The use of diesel in the future raises many questions. How does it affect our air quality? How can emissions limits be met? How will diesel engines run in the future? And how can nitrogen oxide levels be reduced further for diesel vehicles? A technology from Bosch is already today able to provide answers to some of the questions. Bosch has succeeded in improving the already technologically highly advanced diesel powertrain – not through an optimization directly on the engine but with an innovation in the exhaust gas system.
Selective catalytic reduction with double-injection technology (DI-SCR) is a solution with which diesel vehicles can emit less nitrogen oxide and at the same time use AdBlue® more efficiently. A special feature of this innovation is that exhaust emissions can be kept low in all driving situations, whether it be on the freeway or in stop-and-go traffic in the city. The new DI-SCR exhaust-gas treatment can therefore provide a smart answer to the question of how it will be possible to maintain compliance with future emissions limits – and it provides smart answers to questions regarding the future viability of the diesel engine and the air quality in cities.
SCR catalytic converters and ammonia as a reducing agent are used for the reduction of nitrogen oxides. To produce ammonia, an aqueous urea solution with the trade name AdBlue® is injected into the exhaust gas system using Bosch Denoxtronic technology. The urea is converted into ammonia by thermolysis and hydrolysis. In the SCR catalytic converter, the ammonia subsequently reduces the nitrogen oxides in the exhaust gas to water and nitrogen. Temperature is a crucial factor in exhaust-gas treatment. SCR catalytic converters only start working efficiently to reduce the nitrogen oxides in the exhaust gas from a temperature of around 170 °C.
High temperatures, such as those occurring during regeneration of the diesel particulate filter, negatively impact the conversion rate of the close-coupled SCR catalytic converter located near the engine.
The exhaust gas temperatures are dependent on the engine load. Whether a car is driving in city traffic or on the freeway will have an effect on the temperature and therefore on the nitrogen oxide reduction. Sophisticated thermal management that is specially adapted to the vehicle is therefore required. So, what does an exhaust-gas treatment look like that is able to optimally deal with the wide range of temperatures occurring and at the same time improve nitrogen oxide reduction?
Bosch believes the SCR with double-injection technology (DI-SCR) is the way forward. Compared with conventional exhaust-gas treatment, the AdBlue® injection in Bosch’s DI-SCR solution takes place at two points in the exhaust gas system: one injection is in a close-coupled SCR catalytic converter that is located near the engine, and another is in an underfloor SCR catalytic converter situated further away from the engine.
The close-coupled SCR catalytic converter is implemented by means of a coating on the diesel particulate filter; and the underfloor SCR catalytic converter is a pure catalytic converter that is located in the vehicle’s underbody. Thanks to the thermal decoupling, it is possible to deploy the AdBlue® efficiently and effectively independently of the engine load and the diesel particulate filter regeneration phases. Over-injection of urea and the resulting ammonia slip are thus largely prevented.
This is made possible by smart software in the engine control unit that uses a newly developed “kinetic model” to precisely calculate which injection quantity is needed for the respective SCR catalytic converter. Sensors placed upstream and downstream from the SCR catalytic converters supply accurate measurements about the chemical substances present in the exhaust gas system. The two dosing modules inject AdBlue® into the exhaust gas system upstream from the SCR catalytic converters according to the specific needs.
With this technology, it is now possible for diesel vehicles to achieve low nitrogen oxide values. It therefore makes an important contribution to ensuring that compliance with emissions limits can be maintained.
Lower nitrogen oxide levels support the transition to more environmentally friendly mobility
Nitrogen oxide reduction is achieved with less AdBlue®