Page - 50 - in Advanced Chemical Kinetics

α  =  8.3 × 10 −18 × exp  (+22750 / T) × [M] −1 (4) As the temperature increases, τ becomes smaller and also decreases with increasing total con- centration [M] or, equivalently, with increasing pressure at constant temperature. This also means that, as pressure increases, the critical temperature for ignition decreases gradually. Usually, the autoignition occurs at temperature between 1050 and 1100 K. The consistency of this temperature is a recognized feature of HCCI combustion. Not surprisingly, this tem- perature is comparable to the ignition temperature that is observed during engine knock in SI engines [25]. 4. Comparison of combustion characteristic between test fuels in HCCI engine This section will compare the combustion characteristics of the test fuels in HCCI engine. As discussed in conjunction with Figure 3, each respective test fuel shows different autoignition delay times even for the same initial condition due to its fuel autoignition reactivity. Because of this, we should expect quiet different combustion phasing for each fuel depending on the resistance to autoignition under the constant initial condition. The combustion phasing is a critical parameter impacting the thermal efficiency of HCCI engine. If the combustion is too advanced, knocking combustion occurs easily, thus quickly increasing to the risk for engine damage and NOx emissions. On the other hand, excessive combustion-phasing retard leads to unacceptable coefficient of variation (COV) of HCCI combustion with partial-burn and/ or misfire cycles. To facilitate comparison of the combustion characteristics in HCCI engine, the initial temperature is adjusted in the numerical simulation to set the 50% burn point (CA50) at 0degATDC (i.e. TDC). Effectively, the reported combustion phasing refers to CA50 for the main combustion event, starting at the crank angle of minimum heat-release rate between LTHR and HTHR. Presenting the data referring to the main combustion event alone is considered more relevant from the standpoint of quantifying the onset of the main combustion event. With the effects of fuel autoignition reactivity isolated, Figure 6 compares (a) in-cylinder temperature, (b) heat-release rate, (c) magnified view of heat-release rate and (d) accu- mulated heat release for the test fuels. The required initial temperature (To) to maintain CA50 = 0degATDC is 559.5, 301.5, 464.d and 350.5 K for methane, DME, iso-octane and n-heptane, respectively. If the fuel has high resistance to autoignition (i.e. methane and iso- octane), high in-cylinder charge temperature is required during the compression stroke in order to ensure autoignition. As can be seen, methane and iso-octane both require relatively high To, which typically has a negative influence on the peak load that can be obtained. This happens because these two fuels exhibit single-stage ignition at this calculation condition. For a given initial pressure (Po), higher To causes high peak combustion temperature, as shown in Figure 6a, so excessive NOx can become the load-limiting factor. Furthermore, as Figure 6b shows, the increase of peak combustion temperature contributes to the increase Advanced Chemical Kinetics50