reaction mechanism for the overall reaction is known an elementary reaction. The term elementary reaction is used to describe a moment in the reaction when one or more molecules change geometry or perturbed by the addition or omission of another interacting molecule. For methane and DME, a detailed chemical-kinetic mechanism (Mech_56.54; 113 species and 710 reactions) by Burke et al. [17] was used, which has been developed to be capable of predict- ing the combustion of both methane and DME in common combustion environments such as compression ignition engines and gas turbines. For iso-octane and n-heptane, a detailed Figure 2. Chemical formula and illustration of chemical structure with reaction mechanism for methane, DME, n-heptane and iso-octane. Advanced Chemical Kinetics42