## 3 Results and Discussions  
### Considerations for Diagnostics Setup  
First, the engine was installed with balancers and dampers to minimize vibrations and minimize any influences it can have on PIV arrangement. The camera and laser were also positioned on structures that well dampen from the engine vibrations. The results are confirmed to be free of vibration-induced errors which is normally seen as streaks, proving the effectiveness of this strategy. Secondly, to observe the side-view of the pent-roof area, the laser will be impinged on the cylinder head and cannot escape from the combustion cylinder, leading to possible reflections complications. As shown in Figure 3a, the reflection light can create smearing, scatters on the pent-roof and will decrease the clarity of a large portions of the measurement area. In this case, a layer of anti-reflection black paint (commercially available oily black ink) is applied onto the surface of the fire-deck and hence, the reflection light of the \begin{table}
\begin{tabular}{c c} \hline \hline
**Sampling frequency** & 7.2 kHz \\
**Crank angle resolution** & \(1^{\circ}\). CA \\
**Image size** & \(1280\times 800\) (\(59\times 37\) mm) \\
**Spatial resolution of image** & \(0.046\times 0.046\) mm \\
**Seeding particles** & SiO\({}_{2}\) \\
**Measurement cycle number** & 19 (consecutive) \\
**Interrogation area** & \(32\times 32\) px. (\(1.5\times 1.5\) mm) \\
**Overlap (Duplication degree of interrogation area)** & 50\% \\
**Spatial resolution for vector** & \(0.74\times 0.74\) mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: PIV set up specifications. laser sheet is significantly reduced and the valid measurement area is maximized. However, it should be noted that the region around 1 mm away from the fire-deck of the pent-roof is still under the effect of reflection, especially at TDC. While the stoichiometric combustion with port-fuel injection inside the S.I. engine does not produce as strong luminosity as other richer combustion seen in direct injection engine or diesel engine, the luminosity is still strong enough to create complications to the PIV signal. In fact, as seen in Figure 3b, the luminosity will be scattered on the seeds, especially inside the burned region. By adding a bandpass filter at the laser wavelength of 527 nm, it is possible to remove most of the scattered light from luminosity and only capture the dominating scattering signal from the PIV laser sheet. An example PIV image demonstrates the performance of this bandpass filter during combustion is also shown in Figure 3b. For PIV measurement, selection of the seeding particle is crucial to achieve high quality data. As mentioned earlier, the seeds need to be inert (chemically and fluid-dynamically) and resistant to high temperature to be applicable for PIV during a combustion event in engine. We have chosen silica (SiO\({}_{2}\)) particles in this work to ensure the flame will not burn all the particles such as the case when oil seeds were used [13]. However, to be able to distinguish between burned and unburned region from the flame analysis point of view while maintaining flow-field data from all regions, seeds materials will not be the only deciding factor. In fact, as shown in Figure 4a, depends on the amount of particles seeded into the cylinder during a firing cycle, we can either get an underexposed image where the laser scattering is not enough for PIV and flame front detection (top), an overexposed image where seeds amount was too much (middle) or only enough seeds to get PIV data but the flame cannot be detected bottom. The correct amount of seeds allows both PIV data and flame to be detected as shown in the bottom row of Figure 4a. To achieve this correct amount, the seed amount was controlled by adjusting the air flow rate for this seed supply. Once the image is confirmed, the same flow rate was kept for the rest of the experiment. Estimating the appropriate amount of seed particles in this experiment is about 1 mm\({}^{3}\) in bulk volume per cycle.