System and processes for storing an additive and injecting it into the exhaust gases of an engine

Abstract

System for storing an additive solution and injecting it into the exhaust gases of an internal combustion engine, the system comprising a tank for storing the additive and a rotary pump, a metering valve and an injector which are separate devices, this system making it possible to meter and inject the additive solution into the exhaust gases as is, without diluting it with a gas. In this system, the pump outlet pressure is regulated by adjusting the rotational speed of the pump. Process using such a system.

Description

The present invention relates to a system and process for storing an additive and injecting it into the exhaust gases of an engine.

The change in legislation relating to emissions from diesel vehicles especially stipulates a significant reduction in releases of nitrogen oxide in exhaust gases. The SCR (Selective Catalytic Reduction) process in which nitrogen oxides (NOx) are reduced by ammonia is one of the technologies of choice for achieving this pollution control objective. An ammonia precursor solution (generally a solution of urea) is then injected into the exhaust line upstream of the SCR catalyst. The operation of this catalyst requires precise metering of the amount of solution sprayed.

In a known manner, decoupling the pumping, metering and injection operations results in a better precision of the amounts of ammonia precursor solution introduced into the exhaust line.

Thus, Patents U.S. Pat. No. 6,041,594 and U.S. Pat. No. 6,273,120 both describe systems comprising a separate pump, metering valve and injector that enable each of these functions to be carried out. The rapid metering valve (i.e. a valve operating with an adjustable opening frequency and duration) combined with a suitable pump outlet pressure (via a pressure-regulating device) enables precise metering of sprayed amounts of the ammonia precursor solution. These documents however both recommend recourse to compressed air which is mixed with the additive in a mixing chamber prior to its injection into the exhaust gases using an injector. This compressed air would especially prevent clogging of the injector which could be caused by a solid deposit of the additive and/or its degradation products resulting from the high temperature in the vicinity of the exhaust pipe.

These systems have the advantage of a compact, simple and adjustable architecture (for example, the pump may be integrated into the tank; the metering valve may be nearer to or further from the injector depending on the space available beneath the floor of the vehicle). The pump and the metering valve may thus be offset from the exhaust line, which prevents them from overheating and increases the metering accuracy of the valve. Moreover, the injector may be adapted as a function of the size and distribution of the desired droplets. Finally, these systems are particularly suitable for retrofitting to vehicles in circulation.

Such systems however require provision of a compressed-air source, which is furthermore not always available on the vehicle and which therefore has to be purposely provided, which is expensive. Moreover, diluting the additive leads to a reduction in the metering accuracy. Finally, recourse to a mixing chamber adds complexity and expense to these systems.

The present application aims at solving these problems and is based on the fact that recourse to compressed air is in fact not necessary in these “decoupled” systems. Indeed, it would appear that the aforementioned solid deposit disappears very quickly in solution and is easily removed by dissolving during a new injection. In fact, suitable control of the metering pump makes it possible to prevent clogging of the injector, and also makes it possible to prevent the formation of a drop at the outlet orifice of this injector.

Document EP 1 656 986 also proposes a system without compressed air where the pump, metering valve and injector are separate devices, but this system involves a pressure transducer and a solenoid valve for regulating the return flow (if necessary) which is relatively complicated and expensive.

The present application aims at solving this problem by providing a system that is significantly simpler, less expensive and just as effective. Therefore, the present invention relates to a system for storing an additive solution and injecting it into the exhaust gases of an internal combustion engine, said system comprising a tank for storing the additive and a rotary pump, a metering valve and an injector, this system making it possible to meter and inject the additive solution into the exhaust gases as it is, without diluting it with a gas such as compressed air. In particular, the system according to the invention does not comprise a mixing chamber nor any device whatsoever for diluting the additive with a gas such as compressed air. In the system according to the invention, the pump, metering valve and injector are separate devices and the pump outlet pressure is regulated by adjusting the rotational speed of the pump.

The additive concerned by the invention is preferably a reducing agent capable of reducing the NOx present in the exhaust gases of an internal combustion engine. Advantageously it is ammonia, used directly (which has the associated safety and corrosion drawbacks) or generated in situ in the exhaust gases from a precursor, such as urea (which enables the aforementioned drawbacks to be avoided). The invention gives good results with urea and, in particular, with aqueous solutions of urea. Eutectic solutions (comprising 32.5% by weight of urea) are very suitable.

The present invention may be applied to any internal combustion engine likely to generate NOx in its exhaust gases. It may be an engine with or without a fuel return line (i.e. a line returning the surplus fuel not consumed by the engine to the fuel tank). It is advantageously applied to diesel engines, and in particular to vehicle diesel engines and particularly preferably to the diesel engines of heavy goods vehicles.

The system according to the invention comprises at least one tank intended for storing the additive. This tank may be made from any material, preferably one that is chemically resistant to the additive in question. In general, this is metal or plastic. Polyolefin resins, in particular polyethylene (and more particularly HDPE or high-density polyethylene), constitute preferred materials.

The system according to the invention generally comprises an injection line intended to bring the additive to the exhaust pipe of the engine that, in order to do this, connects the tank, the pump, the metering valve and the injector; the pump, metering valve and injector being separate devices placed in series from upstream to downstream (following the flow direction of the additive, i.e. from the tank (upstream) to the injector (downstream)).

The pump of the system according to the invention is used to bring the additive solution to the pressure required for metering and spraying it. Various types of pumps may be suitable for the application: gear pump, piston pump, diaphragm pump, etc. This pump may be located in the additive tank (with the advantage of forming, with it, a compact and integrated module) or, considering the corrosive environment, be located outside of the additive tank. Its constituent materials will preferably be chosen from corrosion-resistant metals (especially certain grades of stainless steel and aluminium). The use of copper, even for connection components, is undesirable.

According to the invention, the pump outlet pressure is regulated by a suitable device which makes it possible to act directly on the rotational speed of the pump. Such a device is described in Application WO 2007/023142 in the name of the Applicant, the content of which is, for this purpose, incorporated by reference in the present application. According to one preferred variant, the system according to the invention comprises a return line equipped with a pressure-limiting device and the pump discharges an excess of additive (by intentionally rotating at too high a rate, i.e. with too high an outlet pressure), this excess (difference between the flow from the pump and that from the injector) being returned to the tank via this line. A simple calibrated spring or diaphragm valve gives good results. A preferred device is formed by a simple restrictor on the return line, preferably combined with a non-return valve. The advantage of this variant consists in allowing effective cooling of the metering valve in a simple manner.

According to a first variant, the pressure-regulating device is located downstream of the metering valve, in a line returning the flow of additive not consumed by this valve to the tank.

According to a second variant, the pressure-regulating device is located downstream of the injector, in a line returning the flow of additive not consumed by this injector to the tank.

The second variant (with excess metering at the injector) is preferred as it also makes it possible to avoid overheating the injector. However, it is advantageous in this case to provide a by-pass line bringing the additive return back upstream of the metering valve (and preferably upstream of the filter, where appropriate: see further on) when the temperature of the additive in the tank exceeds a threshold value (60° C. for example).

This variant also makes it possible to easily integrate a purge function in the conduits, especially to avoid damaging them in case the additive solution freezes. For this it suffices to provide a purge valve with a gas (air for example) inlet in the injection line, upstream of the pump. This valve may then, in a suitable position, suck air though the conduits and force the liquid that they contain back towards the tank.

The injector enabling the injection of the additive into the exhaust gases is generally located at the end of the injection line. This injector may be of any known type. It is advantageously a nozzle or a sprayer making it possible to obtain drops of solution having a diameter between 5 and 100 μm. Such a nozzle is advantageously equipped with an orifice having a diameter of around 150 μm-250 μm. This orifice is supplied by a system of narrow channels (3-4) producing a “swirl” (vortex) effect in the solution upstream of the nozzle. Clogging could be avoided by the purge which removes the last droplets of urea; there is therefore no crystallization by evaporation.

Preferably, the system according to the invention also comprises a filter located between the tank and the pump and making it possible to retain possible impurities present in the solution.

According to the invention, as already explained above, the pump, valve and injector are separate devices located in the injection line between the additive storage tank and the exhaust pipe of the engine. According to one particularly advantageous variant, and as described in Application FR 06/05082 in the name of the Applicant (the content of which is incorporated by reference in the present application), at least the pump is integrated into a flange present in the tank, in the bottom of it. In a particularly preferred manner, the pump, the pressure-limitating device and the metering valve are all integrated into this flange, which is connected to the injector via an injection line. Thus, a particularly compact system is obtained.

The present invention also relates to a process for storing an additive solution and injecting it into an exhaust pipe of an internal combustion engine, said process consisting in conveying the solution stored in a tank to an injector using a rotary pump and a metering valve, injector, pump and metering valve being separate devices and in injecting this solution into the exhaust pipe as is, without diluting it with a gas such as compressed air.

In this process according to the invention, the amount of solution is metered by regulating the opening frequency and duration of the metering valve. This valve may be a piezoelectric or solenoid valve, the regulation of which may be electronic. Thus, most often, the system according to the invention comprises a computer connected to the metering valve and making it possible to convey the required amount of additive (especially as a function of the following characteristics: emission level and degree of conversion of the NOx; temperature and pressure; engine speed and load, etc.) to the injector. This computer may also act on the pressure regulation of the pump. It could, for example, be advantageous to increase the pressure for low flow rates.

In a first variant, the amount of additive discharged by the pump and not consumed by the metering valve (as a function of the control by the computer) is returned to the additive tank by a return line connected between the pump and the metering valve. In another variant, the metering valve conveys an excess of additive to the injector which is returned to the tank by a return line that starts from the injector. In both cases, a pressure-limiting device is located in the return line, which returns the excess flow (resulting from the difference between the flow from the pump and the spraying flow) to the tank.

The present invention is illustrated, in a non-limiting manner, by FIGS. 1 to 3, which each relate to a different variant of the invention. In these FIGS. identical numbers represent identical or similar components. The flow direction of the exhaust gases is indicated therein by an arrow marked “G”.

FIG. 1 represents a simple system where the metering valve (4) conveys exactly the amount of additive required to the injector (8), the excess produced by the pump (3) being returned to the tank (1) via a calibrated valve (6).

The tank (1) contains a reducing agent precursor solution, in this case: an ADBLUE® 32.5% aqueous urea solution. The possible impurities are retained by a filter (2). The pump (3) allows the solution to be withdrawn from the tank and the liquid to be brought to the pressure needed for the required spraying and for the return flow needed for cooling the metering valve. The calibrated valve (6) provides control of the spraying pressure in the line section (3). Metering of the amount of reducing agent precursor is carried out by opening the metering valve (4), which is electronically controlled, with an established opening frequency and duration. The metering valve (4) is connected to the injector (nozzle) (8) by the line (7). The nozzle (8) is designed so as to spray the solution in the form of droplets, of which the size distribution is in general between 5 μm and 120 μm. The excess flow (resulting from the difference between the flow from the pump and the spraying flow) returns to the tank (1) by the line (5). In one variant of FIG. 1, the calibrated valve (6) is replaced by a restrictor combined with a simple non-return valve in the case where the return line (5) is connected to the bottom area of the tank (1).

FIG. 2 illustrates a more complex system which includes a function of cooling the injector (by excess metering of the additive) and a purge function.

This system comprises a purge valve (solenoid valve) (11) which, when the solution is sucked up by the pump (3), is in the open position so as to connect the tank (1) to the filter (2).

It also comprises a by-pass valve (solenoid valve) (10) installed in the return line (5) which this time starts from the injector (8) to its intersection with a by-pass line (13). This return makes it possible to avoid excessive overheating of the nozzle. When the temperature of the solution measured in the tank (1) exceeds 60° C., the solenoid valve (10) switches over and the liquid is conveyed into the by-pass line (13) instead of returning to the tank (1). The valve (11) is a proportional valve. It could also be an on/off valve or a simple bimetallic strip. As shown, the system also comprises a purge function for the feed and return lines. For this purpose, the inlet of the purge valve (11) is connected to an air inlet (12). With a suitable position of this valve (11), air may be sucked in by the pump, forcing the liquid contained in the conduits back towards the tank (1). The lines illustrated are, in addition, equipped with a known heating device.

All the functions of the system are electronically controlled.

FIG. 3 illustrates a system that fulfils the same functions as that of FIG. 2. The components (filter (2), pump (3), metering valve (4), purge valve (11), calibrated valve (6), by-pass valve (10)) have however been integrated inside the tank (1), resulting in a particularly compact system.

Claims

1. A system for storing an additive solution and injecting it into the exhaust gases of an internal combustion engine, said system comprising a tank for storing the additive and a rotary pump, a metering valve and an injector which are separate devices, this system making it possible to meter and inject the additive solution into the exhaust gases as it is, without diluting it with a gas, wherein the pump outlet pressure is regulated by adjusting the rotational speed of the pump.

2. The system according to claim 1, wherein the pump discharges an excess of additive solution (i.e. a higher flow than that of the injector), and wherein the system comprises a return line equipped with a pressure-limiting device that enables the excess of solution to be returned to the tank.

3. The system according to claim 2, wherein the pressure-limiting device is a restrictor which may be combined with a non-return valve.

4. The system according to claim 2, wherein the pressure-limiting device is a calibrated spring or diaphragm valve.

5. The system according to claim 2, wherein the pressure-limiting device is located downstream of the metering valve, in a line that enables the flow of additive not consumed by this valve to be returned to the tank.

6. The system according to claim 2, wherein the pressure-limiting device is located downstream of the injector, in a line that enables the flow of additive not consumed by this injector to be returned to the tank.

7. The system according to claim 6, comprises comprising a by-pass line that enables the flow of additive not consumed by the injector to be brought back upstream of the metering valve instead of to the tank when the temperature of the additive in the tank exceeds a threshold value.

8. The system according to claim 6, comprising a purge valve with a gas inlet upstream of the pump.

9. The system according to any one of the preceding claims claim 1, comprising a filter located between the tank and the pump.

10. The system according to claim 2, wherein the pump, the pressure-limiting device and the metering valve are integrated into a flange which is connected to the injector via an injection line.

11. A process for storing an additive solution and injecting it into an exhaust pipe of an internal combustion engine, said process consisting in conveying the solution stored in a tank to an injector using a rotary pump and a metering valve, injector, pump and metering valve being separate devices, and in injecting this solution into the exhaust pipe as is, without diluting it with a gas, the pump outlet pressure being regulated by adjusting its rotational speed.

12. The process according to claim 11, wherein the pump discharges an excess of additive solution (i.e. a higher flow than that of the injector), and wherein this excess is returned to the tank via a return line equipped with a pressure-limiting device.

13. The process according to claim 11, wherein the metering valve is opened for an adjustable duration and frequency, and wherein the amount of additive solution is metered by adjusting this duration and this frequency electronically.