SCR System and Performance Innovation
The Euro 6/VI regulations concerning greenhouse gas emissions from vehicles present manufacturers with the challenge of reducing several different gases using several different technologies incorporated into the same exhaust system. The reduction of NOX emissions alongside the reduction of CO2 has been a particular challenge for diesel engine manufacturers, and this has accelerated the advances in selective catalytic reduction (SCR) technology to meet NOX emission standards.
The Euro VI legislation due to take effect in 2014 requires an 80% reduction in NOX emissions compared with the Euro V standard. It is also expected that there will be a limit set for ammonia slip to control the amount of ammonia released from an SCR system. Several manufacturers are developing systems and components that aim to achieve such high levels of efficiency.
Delphi SCR dosing system
Delphi Automotive has developed an SCR system that improves performance and reliability of the system, as well as reducing NOX and CO2 emissions. The new system, due to be introduced in 2012, injects aqueous urea solution into the SCR system at up to four times the pressure of previous systems, and reduces CO2 by up to 1g/km when compared with similar systems.
The system is made up of three components. The SCR doser, which is an injector with high pressure pump, injects the urea solution directly into the exhaust at a pressure of 20 bar. The urea solution is fed to the doser by a low pressure pump situated in the Urea Delivery module (UDM). The whole process is controlled by the electronics and software used to manage the system.
Rather than use a timed injector opening, the Delphi system uses a positive displacement solenoid pump, which delivers accurate measured quantities of urea solution regardless of the supply pressure.
The stringent standards that are set out in Euro 6/VI legislation for NOX emissions reduction will be followed by even stricter targets for 2020 when details of the EU’s Euro 7/VII regulations are announced. Manufacturers of vehicles, engines, and SCR systems are researching and developing systems and components which can improve performance and efficiency, to enable them to meet these standards.
New innovations in components are being designed continually as the industry gathers pace, and two of the most recent advances are detailed below.
Delphi SCR ammonia (NH3) sensor technology
Due for initial production in 2012, Delphi has developed an ammonia sensor for the automotive industry, which enables more efficient emissions control. The system allows the SCR to work at optimized NOX conversion rates, which in turn improves fuel efficiency.
The sensor dynamically measures the content of ammonia in emissions and the system can adapt the urea dosage accordingly. Some existing SCR technology use a timed urea dosing process; without accurate means of measurement and control excess and residual ammonia can escape via the exhaust stream. It is anticipated that part of the EU directives will relate to the control of ammonia slip from SCR systems, so the monitoring of ammonia levels will soon be an essential requirement.
The ammonia sensor is placed downstream of the SCR catalyst and NOX sensor; it detects the levels of ammonia in the exhaust stream over a range of 0-100ppm. The sensor converts the ammonia into an electrical signal that the electronic control system uses to control the urea solution dosage.
The ammonia sensor is the first technology that enables an SCR system to have a direct closed loop control system; allowing the optimisation of the urea dosage to be controlled by measurement of the emissions.
Future innovation and design
SCR systems have been developed and improved upon very quickly since the EU implemented their emissions strategy and the standards for the industry to comply with. To develop SCR systems to meet requirements of Euro 6/VI regulations, and systems that can be used beyond 2020, manufacturers will require more innovative designs to provide even greater efficiency.
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