117 • The comparative analysis of the investigation with the use of urea spray and ammonia gas was
described and lead to NO and NO
2
conversion efficiency with the use of different SCR bricks length.
• Insight into the behaviour of the urea droplets in the investigation was obtained. It show that from half to three quarter of droplet from spray remained unconverted to ammonia gas at the
entry of first SCR brick. About 200 ppm ammonia released from droplet react in the SCR brick and between 10 to 100 ppm of potential ammonia passed through the first bricks as droplets.
This occurs from the conditions of NOx matched spray input to excess spray. • The CFD model for gas provide reasonable predictions for the long bricks while the short brick
shows breakthrough of all species due to high space velocity. The reaction kinetics used from literature was able to show some ability to describe the species profiles within the SCR bricks.
• The most significant findings in this study is the higher NO
2
conversion efficiency for 1 SCR brick compared to NO. This cannot be described by the fast SCR kinetic scheme.
5.8 Recommendation for Future Work.
Throughout the investigations, many areas have been identified for future work in order to optimise the SCR system working in the real application. Some of the identified areas include the exhaust gas
analyser, dosing system, more robust spray design, spray position and angle into the exhaust stream, reduced length of the SCR system and also the transient study with the SCR.
5.8.1 Improved gas analyser to measure NOx in presence of ammonia.
Most of the time spent in this investigation involved trying to obtain reliable measurements of NOx, NO and NH
3
upstream and downstream of the SCR brick. The CLD based analyser clearly causes a lot of setback in this investigation and variations in the results. A FTIR Fourier Transfer Infra Red based
analyser was recently identified as better candidates for investigation with the use of urea and ammonia of this magnitude. The response time of the analyser was crucial in getting this information
as the phase changes of the species within the exhaust gases need to be fully captured.
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5.8.2 Spray Dosing System.
Ideally a closed loop feedback spray dosing system would be desirable for this investigation. A manual over-ride system also need to be incorporated, taking into consideration of cold start
condition. The system integrated with the engine ECU unit is under heavy development by many automotive suppliers for this purpose.
5.8.3 Cleaning of spray or continuous spraying
To avoid having to clean the spray injector, a more robust spray design is needed to suit the light duty application. Continuous spraying into the exhaust would definitely not be appropriate, but
should be covered by the closed loop feedback spray dosing system mention earlier. As for the cleaning, perhaps the solution for this lies with the concentration of urea solution used or a better
designed spray to avoid any deposit build up.
5.8.4 Improved warm up and system using sequential program.
The control software for the engine test bed is capable of programming of the sequence for setting up the engine warm up and cool down period, calibrating the analyser, periodic parameters logging
and many other task. As the investigations were conducted, very limited time was spent on this side of the program due to other difficulties and challenges faced with the analyser and the spray system.
The analysers control from the test bed program was not configured for this investigation. In the future, this should be seriously considered to have better control and monitoring sequence.
5.8.5 Signal trigger improvement with level differentiation of spray pulses and gas settings.
