Flame Position and Displacement - Early Flame Analysis and Discussion

CHAPTER 4 RESULTS AND DISCUSSIONS

4.3 Early Flame Analysis and Discussion

4.3.4 Flame Position and Displacement

(a) (b)

Figure 4.28 Flame distortion at 1800 rpm (a) stratified combustion (b) homogeneous combustion

(a) (b)

Figure 4.29 Flame distortion at 2100 rpm (a) stratified combustion (b) homogeneous combustion

the piston approached TDC the flame centers displaced to the direction of the bowl center, to the right. The displacement of the flame, especially in the early stage of flame development, is helpful to avoid or reduce local or global quenching of the small flame ball by eliminating the chance of contact with the spark electrodes.

The displacement and positions of the flame center in reference to the spark center, which was taken to be the center of the coordinate axes, was plotted against time in Figure 4.31 to 4.36 for stratified and homogeneous combustion modes at varying engine speeds and variable induction cases. The position of flame centers and their displacement from the spark center were observed to be affected by the induction process. In the medium tumble induction case, the first flame images of each engine speed captured at 0.15 ms after ignition onset were found displaced 3 to 5.5 mm from the spark center in both combustion modes as shown in Figure 4.31. The displacement in the stratified combustion case showed a linear displacement curve with time and the flame center came nearer to the spark center when flame size increased. In the case of homogeneous combustion, there was little variation in the displacement of flame center within 1.8 ms after ignition timing as shown in Figure 4.31 (b). The exact position of the flame centers were plotted in Figure 4.34 to 4.36 taking axes centers to be the position of the spark center. The x and y coordinate axis used in these figures was the same axis shown in Figure 4.30. It was observed that early flames of tumble induction were located on the left down position from the spark center for both combustion modes. The displacement of the flame was more of radial. This radial displacement of flame center was up to 3:1 to its axial displacement.

Figure 4.30 Schematics of stratified piston at TDC inside engine cylinder

On the other hand, Figure 4.32 to 4.36 portray that flame center positions and displacements for medium swirl and high swirl intake conditions showed different trend from the previous induction system at both combustion modes for all engine speeds under considerations. The space between electrodes of the spark plug is 1 mm.

The very early flame images captured at 0.15 ms after ignition timing for the medium swirl and high swirl induction cases were within 1 mm distance as shown in Figure 4.32 and 4.33. This nearness of the early flames to the electrodes made the flames susceptible to heat losses and quenching (global or local) due to contacts with the relatively colder electrodes. The radial displacement in these inductions was 3 to 4 folds greater than the axial displacement. The other important phenomenon noticed in the medium and high swirl induction cases was that with the flame development time, the flame centers were pushed or displaced towards the right and upward position compared to flame center positions of medium tumble case as shown in Figure 4.34 to 4.36. Generally, the displacement to the right side of the spark center was a function of rise in swirl level at intake process. All flame date captured at medium and high swirl conditions in the homogeneous combustion process had flame centers positioned at the right side of spark center. These flame centers were less scattered than their stratified counterpart. The induction flow structure investigated in earlier sections using PIV system showed the medium and high swirl induction adjustments were capable of forming coherent and strong swirl flows. Zhao et al. [40] mentioned that the swirling flows continue to rotate up to even the end of compression process. The presence of a large bowl-in-piston will have effects on swirl flows during compression process. Heywood [14] discussed about this effect that swirl velocity within the bowl increased as TDC approaching, and outside the bowl it was decreasing. In this study, the bowl axis in the piston has an offset from the cylinder axis as shown in Figure 4.30. This can create an offset swirl flow towards the left portion of the cylinder. This could be the reason for the early flame to be convected to the left portion of the cylinder in medium and high swirl induction cases as depicted in Figure 4.34 to 4.36.

(a) (b)

Figure 4.31 Flame displacement from spark center at medium tumble induction (a) stratified combustion (b) homogeneous combustion

(a) (b)

Figure 4.32 Flame displacement from spark center at medium swirl induction (a) stratified combustion (b) homogeneous combustion

(a) (b)

Figure 4.33 Flame displacement from spark center at high swirl induction (a) stratified combustion (b) homogeneous combustion

0 1 2 3 4 5 6

0 0.5 1 1.5 2 2.5 3 3.5

Displacement [mm]