33 The mixing chamber has a urea spray boss fitted on top and a tiny drainage plug at the bottom. In
the event of running the test with urea spray, the chamber would house the spray injector unit while with the NH
3
gas test the boss was plugged to prevent gas leakage.
3.2.5 Instrumentation module assembly.
Four instrumentation modules were fabricated and assembled. Two of the modules were of diameter 115mm and of length 110 mm. A third had a diameter of 115 mm and was 90 mm long
and a fourth had a diameter of 50 mm was of length 200 mm. The modules were used to house the analysers, thermocouples and lambda sensors. The two 110mm long modules were placed after the
DPF-DOC assembly and before the SCR assembly and the 90 mm long module was placed after the SCR assembly. The 50 mm by 200 mm pipe was placed after the mixing chamber before the long
diffuser. This pipe provided an alternative placement for Urea spray Injector. The instrumentation modules arrangement is as shown in figure 3.2.5.
Figure 3.2.5 Instrumentation modules location along the SCR exhaust system.
Bosses were fabricated to accommodate the urea spray system, lambda sensors, analysers, thermocouples and pressure sensors in the instrumentation modules, urea expansion box and the
end of the exhaust system. 18 inch BSP fittings were used for thermocouples and ΒΌ inch NPT fittings for the Horiba analysers. These ports could be capped during the engine calibration process.
Cleaning of the bosses was required after assembly using respective thread taps. This was done to remove any welding residue left on the bosses to ensure proper fitting for the instrumentation.
34
3.2.6 Long and short diffuser assembly
Four cones were used with different length and cone angles. The longest cone was 410 mm long with a half cone angle of 4.5
O
while the shortest cone was 90 mm long with a half cone angle of 19.9
O
. The inlet and exit cones both were 150 mm long with half cone angles of 12.2
O
. The most important cone in this assembly was the long diffuser cone. This cone was placed after the spray
assembly and before the SCR assembly. This was designed to provide an approximate uniform one dimensional flow from the nozzle to the front face of the SCR catalyst.
3.2.7 Bypass pipe assembly.
