The present invention relates to a novel exhaust after- treatment system for a diesel engine, said system compris- ing a selective catalytic reduction (SCR) converter sec ^¬ tion, an ammonia slip catalyst (ASC) section, a diesel oxi ^¬ dation catalyst (DOC) section and a diesel particulate fil ^¬ ter (DPF) section. Thus, the invention relates to a system for reducing emission of nitrogen oxides (NOx) and particu- lar matter being present in the exhaust from a diesel en ^¬ gine. In particular, the system of the invention provides an improved reduction of NOx during cold start of the en ^¬ gine . The system according to the invention is of special interest for vehicles, such as trucks and passenger cars with diesel post injection options.

Legislation in the field of combustion engine exhausts is requesting very low emissions from HD (heavy duty) trucks and passenger cars. At the same time, the test cycles used are becoming colder. It is normally difficult to reduce the NOx in these cold cycles, especially when the so-called af- tertreatment systems are becoming more complicated with many different functionalities as hydrocarbon oxidation, soot filtration, NOx reduction and ammonia slip reduction. The most general and viable system consists of components in the following order: Oxidation catalyst, soot filter, NOx reduction catalyst and ammonia slip catalyst.

The Euro 6 legislation is focused on exhaust aftertreatment systems that reduce NOx emissions. SCR systems for selec- tive catalytic reduction using a reducing agent (urea/water solution) which is precisely injected and uniformly dis ^¬ tributed in the exhaust system have the potential for high conversion rates. This increases the scope for the treat- ment of nitrogen oxide raw emissions and is likely to have a corresponding positive impact on fuel consumption and CO ₂ emissions. Overall exhaust aftertreatment can be signifi ^¬ cantly optimized by raising the exhaust gas temperature be ^¬ fore the DOC and before the SCR catalysts.

Various factors can affect the efficiency of an SCR system. These include important limiting operating conditions, such as the quality of the evaporation of the urea/water solu ^¬ tion or the degree of uniform distribution of the ammonia formed by the urea/water solution upstream of the SCR cata ^¬ lyst, as well as criteria that affect the catalysts them ^¬ selves, including the DOC and the SCR catalyst.

The conventional exhaust system of modern cars with lean burning engines is equipped with a sequence of an oxidation catalyst, a particulate filter and a catalyst for the se ^¬ lective reduction of NOx (SCR) in presence of a reduction agent . Oxidation catalysts being active in the oxidation of vola ^¬ tile organic compounds and carbon monoxide and SCR cata ^¬ lysts are known in the art and disclosed in numerous publi ^¬ cations . Particulate filters typically used are the so-called wall flow filters with a plurality of inlet and outlet channels. The inlet channels are closed at their outlet and the out- let channels are closed at their inlet, so that the gas flowing into the filter is forced through porous walls de ^¬ fining the channels, whereby particulate matter is filtered off the gas.

In the SCR treatment, ammonia is commonly employed as the reducing agent. Ammonia is a noxious compound and it is preferred to generate ammonia in situ by thermal decomposi ^¬ tion of a urea solution being injected as ammonia precursor into the hot exhaust gas upstream the SCR catalyst.

Even if urea is innocuous and relatively easy to store on board of a car, the use of a liquid solution of urea as a precursor of ammonia reducing agent is problematic, in par- ticular in the cold start phase of the engine, i.e. when the exhaust gas temperature is below 200 C.

When injected as liquid solution in the exhaust gas, urea decomposes to ammonia in sufficient amounts for the SCR on- ly at temperatures from about 200 C.

The most difficult emission to handle is the Ox, and in order to achieve a good cycle result, the NOx reduction needs to start early in the certification cycle. In the conventional exhaust gas cleaning systems as described above, the SCR catalyst is placed as the third exhaust gas cleaning unit in the systems, and it takes some time after the engine has been started, before this catalyst reaches its reaction temperature.

The present invention represents the second generation of the applicant's High Efficiency Exhaust Aftertreatment (HEAT) system developed a few years ago. The system is de ^¬ signed for Euro 6 and beyond, which means that it comprises components for Nox reduction (DeNOx) , ammonia slip removal, oxidation of hydrocarbons and CO and particulate filtra- tion.

Fig. 1 shows the conventional Euro 6 system having the par ^¬ ticulate filtration placed upstream the DeNOx components, while the first generation of the HEAT system shown in Fig. 2 is characterized by having the particulate filtration placed downstream the DeNOx components.

The present invention, i.e. the second generation of the HEAT system, is shown in Fig. 3. The main difference be- tween the first generation of the HEAT system (HEAT-1) and the present invention (HEAT-2) is that in HEAT-1, urea is injected between the engine and the muffler, while the die- sel is injected at a point between SCR-ASC and DOC-DPF. In HEAT-2, both urea and diesel are injected upstream of the SCR-ASC-DOC-DPF system.

A number of prior art publications describe exhaust after- treatment systems. Thus, the applicant's HEAT-1 system is disclosed in EP 2 399 011, where a method for purification of exhaust gas from a diesel engine by removal of parti ^¬ cles, incompletely combusted hydrocarbons, carbon monoxide and nitrogen oxides (NOx) is described with special focus on a simple, but effective method for filter regeneration. In US 6,863,874 a method is described, by which impurities in an exhaust gas are removed by oxidation followed by a filter, where soot is oxidized by nitrogen dioxide and oxy- gen. Further downstream, a reductant is injected into an NOx absorber, and subsequently a three-way catalyst or a catalyst for selective reduction is installed. US 8,484,954 describes a method and a device for selective ^¬ ly purifying the exhaust gas of an internal combustion engine using an SCR catalytic converter, wherein a reducing agent is metered to the exhaust gas prior to entry into the SCR catalytic converter. An overdose of the reducing agent is added to the exhaust gas which, after exiting the SCR catalytic converter, is fed in part through an oxidation catalytic converter and in part to a second SCR catalytic converter via a bypass circumventing the oxidation catalytic converter. In the oxidation catalytic converter, one half of the excess N¾ flowing out of the first SCR cata ^¬ lytic converter is converted to NOx and reacts to form non- hazardous nitrogen after mixing with the other half of the excess N¾ flowing unchanged through a bypass. US 2011/0047994 discloses an exhaust gas purification appa ^¬ ratus including an exhaust gas passage, a first oxidation catalyst, a selective catalytic reduction catalyst, an oxi ^¬ dation-reduction catalyst and a urea/water supply device. Exhaust gas is flowed through the first oxidation catalyst disposed in the exhaust gas passage. The selective catalyt ^¬ ic reduction catalyst is disposed downstream of the first oxidation catalyst. The oxidation-reduction catalyst is disposed downstream of the selective catalytic reduction catalyst. The oxidation-reduction catalyst has reducing property and oxidizing property which are influenced by temperature, wherein the oxidizing property of the oxida ^¬ tion-reduction catalyst is greater than the reducing prop- erty of the oxidation-reduction catalyst under a tempera ^¬ ture that is higher than a temperature under which the re ^¬ ducing property of the oxidation-reduction catalyst is greater than the oxidizing property of the oxidation- reduction catalyst. The urea/water supply device supplies urea/water upstream of the selective catalytic reduction catalyst .

From DE 10 2004 049 289, an exhaust aftertreatment system for a diesel engine is known. The system comprises, in se ^¬ ries, a section for selective reduction of nitrogen oxides, a section with an oxidation catalyst and a section with a diesel particulate filter. Feed pipes to supply auxiliary agents, e.g. ammonia and urea, are arranged upstream of the SCR catalytic converter section in the flow direction of the exhaust gas. The SCR is installed upstream of the oxi ^¬ dation catalyst section, which in turn is installed upstream of the particle filter section in the flow direction of the exhaust gas. So the system configuration is urea- SCR-DOC-DPF in the flow direction of the exhaust gas.

As already mentioned, the configuration of the system ac ^¬ cording to the invention (HEAT-2) is different from both the conventional Euro 6 system, having the particulate fil- tration placed upstream the DeNOx components, and the first generation of the HEAT system (HEAT-1), having the particulate filtration placed downstream the DeNOx components. In the conventional system, exhaust gas flows from the en ^¬ gine to an oxidation catalyst (DOC) . Then the oxidized ex- haust gas is passed to the diesel particulate filter (DPF) . After this, a reductant such as urea, typically an aqueous solution of urea, is injected into the gas, and the mixed gas is then introduced into a catalyst for selective cata ^¬ lytic reduction (SCR) , from where the exhaust gas passes an ammonia slip catalyst (ASC) and leaves the system as a cleaned gas .

The DOC is a diesel oxidation catalyst containing Pt for O ₂ formation, and the DPF is a catalyzed soot filter which also can contain Pt for O ₂ formation. The SCR catalyst is typically a zeolite-based catalyst.

The HEAT-1 system has the reverse configuration compared to the conventional system, i.e. the SCR-ASC upfront the DOC- DPF, and it presents advantages over the conventional sys ^¬ tem, especially as regards effective soot filter regenera- tion. It also secures high NOx conversion at cold start.

The HEAT-2 system of the present invention is novel, both over conventional systems and HEAT-1, in a number of ways: (1) All the exhaust system components ("bricks") can be mounted directly after each other, i.e. SCR-ASC-DOC-DPF .

(2) The bricks can be canned together, i.e. built into the same housing. The following conditions (3) and (4) have to be fulfilled for conditions (1) and (2) to apply:

(3) Both urea and diesel can be injected between the engine and the muffler. (4) The SCR can sustain and partially oxidize diesel hydro ^¬ carbons, which occasionally need to be injected in order to increase the exhaust temperature for DPF regeneration. If sufficient oxidation of hydrocarbons and carbon monoxide over the SCR and ammonia slip catalyst is achieved, it will potentially be possible to avoid the DOC, i.e. remove it. Hence :

(5) The DOC may be removed, leaving a simplified system consisting only of SCR-ASC-DPF. The system according to the present invention presents the following advantages over both its predecessor (HEAT-1) and the conventional systems adapted to Euro 6: a significantly reduced system pressure drop ΔΡ,

- a significantly reduced system volume,

an increased cold start performance compared to

conventional Euro 6 systems,

significantly reduced costs because of simple manufac ^¬ turing and potential removal of the Pt-containing DOC, and - possibility of using the system as extension or modifi ^¬ cation to Euro 4/5 V ₂ 0 ₅ -based SCR systems.