JP4905431B2 - EGR cooler for internal combustion engine - Google Patents

Description

  The present invention relates to an EGR cooler for an internal combustion engine.

  In a diesel engine, EGR for returning a part of exhaust gas to an intake system is performed for the purpose of suppressing combustion temperature and reducing NOx. In order to enhance the NOx reduction effect by EGR, it is effective to cool the EGR gas by providing an EGR cooler on the EGR gas flow path.

If soot in the exhaust gas adheres to the inner wall of the EGR cooler, the cooling efficiency of the EGR gas is reduced and the amount of EGR gas is reduced, so that the NOx reduction effect is reduced. On the other hand, a technique is known in which the exhaust gas flow path of the EGR cooler is coated with a coating material that is excellent in the effect of preventing soot adhesion (see, for example, Patent Document 1).
JP 2001-330394 A

  It is difficult to completely prevent soot accumulation in the EGR cooler even if it is coated with a coating material having a high effect of preventing soot adhesion, and as the soot accumulates, the cooling efficiency of the EGR cooler decreases and the emission is reduced. There was a risk of deterioration. In addition, even when soot accumulates on the EGR cooler and the cooling efficiency of the EGR cooler is the lowest, for example, combustion control is performed to retard the injection timing so that the EGR cooler is retarded so that the fuel efficiency performance when the EGR cooler is new There was also a problem that was limited.

  This invention is made | formed in view of such a problem, and it aims at providing the technique which suppresses the fall of the cooling efficiency of an EGR cooler.

In order to achieve the above object, an EGR cooler for an internal combustion engine of the present invention includes:
An EGR cooler that cools the EGR gas by heat exchange between the EGR gas and the refrigerant,
The EGR cooler is coated with a coating material on the wall surface on the EGR gas distribution side in the thermal contact portion between the EGR gas and the refrigerant,
The coating material is
The degree of increase in the efficiency of heat conduction through the contact portion due to the deterioration of the coating material over time due to the flow of EGR gas,
The degree of decrease in the efficiency of heat conduction through the contact portion due to the exhaust component adhering to the wall surface on the EGR gas flow side in the contact portion due to the flow of EGR gas;
Are coated so that they are substantially equivalent.

  The thermal contact portion between the EGR gas and the refrigerant is a partition that separates the EGR gas flow path from the refrigerant flow path in a configuration in which the refrigerant flow path is provided in the EGR gas flow path, for example.

  The coating material coated on the wall on the EGR gas distribution side in this contact portion deteriorates with time due to acidic exhaust components and exhaust heat and decreases. As the coating material decreases, the resistance to heat exchange between the EGR gas and the refrigerant decreases, so that the efficiency of heat conduction through the contact portion increases. That is, the cooling efficiency of the EGR gas in the EGR cooler is increased.

On the other hand, exhaust components adhere to and accumulate on the wall surface on the EGR gas distribution side in the contact portion.
Examples of exhaust components include soot and unburned fuel (hydrocarbon). As the exhaust component adheres to and accumulates on the wall surface, resistance to heat exchange between the EGR gas and the refrigerant increases, so that the efficiency of heat conduction through the contact portion decreases. That is, the cooling efficiency of the EGR gas in the EGR cooler is lowered.

  In the early stage of use, when the EGR cooler is close to new, the coating material is hardly deteriorated. Therefore, the cooling efficiency of EGR gas is limited by the presence of the coating material, but the exhaust components are hardly adhered or deposited yet. The decrease in the cooling efficiency of the EGR gas due to the presence of exhaust components is small.

  On the other hand, as the EGR cooler usage period becomes longer, the coating material gradually deteriorates, so the EGR gas cooling efficiency increases due to the decrease in the coating material, while the amount of exhaust components attached and accumulated increases. The cooling efficiency of EGR gas becomes lower.

  In the present invention, the degree of increase in the cooling efficiency of EGR gas due to such aging degradation of the coating material, and the degree of decrease in the cooling efficiency of EGR gas due to the attachment / deposition of exhaust components on the inner wall surface of the EGR cooler Since the coating material is coated so as to be substantially equivalent to each other, it is possible to maintain a substantially constant cooling efficiency of the EGR gas from a new EGR cooler over a long period of use.

  Therefore, it is possible to perform combustion control so that the fuel efficiency performance of the EGR cooler when it is new is not limited.

  As the coating material in the present invention, a coating material whose thermal conductivity is lower than the thermal conductivity of the material constituting the contact portion and whose corrosion resistance against EGR gas is lower than a predetermined level may be used.

  If the corrosion resistance is lower than the predetermined level, the degree of decrease in the thermal conductivity at the contact portion due to the attachment / deposition of exhaust components over time associated with the flow of EGR gas over a long period of time and the same increase in thermal conductivity The corrosion resistance is such that a serious deterioration occurs in the coating material over time.

  By coating the wall surface of the contact portion with a coating material having a lower thermal conductivity than the material constituting the contact portion, the efficiency of heat exchange between the EGR gas and the refrigerant through the contact portion is greater than when the coating material is not coated. Lower. And the thermal conductivity in a contact part will increase gradually toward thermal conductivity equal to the thermal conductivity of the material which comprises a contact part with deterioration of a coating material.

  Since the coating material having a relatively weak corrosion resistance against the EGR gas is coated, the deterioration of the coating material with the passage of time during the use of the EGR cooler is surely caused. As a result, it is possible to cause an increase in the thermal conductivity derived from the deterioration of the coating material in the contact portion so as to balance with a decrease in the efficiency of heat conduction due to the adhesion / deposition of exhaust components in the contact portion.

  As described above, the efficiency of heat conduction between the EGR gas and the refrigerant in the contact portion decreases due to the attachment / deposition of exhaust components in the contact portion. The present invention is a technique characterized in that the contact portion is coated with a coating material having relatively low corrosion resistance so as to cause an increase in thermal conductivity that cancels out the degree of the decrease.

Here, the amount of exhaust component adhering to the wall surface of the contact portion varies depending on the position in the EGR cooler and the type of exhaust component. Therefore, it is considered that the degree of decrease in the efficiency of heat conduction caused by the attachment of exhaust components varies depending on the position in the EGR cooler and the type of exhaust components. Therefore, it is preferable that the coating material is coated on the wall surface of the contact portion according to the variation in the degree of decrease in the efficiency of heat conduction according to the position in the EGR cooler and the type of exhaust component.

  For example, the thickness of the coating material may be varied in accordance with the variation in the amount of exhaust component adhering to the wall surface of the contact portion due to the flow of EGR gas depending on the position in the EGR cooler.

  The more the amount of exhaust component adhering to the contact portion, the greater the degree of decrease in heat conduction efficiency. Therefore, the thickness of the coating material is preferably thin so that the efficiency of heat conduction is likely to occur in such a part, in other words, the deterioration of the coating material is likely to proceed. Conversely, at locations where the amount of exhaust component attached to the contact portion is small, the degree of decrease in the efficiency of heat conduction is small, so that the thickness of the coating material is preferably increased so that the deterioration of the coating material is unlikely to proceed.

  For example, soot in the exhaust gas is more likely to accumulate on the wall surface as it is closer to the inlet of the EGR cooler. Therefore, the degree of decrease in the efficiency of heat conduction due to soot deposition is greater as the position is closer to the EGR cooler inlet. Therefore, in order to cause an increase in the efficiency of heat conduction that suitably offsets the decrease in the efficiency of heat conduction caused by soot deposition, the deterioration of the coating material causes the increase in the efficiency from the EGR gas inlet of the EGR cooler. The shorter the distance, the better the thickness of the coating material.

  By doing so, the closer the entrance to the EGR cooler where the efficiency of heat conduction is likely to be reduced due to the accumulation of soot, the easier the deterioration of the coating material proceeds. In addition, the efficiency of heat conduction is likely to increase. Therefore, it becomes possible to make the cooling efficiency of EGR gas constant throughout the entire EGR cooler.

  Hydrocarbons (HC) such as unburned fuel in the exhaust gas are more likely to adhere to the wall surface as the distance from the EGR cooler inlet increases. This is because the temperature is lower as the distance from the inlet of the EGR cooler increases. For example, in the EGR system (for example, LPL-EGR system) configured to take out the exhaust gas after the oxidation catalyst as EGR gas, the adhesion of HC in the EGR cooler is likely to occur under a low temperature condition where the oxidation catalyst is not activated.

  The degree of decrease in the efficiency of heat conduction due to the adhesion of HC is larger as the position is farther from the inlet of the EGR cooler. Therefore, in order to cause an increase in the efficiency of heat conduction that suitably offsets the decrease in the efficiency of heat conduction caused by the adhesion of HC due to the deterioration of the coating material, the EGR gas from the EGR gas inlet of the EGR cooler The longer the distance, the thinner the coating material.

  By doing so, the further away from the EGR cooler inlet the heat transfer efficiency is likely to be reduced due to the adhesion of HC, the deterioration of the coating material is more likely to proceed. In addition, the efficiency of heat conduction is likely to increase. Therefore, it becomes possible to make the cooling efficiency of EGR gas constant throughout the entire EGR cooler.

  In addition, soot is more likely to accumulate near the central axis of the EGR cooler. Therefore, the degree of decrease in the efficiency of heat conduction caused by soot deposition is greater as the position is closer to the central axis of the EGR cooler. Therefore, in order to cause an increase in the efficiency of heat conduction that suitably offsets the decrease in the efficiency of heat conduction caused by soot deposition due to the deterioration of the coating material, the distance from the central axis of the EGR cooler is short. It is better to reduce the thickness of the coating material.

By doing so, the closer to the central axis of the EGR cooler where the efficiency of heat conduction is likely to be reduced due to soot accumulation, the deterioration of the coating material is more likely to proceed. In particular, the efficiency of heat conduction is likely to increase. Therefore, it becomes possible to make the cooling efficiency of EGR gas constant throughout the entire EGR cooler.

  As described above, the cooling efficiency of the EGR gas is constant over the entire EGR cooler by varying the thickness of the coating material according to the variation in the amount of adhesion and deposition of soot and HC depending on the position in the EGR cooler. It becomes possible to. That is, not only the temporal (temporal) variation of the cooling efficiency of the EGR gas but also the spatial variation can be suppressed, and the fuel efficiency can be further improved.

In the present invention, in order to coat the coating material according to the variation in the degree of reduction in the efficiency of heat conduction according to the position in the EGR cooler and the type of exhaust component,
The coating material includes a first component having relatively weak corrosion resistance against EGR gas and a second component having relatively strong corrosion resistance against EGR gas,
The component ratio between the first component and the second component of the coating material is made different according to the variation in the amount of exhaust component adhering to the wall surface of the contact portion due to the flow of EGR gas depending on the position in the EGR cooler. May be.

  The higher the amount of exhaust component adhering to the contact part, the greater the degree of decrease in the efficiency of heat conduction, so that such places are more likely to increase the efficiency of heat conduction, in other words, the deterioration of the coating material In the component composition of the coating material, it is preferable to increase the ratio of the first component having a weak corrosion resistance. Conversely, in places where the amount of exhaust components adhering to the contact portion is small, the degree of decrease in the efficiency of heat conduction is small, so that the coating material composition is highly corrosion resistant so that the coating material does not easily deteriorate. It is preferable to increase the ratio of the second component.

  For example, soot in the exhaust gas is more likely to accumulate on the wall surface as it is closer to the EGR cooler inlet. Therefore, the degree of decrease in the efficiency of heat conduction due to soot deposition is greater as the position is closer to the EGR cooler inlet. Therefore, in order to cause an increase in the efficiency of heat conduction that suitably offsets the decrease in the efficiency of heat conduction caused by soot deposition, the deterioration of the coating material causes the increase in the efficiency from the EGR gas inlet of the EGR cooler. The shorter the distance, the higher the ratio of the first component having a weak corrosion resistance.

  By doing so, the closer the entrance to the EGR cooler where the efficiency of heat conduction is likely to be reduced due to the accumulation of soot, the easier the deterioration of the coating material proceeds. In addition, the efficiency of heat conduction is likely to increase. Therefore, it becomes possible to make the cooling efficiency of EGR gas constant throughout the entire EGR cooler.

  As described above, the HC in the exhaust gas is more likely to adhere to the wall surface as the distance from the inlet of the EGR cooler increases. Therefore, the degree of decrease in the efficiency of heat conduction due to the adhesion of HC is larger as the position is farther from the EGR cooler inlet. Therefore, in order to cause an increase in the efficiency of heat conduction that suitably offsets the decrease in the efficiency of heat conduction caused by the adhesion of HC due to the deterioration of the coating material, the EGR gas from the EGR gas inlet of the EGR cooler The longer the distance, the higher the ratio of the first component with weak corrosion resistance.

  By doing so, the further away from the EGR cooler inlet the heat transfer efficiency is likely to be reduced due to the adhesion of HC, the deterioration of the coating material is more likely to proceed. In addition, the efficiency of heat conduction is likely to increase. Therefore, it becomes possible to make the cooling efficiency of EGR gas constant throughout the entire EGR cooler.

In addition, soot is more likely to accumulate near the central axis of the EGR cooler. Therefore, the degree of decrease in the efficiency of heat conduction caused by soot deposition is greater as the position is closer to the central axis of the EGR cooler. Therefore, in order to cause an increase in the efficiency of heat conduction that suitably offsets the decrease in the efficiency of heat conduction caused by soot deposition due to the deterioration of the coating material, the distance from the central axis of the EGR cooler is short. It is better to increase the ratio of the first component having weak corrosion resistance.

  By doing so, the closer to the central axis of the EGR cooler where the efficiency of heat conduction is likely to be reduced due to soot accumulation, the deterioration of the coating material is more likely to proceed. In particular, the efficiency of heat conduction is likely to increase. Therefore, it becomes possible to make the cooling efficiency of EGR gas constant throughout the entire EGR cooler.

  As described above, the cooling efficiency of the EGR gas is constant over the entire EGR cooler by changing the component ratio of the coating material according to the variation in the amount of adhesion and deposition of soot and HC depending on the position in the EGR cooler. It becomes possible to. That is, not only the temporal (temporal) variation of the cooling efficiency of the EGR gas but also the spatial variation can be suppressed, and the fuel efficiency can be further improved.

The relatively weak first component corrosion resistance to EGR gas, for example, alumina - titanium carbide-based ceramic _{(Al 2} O 3 -TiC) can be mentioned. As the second component is a strong corrosion resistance to EGR gas, for example, it can be exemplified alumina-based ceramic _(Al 2 _{O 3).}

  By this invention, it becomes possible to suppress the fall of the cooling efficiency of an EGR cooler.

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and the like of the components described in the present embodiment are not intended to limit the technical scope of the invention to those unless otherwise specified.

  FIG. 1 is a conceptual diagram schematically showing a schematic configuration of an internal combustion engine to which an EGR cooler according to the present invention is applied and its intake system and exhaust system.

  The engine 1 is a diesel engine having four cylinders 4. Each cylinder 4 is provided with a fuel injection valve 10 for directly injecting fuel into the cylinder.

  Each cylinder 4 communicates with an intake manifold 17 via an intake port (not shown). The intake manifold 17 is connected to the intake passage 2. An EGR passage 26, which will be described later, is connected to the intake passage 2, a throttle valve 9 is provided on the upstream side, an intercooler 6 is provided on the upstream side, and a compressor 11 of the turbocharger 13 is provided on the upstream side. An air flow meter 7 is provided on the upstream side.

  Each cylinder 4 communicates with an exhaust manifold 18 via an exhaust port (not shown). The exhaust manifold 18 is connected to the exhaust passage 3. The exhaust passage 3 is provided with an EGR passage 26, a turbine 12 of the turbocharger 13 on the downstream side thereof, and an exhaust purification device 8 on the downstream side thereof. The exhaust purification device 8 includes a diesel particulate filter, an oxidation catalyst, an NOx storage reduction catalyst, and the like.

The EGR passage 26 communicates the exhaust passage 3 and the intake passage 2, extracts a part of the exhaust from the engine 1, and flows it into the intake passage 2 as EGR gas. The EGR passage 26 is provided with an EGR cooler 27 that cools the EGR gas flowing through the EGR passage 26. An EGR valve 25 that adjusts the amount of EGR gas is provided downstream of the EGR cooler 27 (on the intake passage 2 side).

  The engine 1 includes an accelerator opening sensor 19 that measures the opening of the accelerator pedal 22 and a crank angle sensor 20 that measures the rotation angle of the crankshaft of the engine 1.

  The engine 1 includes an ECU 16 that is a computer unit that controls the operating state of the engine 1. The ECU 16 includes a CPU that executes a control program, a ROM that stores the control program, a RAM that temporarily stores measurement data and calculation results, and the like. In addition to the air flow meter 7, the accelerator opening sensor 19, and the crank angle sensor 20 described above, various sensors are connected to the ECU 16, and measured values from these sensors are input to the ECU 16. In addition to the fuel injection valve 10, the throttle valve 9, and the EGR valve 25 described above, various devices are connected to the ECU 16, and the operations of these devices are controlled by control signals from the ECU 16.

  FIG. 2 is a cross-sectional view of the EGR cooler 27. As shown in FIG. 2, the EGR cooler 27 has an EGR gas flow path 28 through which EGR gas flows and a cooling water flow path 29 through which cooling water of the engine 1 that is a refrigerant flows. The EGR gas is cooled by heat exchange between the EGR gas and the refrigerant via the partition wall 30 therebetween. The partition wall 30 corresponds to the “thermal contact portion between the EGR gas and the refrigerant” in the present invention.

  On the wall surface of the partition wall 30 on the EGR gas flow path 28 side, soot in the EGR gas accumulates over time, reducing the efficiency of heat exchange between the EGR gas and the refrigerant through the partition wall 30 and reducing the amount of EGR gas. It becomes a factor to do. FIG. 3 is a diagram showing the relationship between the period of use of the EGR cooler 27 and the amount of EGR gas. The horizontal axis of FIG. 3 indicates the usage period of the EGR cooler 27 in units of months, and the vertical axis indicates the amount of EGR gas. As shown in FIG. 3, the EGR gas amount decreases as the use period of the EGR cooler 27 becomes longer.

  When the amount of EGR gas decreases, a sufficient NOx reduction effect cannot be obtained. Conventionally, long-term operation of the EGR cooler has been performed by performing combustion control such as retarding the fuel injection timing so as to be compatible with the exhaust regulations even under such a condition that the NOx reduction effect is reduced due to the decrease in the EGR gas amount. In order to meet the exhaust emission regulations in the process of use.

  However, when the combustion control is adapted under the most disadvantageous condition of such NOx reduction effect, there is a problem that the fuel efficiency performance when the EGR cooler is new is limited. FIG. 4 shows how the in-cylinder pressure changes when such combustion control is performed in a state where the EGR cooler is new and a state where soot accumulates on the EGR cooler and the amount of EGR gas decreases. FIG. The horizontal axis in FIG. 4 represents the crank angle, and the vertical axis represents the in-cylinder pressure. As shown in FIG. 4, when the fuel injection timing is retarded so that the NOx emission amount conforms to the exhaust regulations in the state where soot has accumulated in the EGR cooler and the amount of EGR gas has decreased, the EGR cooler will In a new state in which no deposit is accumulated, the in-cylinder pressure does not rise sufficiently, which is disadvantageous in terms of fuel consumption.

  In view of such a problem, in this embodiment, the thermal conductivity of the wall surface on the EGR gas flow path 28 side of the partition wall 30 in the EGR cooler 27 is lower than the thermal conductivity of the material constituting the partition wall 30, and A coating material that is not too strong in corrosion resistance to acidic components such as nitric acid in EGR gas was coated so that the coating material deteriorated over time.

This coating material degrades and decreases over time due to exhaust components such as nitric acid and heat of exhaust. As the coating material decreases, the resistance to heat exchange between the EGR gas and the cooling water decreases, so the efficiency of heat conduction through the partition walls 30 increases. That is, the cooling efficiency of the EGR gas in the EGR cooler 27 increases with time.

  On the other hand, soot in the exhaust gas accumulates on the wall surface of the partition wall 30 on the EGR gas flow path 28 side. As the soot accumulates, resistance to heat exchange between the EGR gas and the cooling water increases, so the efficiency of heat conduction through the partition walls 30 decreases. That is, the cooling efficiency of the EGR gas in the EGR cooler 27 decreases with time.

  When the EGR cooler 27 is close to a new product, the coating material is hardly deteriorated. Therefore, the cooling efficiency of the EGR gas is limited by the presence of the coating material, but the soot is not yet deposited. There is little decrease in the cooling efficiency of the EGR gas due to the deposition of.

  On the other hand, as the usage period of the EGR cooler 27 becomes longer, the coating material gradually deteriorates. Therefore, the cooling efficiency of the EGR gas increases due to the decrease in the coating material, while the EGR gas increases due to the increase in the amount of accumulated soot. The cooling efficiency of the is getting lower.

  In this embodiment, the degree of increase in the cooling efficiency of the EGR gas due to the deterioration of the coating material with time and the accumulation of soot on the wall surface of the partition wall 30 of the EGR cooler 27 on the EGR gas flow path 28 side. The coating material was coated so that the degree of decrease in the cooling efficiency of the EGR gas was substantially equal.

  Here, the relationship between the distance from the inlet of the EGR cooler 27 and the accumulation amount of soot is shown in FIG. In FIG. 5, the horizontal axis represents the distance x from the inlet of the EGR cooler 27, and the vertical axis represents the amount of soot accumulated. As shown in FIG. 6, the distance x from the inlet of the EGR cooler 27 is a value on the coordinate axis with the direction in which the EGR gas flows with the inlet of the EGR cooler 27 as the origin (the direction of the arrow in FIG. 6) being positive. is there. As shown in FIG. 5, soot is likely to accumulate at locations where the distance from the inlet of the EGR cooler 27 is short, and accordingly, the cooling efficiency of the EGR gas due to soot accumulation tends to decrease over time. On the other hand, as the distance from the inlet of the EGR cooler 27 becomes longer, soot is less likely to accumulate, so that the cooling efficiency of the EGR gas due to soot accumulation is less likely to decrease over time. That is, as the inlet side of the EGR cooler 27 is increased, the degree of decrease in the cooling efficiency of the EGR gas due to soot accumulation increases with time.

  Therefore, in this embodiment, the thickness of the coating material is reduced as the distance from the inlet of the EGR cooler 27 is shorter. FIG. 7 is a diagram illustrating the relationship between the distance from the inlet of the EGR cooler 27 and the thickness of the coating material. The horizontal axis in FIG. 7 represents the distance x from the inlet of the EGR cooler 27, and the vertical axis represents the thickness of the coating material.

  In places where the thickness of the coating material is thin, the coating material is likely to be deteriorated by an acidic component in the EGR gas. Therefore, the cooling efficiency of the EGR gas tends to increase with time due to the deterioration of the coating material. On the other hand, since the coating material is hardly deteriorated by the acidic component in the EGR gas at the portion where the coating material is thick, the cooling efficiency of the EGR gas hardly increases with time. Therefore, as shown in FIG. 7, the thickness of the coating material is made thinner toward the inlet side of the EGR cooler 27, so that the cooling efficiency of the EGR gas due to the deterioration of the coating material is increased with time toward the inlet side of the EGR cooler 27. The degree of increase will increase.

In this embodiment, an increase in the cooling efficiency of the EGR gas that is substantially the same as the degree of decrease in the cooling efficiency of the EGR gas over time due to soot deposition is likely to occur over time due to the deterioration of the coating material. In addition, the coating material that is not too strong in corrosion resistance was coated with the coating material having a different thickness depending on the variation in the amount of soot accumulated in the EGR cooler 27 depending on the distance from the inlet of the EGR cooler 27.

  By so doing, the time-dependent decrease in the cooling efficiency of the EGR gas due to deposition of soot is offset by the increase in the cooling efficiency of the EGR gas over time due to the deterioration of the coating material. As a result, the cooling of the EGR gas in the EGR cooler 27 is performed. Efficiency is kept constant over time. Further, since the thickness of the coating material is made different depending on the variation in the amount of soot accumulated due to the difference in the position of the EGR gas flow direction of the EGR cooler 27, the cooling efficiency of the EGR gas of the EGR cooler 27 is reduced. It is kept constant.

  As described above, in the EGR cooler 27 of the present embodiment, it is possible to suppress the variation over time of the cooling efficiency of the EGR gas and the variation due to the position in the EGR cooler 27. The fluctuation of the EGR gas amount can be suppressed after the use process. Therefore, even in a new state of the EGR cooler 27, it is possible to perform combustion control that can optimize fuel consumption and meet exhaust regulations even after a long-term use process.

  In Example 1, an increase in cooling efficiency that offsets the degree of decrease in cooling efficiency over time due to accumulation of soot in the EGR cooler 27 is caused by deterioration of the coating material over time. An example has been described in which the thickness of the coating material is reduced as the position where the deposits are more likely to accumulate.

  In the present embodiment, the degree of increase in the cooling efficiency of the EGR gas due to the deterioration of the coating material over time, and the EGR gas accompanying the accumulation of soot on the wall surface of the partition wall 30 of the EGR cooler 27 on the EGR gas channel 28 side. An example will be described in which the coating material is coated by changing the component ratio of the coating material in accordance with the easiness of deposition of soot so that the degree of decrease in cooling efficiency is substantially equal.

In this example, alumina ceramic (Al ₂ O ₃ ) having strong corrosion resistance to acidic components in EGR gas and alumina / titanium / carbide ceramic having weak corrosion resistance to acidic components in EGR gas compared to alumina ceramics. (Al ₂ O ₃ —TiC) and a coating material containing a plurality of components as main components, the EGR gas flow channel 28 side of the partition wall 30 between the EGR gas flow channel 28 and the cooling water flow channel 29 in the EGR cooler 27. Coat the walls of In the wall surface of the partition wall 30, soot is easily deposited and the component ratio of Al ₂ O ₃ —TiC having relatively low corrosion resistance is increased as the cooling efficiency of the EGR gas is likely to decrease with time. .

FIG. 8 is a diagram showing the relationship between the distance from the inlet of the EGR cooler 27 and the component ratio of Al ₂ O ₃ —TiC of the coating material. The horizontal axis in FIG. 8 represents the distance x from the inlet of the EGR cooler 27, and the vertical axis represents the Al ₂ O ₃ —TiC component ratio of the coating material.

In the portion where the Al ₂ O ₃ —TiC component ratio of the coating material is high, the coating material is likely to be deteriorated by the acidic component in the EGR gas. Therefore, the cooling efficiency of the EGR gas tends to increase with time due to the deterioration of the coating material. On the other hand, in a portion where the Al ₂ O ₃ —TiC component ratio of the coating material is low, deterioration of the coating material due to the acidic component in the EGR gas is unlikely to occur, so that the cooling efficiency of the EGR gas is unlikely to increase with time. Therefore, as shown in FIG. 8, by increasing the component ratio of Al ₂ O ₃ —TiC of the coating material toward the inlet side of the EGR cooler 27, EGR caused by the deterioration of the coating material toward the inlet side of the EGR cooler 27. The degree of increase in the cooling efficiency of the gas with time increases.

In this embodiment, an increase in the cooling efficiency of the EGR gas that is substantially the same as the degree of decrease in the cooling efficiency of the EGR gas over time due to soot deposition is likely to occur over time due to the deterioration of the coating material. Furthermore, the coating material was made such that the component ratio of Al ₂ O ₃ —TiC, which is relatively weak in corrosion resistance in the coating material, was varied according to the variation in the amount of soot accumulated in the EGR cooler 27.

  By so doing, the time-dependent decrease in the cooling efficiency of the EGR gas due to deposition of soot is offset by the increase in the cooling efficiency of the EGR gas over time due to the deterioration of the coating material. As a result, the cooling of the EGR gas in the EGR cooler 27 is performed. Efficiency is kept constant over time. Further, since the ratio of components having relatively weak corrosion resistance in the coating material is made different according to the variation in the amount of accumulated soot due to the difference in the position of the EGR gas flow direction of the EGR cooler 27, the EGR of the EGR cooler 27 is changed. Gas cooling efficiency is kept constant even in space.

  Therefore, also in this embodiment, it is possible to suppress the variation over time of the cooling efficiency of the EGR gas of the EGR cooler 27 and the variation due to the position in the EGR cooler 27. As a result, it is possible to perform combustion control that conforms to exhaust regulations and optimizes fuel efficiency.

  In each of the above-described embodiments, the EGR cooler 27 is made different by changing the thickness of the coating material and the component ratio of the coating material according to the variation in the amount of soot accumulated due to the difference in the position of the EGR gas flow direction in the EGR cooler 27. In the above example, the spatial variation in the EGR gas cooling efficiency is suppressed. However, the variation in the soot accumulation amount also occurs depending on the distance from the central axis of the EGR cooler 27.

  FIG. 9 shows the relationship between the distance from the central axis of the EGR cooler 27 and the amount of soot accumulated. In FIG. 9, the horizontal axis represents the distance r from the central axis of the EGR cooler 27, and the vertical axis represents the soot accumulation amount. As shown in FIG. 2, the distance r from the central axis of the EGR cooler 27 is a value on the coordinate axis in the radial direction toward the outside with the central axis of the EGR cooler 27 as the origin. As shown in FIG. 9, soot is likely to accumulate at a location close to the central axis of the EGR cooler 27, and accordingly, the cooling efficiency of the EGR gas due to soot accumulation tends to decrease over time. On the other hand, the farther from the central axis of the EGR cooler 27 (the closer it is to the outside), the more difficult the soot accumulates, so the time-dependent decline in the cooling efficiency of the EGR gas due to soot accumulation does not easily occur. That is, as the central axis side of the EGR cooler 27 is increased, the degree of decrease in the cooling efficiency of the EGR gas due to soot accumulation increases with time.

  Therefore, in this embodiment, the thickness of the coating material is reduced as the distance from the central axis of the EGR cooler 27 is shorter. FIG. 10 is a diagram illustrating the relationship between the distance from the central axis of the EGR cooler 27 and the thickness of the coating material. The horizontal axis in FIG. 10 represents the distance r from the central axis of the EGR cooler 27, and the vertical axis represents the thickness of the coating material.

  As described above, in a portion where the thickness of the coating material is thin, the coating material is likely to be deteriorated by an acidic component in the EGR gas, and the cooling efficiency of the EGR gas is likely to increase with time due to the deterioration of the coating material. On the other hand, in a portion where the thickness of the coating material is thick, the coating material is hardly deteriorated by an acidic component in the EGR gas, and the cooling efficiency of the EGR gas is hardly increased with time due to the deterioration of the coating material. Therefore, as shown in FIG. 10, by reducing the thickness of the coating material toward the central axis side of the EGR cooler 27, the cooling efficiency of the EGR gas due to the deterioration of the coating material is reduced toward the central axis side of the EGR cooler 27. The degree of increase over time increases.

As described above, in this example, the increase in the cooling efficiency of the EGR gas with the degree of the decrease in the cooling efficiency of the EGR gas with the lapse of time due to the accumulation of soot is caused by the deterioration of the coating material. Thus, the coating material is coated with a coating material that is not too strong in resistance to corrosion depending on the variation in the amount of soot accumulation in the EGR cooler 27 depending on the distance from the central axis of the EGR cooler 27. I did

  By so doing, the time-dependent decrease in the cooling efficiency of the EGR gas due to deposition of soot is offset by the increase in the cooling efficiency of the EGR gas over time due to the deterioration of the coating material. As a result, the cooling of the EGR gas in the EGR cooler 27 is performed. Efficiency is kept constant over time. Further, since the thickness of the coating material is made different according to the variation in the amount of soot accumulated due to the difference in the distance from the central axis of the EGR cooler 27, the cooling efficiency of the EGR gas of the EGR cooler 27 is spatially reduced. Is also kept constant.

  In this embodiment, the thickness of the coating material is varied according to the distance from the central axis of the EGR cooler 27, so that the cooling efficiency of the EGR cooler 27 due to the accumulation of soot in the EGR cooler 27 is reduced. Although the variation due to the distance from the central axis is absorbed, the mixing ratio of the two components having different corrosion resistances in the coating material is made different according to the distance from the central axis of the EGR cooler 27 as in the second embodiment. Thus, the variation in the degree of decrease in the cooling efficiency may be absorbed.

In that case, the shorter the distance from the central axis of the EGR cooler 27, the higher the component ratio of Al ₂ O ₃ —TiC, which is relatively weak in corrosion resistance in the coating material.

  Next, an example in which the present invention is applied to an LPL-EGR system will be described.

  FIG. 11 is a diagram illustrating a schematic configuration of an engine including the LPL-EGR system. In FIG. 11, components that are substantially the same as the engine configuration described in FIG. 1 are not described in detail using the same names and symbols.

  The system shown in FIG. 11 includes an EGR system of a system different from the EGR passage 26 described in FIG. The exhaust passage 3 downstream from the exhaust purification device 8 and the intake passage 2 upstream from the compressor 11 communicate with each other, and a part of the exhaust discharged from the engine 1 and passing through the exhaust purification device 8 is taken out as EGR gas. An LPL-EGR passage 36 that flows into the intake passage 2 is provided. The LPL-EGR passage 36 is provided with an LPL-EGR cooler 37 that cools the EGR gas flowing through the LPL-EGR passage 36. An LPL-EGR valve 35 that adjusts the amount of EGR gas is provided downstream of the LPL-EGR cooler 37 (intake passage 2 side). The opening degree of the LPL-EGR valve 35 is controlled by the ECU 16. The LPL-EGR cooler 37 also has the same structure as the EGR cooler 27 shown in FIG. 2, and has an EGR gas flow path 28 and a cooling water flow path 29 inside thereof, and both are physically separated by a partition wall 30. And are in thermal contact.

  The EGR gas flowing through the LPL-EGR passage 36 is the exhaust gas that has passed through the exhaust gas purification device 8 as described above, and most of the soot contained therein is exhaust gas collected by the diesel particulate filter of the exhaust gas purification device 8. is there. Therefore, in the LPL-EGR cooler 37 provided in the LPL-EGR passage 36, soot accumulation that causes a significant decrease in the amount of EGR gas as described in the above embodiments hardly occurs.

However, in the case where the oxidation catalyst included in the exhaust purification device 8 is not sufficiently activated, unburned fuel (hydrocarbon, HC) in the exhaust gas is not sufficiently removed in the exhaust purification device 8, so that a large amount of HC is produced. The contained exhaust gas flows into the LPL-EGR cooler 37. Accordingly, in the LPL-EGR cooler 37, HC adheres to the wall surface of the partition wall 30 on the EGR flow path 28 side. When HC adheres, as in the case where soot accumulates, it becomes a resistance to heat exchange between the EGR gas and the cooling water through the partition wall 30, which causes a decrease in the cooling efficiency of the EGR gas of the LPL-EGR cooler 37.

  Therefore, in the present embodiment, the degree of decrease in cooling efficiency over time due to HC adhesion in the LPL-EGR cooler 37 and the degree of increase in cooling efficiency of EGR gas due to the deterioration of coating material over time are approximately. The coating material was coated so as to be equivalent.

  Here, the relationship between the distance from the inlet of the LPL-EGR cooler 37 and the amount of HC attached is shown in FIG. In FIG. 12, the horizontal axis represents the distance x from the inlet of the LPL-EGR cooler 37, and the vertical axis represents the amount of HC attached. As in the case of the EGR cooler 27 shown in FIG. 6, the distance x from the inlet of the LPL-EGR cooler 37 is on the coordinate axis with the positive direction of the EGR gas flowing from the inlet of the LPL-EGR cooler 37 as the origin. Is the value of The temperature is higher at the inlet side of the LPL-EGR cooler 37, and the temperature is lower as it is farther from the inlet. Since HC is more likely to adhere to a lower temperature place, as shown in FIG. 12, HC is less likely to adhere at a place where the distance from the inlet of the LPL-EGR cooler 37 is short. Therefore, the cooling efficiency of EGR gas due to the attachment of HC is reduced. Decrease over time is unlikely to occur. On the other hand, the longer the distance from the inlet of the LPL-EGR cooler 37, the more easily HC adheres, and therefore the EGR gas cooling efficiency due to the HC tends to decrease over time. That is, as the distance from the inlet of the LPL-EGR cooler 37 increases, the degree of decrease in the cooling efficiency of the EGR gas due to HC adhesion increases with time.

  Therefore, in this embodiment, the coating material is made thinner as the distance from the inlet of the LPL-EGR cooler 37 is longer. FIG. 13 is a diagram illustrating the relationship between the distance from the inlet of the LPL-EGR cooler 37 and the thickness of the coating material. The horizontal axis in FIG. 13 represents the distance x from the inlet of the LPL-EGR cooler 37, and the vertical axis represents the thickness of the coating material.

  In places where the thickness of the coating material is thin, the coating material is likely to be deteriorated by an acidic component in the EGR gas. Therefore, the cooling efficiency of the EGR gas tends to increase with time due to the deterioration of the coating material. On the other hand, since the coating material is hardly deteriorated by the acidic component in the EGR gas at the portion where the coating material is thick, the cooling efficiency of the EGR gas hardly increases with time. Therefore, as shown in FIG. 13, by reducing the thickness of the coating material toward the outlet side of the LPL-EGR cooler 37, the cooling side of the EGR gas caused by the deterioration of the coating material becomes closer to the outlet side of the LPL-EGR cooler 37. The degree of increase in efficiency over time increases.

  In the present embodiment, an increase in the cooling efficiency of the EGR gas, which is approximately the same as the time-dependent decrease in the cooling efficiency of the EGR gas due to the adhesion of HC, appears to occur over time due to the deterioration of the coating material. Furthermore, the coating material that is not too strong in corrosion resistance is coated with the coating material by varying the thickness according to the variation in the distance from the inlet of the LPL-EGR cooler 37 of the amount of HC adhering to the LPL-EGR cooler 37. I made it.

  By doing so, the time-dependent decrease in the cooling efficiency of the EGR gas due to the adhesion of HC is offset by the increase in the cooling efficiency of the EGR gas over time due to the deterioration of the coating material. As a result, the EGR gas of the LPL-EGR cooler 37 The cooling efficiency is kept constant over time. In addition, since the thickness of the coating material is made different according to the variation in the amount of HC attached due to the difference in the position of the EGR gas flow direction of the LPL-EGR cooler 37, the EGR gas of the LPL-EGR cooler 37 is changed. Cooling efficiency is kept constant even in space.

As described above, in the LPL-EGR cooler 37 according to the present embodiment, it is possible to suppress the variation with time of the cooling efficiency of the EGR gas and the variation due to the position in the LPL-EGR cooler 37. Therefore, the LPL-EGR cooler 37 The fluctuation of the EGR gas amount can be suppressed after the new state and after a long-term use process. Therefore, even when the LPL-EGR cooler 37 is in a new state, it is possible to perform the combustion control that can optimize the fuel consumption and meet the exhaust regulations even after a long-term use process.

Even in the case where the present invention is applied to the LPL-EGR cooler 37, as in the above-described embodiments of the EGR cooler 27, the coating is performed according to the variation in the amount of HC adhering depending on the position in the LPL-EGR cooler 37. it may be varied relatively weak _Al 2 _O 3 ratio of components -TiC corrosion resistance in wood. In that case, as the distance from the inlet of the LPL-EGR cooler 37 becomes longer as described above, the adhesion amount of HC tends to increase. Therefore, as the distance from the inlet of the LPL-EGR cooler 37 becomes longer, the corrosion resistance becomes relatively higher. It is preferable to increase the weak Al ₂ O ₃ —TiC component ratio.

  In addition, depending on the variation in the amount of HC deposited due to the difference in the distance from the central axis in the LPL-EGR cooler 37, the component ratio that is weak in the thickness of the coating material and the corrosion resistance may be varied.

The above-described embodiment is an example for explaining the present invention, and various modifications can be made to the above-described embodiment without departing from the gist of the present invention. For example, in each of the above-described embodiments, the example in which the coating thickness and the Al ₂ O ₃ —TiC component ratio are continuously and linearly changed with respect to the distance x from the inlet of the EGR cooler has been described. These amounts may be changed stepwise or may be changed in a non-linear functional relationship. For example, in the case of Example 1, the coating thickness is increased with increasing distance x, and in the case of Example 2, the component ratio of Al ₂ O ₃ —TiC is decreased with increasing distance x. Any change may be used as long as the change tendency is.

It is a figure which shows schematic structure of the engine in Examples 1-3, and its intake system / exhaust system. It is sectional drawing of an EGR cooler and a LPL-EGR cooler. It is a figure which shows the relationship between the use period of an EGR cooler, and the amount of EGR gas. It is a figure showing the change of the in-cylinder pressure in each of the case where the EGR cooler is in a new state and the state where soot has accumulated on the EGR cooler. It is a figure showing the relationship between the distance from the entrance of an EGR cooler, and the accumulation amount of soot. It is a figure which shows the coordinate showing the distance from the inlet_port | entrance of an EGR cooler. It is a figure showing the relationship between the distance from the entrance of an EGR cooler, and the thickness of a coating material. It is a diagram representing the relationship between the _Al 2 _O 3 component of -TiC ratio in distance and the coating material from the inlet of the EGR cooler. It is a figure showing the relationship between the distance from the center axis | shaft of an EGR cooler, and the accumulation amount of soot. It is a figure showing the relationship between the distance from the center axis | shaft of an EGR cooler, and the thickness of a coating material. It is a figure which shows schematic structure of the engine in Example 4, and its intake system / exhaust system. It is a figure showing the relationship between the distance from the inlet of a LPL-EGR cooler, and the adhesion amount of HC. It is a figure showing the relationship between the distance from the inlet of a LPL-EGR cooler, and the thickness of a coating material.

Explanation of symbols

1 Engine 2 Intake Passage 3 Exhaust Passage 4 Cylinder 6 Intercooler 7 Air Flow Meter 8 Exhaust Purification Device 9 Throttle Valve 10 Fuel Injection Valve 11 Compressor 12 Turbine 13 Turbocharger 16 ECU
17 Intake manifold 18 Exhaust manifold 19 Accelerator opening sensor 20 Crank angle sensor 22 Accelerator pedal 25 EGR valve 26 EGR passage 27 EGR cooler 35 LPL-EGR valve 36 LPL-EGR passage 37 LPL-EGR cooler

Claims (11)

An EGR cooler that cools the EGR gas by heat exchange between the EGR gas and the refrigerant,
The EGR cooler is coated with a coating material on the wall surface on the EGR gas distribution side in the thermal contact portion between the EGR gas and the refrigerant,
The coating material is
The degree of increase in the efficiency of heat conduction through the contact portion due to the deterioration of the coating material over time due to the flow of EGR gas,
The degree of decrease in the efficiency of heat conduction through the contact portion due to the exhaust component adhering to the wall surface on the EGR gas flow side in the contact portion due to the flow of EGR gas;
Is an EGR cooler for an internal combustion engine, which is coated so as to be substantially equivalent. In claim 1,
The EGR cooler for an internal combustion engine, wherein the coating material has a thermal conductivity lower than a thermal conductivity of a material constituting the contact portion, and has a corrosion resistance against EGR gas lower than a predetermined level. In claim 1 or 2,
An EGR cooler for an internal combustion engine, wherein the thickness of the coating material is made different according to variations in the amount of exhaust components adhering to the wall surface of the contact portion due to the flow of EGR gas depending on the position in the EGR cooler. In claim 3,
An EGR cooler for an internal combustion engine, wherein the coating material is made thinner as the distance from the EGR gas inlet of the EGR cooler is shorter. In claim 3,
An EGR cooler for an internal combustion engine, wherein the coating material is made thinner as the distance from the EGR gas inlet of the EGR cooler is longer. In claim 3,
An EGR cooler for an internal combustion engine, wherein the coating material is made thinner as the distance from the central axis of the EGR cooler is shorter. In claim 1 or 2,
The coating material includes a first component having relatively weak corrosion resistance against EGR gas and a second component having relatively strong corrosion resistance against EGR gas,
The component ratio between the first component and the second component of the coating material is made different according to the variation in the amount of exhaust component adhering to the wall surface of the contact portion due to the flow of EGR gas depending on the position in the EGR cooler. An EGR cooler for internal combustion engines. In claim 7,
An EGR cooler for an internal combustion engine, wherein the ratio of the first component is increased as the distance from the EGR gas inlet of the EGR cooler is shorter. In claim 7,
The EGR cooler for an internal combustion engine, wherein the ratio of the first component is increased as the distance from the EGR gas inlet of the EGR cooler is longer. In claim 7,
An EGR cooler for an internal combustion engine, wherein the ratio of the first component is increased as the distance from the central axis of the EGR cooler is shorter. In claim 7,
The first component is an alumina-titanium carbide ceramic,
The EGR cooler for an internal combustion engine, wherein the second component is an alumina-based ceramic.

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