In currently available vehicles, urea, for example, a reductant is used in SCR systems (where “SCR” is an abbreviation for “selective catalytic reduction”) comprising an SCR catalyst. The reductant and NOx gas can react in which the catalyst and be converted to nitrogen gas and water. Different types of reductants can be used in SCR systems. One commonly used reductant is, for example, a reductant under the trademark, AdBlue. Such an SCR system may be applied also at a stationary application.
The SCR system may also include also a diesel oxidation catalyst (DOC). The DOC is arranged, among other things, to convert NO gas from an engine to NO2 gas. In prior art technology, the DOC is arranged upstream of the SCR catalyst and upstream also of the position for the dosage of reduction agent, where relevant.
One type of SCR system includes a container that contains a reductant. The SCR system also has a pump configured to pump up the reductant from the container through a suction pipe and to supply it through a pressurized pipe to a dosage unit that is arranged at an exhaust gas system on the vehicle, for example, at an exhaust pipe at the exhaust gas system. The dosage unit is configured to inject a required amount of reductant into an exhaust gas system upstream of the SCR catalyst according to drive routines that are stored in a control unit in the vehicle.
There is an ever-present need to reduce the amount of emissions from engines in motor vehicles. This applies to heavy motor vehicles such as, trucks and buses, since legal requirements for emissions are continuously being tightened.
During a “cold start” of an SCR system at the motor vehicle, it takes a certain period until the components that make up the system achieve an appropriate operating temperature. No reducing agent is dosed during this period, which means that it is then not possible to reduce undesired emissions to a suitable extent. Dosage during this period does not take place in order to avoid formation of deposits of reducing agent in the exhaust gas cleaning system upstream of the SCR catalyst. In certain cases, it may be a period of up to 10 minutes, during which an engine creates emissions without reducing agent being dosed.
US20050069476 describes a system in which the dosage of reducing agent is carried out in association with switching off an engine of a vehicle to achieve the storage of ammonia in an SCR catalyst.
US20120023906 also describes a system in which the dosage of reducing agent is carried out in association with switching off an engine a vehicle to achieve the storage of ammonia in an SCR catalyst.
These solutions, however, have a number of disadvantages. It is necessary, for example, for an exhaust gas treatment system in this case to demonstrate a rather high temperature in order for the reducing agent to be vaporized in an appropriate manner. Furthermore, the engine will need to be operated, and in this case to produce a flow of exhaust gas, such that it is possible to add the dosed reducing agent to the SCR catalyst. There is also an imminent risk with the prior art systems that that unreasonably large quantities of “urea deposits” are formed. These may persist permanently or they may often increase in size if dosage is continuous, which will reduce the performance of the SCR system and may sometimes lead to a sound absorber, including the SCR catalyst, needing to be repaired or exchanged, with very high costs as a consequence.