FR3029967A1 - POST-TREATMENT SYSTEM FOR POLLUTANT EMISSIONS OF A TWO-STROKE ENGINE BY SELECTIVE CATALYTIC REDUCTION WITH HYDROCARBONS - Google Patents

Abstract

The main object of the invention is a system (14) for post-treatment of pollutant emissions of a two-cycle engine (12), implementing a selective catalytic reduction (SCR) of nitrogen oxides (NOx). ) polluting emissions, using hydrocarbons (HC), comprising: - a selective catalytic reduction unit (16), at the engine outlet, in which the selective catalytic reduction (SCR) is implemented, - an injection unit hydrocarbons (20) in the engine, the unburned hydrocarbons at the outlet of the engine being reductants, - control means (22) for adjusting the quantity of hydrocarbons introduced by the hydrocarbon injection unit (20) in the engine so that: ▪ the ratio of the amount of unburned hydrocarbons, expressed in part per million carbon (ppmC), to the amount of nitrogen oxides, expressed in parts per million (ppm), is between 2 and 10, ▪ the exhaust gas temperature is between 300 and 500 ° C.

Description

FIELD OF THE INVENTION The present invention relates to the field of the treatment of pollutant emissions from two-stroke engines, and in particular to the engines of two-cycle engines. two light periods, particularly of the spark ignition type, for example gasoline. The invention can find applications in various fields of industry, and in particular for two-stroke engines used in particular in the field of motor vehicles. More specifically, it relates to a two-cycle engine emission aftertreatment system configured to implement a selective catalytic reduction using hydrocarbons as reducing agents, an assembly comprising such a post-treatment system and a two-stroke engine, and a pollutant post-treatment process of an associated two-stroke engine. STATE OF PRIOR ART Nowadays, the need to design compact heat engines, especially for the hybridization of vehicles, and the need to provide low-cost engines for light-duty vehicles, particularly intended for emerging markets, revive the interest of two-stroke engines for applications in the field of motor vehicles. Indeed, the two-stroke engine has many advantages, particularly in terms of simplicity (few moving parts), high power density and limited cost. However, the use of two-stroke engines involves the production of significant emissions that generally do not meet the standards imposed.

Also, even though the two-stroke engine has the advantages mentioned above and also a certain interest in terms of fuel consumption, and therefore of carbon dioxide emissions, the management of its pollutant emissions remains problematic.

It is therefore necessary to provide a two-cycle engine emission aftertreatment system which must particularly take into account the exhaust gas conditions described below. On the one hand, the hydrocarbons (HC) resulting from incomplete combustion (low loads), short-circuiting of a part of the charge directly to the exhaust or from the combustion of the lubricant reach altogether levels which are two three times higher than those from a traditional four-stroke engine. On the other hand, due to a significant short-circuiting, the gas mixture at the exhaust is generally poor, which makes inoperative a three-way catalyst, particularly for the treatment of nitrogen oxides (NOx) . Solutions have already been proposed in the prior art to allow the depollution of a two-stroke engine. Thus, for example, the European patent EP 0 497 034 B1 proposes to carry out a reduction of nitrogen oxides on a catalyst based on zeolites exchanged using hydrocarbons from combustion. However, the injection technologies described in this patent are not really suited to the applications contemplated by the invention, and no management of the ratio between hydrocarbons and nitrogen oxides, nor the temperature of the catalyst, is mentioned. In addition, US patent application US 2010/0293926 A1 proposes to use selective catalytic reduction technology with a four-stroke spark ignition engine by selecting hydrocarbons as reducers. In particular, it describes the various means that can be used to generate the hydrocarbons by adjusting the engine. However, the type of engine and the applications concerned by this document diverge from the invention.

SUMMARY OF THE INVENTION There is therefore a need for designing an alternative system for post-treatment of pollutant emissions, in particular nitrogen oxides (NOx), of a two-stroke engine.

The object of the invention is to at least partially overcome the above mentioned needs and disadvantages relating to the embodiments of the prior art. The invention thus has, according to one of its aspects, a pollutant emission after-treatment system of a two-cycle engine, configured to implement a selective catalytic reduction of the nitrogen oxides present in the polluting emissions of the engine by using hydrocarbons as reducing agents, characterized in that it comprises: a selective catalytic reduction unit, intended to be located at the outlet of the two-stroke engine, in which the selective catalytic reduction of the nitrogen oxides present in said polluting emissions is implemented, 15 - a hydrocarbon injection unit for introducing hydrocarbons into the engine whose unburnt hydrocarbons at the engine outlet are reducing agents during the selective catalytic reduction, control means for adjusting the quantity of hydrocarbons introduced by the hydrocarbon injection unit into the engine so that: contribution between the amount of unburned hydrocarbons, expressed in part per million carbon (ppmC), and the quantity of nitrogen oxides, expressed in parts per million (ppm), is between 2 and 10, in particular between 2 and 4 the temperature of the exhaust gas is between 300 and 500 ° C. The control means can be configured so that the ratio between the amount of unburned hydrocarbons and the amount of nitrogen oxides is greater than 3, in particular of the order of 4. The hydrocarbons, used as reducing agents for selective catalytic reduction, comprise light cuts less than C5 and / or paraffins (n-deca ne).

The hydrocarbons used as reducing agents for selective catalytic reduction may include alcohols, such as, for example, ethanol. The selective catalytic reduction unit can use, as catalysts, silver (Ag) on alumina (Al 2 O 3).

The system may further comprise an oxidation catalyst unit, at the outlet of the selective catalytic reduction unit, for the oxidation treatment of pollutant emissions other than nitrogen oxides, in particular the residual hydrocarbons leaving the catalytic reduction unit. the selective catalytic reduction unit. The oxidation catalyst unit may be based on precious metals such as platinum (Pt) and / or palladium (Pd) on alumina. The invention also relates to an assembly which may comprise: a two-stroke engine; a post-treatment system according to any one of the preceding claims, of the pollutant emissions of said two-stroke engine.

The two-stroke engine may be a gasoline-type light engine. Thanks to the invention, it may be possible to efficiently carry out the depollution of a two-stroke engine, in particular a two-stroke engine of the gasoline light type, by the implementation of a catalytic reduction with selective reduction by hydrocarbons (HC-SCR), that is to say a selective catalytic passive reduction of the nitrogen oxides present in the polluting emissions by hydrocarbons emitted by the engine. In particular, it may be possible to optimize the reduction of nitrogen oxides present in the emission emissions of a two-stroke engine by acting via combustion on the ratio between hydrocarbons (HC) and nitrogen oxides (NOx). , and also on the temperature of the exhaust gas. It is thus possible to optimize the engine torque 2T / catalyst HC-SCR thanks to a specific adjustment of the engine 2T favoring temperatures and ratios HC on NOx favorable to the reaction of HC-SCR. In addition, the use of the HC-SCR for a two-stroke engine has a reduced cost (possibility of absence of dedicated injection), a passivity in the use (no actuator or dedicated management of the system). post-treatment), good durability and good efficiency, which can be greater than 50% for high nitrogen oxides feeds. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood on reading the detailed description which follows, of an example of a non-limiting principle thereof, as well as on the examination of FIG. schematic and partial, of the accompanying drawing, illustrating an exemplary assembly comprising a two-stroke engine and a post-processing system according to the invention. In this single FIG. 1, the different parts shown are not necessarily in a uniform scale, to make the figure more readable. DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT FIG. 1 is a schematic diagram of an example of a general principle of an assembly 10 comprising a two-cycle engine 12 and a system 14 for post-treatment of the polluting emissions of this engine. two-stroke.

This engine is advantageously a two-stroke engine, particularly with lights, particularly of the spark ignition type, for example gasoline. Advantageously, this post-processing system 14 according to the invention is configured to implement selective hydrocarbon reduction catalysis (HC-SCR) at the output of the two-stroke engine 12, that is to say selective catalytic reduction (SCR) of the nitrogen oxides (NOx) present in the pollutant emissions of the two-stroke engine, using hydrocarbons (HC) as reducing agents. To do this, as can be seen in FIG. 1, the system 14 comprises a selective catalytic reduction unit 16, which is located, at the outlet of the two-stroke engine, in the exhaust line 18. In this unit of Selective catalytic reduction 16, the nitrogen oxides from the two-stroke engine are reduced by HC-SCR. Selective Catalytic Reduction (SCR) is commonly referred to by the acronym SCR for "Selective Catalytic Reduction". In addition, the implementation of selective catalytic reduction (SCR) with hydrocarbons (HC) as reducing agents is commonly referred to by the acronym HC-SCR, which is used in the following description. In addition, the system 14 also comprises a hydrocarbon injection unit 20. This unit makes it possible to inject the HC hydrocarbons, either directly into the combustion chamber of the engine so that they mix with the oxidant (generally the air) which is present (direct injection) forming a fuel mixture in this chamber, or in the intake manifold of this engine so that they mix with the oxidant which circulates to form a carburetted mixture which is then directed into the combustion chamber of the engine (indirect injection). This unit thus makes it possible to inject HC hydrocarbons whose unburned hydrocarbons will be used as reducing agents during the selective catalytic reduction for the selective catalytic reduction unit 16.

Advantageously, the system 14 also comprises control means 22 for controlling the quantity of hydrocarbons introduced into the combustion chamber of the engine by the hydrocarbon injection unit 20 through a line 24. These control means also receive information from means for measuring the temperature of the exhaust gas 26. These means comprise a temperature sensor 28 advantageously placed in the exhaust line, a transmission line 30. the signal from the sensor to a computing unit 32 and a transmission line 34 from the sensor information to the control means 22.

Thus, depending on the demand by the power demand user for the engine, the control means will control the amount of hydrocarbons to be introduced into the engine by the unit 20 so as to meet this demand. power of the user, so that the ratio between the amount of unburned hydrocarbons from the engine, expressed in ppmC, and the amount of nitrogen oxides, expressed in ppm, emitted by this engine is between 2 and 10, and 3029967 7 in particular between 2 and 4, or even of the order of 4 for optimum configuration, and so that the temperature of the exhaust gas introduced into the selective catalytic reduction unit 16 is included between 300 and 500 ° C. This hydrocarbon injection unit is thus connected to the two-stroke engine, so that the hydrocarbons introduced into the selective catalytic reduction unit 16 come directly from the two-stroke engine. Advantageously, hydrocarbons of the hydrocarbon injection unit, used as reducing agents for selective catalytic reduction, comprise light cuts smaller than C5 and / or paraffins (n-decane) and / or alcohols such as ethanol. In addition, as a catalyst for HC-SCR, the selective catalytic reduction unit 16 preferably uses silver (Ag), for example 2% by weight, on alumina (Al 2 O 3). Furthermore, as shown, the post-treatment system 14 according to the invention may comprise an oxidation catalyst unit 36 at the outlet of the selective catalytic reduction unit 16. This oxidation catalyst unit 36 allows the oxidation treatment of polluting emissions other than nitrogen oxides, and in particular that of unburned residual hydrocarbons leaving the selective catalytic reduction unit 20 and undesirable by-products, such as for example ammonia (NH 3 ), hydrogen cyanide (HCN), aldehydes or partially oxidized hydrocarbons. This oxidation catalyst unit may advantageously be composed of precious metals such as platinum (Pt) and / or palladium (Pd) on alumina (Al 2 O 3).

It has been found that, with respect to the use of hydrocarbons as reducing agents, aromatic chain hydrocarbons are poor reducing agents and can cause slow deactivation; short chain paraffins are good high temperature reducers and cause moderate deactivation; long chain paraffins are good reducers; The alcohols are very good reducing agents but may give rise to the creation of ammonia (NH3) and hydrogen cyanide (HCN). A commercial fuel being an assembly of all the molecules mentioned above, the performance can be more or less interesting depending on the choice of this fuel. It should also be noted that the greater the volume of catalyst used, the better the reduction of nitrogen oxides.

Of course, the invention is not limited to the embodiment which has just been described. Various modifications may be made by the skilled person.

Claims (9)

REVENDICATIONS1. A two-cycle engine emission (12) aftertreatment system (14) configured to perform selective catalytic reduction (SCR) of nitrogen oxides (NOx) present in the engine exhaust emissions using hydrocarbons (HC) as reducing agents, characterized in that it comprises: - a selective catalytic reduction unit (16), intended to be located at the outlet of the two-stroke engine (12), in which the reduction selective catalytic converter (SCR) of the nitrogen oxides (NOx) present in said pollutant emissions is implemented, - a hydrocarbon injection unit (20) for introducing hydrocarbons (HC) into the engine whose unburned hydrocarbons in motor output are reducers during the selective catalytic reduction (SCR), - control means (22) for adjusting the amount of hydrocarbons (HC) introduced by the hydrocarbon injection unit (20) into the engine so that The ratio of the amount of unburned hydrocarbons (HC), expressed in part per million carbon (ppmC), to the amount of nitrogen oxides (NOx), expressed in parts per million (ppm), is between 2 and 10, especially between 2 and 4, the temperature of the exhaust gas is between 300 and 500 ° C. 2. System (14) according to claim 1, characterized in that the control means (22) are configured so that the ratio between the amount of unburned hydrocarbons (HC) and the amount of nitrogen oxides (NOx ) greater than 3, in particular of the order of 4. 3. System (14) according to one of the preceding claims, characterized in that the hydrocarbons (HC), used as reducing agents for selective catalytic reduction (SCR), comprise light cuts of less than C5 and / or paraffins (n-deca ne). 4. System (14) according to one of claims 1 or 2, characterized in that the hydrocarbons (HC) used as reducing agents for the selective catalytic reduction comprise alcohols, such as ethanol. 5. System (14) according to any one of the preceding claims, characterized in that the selective catalytic reduction unit (16) uses, as catalysts, silver (Ag) on alumina ( A1203). 6. System (14) according to any one of the preceding claims, characterized in that it further comprises an oxidation catalyst unit (36) at the outlet of the selective catalytic reduction unit (16), for the oxidative treatment of polluting emissions other than nitrogen oxides (NOx), especially residual hydrocarbons (HC) leaving the selective catalytic reduction unit (16). 7. System (14) according to claim 6, characterized in that the oxidation catalyst unit (36) is based on precious metals such as platinum (Pt) and / or palladium (Pd) on alumina. 8. Assembly (10), characterized in that it comprises: a two-stroke engine (12), a system (14) of post-treatment according to any one of the preceding claims 25 polluting emissions of said two-stroke engine (2). 9. An assembly according to claim 8, characterized in that the two-stroke engine (12) is a gasoline-type light engine.