Note that the PSHR effects qualitatively agree with the simulated ignition delay contours, where further progression into the ignition delay contours coincides with increased PSHR magnitude.
This subsection investigates the effect of engine pressure differential (and the accompanying trapped residual) changes on PSHR behavior at a constant intake temperature.
The ensemble-average PSHR of the three conditions listed in Table 7 is displayed in Figure 6, where ensemble-average AHRR of the backpressure sweep (dashed data) is overlaid on similar corresponding cases from the intake temperature sweep (solid lines).
The trends in Figure 6 illustrate that either backpressure or intake temperature can be used to increase PSHR. This is not unexpected, as for a given cam phasing either increased intake temperature or trapped residual fraction will increase the temperature at IVC ([T.sub.ivc]), as calculated by ideal gas law.
For a given [T.sub.ivc] the TDC pressure and temperature increase with increased n from backpressure, and if PSHR is present, the temperature continues to increase up to spark.
Not only is the effect of backpressure on TDC reactivity captured by the proportional relation to PSHR, but it is also present in the SPI tendency of the operating conditions.
Although the measurements of SPI count and PSHR magnitude correlated with reduced TDC ignition delay, the order of magnitude increase in SPI event count with the backpressure sweep is not well represented by the ensemble traces in Figure 7.
The comparative results of Figure 8 and Figure 9 illustrate the increase in cyclic dispersion in PSHR and overall AHRR present in the backpressure sweep.
The SPI counts in Table 9 illustrate that no SPI events were recorded without PSHR. This is not to say that PSHR is required for SPI events to occur, but suggests that in order for pre-ignition events to transition into a follow-on super-knock event, the charge thermodynamic state must be such that a large autoignition or ignition without the spark discharge can be supported.
In the present subsection, comparison between the backpressure and intake temperature sweeps highlights that not only is there an appreciable difference in the ensemble average bulk gas temperature-pressure trajectory, but also the cyclic dispersion in the PSHR. Since the backpressure sweep approach links the thermodynamic state of a given cycle to the previous cycle, increased dispersion is not unexpected.
Data show that although the pressure-temperature trajectory approaching TDC is nearly identical between the two GDI orientations, the modified (i.e., dashed data, [theta] injector=180[degrees]) orientation begins to express PSHR before and more fully than the solid data.
The combined effect of increased spark advance and earlier PSHR with the modified GDI orientation suggest that increased inhomogeneity exists in the modified GDI orientation, where overly lean regions exist near the spark plug, and richer regions exist elsewhere in the chamber.