WO2008002258A1

WO2008002258A1 - Method and device for treatment of exhaust gases, and vehicle

Info

Publication number: WO2008002258A1
Application number: PCT/SE2007/050437
Authority: WO
Inventors: Vesa Hokkanen
Original assignee: Scania Cv Ab (Publ)
Priority date: 2006-06-28
Filing date: 2007-06-20
Publication date: 2008-01-03
Also published as: SE530028C2; SE0601418L; DE112007001502T5

Abstract

The invention relates to a method and a device for treatment of exhaust gases from a combustion engine (1), e.g. a diesel engine. Exhaust gases from the engine are led to two catalysts (5, 7) which are arranged in series and in which nitrogen oxides in the exhaust gases are treated by selective catalytic reduction. Proportioning of reduction agent for the respective catalysts is effected by means of proportioning devices (6, 8) through the action of feedback control systems (9, 10) separately for each catalyst. The invention also relates to a vehicle provided with a device according to the invention.

Description

METHOD AND DEVICE FOR TREATMENT OF EXHAUST GASES, AND VEHICLE

Technical field

The present invention relates to a method for treatment of exhaust gases from a combustion engine according to the introductory part of the attached claim 1. The invention further relates to a device for treatment of exhaust gases according to the introductory part of the attached claim 6. The invention also relates to a vehicle according to the introductory part of the attached claim 12.

State of the art

In combustion engine technology, a high combustion temperature is often associated with better performance, less fuel consumption and lower emissions of both particles and hydrocarbons and carbon monoxide. However, a high combustion temperature also causes an undesirable high discharge of nitrogen oxides (NO_x).

Previously known technology for cleaning the exhaust gases of combustion engines such as diesel engines caters inter alia for the cleaning of nitrogen oxides (NO_x) and particulate discharges.

Under today's and still more under expected future legislation concerning exhaust gas emissions, high NO_x contents are not acceptable. Techniques have been developed for selective catalytic reduction, so-called SCR technology, whereby nitrogen oxides (NO_x) in exhaust gases are reduced by means of ammonia, e.g. in the form of urea such as the kind traded under the name AdBlue^®, which is supplied in liquid form to an SCR unit.

However, today's SCR technology is not sufficiently effective to achieve desired low emissions OfNO_x with respect to future legislation, so development has now been directed towards reducing the NO_x content from the engine by using a combination of SCR technology with exhaust gas recirculation, so-called EGR (Exhaust Gas Recirculation), whereby recirculating exhaust gases are in most cases also cooled before being led back to the engine. EGR reduces the temperature in the combustion chamber, and the lower temperature results in lower NO_x contents in the exhaust gases, which are thereafter cleaned by SCR technology. A substantial disadvantage, however, is that EGR causes low temperature of exhaust gases, making it difficult for the SCR catalyst to work at sufficiently high degrees of efficiency, since SCR catalysts require a certain temperature if they are to function satisfactorily. Known EGR technology also reduces engine efficiency in that a certain amount of energy is devoted to the EGR function.

The object of the present invention is to provide for combustion engines, particularly for diesel engines, an effective and inexpensive form of exhaust gas cleaning which also results inter alia in lower strength requirements for engine components such as cylinders etc.

Brief description of the invention

The object as above is achieved with a method, a device and a vehicle with features according to the independent claims 1, 6 and 12 respectively. Further advantages are achieved with features according to the respective dependent claims.

Brief description of the drawings

The invention is described below in more detail with respect to embodiment examples and the attached drawings, in which

Fig. 1 in the form of a block diagram depicts schematically a first embodiment of a device according to the invention. Description of embodiment examples

Fig. 1 depicts a combustion engine exemplified by a supercharged diesel engine 1, which in the embodiment depicted comprises a charge air cooler 2 and a turbo unit 3 of a conventional kind. Versions without a charge air cooler 2 are conceivable and exist.

The exhaust gases from the engine, represented by an arrow 4, are led through the exhaust line to a first SCR catalyst 5 for selective catalytic reduction through the reducing influence of a reduction agent, preferably urea in a known manner, whereby NO_x is reduced to N₂ and/or N₂O. The use of other reduction agents, e.g. ammonia, is also conceivable. The form of urea preferred is the substance AdB lue^®, which is a 32.5% solution of urea in water and is specified in standard DIN V 70 070.

A device 6 for proportioning and adding urea to the exhaust gases is arranged upstream of the catalyst 5 in the exhaust line. The schematic diagram depicts the device 6 and the catalyst 5 as separate, but alternative embodiments in which they are put together to form a single unit are also conceivable.

A second SCR catalyst 7 is arranged in the exhaust line in series with the first SCR catalyst 5 in order to subject the exhaust gases already treated in the first SCR catalyst 5 to treatment in order to further reduce their NO_x content. A device 8 for proportioning and adding urea is situated upstream of the second SCR catalyst 7 in a manner similar to the arrangement pertaining to the first catalyst 5. Here again, embodiments in which the device 8 and the catalyst 7 are put together to form a single unit are also conceivable.

For each of the catalysts 5, 7 there is a feedback control system 9, 10 respectively, adapted to controlling, on the basis of the results of the treatment in the respective catalyst, the proportioning of urea to the respective catalyst by means of the respective proportioning and mixing device 6, 8. To this end, there are sensors downstream of the respective catalysts 5, 7 to detect the content of representative parameters in the exhaust gases, which sensors are connected to the respective control systems 9, 10 and thereby provide information about relevant parameters. For them to be able to detect correct values, it is important that the sensors pertaining to the respective catalysts are arranged at positions where the exhaust gases are homogeneously distributed as regards the parameters being detected. It is therefore an advantage if the exhaust line is provided with a relatively long distance from catalyst to sensor so that the exhaust gases are thoroughly mixed after leaving the catalyst. In some cases they may need to be further mixed by means of guide rails, pipe bends or other means of mixing the exhaust gases.

According to preferred embodiments, the treatment result is detected as a surplus of reduction agent, urea, in the exhaust gases and the respective feedback control system is adapted to controlling the proportioning with respect to a predetermined content of reduction agent such as urea in the exhaust gases, e.g. a minimisation of the reduction agent content in the exhaust gases.

Embodiments are also conceivable where the treatment result is detected as the content of NO_x in the exhaust gases, whereby the respective control systems are adapted to controlling the proportioning with respect to a predetermined content of NO_x in the exhaust gases.

In either case, reduction agent surplus or NO_x content, adaptive control systems are conceivable in which the proportioning is controlled on the basis of empirical values for reduction agent surplus or NO_x content respectively.

According to an in certain cases particularly preferred embodiment, the treatment result is detected through a combination of what is described above, i.e. both the NO_x content and the reduction agent surplus in the exhaust gases, whereby the respective feedback control systems are adapted to controlling the proportioning with respect to a predetermined result for one or both of these magnitudes. It is thus possible to cater for shortcomings of a catalyst. It may thus happen, for example, that urea accumulates, or even accumulates locally, in the catalyst, with the result that a detected zero surplus of urea in the exhaust gases does not necessarily mean that urea supplied has been consumed.

It is thus possible for a detected NO_x content to provide information for use in achieving more effective and more appropriate control of proportioning and at the same time to cater for technical shortcomings of the kind described above.

It is conceivable for different forms of detection to be used for different catalysts.

Embodiments of the device are conceivable in which at least one further SCR catalyst is connected in series with the first two.

According to preferred embodiments, the treatment of the exhaust gases is intended to be effected without the engine's exhaust gases being recirculated to the engine inlet according to known EGR technology. The engine will thus lack a return line connecting its exhaust side to its inlet side. This makes it impossible for the treatment of exhaust gases according to the invention to be combined in certain conditions with an engine which also uses EGR technology for reduction of exhaust gas emissions.

The method for the treatment of exhaust gases and the function of the device according to the invention are perhaps already substantially indicated by the above part of the description.

The exhaust gases from the engine are led first to the first SCR catalyst 5 where they are subjected to a first treatment step whereby urea injected from the device 6 reacts with the nitrogen oxides in the exhaust gases and thereby reduces their content. This step certainly reduces the content of NO_x in the exhaust gases but a certain residual amount still remains. The exhaust gases treated in the first step are then led on to the second SCR catalyst 7 which is arranged in series with the first catalyst and in which the exhaust gases are subjected to a second treatment step whereby urea injected from the device 8 reacts with the nitrogen oxides in the exhaust gases in order to further reduce their NO_x content. The proportioning of urea at each step is controlled by the feedback control systems 9, 10 at each step on the basis of the results of the respective step, where the result may be detected as NO_x content and/or reduction agent surplus in the exhaust gases after the respective step.

The invention affords advantages in the form of effective exhaust gas treatment.

Eliminating the need for EGR affords advantages in the form of acceptance of high exhaust temperatures, resulting in more effective catalyst function. It also reduces the strength requirements for the engine through less need for tolerance of high cylinder pressures. Elimination of EGR does of course eliminate the need for an EGR system and also results in the vehicle's existing cooling capacity being sufficient to allow more power to be extracted from the engine.

There is also technical intrinsic value in that serial use of SCR catalysts results in a uniform treatment technology which does not affect the engine.

The invention is described above in connection with preferred embodiments and embodiment examples.

Further embodiments and minor modifications and additions are of course conceivable without departing from the fundamental idea of the invention.

Thus two or more SCR catalysts with associated proportioning and mixing devices might be put together to form a single unit.

The above description does at certain points use urea as an example of a reduction agent, but in the case of a reduction agent other than urea the description is still applicable, e.g. as regards proportioning and mixing devices etc.

Embodiments are conceivable where the reduction agent used is not the same in all the SCR catalysts. Moreover, the feedback control systems may be an integral part of the vehicle's overall control and monitoring system. A common control system for both/all of the catalysts is also conceivable.

It has here been assumed that for one skilled in the art SCR catalysts and proportioning and mixing devices are known components with known functions. This also applies to technology for measuring NO_x contents and reduction agent contents in exhaust gases.

The invention is thus not to be regarded as limited to the embodiments indicated above but may be varied within its scope indicated by the attached claims.

Claims

1. A method for treatment of exhaust gases from a combustion engine, comprising a first step of reducing the content of nitrogen oxides NO_x in the exhaust gases by treatment in a first catalyst through the reducing influence of a reduction agent, and a second step of further reducing the content of nitrogen oxides NO_x in the exhaust gases, by treatment of the exhaust gases thus treated in the first catalyst (5), in at least one further catalyst (7) connected in series with the first catalyst (5), through the reducing effect of a reduction agent, characterised by steps of proportioning the supply of reduction agent for the each catalyst (5, 7) on the basis of the results of the treatment in the respective catalyst, by means of feedback control systems (9, 10) connected accordingly to sensors which are arranged after the respective catalysts (5, 7) and detect the content in the exhaust gases of representing parameters and thereby provide information about relevant parameters.

2. A method according to claim 1, characterised by the step of determining the treatment result as NO_x content and/or reduction agent surplus in the exhaust gases.

3. A method according to claim 1 or 2, characterised in that urea is used as reduction agent, preferably in the form of AdBlue^®.

4. A method according to claim 1, 2 or 3, characterised by the step of providing information about the NO_x content of the exhaust gases after at least one catalyst (5, 7) as a basis for proportioning of reduction agent for the catalyst (5, 7), and the step of proportioning reduction agent for the catalyst (5, 7) with respect to a predetermined content of NO_x in the exhaust gases.

5. A method according to claim 1, 2 or 3, characterised by the step of providing information about the reduction agent surplus of the exhaust gases after at least one catalyst (5, 7) as a basis for proportioning of reduction agent for the catalyst (5, 7), and the step of proportioning reduction agent for the catalyst (5, 7) so that the reduction agent surplus is minimised.

6. A device for treatment of exhaust gases from a combustion engine, comprising devices for reducing the content of nitrogen oxides NO_x in the exhaust gases by treatment in a first catalyst through the reducing effect of a reduction agent, and at least one further catalyst (7) for subjecting the exhaust gases thus treated in the first catalyst (5) to treatment through the reducing effect of a reduction agent, which further catalyst (7) is connected in series with the first catalyst (5), characterised by devices (6, 8) for proportioning of reduction agent for supply to the respective catalyst (5, 7) on the basis of the result of the treatment in the respective catalyst (5, 7), by means of a feedback control system (9, 10) connected accordingly to sensors which are arranged after the respective catalysts (5, 7) and detect the content in the exhaust gases of representing parameters and thereby provide information about relevant parameters.

7. A device according to claim 6, characterised by devices for determining the treatment result as NO_x content and/or reduction agent surplus in the exhaust gases after the respective catalyst (5, 7).

8. A device according to claim 6 or 7, characterised in that said reduction agent is urea, preferably in the form of AdBlue^®.

9. A device according to claim 6, 7 or 8, characterised by devices for providing information about the NO_x content of the exhaust gases as a basis for proportioning of reduction agent to at least one catalyst (5, 7), and in that the control system (9, 10) is adapted to proportioning of reduction agent for the relevant catalyst (5, 7) with respect to a predetermined content of NO_x in the exhaust gases.

10. A device according to claim 6, 7 or 8, characterised by devices for providing information about the reduction agent content of the exhaust gases as a basis for proportioning of reduction agent to at least one catalyst (5, 7), and in that the control system (9, 10) is adapted to proportioning of reduction agent for the relevant catalyst (5, 7) so that reduction agent surplus is minimised.

11. A device according to claim 7, 8, 9 or 10, characterised in that at least two catalysts (5, 7) are arranged as a unit for serial treatment of the exhaust gases.

12. A vehicle, characterised in that it comprises devices according to any one of claims 6-11.