GB2521425A

GB2521425A - Exhaust system for a diesel engine

Info

Publication number: GB2521425A
Application number: GB1322570.1A
Authority: GB
Inventors: Davide Mercuri; Claudio Di Pierro
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2013-12-19
Filing date: 2013-12-19
Publication date: 2015-06-24
Also published as: GB201322570D0

Abstract

An exhaust system 270 for a Diesel engine comprises an exhaust pipe 275, a selective catalytic reduction (SCR) system 283 located in the exhaust pipe, a first Diesel oxidation catalyst (DOC) 280 located in the exhaust pipe upstream of the SCR, and a second DOC 281 located in the exhaust pipe between the first DOC and the SCR. The first DOC is configured to oxidise hydrocarbon (HC) and carbon monoxide (CO) into carbon dioxide (CO2), and the second DOC is configured to oxides nitrogen monoxide (NO) into nitrogen dioxide (NO2). Each DOC may contain a mixture of Platinum (Pt) and Palladium (Pd), the ratio of Platinum to Palladium in the first DOC being smaller than that in the second DOC. The first DOC may also be an NOx adsorber. The DOCs may be on a single substrate, or on separate substrates, and in a single casing. A Diesel particulate filter (DPF) may be located downstream of the second DOC, and may be integrated with the SCR as an SCRF.

Description

EXHAUST SYSTEM FOR A DIESEL ENGINE

TECHNICAL FIELD

The present disclosure generally relates to an exhaust system for an internal combustion engine, particularly a Diesel engine of a motor vehicle such as a passenger car.

BACKGROUND

It is known that a Diesel engine generally comprises an engine block defining one or more cylinders, each of which is closed by a cylinder head and accommodates a reciprocating piston that cooperates with the cylinder head to define a combustion chamber. A fuel and air mixture is cyclically disposed in the combustion chamber and ignited, thereby resulting in hot expanding exhaust gases causing reciprocal movement of the piston that rotates a crankshaft.

After the expansion, the exhaust gases exit the combustion chamber and are directed into an exhaust system, which generally includes an exhaust pipe having one or more after-treatment devices configured to filter and/or change the composition of the exhaust gases, such as for example a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (e.g. DPF), and a Selective Catalytic Reduction (5CR) system.

The DOC is a device designed to oxidize hydrocarbons (HC) and carbon monoxides (CO) into carbon dioxides (C02) and water. To perform this task, the DOC conventionally com-prises a catalyst substrate, a washcoat applied to the catalyst substrate and a mixture of platinum group materials (PGM) dispersed in the washcoat. The catalyst support is usually a ceramic or metallic monolith (brick) with a honeycomb structure, which is designed to provide a large surface area. The washcoat is a carrier for the PGM mixture and is used * 25 to disperse these materials over a large surface area. The washcoat may be realized with different technologies and materials, such as aluminum oxide, titanium dioxide, silicon di- * oxide, or a mixture of silica and alumina. Washcoat materials are selected to form a rough, *.* irregular surface, which greatly increases the surface area compared to the smooth sur- * ** face of the bare substrate. This maximizes the catalytically active surface available to react *:*.; 30 with the engine exhaust gases. The PGM mixture is suspended in the washcoat prior to applying the washcoat to the catalyst substrate. The PGM mixture generally comprise plat-*1 mum (Pt), which is mainly used to increase the oxidation capacity of the catalyst, and pal-ladium (Pd), which is mainly used to increase the thermal stability of the catalyst. The specific quantities of palladium and platinum in the PGM mixture are chosen to achieve a trade-off between these two effects and may vary on the basis of specific engine manu-facturer requests.

The DPF is a device designed to remove Diesel particulate matter or soot from the exhaust gases. The most widely used DPFs comprise a ceramic or polymeric core, which is very similar to the catalyst substrate of the DCC. However, the DPF core has alternate channels plugged, so that the plugs force the exhaust gases to flow through the walls and the par- ticulate collects on the inlet face. Other DPFs may comprise porous media made of ce-ramic or metallic materials.

The 5CR system is finally used to convert nitrogen oxides (NOr), namely nitrogen monox-ides (NC) and nitrogen dioxide (NO2), into diatomic nitrogen (N2) and water. The SCR system generally comprises a 5CR catalyst located in the exhaust pipe downstream of a SCR injector provided for injecting in the exhaust pipe a Diesel Exhaust Fluid (DEF), typi-cally a solution of Urea. The DEF mixes with the exhaust gases and is absorbed inside the 5CR catalyst, where it allows the reactions that cause the conversion of the nitrogen ox-ides. However, in order to achieve highest NO conversion efficiency, particularly at low operating temperatures, it is has been found that the exhaust gases entering the selective reduction catalyst should contain a quantity of NO2 as near as possible to an half of the overall quantity of NON: NO2

NO

According to a standard exhaust system layout, the SCR system is located in the exhaust pipe in an under-floor position, downstream of a closed coupled aftertreatment device that is located near the Diesel engine and that include a DOC combined with (i.e. immediately followed by) a DPF. With this configuration, it is pretty easy to achieve a good NO con-version efficiency in the 5CR system. Indeed, even if the DCC may have the side effect of reducing the incoming NC2 into NO (due to the presence of HC at the DCC inlet), the DPF * * has the opposite effect of promoting the oxidation of the NC into NO2, so that the percent-age of NO2 at the SCR catalyst inlet is generally high enough to guarantee a good NO conversion efficiency.

By contrast, this kind of exhaust system layout may not be able to achieve a sufficient reduction of NO emissions in case of very low CO2 emission level that will be required in the future by the most severe legislation.

Other exhaust system layouts designed to solve this drawback comprise a closed coupled DOC immediately followed by an SCR system and a DPF combined with the SCR catalyst.

This layout allows increasing the temperature on the SCR catalyst quicker, thereby leading to higher a NO conversion efficiency, which also allows a reduction in CO2 emissions.

However, as long as the Diesel engine operates at low temperatures, this layout is not able to guarantee the right percentage of NO2 at the inlet of the SCR catalyst, thereby significantly worsening the NO conversion efficiency in that conditions. Besides, the use of warm-up strategies would not be of any help to achieve the right NO2INOx ratio, due to HG presence into the exhaust flow in front of the DOG. Indeed, conventional warm-up strategies would not raise the temperature up enough to meet the required efficiency (for the above-mentioned reasons), whereas stronger warm-up strategies would lead to an unacceptable increase of CO2 emissions.

SUMMARY

In view of the above, an object of an embodiment of the present invention is that of provid-ing an exhaust system that can solve, or at least positively reduce, the above-mentioned drawbacks.

Another object is that of reaching this goal with a simple, rational and rather inexpensive solution.

These and other objects are achieved by the characteristics of the embodiments of the invention as reported in the independent claims. The dependent claims contains preferred * :* * 25 or particularly advantageous secondary features of the various embodiments of the inven-tions.

In particular, an embodiment of the invention provides an exhaust system for a Diesel engine comprising an exhaust pipe, a selective catalytic reduction system, a first Diesel oxidation catalyst upstream of the selective catalytic reduction system, and a second Die-****** * 30 sel oxidation catalyst located between the first Diesel oxidation catalyst and the selective * S. catalytic reduction system, wherein the first Diesel oxidation catalyst is configured to oxi-dize hydrocarbons and carbon monoxides into carbon dioxides, and the second Diesel oxidation catalyst is configured to oxidize nitrogen monoxides into nitrogen dioxides.

By virtue of this solution, the first Diesel oxidation catalyst performs the functions of tradi-tional Diesel oxidation catalysts, whereas the second oxidation catalyst has the effect of increasing the percentage of NO2 in the exhaust gases upstream of the 8CR catalyst, particularly at low operating temperatures, so that the NO2/NO ratio is high enough to enhance the NO conversion efficiency.

According to an embodiment of the invention, the first and the second Diesel oxidation catalyst may differ from each other in at least one of the following characteristics: mixture of platinum group metals, washcoat carrying the mixture, catalyst substrate material and volume.

This embodiment provides a simple solution to configure the two Diesel oxidation catalysts to perform their respective tasks.

According to another embodiment of the invention, the platinum group metals mixture of both the first Diesel oxidation catalyst and the second Diesel oxidation catalyst may contain platinum and palladium, wherein the ratio of platinum to palladium in the first Diesel oxida-tion catalyst is smaller than the ratio of platinum to palladium in the second Diesel oxidation catalyst (each of these ratios may be more precisely defined as the ratio of the weight of platinum to the weight of palladium).

In this way, the high quantity of palladium in the first Diesel oxidation catalyst has the advantage of reducing the impact of aging on the catalyst efficiency, whereas the high quantity of platinum in the second Diesel oxidation catalyst has the advantage of prompting the oxidation of NO into NO2.

According to another embodiment of the invention, the first Diesel oxidation catalyst may *.... 25 be configured as a NO adsorber.

In other words, the catalytic substrate of the first Diesel oxidation catalyst may be coated * with a special washcoat containing materials, such as zeolites and/or Alkali/alkaline oxides (carbonate), which are able to trap NO and NO2 molecules, acting as molecular sponges.

This solution has the advantage of allowing the first Diesel oxidation catalyst to retain the fl.

* 30 NO as long as the engine operating temperature is low, and to let them pass only when ** the temperature is sufficiently high to guarantee a high NO conversion efficiency in the 8CR system.

According to a different embodiment of the invention, the first Diesel oxidation catalyst and the second Diesel oxidation catalyst may be embodied in a single catalyst substrate.

In other words, a single catalyst substrate may be provided having two portions with dif-ferent washcoat and/or PGM mixture and/or volume, so that one of these portions has the capabilities of the first Diesel oxidation catalyst, particularly the capability of oxidizing hy- drocarbon and carbon monoxide into carbon dioxides and water, whereas the second por-tions has the capabilities of the second Diesel oxidation catalyst, particularly the capability of oxidizing nitrogen oxides into nitrogen dioxides. This solution has the advantage of providing an aftertreatment device that is very compact and easy to install in the exhaust system.

According to an alternative embodiment of the invention, the first Diesel oxidation catalyst and the second Diesel oxidation catalysts may be embodied in two separated catalyst substrates.

This embodiment of the invention is advantageous in that the two separate Diesel oxidation catalysts may be easier to manufacture.

According to another embodiment of the invention, the first Diesel oxidation catalyst and the second Diesel oxidation catalyst may be accommodated in a same casing.

This embodiment of the invention has the advantage of simplifying the packaging of the Diesel oxidation catalysts.

Another embodiment of the invention provides that the selective catalytic reduction system may comprise a mixer located in the exhaust pipe between a Diesel exhaust fluid injector and a selective reduction catalyst.

The advantage of this embodiment of the invention is that of improving the efficiency of the SCR system, since the mixer enhance dispersion and vaporization of DEF in the ex-haust gases. * *

According to another embodiment of the invention, the exhaust system may further corn- * prise a Diesel particulate filter located in the exhaust pipe downstream of the second Diesel oxidation catalyst.

In this way, the exhaust system is advantageously able to remove Diesel particulate matter *.*.*.

* 30 or soot from the exhaust gases.

* Still another embodiment of the invention provides that the Diesel particulate filter and the selective reduction catalyst may be integrated in a single aftertreatment device.

In practice, the selective catalytic reduction washcoat may be applied directly onto the Diesel particulate filter substrate. In this way, the exhaust system is smaller and more compact.

The invention can be also embodied as a Diesel engine equipped with the exhaust system above and as a motor vehicle including such Diesel engine.

Another embodiment of the invention may provide a method of treating exhaust gases of a Diesel engine comprising the steps of: -oxidizing hydrocarbon and carbon monoxides contained in the exhaust gases into carbon dioxides by means of a first Diesel oxidation catalyst, -oxidizing nitrogen monoxides contained in the exhaust gases exiting the first Diesel oxi-dation catalyst into nitrogen dioxides by means of a second Diesel oxidation catalyst, -converting nitrogen monoxides and the nitrogen dioxides contained in the exhaust gases exiting the second Diesel oxidation catalyst into diatomic nitrogen by means of an SCR system.

This embodiment of the invention basically achieves the same advantages of the exhaust system described above, particularly that of increasing the percentage of NO2 at the SCR catalyst inlet, thereby enhancing its conversion efficiency.

Also in this embodiment, the first and the second Diesel oxidation catalyst may differ one another for at least one of the following characteristics: mixture of platinum group metals, washcoat carrying the mixture, catalyst substrate material and volume. Particularly, the platinum group metals mixture of both the first Diesel oxidation catalyst and the second Diesel oxidation catalyst may contain platinum and palladium, wherein the ratio of platinum to palladium in the first Diesel oxidation catalyst is smaller than the ratio of platinum to palladium in the second Diesel oxidation catalyst (each of these ratios may be more pre-cisely defined as the ratio of the weight of platinum to the weight of palladium). Moreover, the first Diesel oxidation catalyst may be configured as a NO adsorber. The first Diesel oxidation catalyst and the second Diesel oxidation catalyst may be embodied either in a single catalyst substrate or in two separate catalyst substrate, and may also be accommo-dated in a same casing.

The SCR system may provide for injecting a Diesel exhaust fluid in the exhaust gases exiting the second Diesel oxidation catalyst and the for conveying the exhaust gases con-taining the Diesel exhaust fluid into a SCR catalyst, possibly after having conveyed them through a mixer.

The method may finally comprise the step of removing the particulate mailer from the ex-haust gases exiting the SCR system by means of a Diesel particulate filter. The Diesel particulate filter be integrated in a single aftertreatment device with the selective reduction catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings.

Figure 1 schematically shows an automotive system.

Figure 2 is the section A-A of figure 1.

Figure 3 is a schematic representations of an exhaust system belonging to the automotive system of figure 1.

DETAILED DESCRIPTION

Some embodiments may include an automotive system 1001 as shown in Figures 1 and 2, that may belong to a motor vehicle, such as a car or a truck. The automotive system 100 includes an intemal combustion engine (ICE) 110, for instance a Diesel engine, having an engine block 120 defining at least one cylinder 125 having a piston 140 coupled to rotate a crankshaft 145. A cylinder head 130 cooperates with the piston 140 to define a combus- tion chamber 150. A fuel and air mixture (not shown) is disposed in the combustion cham- ber 150 and ignited, resulting in hot expanding exhaust gases causing reciprocal move-ment of the piston 140. The fuel is provided by at least one fuel injector 160 and the air through at least one intake port 210. The fuel is provided at high pressure to the fuel injec- : 25 tor 160 from a fuel rail 170 in fluid communication with a high pressure fuel pump 180 that increases the pressure of the fuel received from a fuel source 190. Each of the cylinders has at least two valves 215, actuated by a camshaft 136 rotating in time with the crankshaft 145. The valves 215 selectively allow air into the combustion chamber 150 from the port 210 and alternately allow exhaust gases to exit through a port 220. In some ex- ! 1 30 amples, a cam phaser 155 may selectively vary the timing between the camshaft 135 and . .; the crankshaft 145.

The air may be distributed to the air intake port(s) 210 through an intake manifold 200. An air intake dud 205 may provide air from the ambient environment to the intake manifold 200. In other embodiments, a throttle body 330 may be provided to regulate the flow of air into the manifold 200. In still other embodiments, a forced air system such as a turbo- charger 230, having a compressor 240 rotationally coupled to a turbine 250, may be pro-vided. Rotation of the compressor 240 increases the pressure and temperature of the air in the duct 205 and manifold 200. An intercooler 260 disposed in the dud 205 may reduce the temperature of the air. The turbine 250 rotates by receiving exhaust gases from an exhaust manifold 225 that directs exhaust gases from the exhaust ports 220 and through a series of vanes prior to expansion through the turbine 250. The exhaust gases exit the turbine 250 and are directed into an exhaust system 270. This example shows a variable geometry turbine (VGT) with a VGT actuator 290 arranged to move the vanes to alter the flow of the exhaust gases through the turbine 250. In other embodiments, the turbocharger 230 may be fixed geometry and/or include a waste gate.

The exhaust system 270 may include an exhaust gas recirculation (EGR) system 300 cou-pled between the exhaust manifold 225 and the intake manifold 200. The EGR system 300 may include an EGR cooler 310 to reduce the temperature of the exhaust gases in the EGR system 300. An EGR valve 320 regulates a flow of exhaust gases in the EGR system 300.

The exhaust system 270 may also include an exhaust pipe 275 conveying the exhaust gases from the exhaust manifold 225 (or the turbine 250 of the turbocharger 230, if pre-sent) to the extemal environment. The exhaust pipe 275 may have one or more exhaust aftertreatment devices, namely devices configured to filter and/or change the composition of the exhaust gases.

In the present example, the aftertreatment devices include at least two Diesel oxidation : 25 catalysts (DOCS) of different kinds, namely a first DOC 280 and a second DCC 281 located * in series in the exhaust pipe 275. Each one of these Diesel oxidation catalysts 280 and 281 generally comprises a catalyst substrate that may be a ceramic or metallic monolith (brick) with a honeycomb structure, which is designed to provide a large surface area. The catalyst substrate is coated with a washcoat, which is a carrier for the catalytic materials **S**. . * . 30 and is used to disperse these materials over a large surface area. The washcoat may be realized with different technologies and materials, such as aluminum oxide, titanium diox-ide, silicon dioxide, or a mixture of silica and alumina. Washcoat materials are selected to form a rough, irregular surface, which greatly in-creases the surface area compared to the smooth surface of the bare substrate. This maximizes the catalytically active surface avail-able to react with the engine exhaust. Each Diesel oxidation catalyst 280 and 281 further comprises a mixture of platinum group materials (PGM), which are the catalytic materials dispersed in the washcoat. The PGM mixture is suspended in the washcoat prior to apply-ing the washcoat to the catalyst substrate. The PGM mixture generally comprise palladium (Pd), which increase the thermal stability of the catalyst thereby reducing its deterioration due to aging effects, and platinum (Pt), which is mainly used as an oxidation catalyst.

The first DCC 280 is configured mainly to oxidize hydrocarbon (HC) and carbon monoxides (CC) into carbon dioxides (CC2) and water. In other words, the catalytic substrate (i.e. its constitutive material, dimensions, volume, etc.), as well as the washcoat (i.e. its constitu-tive material, production technology, etc.) and the PGM mixture (i.e. materials, quantities, etc.) of the first DCC 280 are chosen to achieve mainly the above mentioned effect, namely that of oxidizing hydrocarbon (HG) and carbon monoxides (CC) contained in the exhaust gases into carbon dioxides (C02) and water. In particular, the PGM mixture of the first DCC 280 may contain a quantity of palladium (Pd) and a quantity of platinum (Pt), wherein the ratio (Pt:Pd) of platinum to palladium (expressed in weight) is quite low, so as to en-hance the thermal stability and reduce the aging deterioration of the first DOC 280. By way of example, the ratio Pt:Pd of the first DOC 280 may be equal to 1:1. The catalyst substrate of the first DOG 280 may be metallic. The washcoat of the first DCC 280 may be a normal oxidation washcoat or even a NO adsorber. In the latter case, the washcoat may contain materials, such as zeolites and/or Alkali/alkaline oxides (carbonate), which are able to trap NO and NO2 molecules, acting as molecular sponges that retain the NC contained in the exhaust gases. *0e

: 25 This first DCC 280 may be located in the exhaust pipe 275 in a closed coupled position, * namely near to the Diesel engine 110, immediately downstream of the turbocharger 230 if present. The first DOC 280 may be accommodated inside an external casing 282, which locally enlarges the exhaust pipe 275 and defines a flow path for the exhaust gases that flow from the Diesel engine 110 towards the environment. Inside this external casing 282, : " 30 the first DOC 280 is placed to entirely intercept the flow path, so that all the exhaust gases are forced to flow through it.

Differently, the second DOC 281 is configured mainly to oxidize nitrogen monoxides (NO) into nitrogen dioxides (NO2). In other words, the catalytic substrate (i.e. its constitutive material, dimensions, volume, etc.), as well as the washcoat (i.e. its constitutive material, production technology, etc.) and the POM mixture (i.e. materials, quantities, etc.) of the second DOC 281 are chosen to achieve mainly the above mentioned effect, namely that of oxidizing the nitrogen monoxides (NO) contained in the exhaust gases into nitrogen dioxides (NO2). In this regard, the second DCC 281 may thus differ from the first DOG 280 for at least one of the above mentioned characteristics. In particular, the PGM mixture of the second DOG 281 may contain a quantity of palladium (Pd) and a quantity of platinum (Pt), wherein the ratio (Pt:Pd) of platinum to palladium (expressed in weight) is bigger than the ratio Pt:Pd of the first DOG 280, so as to enhance the oxidation effects and increase the quantity of NO2 in the exhaust gases. By way of example, the ratio Pt:Pd of the second DOC 281 may be equal to 4:1.

This second DOG 281 is located in the exhaust pipe 275 downstream of the first DOC 280, so that all the exhaust gases exiting the first DOC 280 are forced to pass also through the second DOG 281. In the present example, the second DOG 281 is located immediately downstream of the first DOG 280. In particular, the second DOC 281 may be accommo- dated inside the external casing 282, where it may completely intercept the flow path down-stream of the first DOG 280, thereby defining a single/integrated aftertreatment device.

The second DOG 281 may comprise a catalyst substrate that is separated from the catalyst substrate of the first DOG 280 (i.e. they may be realized as two separated bricks, which may be placed in mutual contact or with an air-gap). In other embodiments, the first and the second DCC 280 and 281 may share the same catalytic substrate. In other words, they may be embodied in a single monolithic body (brick) having two portions with different * washcoat and/or PGM mixture and/or volume, so that one of these portions has the capa-*Ia.

bilities of the first DOG 280, particularly the capability of oxidizing hydrocarbon and carbon monoxide into carbon dioxides and water, whereas the second portions has the capabili- *:"4 ties of the second DOC 281, particularly the capability of oxidizing nitrogen oxides into nitrogen dioxides.

The aftertreatment devices may further include an SCR system 283 located in the exhaust pipe 275 downstream of the second DCC 281, to convert nitrogen oxides (NOr) contained ** *j in the exhaust gases, namely nitrogen monoxides (NO) and nitrogen dioxide (NO2), into diatomic nitrogen (N2) and water. The SGR system 283 comprises a SGR catalyst 284, which is connected to the second DOC 281 through a portion 285 of the exhaust pipe 275 that is usually referred as connection tube. The 8CR catalyst 284 may be accommodated inside an external casing 286, which locally enlarges the exhaust pipe 275 and defines a flow path for an exhaust gases that flows towards the environment. Inside this external casing 286, the SCR catalyst 284 is placed to entirely intercept the flow path, so that all the exhaust gases are forced to flow through it. The 8CR system 283 may further comprise an DEF injector 287 located in the conneclion tube 285, to inject in the exhaust pipe 275 a Diesel Exhaust Fluid (DEF), typically a solution of Urea. In this way, the DEF mixes with the exhaust gases and is absorbed inside the SCR catalyst 284, where it allows the reac-tions that lead to the conversion of the nitrogen oxides. In order to improve the mixing and the vaporization of the DEF, the SCR system 283 may also comprise a mixer 288 located in the connection tube 285 between the DEF injector 287 and the 8CR catalyst 284. In particular, the mixer 288 may be located at the inlet of the external casing 286. The mixer 286 may be embodied as a stationary swirler, namely a sort of stationary vane wheel that radially deviates the oncoming exhaust gases, thereby generating a turbulence that en-hance the mixing and vaporization of the DEF, particularly in the cross stream direction.

The aftertreatment devices may finally include a Diesel particulate filter (DPF) 289 located in the exhaust pipe 275 downstream of the second DOC 281, to remove Diesel particulate mailer or soot from the exhaust gases. In general, The DPF 289 may comprise a ceramic or polymeric core, which is very similar to the catalyst substrate of a conventional DOG.

However, the DPF core has alternate channels plugged, so that the plugs force the ex-haust gases to flow through the wall and the particulate collects on the inlet face. In other embodiments, the DPF 289 may comprise porous media made of ceramic or metallic ma-terials. In the present example, the DPF 289 and the SCR catalyst 284 are integrated to form a single aftertreatment device. In practice, the washcoat of the 8CR catalyst 284 is applied directly onto the core of the DPF 289.

Thanks to the above described layout, the exhaust system 270 operates as follows. The exhaust gases coming from the Diesel engine 110 pass first through the first DOG 260. At low operating temperature, the first DOG 280 absorbs the hydrocarbon (HG) contained in * * .. 30 the exhaust gases and, as soon as a light off temperature is achieved, oxidizes the trapped *:*.; HG and CO into GO2 and water. In this way, the second DOC 281 receives exhaust gases having a low HG content, so that it is advantageously able to oxidize part of the nitrogen oxides (NO) contained in the exhaust gases into nitrogen dioxides (NO2). As a conse-quence, the ratio between NO2 and NO (i.e. NO2 and NO) in the exhaust gases at the inlet of the SCR catalyst 284 is always sufficiently high and near to the optimal value of 0.5 which guarantees high NO conversion efficiency, and which can also be used to reduce CO2 emissions.

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary em- bodiment, it being understood that various changes may be made in the function and ar-rangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents. a.. * * * * * * * S. S

S S S S 55

REFERENCES

automotive system Diesel engine 120 engine block cylinder cylinder head camshaft piston 145 crankshaft combustion chamber cam phaser fuel injector fuel rail 180 fuel pump fuel source intake manifold 205 air intake duct 210 intake port 215 valves 220 exhaust port 225 exhaust manifold 230 turbocharger 240 compressor 250 turbine * 260 intercooler 270 exhaust system 275 exhaust pipe 280 first DOC *:*.* 30 281 second DOC *". : 282 external casing * 0 283 SCR system 284 5CR catalyst 285 connection tube 286 external casing 287 DEF injector 288 mixer 289 DPF 290 VGT actuator 300 exhaust gas recirculation system 310 EGR cooler 320 EGR valve 330 throttle body S...

S..... * . * *

*.S..* * . *5 * * . S *5

Claims

CLAIMS1. An exhaust system (270) for a Diesel engine (110) comprising an exhaust pipe (275), a selective catalytic reduction system (283), a first Diesel oxidation catalyst (280) located upstream of the selective catalytic reduction system (283), and a second Diesel oxidation catalyst (281) located between the first Diesel oxidation catalyst (280) and the selective catalytic reduction system (283), wherein the first Diesel oxidation catalyst (280) is config-ured to oxidize hydrocarbons and carbon monoxides into carbon dioxides, and the second Diesel oxidation catalyst (281) is configured to oxidize nitrogen monoxides into nitrogen dioxides.
2. An exhaust system (270) according to claim 1, wherein the first and the second Die- sel oxidation catalysts (280, 281) differ from each other in least one of the following char- acteristics: mixture of platinum group metals, washcoat carrying the mixture, catalyst sub-strate material and volume.
3. An exhaust system (270) according to claim 2, wherein the platinum group metals mixture of both the first Diesel oxidation catalyst (280) and the second Diesel oxidation catalyst (281) contain platinum and palladium, and wherein the ratio of platinum to palla- dium in the first Diesel oxidation catalyst (280) is smaller than the ratio of platinum to pal-ladium in the second Diesel oxidation catalyst (281).
4. An exhaust system (270) according to any of the preceding claims, wherein the first Diesel oxidation catalyst (260) is configured as a NO adsorber.
5. An exhaust system (270) according to any of the preceding claims, wherein the first Diesel oxidation catalyst (280) and the second Diesel oxidation catalyst (281) are embod-ied in a single catalyst substrate.
6. An exhaust system (270) according to any of the claims from I to 4, wherein the first * *..** * Diesel oxidation catalyst (280) and the second Diesel oxidation catalyst (261) are embod-ied in two separated catalyst substrates.
7. An exhaust system (270) according to any of the preceding claims, wherein the first Diesel oxidation catalyst (280) and the second Diesel oxidation catalyst (281) are accom- *:* 30 modated in a same casing (282).

* : *
8. An exhaust system (270) according to any of the preceding claims, wherein the se-lective catalytic reduction system (283) comprises a mixer (288) located in the exhaust pipe (275) between a Diesel exhaust fluid injector (287) and a selective reduction catalyst (284).
9. An exhaust system (270) according to claim 8, comprising a Diesel particulate filter (289) located in the exhaust pipe (275) downstream of the second Diesel oxidation catalyst (281).
10. An exhaust system (270) according to claim 9, wherein the Diesel particulate filter (289) and the selective reduction catalyst (284) are integrated in a single aftertreatment device.
II. A Diesel engine (110) equipped with an exhaust system (270) according to any of the preceding claims.
12. A motor vehicle comprising a Diesel engine (110) according to claim 11.
13. A method of treating exhaust gases of a Diesel engine (110) comprising the steps of: -oxidizing hydrocarbon and carbon monoxides contained in the exhaust gases into carbon dioxides by means of a first Diesel oxidation catalyst (280), -oxidizing nitrogen monoxides contained in the exhaust gases exiting the first Diesel oxi-dation catalyst (280) into nitrogen dioxides by means of a second Diesel oxidation catalyst (281), -converting nitrogen monoxides and the nitrogen dioxides contained in the exhaust gases exiting the second Diesel oxidation catalyst (281) into diatomic nitrogen by means of an 8CR system (283). * .*..*.. * . * *** * * * ** * * * . * *.