Exhaust purification system of an internal combustion engine

An exhaust treatment catalyst (5) is arranged in the engine exhaust passage, and hydrogen generated in the reformer (6) is supplied through the hydrogen supply pipe (13) to the inside of the engine exhaust passage upstream of the exhaust treatment catalyst (5). Heat exchange fins (15) for heat exchange with exhaust gas flowing through the inside of the engine exhaust passage are formed on the outer circumferential surface of the hydrogen supply pipe (13) inserted inside the engine exhaust passage.

TECHNICAL FIELD

The present invention relates to an exhaust purification system of an internal combustion engine.

BACKGROUND ART

Known in the past has been an internal combustion engine in which an NOXpurification catalyst is arranged in an engine exhaust passage, a fuel reforming device for generating reformed gas containing hydrogen is provided, high temperature reformed gas containing hydrogen produced in the fuel reforming device is fed to the engine exhaust passage upstream of the NOXpurification catalyst at the time of engine start, and the hydrogen in the supplied reformed gas is used to raise the NOXpurification rate of the NOXpurification catalyst (for example, see Japanese Unexamined Patent Publication No. 2010-270664).

SUMMARY OF INVENTION

Technical Problem

In this regard, in the case of arranging an exhaust treatment catalyst like an oxidation catalyst in an engine exhaust passage, supplying high temperature reformed gas containing hydrogen generated in a fuel reforming device to the inside of the engine exhaust passage upstream of the exhaust treatment catalyst, and trying to make the temperature of the exhaust treatment catalyst rapidly rise by the heat of oxidation reaction of the hydrogen on the exhaust treatment catalyst, if the hydrogen supplied to the inside of the engine exhaust passage ends up reacting with the oxygen contained in the exhaust gas and will be consumed by self igniting before reacting with oxygen on the exhaust treatment catalyst, no heat of oxidation reaction of hydrogen will be generated any longer on the exhaust treatment catalyst and it will become difficult to make the temperature of the exhaust treatment catalyst rapidly rise.

In this case, to make the temperature of the exhaust treatment catalyst rapidly rise, it is necessary to keep the hydrogen supplied to the inside of the engine exhaust passage from reacting with oxygen contained in the exhaust gas and being consumed by self igniting before reacting with oxygen on the exhaust treatment catalyst. However, in the above-mentioned internal combustion engine, this was not considered at all.

Solution to Problem

According to the present invention, there is provided an exhaust purification system of an internal combustion engine comprising: a reformer, an exhaust treatment catalyst arranged in an engine exhaust passage, a hydrogen supply pipe inserted inside the engine exhaust passage upstream of the exhaust treatment catalyst, hydrogen generated in the reformer being supplied to the engine exhaust passage upstream of the exhaust treatment catalyst via the hydrogen supply pipe, and heat exchange fins formed on an outer circumferential surface of the hydrogen supply pipe for heat exchange with exhaust gas flowing through an inside of the engine exhaust passage.

Advantageous Effects of Invention

By forming heat exchange fins for heat exchange with exhaust gas on the outer circumferential surface of the hydrogen supply pipe, the temperature of the reformed gas falls. Due to this, hydrogen contained in the reformed gas is kept from being consumed by self igniting, so it is possible to make the temperature of the exhaust treatment catalyst rapidly rise. Furthermore, by forming heat exchange fins on the outer circumferential surface of the hydrogen supply pipe, the heat of the reformed gas is efficiently transmitted to the exhaust gas. As a result, the temperature of the exhaust gas rises and accordingly a rise in temperature of the exhaust treatment catalyst is promoted.

DESCRIPTION OF EMBODIMENTS

FIG. 1is an overall view of a compression ignition type internal combustion engine. Referring toFIG. 1, 1indicates an engine body,2an exhaust manifold,3an exhaust pipe,4an exhaust treatment device connected to the exhaust pipe3,5an exhaust treatment catalyst held inside the exhaust treatment device4, and6a reformer for forming reformed gas containing hydrogen. The reformer6is provided with a reforming catalyst7, a burner combustion chamber8formed at one side of the reforming catalyst7, a reformed gas outflow chamber9formed at the other side of the reforming catalyst7, and a burner10. The burner10is connected to a fuel tank11and an air pump12. Fuel supplied from the fuel tank11and air supplied from the air pump12are supplied from the burner10to the inside of the burner combustion chamber8.

The fuel supplied from the burner10is made to burn inside the burner combustion chamber8. Next, the produced combustion gas is sent into the reforming catalyst7and reformed whereby reformed gas containing hydrogen is produced in the reforming catalyst7. The reformed gas containing hydrogen produced at the reforming catalyst7is sent into the reformed gas outflow chamber9. The reformed gas containing hydrogen sent into the reformed gas outflow chamber9is supplied through a hydrogen supply pipe13extending from the reformed gas outflow chamber9to the inside of the exhaust pipe3, to the inside of the exhaust pipe3upstream of the exhaust treatment catalyst5, that is, to the inside of the engine exhaust passage upstream of the exhaust treatment catalyst5. This exhaust treatment catalyst5is comprised of an oxidation catalyst, NOXstorage catalyst, or catalyst-equipped particulate filter.

FIG. 2Ais an enlarged cross-sectional side view of the surroundings of the exhaust treatment device4shown inFIG. 1. Referring toFIG. 2A, the hydrogen supply pipe13is comprised of a hollow metal pipe. The front end part of this hydrogen supply pipe13extends from an outside of the exhaust pipe3through a wall of the exhaust pipe3to an inside of the exhaust pipe3. The front end part of the hydrogen supply part13is bent at the center part at the inside of the exhaust pipe3in the axial direction of the exhaust pipe3so that the front end opening part14of the hydrogen supply pipe13faces the upstream side end face of the exhaust treatment catalyst5. Note that, in the example shown inFIG. 2A, the front end part of the hydrogen supply pipe13forms an L-shape inside the exhaust pipe3.

On the other hand, as shown inFIG. 2AandFIG. 2B, a plurality of heat exchange fins15for heat exchange with the exhaust gas flowing through the inside of the exhaust pipe3are formed on the outer circumferential surface of the hydrogen supply pipe13positioned inside the exhaust pipe3. In other words, a plurality of heat exchange fins15for heat exchange with the exhaust gas flowing through the inside of the engine exhaust passage are formed on the outer circumferential surface of the hydrogen supply pipe13inserted inside the engine exhaust passage. As will be understood fromFIG. 2AandFIG. 2B, these heat exchange fins15are comprised of thin fins extending in the direction of flow of exhaust gas at the inside of the exhaust pipe3. Further, in the example shown inFIG. 2AandFIG. 2B, heat exchange fins15are formed over the entire outer circumferential surface of the hydrogen supply pipe13positioned inside the exhaust pipe3.

On the other hand, in the example shown inFIG. 2AandFIG. 2B, a plurality of heat exchange fins16for heat exchange with the hydrogen flowing through the inside of the hydrogen supply pipe13, more accurately for heat exchange with the reformed gas containing hydrogen, are also formed on the inner circumferential surface of the hydrogen supply pipe13positioned inside the exhaust pipe3. Note that, in the example shown inFIG. 2AandFIG. 2B, these heat exchange fins16are formed in the hydrogen supply pipe13positioned inside the exhaust pipe3only at the part extending along the axial line of the exhaust pipe3.

FIG. 3shows a modification of the hydrogen supply pipe13. In the example shown inFIG. 3, in the same way as the example shown inFIG. 2AandFIG. 2B, a plurality of heat exchange fins15for heat exchange with the exhaust gas flowing through the inside of the exhaust pipe3are formed on the outer circumferential surface of the hydrogen supply pipe13positioned at the inside of the exhaust pipe3. As opposed to this, in the example shown inFIG. 3, unlike the example shown inFIG. 2AandFIG. 2B, heat exchange fins are not formed on the inner circumferential surface of the hydrogen supply pipe13positioned inside the exhaust pipe3. Instead of this, in the example shown inFIG. 3, a swirl flow generator17for imparting a swirl flow around the axis of the hydrogen supply pipe13to the hydrogen flowing through the inside of the hydrogen supply pipe13, more accurately to the reformed gas containing hydrogen, is arranged at a position inside the hydrogen supply pipe13and outside of the exhaust pipe3.

FIG. 4AandFIG. 4Bshow another modification of the hydrogen supply pipe13. In the example shown inFIG. 4AandFIG. 4B, the front end part of the hydrogen supply pipe13extends inside of the exhaust pipe3in a vortex shape around the axis of the exhaust pipe3up to the axis of the exhaust pipe3and the front end opening part14of the hydrogen supply pipe13is oriented toward the upstream side end face of the exhaust treatment catalyst5. In this modification as well, as shown inFIG. 4AandFIG. 4B, a plurality of heat exchange fins15for heat exchange with the exhaust gas flowing through the inside of the exhaust pipe3are formed on the outer circumferential surface of the hydrogen supply pipe13positioned at the inside of the exhaust pipe3. These heat exchange fins15are comprised of thin fins extending in the direction of flow of the exhaust gas inside the exhaust pipe3.

As explained above, in the reformer6, hydrogen is formed by reforming the fuel. Therefore, next, referring toFIG. 5, the reform reaction in the case of using diesel fuel as fuel will be simply explained.FIGS. 5(a) and (b)show the reaction formula when a complete oxidation reaction is performed and the reaction formula when a partial oxidation reform reaction is performed in the case of using the generally used diesel fuel as fuel. Note that, the amounts of heat generated ΔH0in the reaction formulas are shown by the lower heating value (LHV). In the reformer6shown inFIG. 1, the fuel and air supplied from the burner10react at the reforming catalyst7by the partial oxidation reform reaction shown inFIG. 5(b)whereby hydrogen is formed. This partial oxidation reform reaction, as shown by the reaction formula of the partial oxidation reform reaction ofFIG. 5(b), is performed by a rich air-fuel ratio of an O2/C molar ratio 0.5 indicating the ratio of the air and fuel which are made to react. At this time, CO and H2are formed.

FIG. 6Ashows the relationship between the reaction equilibrium temperature TB when the air and fuel are made to react at the reforming catalyst7and reach an equilibrium and the O2/C molar ratio of the air and fuel. Note that, the solid line ofFIG. 6Ashows the theoretical value when the air temperature is 25° C. As shown by the solid line ofFIG. 6A, when the partial oxidation reform reaction is performed by a rich air-fuel ratio of an O2/C molar ratio=0.5, the equilibrium reaction temperature TB becomes about 830° C. At this time, substantially 830° C. reformed gas flows out from the reforming catalyst7to the inside of the reformed gas outflow chamber9and the reformed gas flowing out to the inside of the reformed gas outflow chamber9is sent through the hydrogen supply pipe13to the inside of the exhaust pipe3. Note that, the actual equilibrium reaction temperature TB at this time is somewhat lower than 830° C., therefore, actually, the temperature of the reformed gas flowing out to the inside of the reformed gas outflow chamber9is somewhat lower than 830° C.

On the other hand, as will be understood from the reaction formula of the complete oxidation reaction ofFIG. 5(a), the ratio of the air and fuel when the O2/C molar ratio=1.4575 becomes the stoichiometric air-fuel ratio. As shown inFIG. 6A, the reaction equilibrium temperature TB becomes the highest when the ratio of the air and fuel becomes the stoichiometric air-fuel ratio. When the O2/C molar ratio is between 0.5 and 1.4575, in part, the partial oxidation reform reaction is performed, while in part, the complete oxidation reaction is performed. In this case, the larger the O2/C molar ratio, the larger the ratio by which the complete oxidation reaction is performed compared with the ratio by which the partial oxidation reform reaction is performed, so the larger the O2/C molar ratio, the higher the reaction equilibrium temperature TB.

On the other hand,FIG. 6Bshows the relationship between the number of molecules (H2and CO) formed per atom of carbon and the O2/C molar ratio. As explained above, the larger the O2/C molar ratio than 0.5, the smaller the ratio by which the partial oxidation reform reaction is performed. Therefore, as shown inFIG. 6B, the greater the O2/C molar ratio than 0.5, the smaller the amounts of formation of H2and CO. Further, as shown inFIG. 6A, if the O2/C molar ratio becomes larger than 0.5, the equilibrium reaction temperature TB rapidly rises and the temperature of the reforming catalyst7also rapidly rises. Therefore, if making the O2/C molar ratio larger than 0.5, the reforming catalyst7ends up deteriorating due to heat. On the other hand, as shown inFIG. 6B, if the O2/C molar ratio becomes smaller than 0.5, the excess carbon C not able to be reacted with increases. This excess carbon C deposits inside the pores of the substrate of the reforming catalyst7to cause so-called “coking”. If coking occurs, the reform ability of the reforming catalyst7remarkably falls. Therefore, to avoid the occurrence of coking, the O2/C molar ratio has to be kept from becoming smaller than 0.5.

Further, as will be understood fromFIG. 6B, when the O2/C molar ratio is 0.5, the amount of formation of hydrogen becomes the largest in the range where no excess carbon C is formed. Therefore, when performing the partial oxidation reform reaction for forming hydrogen, to avoid coking and heat deterioration of the reforming catalyst7while enabling the most efficient formation of hydrogen, the O2/C molar ratio is made 0.5 or slightly higher than 0.5. The reformed gas containing hydrogen formed at this time falls somewhat in temperature up to reaching the exhaust pipe3and becomes 700° C. to 920° C. or so.

Next, for example, the case of making the temperature of the exhaust treatment catalyst5rise when, like at the time of engine warm-up operation, the temperature of the exhaust treatment catalyst5is low will be explained. Now then, when the temperature of the exhaust treatment catalyst5is low, if high temperature reformed gas containing hydrogen is supplied from the hydrogen supply pipe13, the exhaust treatment catalyst5is heated by not only the heat of the exhaust gas, but also the heat of the supplied reformed gas and rises in temperature. At this time, the exhaust treatment catalyst5rises in temperature due to the heat of exhaust gas and heat of reformed gas transferred by heat transfer to the exhaust treatment catalyst5. On the other hand, as explained above, the exhaust treatment catalyst5is comprised of an oxidation catalyst, NOXstorage catalyst, or catalyst-equipped particulate filter. This exhaust treatment catalyst5carries a precious metal catalyst such as platinum Pt, palladium Pd, or rhodium Rh. If in this way the exhaust treatment catalyst5carries a precious metal catalyst, the hydrogen contained in the reformed gas supplied from the hydrogen supply pipe13will be made to react with oxygen on the precious metal catalyst, and the exhaust treatment catalyst5will further rise in temperature due to the heat of oxidation reaction generated at this time.

In this regard, when the exhaust treatment catalyst5is heated by the heat of oxidation reaction of hydrogen in this way, the exhaust treatment catalyst5itself is directly heated by the heat of oxidation reaction of hydrogen. Therefore, if the exhaust treatment catalyst5is heated by the heat of oxidation reaction of hydrogen, the temperature of the exhaust treatment catalyst5is made to rise far more rapidly compared with the case where the exhaust treatment catalyst5is heated due to heat transfer of the heat of exhaust gas and heat of reformed gas. Therefore, to make the temperature of the exhaust treatment catalyst5rise, utilizing the heat of oxidation reaction of hydrogen is extremely effective. For this reason, it is necessary to send as much hydrogen as possible from the hydrogen supply pipe13to the inside of the exhaust treatment catalyst5.

In this regard, hydrogen reacts with oxygen (2H2+O2→2H2O) and is consumed by self ignition if there is oxygen present in the surroundings and the temperature of the surroundings becomes high. The hatched region ofFIG. 7shows the region where hydrogen reacts with oxygen and hydrogen is consumed by self ignition in this way. Note that, inFIG. 7, the abscissa shows the temperature around the hydrogen, that is, the ambient air temperature (° C.), while the ordinate shows the pressure (mmHg). Further, inFIG. 7, the broken line shows the atmospheric pressure. Therefore, fromFIG. 7, it will be understood that if the ambient air temperature becomes 550° C. or more, hydrogen will be consumed by self ignition. On the other hand, the exhaust gas pressure inside the exhaust pipe3upstream of the exhaust treatment catalyst5is substantially atmospheric pressure. Therefore, when hydrogen is supplied from the hydrogen supply pipe13to the inside of the exhaust pipe3, if the ambient air temperature is about 550° C. or more, this hydrogen will be consumed by self ignition.

Now then, to send as much hydrogen as possible from the hydrogen supply pipe13to the inside of the exhaust treatment catalyst5, it is necessary to make the hydrogen supplied from the hydrogen supply pipe13to the inside of the exhaust pipe3reach the exhaust treatment catalyst5without being consumed by self ignition. For this, when hydrogen is supplied from the hydrogen supply pipe13to the inside of the exhaust pipe3, the temperature around the supplied hydrogen, that is, the ambient air temperature, has to be made to decrease to about 550° C. or less. On the other hand, as explained above, the reformed gas containing hydrogen formed at the reformer6becomes 700° C. to 920° C. or so around when reaching the exhaust pipe3. Therefore, in order to send as much hydrogen as possible from the hydrogen supply pipe13to the inside of the exhaust treatment catalyst5, it is necessary to lower the temperature of the hydrogen, which is 700° C. to 920° C. or so while hydrogen is flowing through the hydrogen supply pipe13, so that the temperature around the hydrogen supplied from the hydrogen supply pipe13, that is, the ambient air temperature, becomes about 550° C. or less.

Therefore, in an embodiment according to the present invention, a plurality of heat exchange fins15for heat exchange with exhaust gas flowing through the inside of the exhaust pipe3are formed on at least the outer circumferential surface of the hydrogen supply pipe13positioned inside the exhaust pipe3. If in this way a plurality of heat exchange fins15are formed on the outer circumferential surface of the hydrogen supply pipe13, due to the heat exchange action with the exhaust gas, which is lower in temperature than the temperature of the hydrogen flowing through the hydrogen supply pipe13, the temperature of the hydrogen flowing through the hydrogen supply pipe13is made to fall so that the temperature around the hydrogen supplied from the hydrogen supply pipe13to the inside of the exhaust gas, that is, the ambient air temperature, becomes about 550° C. or less. As a result, the hydrogen supplied from the hydrogen supply pipe13to the exhaust pipe3will be sent into the exhaust treatment catalyst5without being consumed by self ignition, and the temperature of the exhaust treatment catalyst5is made to rapidly rise due to the heat of oxidation reaction of the hydrogen generated in the exhaust treatment catalyst5.

On the other hand, the exhaust gas flowing around the hydrogen supply pipe13is heated by the heat exchange action with the hydrogen flowing through the hydrogen supply pipe13and rises in temperature. This raised temperature exhaust gas flows into the exhaust treatment catalyst5whereby the temperature of the exhaust treatment catalyst5is made to further rise. That is, the amount of heat used for cooling the hydrogen flowing through the inside of the hydrogen supply pipe13can be effectively utilized for raising the temperature of the exhaust treatment catalyst5. Note that, the cooling action of the hydrogen flowing through the hydrogen supply pipe13is further promoted by formation of the heat exchange fins15over the entire outer circumferential surface of the hydrogen supply pipe13and, as shown inFIG. 2AandFIG. 2B, is further promoted by formation of a plurality of the heat exchange fins16over the inner circumferential surface of the hydrogen supply pipe13positioned inside of the exhaust pipe3.

In the modification shown inFIG. 3, the swirl flow generator17is arranged inside the hydrogen supply pipe13positioned at the outside side part of the exhaust pipe3, that is, at the entrance of the hydrogen supply pipe13to the inside of the exhaust pipe3. Due to this swirl flow generator17, a swirl flow about the axis of the hydrogen supply pipe13is imparted to the reformed gas containing hydrogen flowing through the hydrogen supply pipe13. As a result, heat exchange between the hydrogen flowing through the hydrogen supply pipe13and the exhaust gas is promoted and the cooling action of the hydrogen flowing through the hydrogen supply pipe13is promoted. Further, in the modification shown inFIG. 4AandFIG. 4B, the contact area of the hydrogen supply pipe13with the exhaust gas is increased and the heat exchange time with the exhaust gas is also increased, so the cooling action of the hydrogen flowing through the hydrogen supply pipe13is further promoted.