66 the engine was about 20 NO
2
to 80 NO. This configuration was finalized and used throughout the investigation.
Table 3.8.3 NONO
2
ratio based on DPF-DOC assembly with EGR off. Sampling Location
NOx ppm
NO ppm
NO
2
ppm NO
2
NOx percentage
Upstream DPF-DOC 525
420 105
20 Downstream DPF-DOC
530 205
325 60
3.9 Measurement using various sampling probe length.
Prior to designing the experimental test matrix, a brief investigation of various sampling probe lengths was also conducted. The assumption throughout was that the flow was essentially one
dimensional within the SCR. To assess the validity of this assumption, measurements were taken inside the exhaust pipe using 3 different lengths of sampling probes. The sampling probe was
connected directly to the end of the heated line from the MEXA 1170Nx. Based on the inside diameter of the instrumentation module 120 mm, the centre stream was 60 mm from the pipe
wall. The three sampling probes used were at 55 mm, 25 mm and 10 mm from the wall as shown in figures 3.9a, b and c.
Figure 3.9a Variation of sampling probe length for profile measurement Experiments were conducted using 4 NH
3
gas. The experiments were conducted at engine speed of 1500 rpm and BMEP of 6 bars. The quadruple SCR catalyst was used. Initially the NOx measurement
was taken upstream of the SCR catalyst without injecting NH
3
gas. Then, the MEXA sampling probe was moved to the location downstream of the SCR catalyst and NH
3
gas was injected. The analyser
67 was thus placed to measure NH
3
or NOx slippage after the SCR. The same procedure was used for all probes.
Figure 3.9b Long 55 mm sampling probe Figure 3.9c Medium 25 mm sampling probe
The comparison of results between the medium length probe and the long probe at a setting of 100 mm of the glass float are tabulated in table 3.9.
Table 3.9 Profile Measurements inside the exhaust stream. Date
Probe length
SCR brick
Glass float
mm Gas
Pressure psi
NOx in upstream
SCR NH
3
in upstream
SCR NH
3
out downstream
SCR NOx out
downstream SCR
16jun 55 mm
4 100
1.2 579
510 6
150 24Jun
25 mm 4
100 1.2
576 535
7 128
24Jun 25 mm
4 100
1.2 576
534 5
125
From table 3.9, NOx in and NH
3
out measurement do not show much variation between long 55 mm and medium25 mm sampling probe. The slight variation is due to the NH
3
distribution being non-uniform upstream and hence NOx consumption is not uniform downstream. A plot of results for
the long sampling probe measurements at various gas flow rates are shown by the blue line in figure 3.9d. The two measurements using the medium sampling probe are shown in green.
55 mm 25 mm
68 Figure 3.9d Check point with medium sampling probe for gas measurement.
The result shows that the medium sampling probe on the MEXA ammonia analyser was producing similar result as the long sampling probe. So it was concluded that the sampling probe length inside
the exhaust assembly does not have much impact on the measurement of the NOx and NH
3
level in the exhaust flow. As a result of this, the experiment with the short probe 10 mm was not
necessary for the investigation. Therefore all further measurements used the long probe.
3.10 Problems associated with the MEXA Analyser
