*4.2. Knocking Combustion Visualization*

After the validation of the methodology, an analysis of the knock onset is performed in this section. The study is conducted for two extreme knocking cases. On the one hand, the simulated cycle number 8, highlighted in Figure 4, is selected as the upper limit since it exhibit the higher MAPO value. And, on the other hand, cycle number 9 is chosen as the lower limit, being the cycle with the slightest knock.

A series of snapshots were conscientiously chosen and plotted together in Figure 5 for studding the combustion process. They were specifically selected considering different stages of combustion to allow a proper visualization of the endgas auto-ignition.

In this figure, the combustion tracking is done by clipping the temperature field at 2000 K and colouring it by the fuel energy release, thus showing the location of the reaction zones. Besides, pressure profiles registered at the transducer position are included to distinguish at which cycle step is located each snapshot while allowing to relate the combustion with its corresponding pressure effects [51,52].

As can be seen in both sequences of snapshots, combustion starts spontaneously as a result of a first auto-ignition event located inside the piston bowl where local temperature and mixing conditions are more favourable. Then, the combustion rapidly progresses by consuming the charge located within the bowl. At this point, the pressure rise due to combustion compresses the unburned gases at the squish region, causing the appearance of additional hot spots as a result of a second auto-ignition event. Looking at the colour scale, the rate of energy released by these hot spots is very localized and noticeably greater than that released during the previous combustion phase.

Examination of the pressure profiles also reveals that the onset of these hot spots coincides with the instant at which pressure oscillations become apparent, showing again that this abnormal combustion event has many similarities with the traditional knocking combustion in SI engines. Furthermore, it is possible to see that the knock intensity decreases as combustion is shifted towards the expansion stroke.

Besides the visualization of the combustion progress, Figure 5 shows a comparison between two cycles with heavy and slight knocking conditions. It is clear from this figure that, while the ignition is produced 2 cad after TDC in the cycle with heavy knock, combustion is delayed almost 2 cad in the slight knock cycle. Moreover, the burning rate is significantly higher in the former case since temperature iso-volumes grow faster. Thereby, the endgas auto-ignition is produced closer to the TDC, contributing to increase the pressure rise and its associated local oscillations to a greater extent. However, despite these differences, it is important to note that spatial patterns are quite similar at the knock onset.

**Figure 5.** Visualization of the combustion process. A sequence of snapshots is depicted in order to identify the differences between two extreme cases with a remarkable knock level variation. The combustion process is visualized by clipping the temperature field at 2000 K and colouring it by the fuel energy release.
