EP2025912A1 - Engine exhaust gas circuit - Google Patents

Abstract

The circuit has an exhaust line (6) provided with a particle filter (10), and a gas recirculation channel (14) connected to the line at one of its end in downstream of the filter and to an engine inlet circuit (4) at another end. A heat exchanger (17) cools gas in the channel. A tube (18) playing role of bypass is arranged at downstream of the exchanger for returning the channel to the exhaust line. Control units control flow of gas admitted in the channel in a selective manner, and orientation units orient the gas towards an inlet circuit or towards the tube.

Description

Technical area

The present invention relates to an exhaust gas circuit of a combustion engine provided with exhaust gas recirculation means. The invention is more particularly suitable for compression ignition engines, said diesel and gasoline engines with an EGR function.

State of the art

To reduce emissions of pollutants at the source, including the production of monoxides and nitrogen oxides, the fuel-fuel mixture (air-gas oil) in the combustion chamber is diluted with an inert gas, normally constituted by gases. burned. For this, a fraction of the exhaust gases are taken at the exhaust manifold or the exhaust line and reinjected at the intake. This technique is most often known by its Anglo-Saxon acronym EGR (Exhaust Gas Recirculation).

The exhaust gases are by definition hot. Now, it is well known that the emissions of nitrogen oxides from an engine are normally all the better when the intake gases are cold. Also, the recirculation line usually admits a heat exchanger so as to cool the recirculated gas fraction before mixing with the fresh gas.

As, moreover, there are phases of operation of the engine, in particular during a cold start, where such cooling is not desirable, the recirculation line most often comprises two branches, one of which is cooled, and means to control the respective flows of recirculated gases in these two branches.

Finally, the recirculation line also comprises means for modifying the recirculated flue gas fraction.

When the engine is equipped with a turbocharger, that is to say that the fresh gases are compressed by means of a compressor energized by a turbine placed in the exhaust line just downstream of the exhaust manifold, two major types of architecture are possible: a so-called high-pressure architecture, according to which the bypass for the recirculation of gases is placed upstream of the turbine, or a so-called low-pressure architecture according to which the bypass is placed downstream of this turbine.

Examples of recirculation line architectures, high or low pressure are notably presented in the publication EP 0 596 855 . Several of the proposed architectures provide for equipping the recirculation line with a particulate filter, specific to this line or disposed upstream of the bypass, this type of architecture advantageously making it possible to avoid injecting particles with can in the long run foul the intake line between the compressor and the intake manifold. In practice, it is clear that a filter on the recirculation line itself does not make it possible to do without a second filter for non-recirculated gases, this variant of the invention is therefore particularly expensive, how much it could even to be implemented, which is far from obvious considering that the volumes under hoods are becoming smaller, in particular because of the development of the supply of diesel engines even for vehicles of small sizes. In addition, the aforementioned publication does not propose steering strategies corresponding to the different architectures proposed and especially to the different phases of driving.

Regardless of the control of pollutant emissions, which at best is essentially transparent to the occupants of the vehicle, at worst generates a slight decline in driving enjoyment, it is always desirable to improve the comfort of the occupants. A situation often considered uncomfortable occurs shortly after starting the vehicle in cold weather: firstly, the occupants of the vehicle expect that the desired temperature (for example 20 ° C) is quickly reached in the passenger compartment, but on the other hand, the heating to the cabin is relatively inefficient as the engine temperature has not reached the normal operating range.

During this startup phase, heating the coolant would thus both improve the heating of the passenger compartment and accelerate the temperature rise of the engine.

It is known to use the heat of the exhaust gases to provide this intake of calories to the coolant by means of a heat exchanger placed on the exhaust line, known as RTE, acronym for heat recovery at the same time. Exhaust. On vehicles equipped with such a RTE function, the exchanger is added to that provided for cooling the exhaust gases reintroduced into the engine. Obviously, this doubling of equipment increases the cost of the vehicle, and more is only possible if the space under the hood is sufficient, which is far from always being the case, especially for small vehicles. cut.

An exhaust line in which the exchanger intended for the cooling of the exhaust gases admitted into the recirculation branch is also used outside this function is known from the patent FR 2,770,582 but in a rather different concept. Indeed, this document describes an exhaust line comprising an exhaust gas recycle line provided with a gas heat exchanger and removing gases upstream of a catalytic converter, a bypass of the gases, bypassing the circuit. intake and returning to the line upstream of the catalytic converter being provided. According to this teaching, the exhaust gases are admitted into the recycling pipe before being directly re-admitted into the exhaust line during engine start-up phases, in which case the gases help to warm the cooling liquid circulating in the engine. heat exchanger, which accelerates the engine warm-up, and in a second time, to heat the cabin, but especially during fully loaded phases, when the temperature of the exhaust gas is maximum, to cool the gases so that their temperature does not exceed the conditions of use of the catalytic converter.

This patent FR 2,770,582 therefore requires that the diversion to the exhaust gas recirculation pipe is placed upstream of the catalytic means. Such a configuration assumes that the recirculated gases are unpurified gases. In the case of engines equipped with a particulate filter, especially diesel engines, this assumes that the recirculated gases are charged in particles.

This is not perfectly satisfactory from the point of view of the proper operation of the engine. In fact, ideally all fuel and oxidant inputs should be perfectly controlled, and the recirculated gases perfectly inert. To allow a recirculation of unpurified gases is therefore not in itself an optimal one. Moreover, and most importantly, the recirculation circuit comprises a series of means for opening or closing conduits, in a binary (all-or-nothing) or proportional manner. However, these means can be deteriorated if they are exposed to unpurified gases, hence a risk of failure all the more important that these parts are exposed to very high temperature and therefore already relatively fragile.

Moreover, the soot present in the unpurified exhaust gas can form insulating deposits on the surface of the exchanger so that the efficiency of the latter is lower, which can be particularly damaging during the phases of operation with recirculation of the gases at the intake, phases during which the exchanger has the function of preventing the fresh gases from being mixed with too hot gases, to the detriment of a good efficiency of the engine.

The present invention aims at a new architecture of exhaust line more robust, and therefore more economical than the architectures known in the art.

According to the invention, this object is achieved by an exhaust gas circuit of an engine comprising an exhaust line provided with a particle filter, a gas recirculation pipe connected to one of its ends at the line. exhaust system, downstream of the particulate filter, and at its other end, to the engine intake circuit and a heat exchanger for cooling the gases in the recirculation line; this circuit being remarkable in that it comprises, downstream of the heat exchanger, a tube connected to the exhaust line (called 'Tube RTE') and means for selectively controlling the flow rate of gases admitted into the pipe of recirculation and means for directing the gases to the intake circuit or to the RTE Tube.

In this document, the concepts of downstream and upstream are given by reference to the direction of gas flow, the engine being on the exhaust side the source.

In an alternative embodiment, the circuit also comprises means for passing the heat exchanger for at least a fraction of the gases reinjected at the inlet. These means can be constituted by a high pressure EGR circuit, with a bifurcation of the exhaust line upstream of the turbocharger turbine (or at least upstream of the particulate filter in the event that the engine is not equipped with such a turbocharger) and / or by a bypass of the heat exchanger.

Different combinations of valves make it possible to control the respective flow rates in the various pipes of the low pressure EGR circuit. Thus, it is possible to use a proportional valve placed in the recirculation pipe, downstream of the junction point between the RTE pipe and the recirculation pipe, a second valve placed in the RTE pipe and a proportional valve placed in the line. exhaust, downstream of the diversion constituted by the recirculation pipe and upstream of the return of the Tube RTE. The use of a proportional valve for the second valve makes it possible not to cool all the recirculated gases, variant all the more advantageous that the engine would not be equipped with a high pressure EGR circuit. If the use of the RTE Tube is not desired in an EGR mode, the second valve will preferably be an all-or-nothing type valve, more robust and less expensive.

According to another variant of the invention, the means for selectively controlling the flow rate of the gases admitted into the recirculation pipe and the means for directing the gases towards the intake circuit or to the bypass comprise a 3-way valve to the junction between the bypass and the exhaust line and a proportional valve, placed in the recirculation pipe, downstream of the branch to the RTE tube.

Another particularly advantageous possibility is to use a 3-way / proportional mixed valve at the junction between the RTE tube and the exhaust line and a proportional valve, placed in the pipeline of recirculation, downstream of the derivation to the RTE Tube. The mixed valve comprises for example a pivoting flap, slaved to an actuator and a free flap provided with a return spring.

In another variant of the invention, the circuit comprises a 3-way valve at the junction between the RTE tube and the recirculation pipe and a proportional valve between the junction between the line and the pipe and the junction between the line and the pipe. RTE tube.

Brief description of the figures

Other advantages and particularities of the invention appear from the description of embodiments given below with reference to the appended drawings in which:

The figure 1 illustrates different heating requirements depending on the engine water temperature and the outside temperature;

The figure 2 is a schematic view of an exhaust gas circuit according to the invention;

The figure 3 details the low pressure EGR sub-circuit of the figure 2 in a depollution mode;

The figure 4 details the low pressure EGR sub-circuit of the figure 2 in an RTE mode;

The figure 5 illustrates a variant of an EGR sub-circuit comprising only two valves;

The figure 6 is a detailed view of a valve particularly suitable for the configuration of the figure 5 ;

The figure 7 illustrates another variant with two valves of a subcircuit according to the invention.

Detailed description of embodiments of the invention

The figure 1 is a graph on which a number of rolling situations have been positioned as a function of the outside ambient temperature (ordinate) and the water temperature of the engine cooling circuit (abscissa).

If the outside temperature is above a certain threshold, typically between 5 and 10 ° C, the cabin air can be easily warmed by recovering the heat from the engine by means of the heater (heat exchanger to the engine). passenger). For lower temperatures, this heating mode is often considered insufficient, especially if the vehicle is traveling relatively slowly (and therefore the temperature of the coolant is low, for example lower than about 40 ° C). In this case, additional heating is desirable, which heating can be obtained by recovering a portion of the heat of the exhaust gas, which is indicated on the graph by the zone denoted "area need additional heating cabin".

On the other hand, from the strict motorist point of view, it is not desirable to cool a possible recycled fraction of the exhaust gases if the outside temperature is low. In fact, the exhaust gases consist essentially of a mixture of water vapor and carbon dioxide (the products of the reaction of combustion of air with the fuel). However, if the water circulating in the heat exchanger equipping the recirculation pipe is very cold, the cooling can be somehow "too effective" and cause condensation of a portion of the water vapor. The water droplets thus formed could then be injected into the compressor of the intake line at the risk of causing it to break. Therefore for these low temperatures, whatever the temperature of the engine water, it is not desirable to proceed with a cooling of the EGR gas for the depollution. The area of additional heating need of the passenger compartment therefore corresponds to conditions other than those of the EGR zone.

As long as the engine is really "cold", so as long as the water temperature is low, the combustion chamber is at a relatively low temperature, which does not induce a need to cool the EGR gas for questions of emission of nitrogen oxides, the need for a heat exchanger being justified on the heavily loaded engine operating points, hot engine. On the other hand, it is always desirable to help the temperature rise of the engine by not cooling too much oil to a stage where its temperature has to increase further, hence the indicated area of "need heating water motor".

In the absence of a return conduit for circulating the exhaust gas in the EGR exchanger without returning them to admission, we are in a phase or paradoxically, there is a strong need for heating the coolant to favor the heating of the passenger compartment but during which no fraction of the exhaust gas circulates at the level of the heat exchanger EGR so that there is no possible recovery of the available heat in the exhaust.

If now it is provided, as according to the invention means for circulating the exhaust gas at the EGR exchanger without returning them to admission, it becomes possible to use these calories. Moreover, since these gases are no longer reinjected, the fraction of the exhaust gases admitted to the EGR branch can be 100%, whereas as long as this fraction is intended to be reinjected at intake, it can only to be partial under pain of an immediate engine failure.

Having these means of direct return is therefore particularly advantageous. However, it is clear that this assumes at least one additional valve compared to a conventional installation and that all the valves of the EGR circuit will be exposed to larger amounts of exhaust gas. So if these gases are not purified, the risk is very great that the valves are quickly damaged by the gradual accumulation of soot. Suies, moreover, will form an insulating layer on the walls of the exchanger and degrade its effectiveness. Placing the return on a low pressure EGR circuit, downstream of the particulate filter, is therefore particularly advantageous.

An example of a gas circuit according to the invention is more particularly represented in FIG. figure 2 . For clarity, on this figure 2 , the intake gases are schematized by simple arrows while the exhaust gases are represented by solid arrows.

The engine 1 is provided with means for admitting the fresh gases into the cylinders, here schematized by an intake distributor 2 and means for evacuating the combustion gases, here shown schematically by an exhaust manifold 3.

Fresh air, filtered and dried, sucked by the engine, is led through the intake pipe 4 to a compressor 5 which compresses the fresh air and thus supercharging the engine, thus allowing maximum torque of the engine. bigger engine. After this compressor, the supply circuit normally comprises a heat exchanger, not shown here, for cooling the fresh gases heated by the compressor. Then these cooled fresh gases are led to the intake manifold.

The exhaust line 6 starts at the exhaust manifold. The flue gases drive a turbine 7 which actuates the compressor 4. A bypass 8 associated with a valve 9 (wastegate), regulates the flow of gas at the turbine to control the boost pressure level. Downstream of the turbine 7 is arranged a particulate filter 10 or more exactly an assembly comprising an oxidation catalyst and a particulate filter, the oxidation catalyst for oxidizing carbon monoxide and unburned hydrocarbons, and the filter to accumulate soot, consisting essentially of carbon, outside the regeneration phases during which the temperature of the exhaust gas is greater than the soot combustion temperature. Downstream of the particulate filter, the exhaust gases continue to run to the outside of the vehicle as shown by the arrow 11.

The circuit further comprises means for recycling a fraction of the exhaust gases by reinjecting them on admission. On the circuit shown here, these means consist of two sub-circuits called high and low pressure.

The high pressure sub-circuit 12 (EGR-HP) comprises a pipe connecting the exhaust manifold 3 to the inlet manifold 2, a proportional valve 13 allowing a flow control. Note that the connection with the exhaust part could also be carried out at any point upstream of the turbine 7. In the configuration proposed here, the gases recirculated by this high pressure circuit are not cooled, the high pressure valve can be closed EGR when it is desired to cool the gases, and adjusting the respective flow rates between the low-pressure and high-pressure branches, a fine adjustment of the temperature is possible if desired. Of course, it would also be possible to provide a cooling exchanger in the high pressure branch even if in practice, this solution is obviously more expensive.

The low pressure EGR sub-circuit is for its part constituted by a pipe 14 which connects the exhaust line 6 to the intake pipe 4, between a point (or junction) 15 of the exhaust line downstream of the filter particle and a point 16 of the intake pipe upstream of the compressor. The pipe 14 passes through a heat exchanger 17 for cooling the gases flowing in the pipe. This heat exchanger 17 is part of the cooling circuit, which also has the main function of cooling the engine and which passes through different heat transfer zones, heating up when it cools the oil and the internal engine material or gases. exhaust and being cooled at the radiator of the vehicle and possibly at the fresh air intake into the passenger compartment when heating is controlled.

Furthermore, a tube (called RTE tube) 18 connects a point (junction) 19 of the exhaust line 6, downstream of the point 15, and a point 20 (junction) of the pipe 14 downstream of the heat exchanger 17.

The low pressure EGR circuit shown in FIG. figure 2 has 3 valves. A first valve 21, proportional valve type, is placed for example just upstream of point 16. In the open position, it allows the recycling of gases, and will subsequently be called EGR valve (or BP EGR valve). The second valve 22, also referred to hereafter as RTE valve, may be of the all-or-nothing type as will be explained later. Finally, between point 15 and point 19 there is provided a valve 23 called pressure drop valve, of the proportional type.

When the engine is warm and EGR mode operation is desired, the gas streams will typically be as shown in FIG. figure 3 , that is to say that the BP EGR valve (21) is partially open and the valve 23 partially open (it being understood that the flow towards the terminal part of the pot exhaust of the vehicle can only be partially and not completely closed), so that a portion of the gas is diverted to the pipe 14.

If the valve 22 is in the closed position, all the gases reintroduced at the intake upstream of the compressor is cooled by the EGR exchanger. In the hypothesis here figured, the HP EGR line is also functional and the gases that pass through it are not cooled, considering that in the architecture proposed here, the gases are directly readmitted into the intake manifold and are therefore not not overheated by the compressor. Without departing from the scope of the invention, it is also possible to provide cooling, preferably optional with a bypass, for the line EGT HP - or in any other variant, opt for an architecture without HP line.

If starting from the configuration illustrated in the figure 3 , the valve 22 is open, a part of the recirculated gases in the low pressure line can pass directly through the tube 18 which thus acts as a bypass of the EGR exchanger which allows a better control of the temperature of the gases. exhaust at the time of their reintroduction into the intake pipe.

In the mode shown in figure 4 , the EGR valves 21 and 23 are now closed while the RTE valve 22 is open. With the exception of the fraction of the gases "leaking" through the proportional valve 23, it can be considered that all the exhaust gases arriving at the compressor outlet and then the particulate filter are directed towards the pipe 14, cooled by the EGR exchanger and return to the exhaust line through the tube 18 which now serves as a return tube. Considering that the quantity of gas to be cooled is greater than for a simple EGR exchanger, it is advantageous to size this exchanger with respect to the flows treated in this operating mode, and not in the EGR mode illustrated in FIG. figure 3 , taking care as far as possible to minimize the pressure drops.

In this mode of operation, corresponding to a cold engine configuration, the heating of the coolant at the level of the EGR exchanger is then maximum, which makes it possible to promote the temperature rise of the engine and to heat the cabin more quickly. This mode of operation is therefore advantageous in winter to help warm the cabin. Whatever outside temperature, it can also be used as long as the temperature of the coolant is relatively low, for example below 60 ° C, this in particular to allow a regeneration of the particulate filter quickly after startup - which can be desired for example when a regeneration has been interrupted following the stopping of the vehicle. → Another mode of operation exists: valve 23 closed and valves 21 and 22 in regulation in order to maximize the calories transmitted to the engine water while cooling the EGR gases.

As indicated above, it is important to have means of throttling for switching between 3 operating modes: a basic operation, in which the exhaust gases do not circulate in the pipe 14 (and therefore no longer in the pipe 18), an EGR mode - with a variable fraction of recirculated gas to the intake, and an RTE mode.

To obtain these 3 modes, one can use an architecture with 1 on-off valve and 2 proportional valves as indicated previously or other architectures involving a smaller number of valves.

In the variant illustrated in figure 5 a 3-way valve 30 is disposed at the point 19 and a proportional valve 31 is disposed between the points 20 and 16 of the pipe 14. When the 3-way valve 30 closes the pipe 18 (Figure 5A and 5B), the circuit is analogous to a "normal" circuit (neither RTE nor EGR) when the proportional valve 31 is closed (diagram 5A) or similar to an EGR circuit when this valve is partially open (diagram 5B). When the 3-way valve closes the line 4, the proportional valve being closed, all the gases pass through the pipe 14 and return to the line 6 by means of the tube 18, the circuit is then in position RTE (diagram 5C).

In practice, however, it is difficult to design a proportional valve whose closed position is effectively sealed. One solution is then to use a simple on-off valve to replace the proportional EGR valve and to place at point 19 a double-vane valve, playing both a 3-way valve and proportional valve role. as for example shown in the figure 6 .

This double-shutter valve thus comprises a pivoting flap 115 in the clockwise direction about an axis of rotation perpendicular to the flap, the movement of the flap being slaved to a pneumatic or electric actuator, not shown here. The actuator thus makes it possible to open the flap 115 to control the fraction of the exhaust gas redirected towards the duct 14. A second flap 121 is also mounted around the axis of rotation, this flap not being controlled by a actuator but simply provided with a return spring tending to press it in the closed position of the tube 18. When the flap 115 closes the line 6, the pressure of the exhaust gas forces the flap 121 to open the outlet of the duct 18. Note that in this variant, if actuated flap 115 "leaks" in the closed position, we simply have a small fraction of the exhaust gas that will not be directed to the exchanger in the RTE mode but this will only have to a very limited effect, that of slowing down slightly the rise in temperature of the cooling water, which is much less critical than at the level of the EGR gases. In addition this variant has the advantage of ensuring a constant flow to the exhaust even in the case of a non-functional valve 30.

Another variant, illustrated in figure 7 consists in using at point 20 a 3-way valve 41 and between point 15 and point 19 a proportional valve 42.

In these different variants, the invention makes it possible both to make the best use of the heat of the exhaust gases when there is a need to recover it without dedicating a heat exchanger specific to this function while being compatible with recirculation particularly high which is very favorable from the point of view of depollution.

Claims (12)

An exhaust gas circuit of an engine comprising: An exhaust line (6) provided with a particulate filter (10); A gas recirculation pipe (14) connected at one of these ends to the exhaust line, downstream of the particulate filter (14), and at its other end to the intake circuit of the engine (4); A heat exchanger (17) for cooling the gases in the recirculation pipe (14); characterized in that it comprises, downstream of the heat exchanger (17), a RTE tube (18) for a return of the recirculation line to the exhaust line (6) and means for selectively controlling the flow rate gases admitted into the recirculation pipe and means for directing the gases to the intake circuit or to the RTE pipe. Circuit according to claim 1, characterized in that it comprises means for bypassing the heat exchanger for at least a fraction of the recirculated gases and readmitted to the intake of the engine. Circuit according to Claim 2, characterized in that these means for bypassing the heat exchanger comprise a recirculation circuit (12) connected on the one hand to the engine intake circuit and on the other hand to the exhaust manifold or to the exhaust manifold. a point of the exhaust line upstream of the particulate filter and a possible turbine of a supercharging system. Circuit according to Claim 2 or Claim 3, characterized in that the means for passing the heat exchanger comprise the RTE tube. Circuit according to any one of the preceding claims, characterized in that the means for selectively controlling the flow rate of the gases admitted into the recirculation pipe and the means for directing the gases towards the intake circuit or to the bypass comprise a valve proportional (21) located in the pipe 14, downstream of the junction point between the bypass (18) and the pipe (14), a valve (22) placed in the RTE pipe (18) and a proportional valve (23). ) placed in the exhaust line (6), downstream of the diversion to the recirculation pipe (14) and upstream of the return of the RTE pipe. Circuit according to Claim 5, characterized in that the valve (22) is an all-or-nothing type valve. Circuit according to Claim 5, characterized in that the valve (22) is a proportional type valve. Circuit according to any one of Claims 1 to 4, characterized in that the means for selectively controlling the flow rate of the gases admitted into the recirculation pipe and the means for directing the gases towards the intake circuit or towards the RTE pipe comprise a 3-way valve (30) at the junction between the RTE tube (18) and the exhaust line (6) and a proportional valve (31), placed in the recirculation line (14), downstream of the bypass to the bypass (18). Circuit according to any one of Claims 1 to 4, characterized in that the means for selectively controlling the flow rate of the gases admitted into the recirculation pipe and the means for directing the gases towards the intake circuit or towards the RTE pipe comprise a mixed 3-way / proportional valve (30) at the junction between the RTE tube (18) and the exhaust line (6) and an on-off valve (31) placed in the recirculation pipe (14). ), downstream of the bypass to the bypass (18). Circuit according to claim 9, characterized in that the mixed valve comprises a pivoting flap (115), slaved to an actuator and a free flap (121) provided with a return spring. Circuit according to any one of Claims 1 to 4, characterized in that the means for selectively controlling the flow rate of the gases admitted into the recirculation pipe and the means for directing the gases towards the intake circuit or to the bypass. comprise a 3-way valve (41) at the junction (20) between the bypass (18) and the recirculation pipe (14) and a proportional valve (42) between the junction between the line (6) and the pipe (14) and the junction (19) between the line (6) and the bypass (18). The driving strategies may need to be explained: activate the valves according to water data, external air, speed, load, need for interior heating.

Images