The first step when designing an exhaust emission control system is to decide which pollutants need to be reduced, and by how much.
It is generally not a good idea to attempt to reduce all types of pollutants unless absolutely necessary since this requires expensive, complicated exhaust aftertreatment systems which take up a lot of space. Where the priority is to obtain a very large reduction in nitrogen oxides (‘Nox’) levels, a standalone SCR sytem such as the example shown below is ideal.
As can be seen, the system is fairly complex and incorporates numerous components including an ECU.
This contains the software to control the Adblue dosing pump, as well as various diagnostic functions.
After installation the system needs to be commissioned which involves programming the ECU and testing the operation of the whole system. Once in the field, the ECU will monitor the performance of the system and generate error messages if anything unexpected occurs.
Ideally an SCR system should be regularly inspected by the company which supplied and installed it, which provides an opportunity to download error messages and make adjustments to the settings if necessary. However in some cases it may be possible to monitor the system remotely using telemetry.
Although the primary aim of a standalone SCR system is to reduce Nox, it can also achieve worthwhile reductions in the emissions of carbon monoxide and hydrocarbons, typically in the range of 50-75%.
In the case of diesel particulate matter, a standalone SCR system can reduce the ‘soluble organic fraction’ but not the elemental carbon. Therefore the overall reduction in diesel particulate matter resulting from this type of system is usually in the range of 10-40%.
Standalone SCR systems are suitable for both OE and retrofit applications, and are often specified for mobile machinery used in confined spaces as well as large stationary engines which are located in sensitive areas.
(SCR Only Schematic)
