FR2755181A1 - METHOD AND DEVICE FOR REDUCING THE POLLUTANTS CONTAINED IN THE EXHAUST GASES OF AN INTERNAL COMBUSTION ENGINE - Google Patents

Abstract

The method consists in introducing a predetermined amount of oxygenated air (synthetic air) into the exhaust system of a combustion engine. The invention is applicable to all catalytic exhaust systems and to particle filters.

Description

Method and device for reducing pollutants contained in the exhaust gases of an internal combustion engine.

 The present invention relates generally to a method for reducing pollution due to the exhaust gases of an internal combustion engine and to a device for implementing the method.

 Global standards against vehicle pollution have become increasingly stringent in recent years.

 To meet these emission standards, car manufacturers have developed increasingly effective solutions for removing pollutants from engine exhaust gases, such as, for example, catalytic exhaust pipes for petrol engines and fuel filters. particles for diesel engines.

 Engine exhaust includes, as main pollutants, unburnt hydrocarbons, carbon monoxide, nitrogen oxides (NO and NO2) and soot (mainly in the case of a diesel engine).

 Catalytic converters were used, among other things, to purify the exhaust gases of petrol engines of a large part of their compounds which are harmful to health. These catalytic converters, well known in themselves, comprise a system of catalysts which have the purpose of oxidizing unburnt hydrocarbons as well as carbon monoxide and reducing nitrogen oxides. These catalytic converters have been found to be effective, having conversion rates of approximately 95%.

However, the action of the catalyst system only starts from a relatively high temperature of around 250 C and is only optimal at temperatures of around 450 "C. During departures when vehicles are cold, of course, the catalytic converter is at a temperature well below 250 "C and is therefore inactive. It is the heat of the exhaust gases that will gradually increase the temperature of the catalyst system to the value required to make it efficient. During this entire period of inactivity of the catalyst system (which lasts about 1 minute for the Europe cycle which includes 20), 80% of the pollutant emissions recorded are produced. We can therefore understand the interest for car manufacturers in reducing the start-up time of the catalyst system.

 One solution to reduce this priming time is to introduce air at room temperature into the vehicle exhaust system during the cold phase, that is to say the engine operating phase before the system catalytic converter catalysts does not come into action. Such a solution has two advantages. A first advantage is to produce a lean mixture of exhaust gases which, at the level of the catalyst system, is favorable for the oxidation of carbon dioxide and unburnt hydrocarbons. These oxidation reactions being exothermic, the temperature rise of the pot catalyst system is thus accelerated. The second advantage is to carry out post-oxidation in the gas phase of unburnt hydrocarbons and carbon dioxide before they enter the catalytic converter. This postoxidation not only decreases the rate of pollutants but also allows the exhaust gases to heat up due to the exothermicity of the oxidation reactions. The exhaust gases thus heated allow the temperature of the catalytic converter system to increase more quickly.

 The injection of air into the exhaust (I.A.E.) is generally provided by a pump whose flow rate is predetermined for a given engine.

 This air flow is interesting because of the oxygen it provides.

Nitrogen, which is the main constituent of air (79%), does not participate in any way in this post-treatment and on the contrary hinders the activation of the catalytic converter by the fact that it absorbs part of the heat of the exhaust gases, dilutes the oxygen and decreases its partial pressure, increases the auto-ignition temperatures of products liable to be oxidized, and decreases the speeds of chemical oxidation reactions. In addition, the injection of exhaust air is only functional in cold start-up and no system is provided to reduce the emissions of unburnt hydrocarbons and carbon dioxide after the catalytic converter when the latter operational.

Furthermore, in the case of diesel engines, problems are encountered with the treatment of particles, and this, on all of the operating points of a vehicle. Current filter systems accumulate soot which must be burned in order to regenerate it. The studies in progress aim to optimize the combustion / regeneration parameters so as to obtain
- regeneration at low temperature (below 400 "C),
- complete regeneration by combustion of all the soot trapped in the filter or
- high regeneration frequencies, that is to say for relatively low soot contents in the filter.

 The object of the present invention is therefore to provide a process for the reduction of pollutants, in particular unburnt hydrocarbons, carbon dioxide, and particles, contained in the exhaust gases of an internal combustion engine which overcomes the above drawbacks. above.

 Another object of the present invention is a device for implementing the method according to the invention.

 According to the invention it has been found that by directly introducing into the exhaust system of an internal combustion engine a predetermined quantity of synthetic air, that is to say air enriched with oxygen, an oxidation is obtained. more complete with unburnt hydrocarbons and carbon dioxide.

 The present invention therefore relates to a method for reducing these pollutants contained in the exhaust gases of an internal combustion engine comprising an exhaust system which comprises the introduction of a predetermined quantity of synthetic air directly into the system. exhaust.

 In the present description, synthetic air means any air enriched in oxygen containing more than 21% of oxygen, including pure oxygen.

 In the case of a catalytic exhaust system which generally comprises a catalyst system comprising a priming catalyst or precatalyst and a main catalyst (main catalytic converter) the introduction of the predetermined quantity of synthetic air can be carried out upstream of the precatalyst and / or between the precatalyst and the main catalyst and / or downstream of the main catalyst, and / or in the main catalyst, in particular between the last two catalytic rolls of the main catalyst.

 This introduction can be carried out continuously or discontinuously depending on the type of application envisaged.

 When the introduction of synthetic air takes place upstream of the precatalyst or between the precatalyst and the main catalyst, faster, or even more complete, oxidation is obtained, and therefore greater heat generation. Due to the reduced nitrogen content, less heat is absorbed by the nitrogen and therefore a higher temperature of the exhaust gases. Consequently, a faster priming of the catalyst system is obtained and consequently a shorter period of inactivity of the latter and thus a reduction in polluting emissions during the cold start of the engine.

 When the introduction of synthetic air takes place downstream of the main catalyst, a post-oxidation of the last unburnt hydrocarbons and carbon dioxide not treated by the catalyst system is obtained.

 In the case of an exhaust system for a diesel engine comprising a particulate filter and / or an oxidation catalyst, the introduction of synthetic air can be carried out either upstream or downstream of the particulate filter and / or the oxidation catalyst. When the introduction of synthetic air takes place upstream of the particulate filter, this introduction has the additional advantage of regenerating the particulate filter, thereby reducing their emissions.

 When the introduction takes place downstream of the particulate filter, the same advantages are obtained as in the case of a catalytic converter.

 When the introduction of synthetic air takes place upstream of the precatalyst or between the precatalyst and the main catalyst in the case of a catalytic exhaust system, this introduction is preferably carried out only when the engine is cold started until the operation of the catalyst system begins. Preferably the introduction time is short, of the order of about 40 seconds to initiate the operation of the catalyst system and the flow rates are relatively low. When the introduction of synthetic air takes place upstream of a particulate filter, this introduction is preferably intermittent in order to regenerate the filter periodically. The flow rates used are also relatively low.

 In all cases the temperatures obtained are not high enough for there to be an effective formation of nitrogen oxides.

 The present invention also relates to a device for implementing the method of the invention which comprises a device for producing synthetic air connected to an air source, for example a compressor compressing atmospheric air and a device receiving air synthetic delivered by the production device and introducing a predetermined amount of synthetic air directly into an exhaust system of an internal combustion engine.

 The devices and methods for enriching the air with oxygen are known. Three different known methods and devices are described below for enriching the air with oxygen, preferential adsorption with commutation (swing adsorption), membranes with relative permeability and conductive ceramics.

Preferential adsorption with switching (Swing Adsorption)
The principle is based on the preferential adsorption of nitrogen on certain materials such as Zeolites (aluminosilicate compound).

The air in contact with these zeolites thus discharges part of the nitrogen constituting it and sees its oxygen concentration increase.

After a certain operating time the zeolites are saturated with nitrogen and it is therefore necessary to regenerate them. This regeneration is done by switching (Swing) from the column of zeolites to ambient air using a pressure gradient and / or a temperature gradient.

When using a Pressure Swing Adsorption, at least two columns are used; one operates under a certain air flow and at a pressure of 2 to 10 bars; the zeolite in this column is gradually loaded with nitrogen and the oxygen concentration at the outlet is greater than 21%. A fraction of this flow (elution rate) is directed to the second column and allows its regeneration. When the first column is saturated the air flows are reversed allowing the oxygen concentration in the second column and the regeneration of the first. The duration of use of a column or of its regeneration is called the cycle time. The concentration of oxygen leaving the system depends on many factors:
- air: pressure, flow, temperature
- system: zeolite weight, cycle time.

The system using a temperature gradient (Temperature
Swing Adsorption) is similar to the previous one and uses the temperature differences to allow column regeneration.

Other switching adsorption systems (PTV Swing
Adsorption) combine different operating parameters (pressure or vacuum, temperature), these more efficient systems adapt more easily to the need: better efficiency, reduction in volume, etc.

Selective permeability membranes
In this case, it is the difference in permeation of oxygen and nitrogen through a membrane which ensures the separation of these two constituents. Air is introduced under pressure on one face of the membrane; part of the gases, preferably oxygen, crosses the membrane and is recovered at ambient pressure; the other part, richer in nitrogen, is collected downstream, without significant loss of pressure.

 This system is generally used to recover nitrogen and not oxygen considered as a by-product.

Ionic conductive ceramics
It is an electrochemical operation which makes it possible to recover oxygen from the air. The air is in contact with one face of the ceramic (the cathode) on which the oxygen is reduced. The ions thus formed O- selectively pass through the conductive ceramic (at high temperature) under the effect of an electrical potential. On the other side (the anode) the O-- ions recombine into oxygen. The system developed is a ceramic cylinder bathed by ambient air on its external face and which accumulates pure oxygen under pressure in its internal volume.

 The introduction device can be any device of conventional type, such as for example an injector. If necessary, a buffer storage device can be provided between the synthetic air production device and the introduction device, which allows better dimensioning of the synthetic air production device.

 The air source can be atmospheric air or air taken from the exhaust system in the case of a diesel or gasoline engine with lean operation (high temperature). In the latter case, the synthetic air production device can be placed in line with the exhaust system.

 The air source can also be constituted by a turbocharger driven by the work of expansion of the exhaust gases and which provides all or part of the energy necessary for the compression of the ambient air supplied to the inlet of the device. production of synthetic air.

 If necessary, the nitrogen separated from the synthetic air by the synthetic air production device can be introduced into the intake pipe of the engine upstream of the intake throttle.

 There is shown schematically in Figure 1, an embodiment of a device for implementing the method of the invention.

 The device comprises an air source 10 which in the embodiment shown consists of a turbocharger driven by the exhaust gas from the engine to compress the ambient air.

The ambient air compressed by the turbocharger 10 is delivered to a device for producing synthetic air 20, for example a device as described above, which separates the compressed air received into a stream of synthetic air (enriched in oxygen) 21 and a stream of nitrogen 22. The stream of synthetic air 21 is introduced by an introduction device, such as an injector (not shown), upstream of a catalytic exhaust pipe or particles (40), while the nitrogen stream is reintroduced into the intake duct 31 of the engine upstream of the intake throttle 32.

Claims (11)

 1. Method for reducing pollutants contained in the exhaust gases of an internal combustion engine comprising an exhaust system characterized in that it comprises the introduction of a predetermined quantity of synthetic air into the system d 'exhaust.  2. Method according to claim 1, characterized in that the exhaust system is a catalytic exhaust system comprising a catalyst system.  3. Method according to claim 2, characterized in that the introduction of the predetermined amount of synthetic air takes place upstream and / or downstream of the catalyst system and / or into the catalyst system.  4. Method according to claim 2, characterized in that the catalyst system comprises a priming catalyst and a main catalyst, the introduction of the predetermined quantity of synthetic air taking place upstream of the priming catalyst and / or between the priming catalyst and the main catalyst and / or downstream of the main catalyst and / or in the main catalyst.  5. Method according to claim 3 or 4, characterized in that the introduction of synthetic air elsewhere than downstream of the catalyst system and / or into the main catalyst takes place during the cold start of the engine and up to at the start of the operation of the catalyst system.  6. Method according to claim 1, characterized in that the exhaust system is an exhaust system of a diesel engine which comprises a particulate filter.  7. Method according to claim 6, characterized in that the introduction of the predetermined amount of synthetic air takes place upstream and / or downstream of the particulate filter.  8. Method according to claim 7, characterized in that the introduction of synthetic air upstream of the particulate filter is carried out intermittently.  9. Device for implementing the method according to claim 1, characterized in that it comprises a device for producing synthetic air (20) connected to an air source and a device receiving the synthetic air produced and introducing a predetermined amount of synthetic air into an exhaust system of an internal combustion engine.  10. Device according to claim 9, characterized in that it comprises a device for storing synthetic air disposed between the production device and the introduction device.  11. Device according to claim 2 or 10, characterized in that the air source comprises a turbocharger.