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.
The system was designed to have the option of a bypass system, but it was not used in the experiments described in this thesis so the pipes were capped. Pressure tapping was installed at the
cap for measuring the exhaust backpressure for the system. The bypass T-joint with pressure tapping is shown in figure 3.2.7.
Figure 3.2.7 Capped T-joint with pressure tapping.
3.2.8 DPF Monitoring and Preconditioning
The DOC, DPF and SCR catalysts were supplied by Johnson Matthey along with technical data and procedure for monitoring and preconditioning. As the engine ran an increase in backpressure
indicated that the DPF was being loaded with soot. Hence during the project the DPF was periodically cleared by blowing it out using a high pressure air supply. With a DPF system, it is
important to avoid uncontrolled regeneration especially under severe conditions such when the engine load is rapidly reduced. This could result in damage to the DPF due to overheating especially
when there the DPF is heavily loaded with soot. Throughout the experiment, close monitoring of the temperature and pressure across the SCR exhaust was undertaken using the thermocouples placed
at various locations across the exhaust. Monitoring and data logging was done using the Froude Consine Texcel v10 software.
35
3.2.10 SCR Catalyst Monitoring and Preconditioning
In the beginning of this test programme, the engine was run for sometime and it is assumed that the bricks were effectively de-greened.
3.3 EXSA 1500 NOx Analyser Setup
The EXSA 1500 NOx Analyser was supplied by Horiba Instruments limited. The operation of this analyser is described in the operating manual and is targeted for measuring emissions from small
engines ranging from two or four stroke gasoline and also diesels. It is capable of measuring CO, CO
2
, NOx, O
2
and THC simultaneously. This equipment is compatible with the SAE J1088 R standard. The
standard is a SAE recommended practice and Test Procedure for the Measurement of Gaseous Exhaust Emissions from Small Utility Engines. In this investigation, the EXSA 1500 NOx analyser was
used mainly to measure the engine out NOx level in the first instrumentation module as shown in figure 3.3.2. The EXSA 1500 was also used to measure NO in other locations of the SCR exhaust
system based on the test matrix.
3.3.1 EXSA 1500 Specifications and Resolutions
The EXSA 1500 utilizes a cross flow type Non Dispersive Infra Red NDIR sensor at normal temperature for measuring CO and CO
2
. For measurement of NO and NOx, a chemiluminescence detector CLD is used while O
2
is measured with a single coil type magnetic pressure. THC on the other hand is measured using a heating type Flame Ionization Detector FID. The specification of
EXSA 1500 is given in the table 3.3.1.
36 Table 3.3.1 Technical Specifications of EXSA 1500 Common gas analyser.
[Extracted from the Horiba Ltd, EXSA 1500 operating manual Oct 2004]
Detection Target: Gasoline engine 2-stroke, 4-stroke exhaust, GM diesel engine
exhaust gas Detection:
COCO
2
:NDIR - Non Dispersive Infra Red Detector NO NOx :CLD - Chemiluminescence Detector
O
2
:MPD – Magnetic Pressure Detector THC :HFID - Heated Flame Ionization Detector
Measurable Ranges Used CO: 0
～ 5000ppm
CO
2
: 0 ～
20 vol. THC: 0
～ 500 ppm C
NOx: 0 ～
1000 ppm O
2
: 0 ～
25 vol. AFR: 10-20
λ : 0.5 – 2.5 Repeatability:
±1 of Full Scale 90
％ percent respond:
15 seconds
3.3.2 Gas requirements and Calibration Gases
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