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B OUNDARY LAYER AND NEAR WALL EFFECTS

6 Results

6 Results

square of the correlation coefficient and denotes the strength of the linear relation between chemiluminescence intensity and RoHR or, x and y as shown in (6.2).

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(6.2)

−300 −20 −10 0 10 20

10 20 30 40

dQ [J/CAD]

Crank Angle [CAD]

−30 −20 −10 0 10 200

300 600 900 1200

Intensity [a.u]

dQdCAD Intensity

Figure 42. Rate of heat release and maximum chemiluminescence intensity for the base case

R2= 0.9869

0 5 10 15 20 25 30

0 200 400 600 800 1000 1200

Intensity [a.u.]

Rateofheatrelease[J/CAD]

Figure 43. Variation of image average maximum intensity versus rate of heat release for the base case.

The steep temperature gradient in the boundary layer is indicated by a fast decrease in chemiluminescence intensity towards the combustion chamber surface. Therefore chemiluminescence imaging can be used to calculate the boundary layer behaviour.

For these calculations the boundary layer is defined as the area from 15 to 85 % of maximum intensity at each crank angle. These intensities are calculated from the mean images of 50 cycles taken through the quartz liner. One such image is shown in Figure 44.

Since the boundary layer calculations are based on averaged information it is of great interest to have a look at the cycle to cycle variations. Figure46shows the variation coefficient for single images of the base case. What is obvious is the high cycle to cycle variation at the start of main combustion, especially where the boundary layer is situated. Comparing to the rate of heat release in Figure 42 it can be seen that for increasing burn rate the variations from cycle to cycle decrease rapidly, becoming low during the main combustion. For late crank angles the cycle to cycle variations increase slightly again, now mainly at the area of the main combustion while the boundary layer shows less variations.

The change in combustion duration for the different sweeps can be seen in Figure 45.

For later injection, increased load and increased speed, the burn duration goes down.

Yet for the swirl case no effect in any direction can be seen. Since both later direct injection and higher load will give zones of richer mixture these are expected to burn faster. For increased engine speed heat losses are expected to decrease, thus enhancing the burn rate. Here constant burn duration in CAD means shorter burn duration in time. For the 1500 rpm case an even faster burn duration is seen, decreased also in CAD.

6 Results

The closing of one of the intake ports changes the swirl number from 2 to 2.6 for fully closed port and should then also increase the turbulence. Geometry generated turbulence by modifying the piston crown has shown longer burn duration in earlier studies [61, 62]. A later simulation suggested that the main reason for the increased burn duration was due to changes in heat transfer and thermal stratification in the piston bowl [63, 64]. However no such behaviour can be seen for turbulence generated by the intake flow in a pancake shaped combustion chamber. The pancake combustion chamber simply doesn’t allow the same conditions for temperature stratification as the bowl. Another reason for not seeing any change in burn duration can be that at low speed the generated turbulence variation is too small. Also the effect of turbulence should increase for increased fuel stratification, with more fuel rich zones. If the PFI charge is well mixed this effect should be small, so the effect of increased swirl should have greater impact on DI HCCI.

Figure 44. Averaged horizontal view of combustion through the quartz liner with the injector clearly visible in the center

15 20 25 30 35 40

Duration 10−90% heat released [CAD]

Swirl 0 25 50 75 100 % Blocked DI 180, 130, 90, 50, 30 BTDC Load 1, 1.5, 2, 2.5, 3 bar Speed 700, 900, 1200, 1500 rpm

Figure 45. 10 to 90 percent burnt duration for the different sweeps. Different conditions from left to right side of the figure.

Figure 46. Coefficient of variation of

chemiluminescence intensity for single images at each CAD (ATDC) for one of the base cases.

Although it is not possible to see a difference in burn duration due to turbulence the boundary layer thickness is still affected. Figure 47 shows the change in boundary layer thickness over crank angle for increasingly blocked intake port. During the early combustion the boundary layer thickness actually decreases with increased swirl, and then in the later part it instead increases. The over all increase in boundary layer thickness for very early CADs can be disregarded since the overall signal level is very low and no clear conclusions can be drawn.

6 Results

What can be seen though is a constant increase in boundary layer thickness during combustion. A thicker boundary layer is slightly unexpected since the reacting core is expected to increase the temperature closer to the walls and increase the reaction rate of the inner part of the boundary layer. This is actually also the case. During combustion the chemiluminescence intensity increases closer to the combustion chamber walls above the 15 % level set as the lower level for the boundary layer calculation. The intensity level of the core is not uniform so although the reaction rate increases from the centre towards the piston and fire deck the chemiluminescence intensity still does not reach above the upper level for the boundary layer definition, thus the boundary layer is seen to increase during combustion.

For the DI sweep and late injection, for the load sweep at high load and for the speed sweep at high speed all show shorter burn duration and a corresponding thinner boundary layer. A correlation between boundary layer thickness and the burn duration can be found.

For increasing speed a slight decrease in boundary layer thickness can be seen, from 700 to 1500 rpm, shown in Figure 48. For the 1500 rpm case a strange phenomenon can be seen during the late combustion. A sharp increase in boundary layer thickness seams to occur. This is however not completely true. Figure 49 shows the intensity distribution from the cylinder head to the piston crown for different CADs for the 1500 rpm case. It shows a plateau at an intensity level close to what is used to define the boundary layer. This can be explained by the later combustion moving closer to the piston as the boundary zone is heated simultaneously as the core of the charge is already partially oxidized with a decreasing reaction rate. The effect is even more pronounced when doing tests with high levels of EGR as shown in Figure 50.

−100 −5 0 5 10 15

1 2 3 4 5

Crank angle [CAD]

Boundary layer thickness [mm]

0% (open) 25%

50%

75%

100% (blocked)

Figure 47. Boundary layer thickness for swirl sweep.

−100 −5 0 5 10 15

1 2 3 4 5

Crank angle [CAD]

Boundary layer thickness [mm]

700 RPM 900 RPM 1200 RPM 1500 RPM

Figure 48. Boundary layer thickness for speed sweep.

6 Results

4 8 12 16

0 15 50 85 100

Distance [mm]

Scaled intensity [%]

−5° ATDC 0° ATDC 9° ATDC 12° ATDC 15° ATDC

Figure 49. Scaled intensity profile for 1500rpm case from speed sweep at different CADs.

Cylinder head located at ~4 mm and piston at

~13 at TDC.

4 8 12 16

0 15 50 85 100

50% EGR

Distance [mm]

Scaled intensity [%]

0° ATDC 5° ATDC 10° ATDC 12° ATDC 15° ATDC

Figure 50. Scaled intensity profile for 50 % EGR at different CADs. Cylinder head located at ~4 mm and piston at ~13 at TDC.