28 On the engine the mass flow rate was measured with a Testo digital manometer in mmH
2
0 and later converted to gramseconds and was recorded throughout the investigation. The Ricardo mass flow
meter configuration with digital manometer is shown in figure 3.1.4
Figure 3.1.4 Ricardo mass flow meter measuring engine Mass Flow Rate MFR
3.2 Final SCR Exhaust build and commissioning.
The Selective Catalyst Reduction SCR exhaust system was built based on the parts supplied by EMCON Technologies Incorporated and catalysts supplied by Johnson Matthey and the finalized
drawing agreed in a quarterly review meeting at Coventry University. The details of the parts supplied are listed in appendix 3.2. The SCR exhaust system comprises a Diesel Particulate Filter
DPF, Diesel Oxidation Catalyst DOC, expansion chamber and nozzle, a narrow angled diffuser, SCR catalyst, bypass pipe and instrumentation modules. Figure 3.2 shows a schematic of the final
assembly. It has been designed in such a way so to provide approximately 1D flow for comparison with a 1D computational model. Details of the components are discussed later.
From the engine exhaust manifold outlet, the exhaust was connected to the Diesel Oxidation Catalyst DOC for NO, CO and HC oxidation. Diesel oxidation catalysts can reduce emissions of
particulate matter PM from 15 to 30 percent while hydrocarbons HC and carbon monoxide CO by over 90 percent within temperature interval of 20 to 30
C45.These processes can be described by the following chemical reactions.
Digital manometers
Ricardo mass flow meter
29 [HC] + O
2
CO
2
+ H
2
O Equation 3.2a
CO + 12O
2
CO
2
Equation 3.2b HC are oxidized to form carbon dioxide and water vapour. The reaction in equation 3.2a represents
two processes: the oxidation of gas phase HC and the oxidation of organic fraction of diesel particulates SOF compounds. Reaction in equation 3.2b describes the oxidation of carbon
monoxide to carbon dioxide. Since carbon dioxide and water vapour are considered harmless, the above reactions bring an obvious emission benefit. The most significant contribution of the DOC is to
oxidize incoming NO to NO
2
which allow fast SCR reaction to reduce NOx as described in the equation 3.2c
2NH
3
+ NO + NO
2
2N
2
+ 3H
2
O Equation 3.2c
Therefore, the arrangement where DPF and DOC were designed in this investigation was crucial to provide sufficient NONO
2
ratio for optimum SCR reaction. The first instrumentation module was connected to the DOC to accommodate the EXSA, MEXA analyser, lambda sensor and
thermocouples for measuring the exhaust emissions downstream of the DPF and DOC and also monitoring exhaust temperature.
Figure 3.2 Final Assembly of the SCR Exhaust System.
Bypass pipe
DOC DPF
30
3.2.1 SCR Exhaust Fabrications and Specifications.