Exhaust purification device of an internal combustion engine

In an internal combustion engine, an NOX selective reducing catalyst is arranged in the engine exhaust passage, and an NOX storage catalyst able to store NOX contained in the exhaust gas is arranged at the upstream of the NOX selective reducing catalyst. The NOX storage catalyst is fed with mist fuel, and the NOX stored in the NOX storage catalyst and the fed fuel are used to produce an intermediate product comprising bonded molecules comprised of NH2 and a hydrocarbon molecule more than an equivalent ratio with respect to one NOX molecule. These intermediate products are adsorbed at the NOX selective reducing catalyst, whereby the adsorbed intermediate product reduces the NOX in the exhaust gas.

TECHNICAL FIELD

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

BACKGROUND ART

Known in the art is an internal combustion engine arranging in the engine exhaust passage an NOXstorage catalyst which stores NOXcontained in the exhaust gas when the air fuel ratio of the inflowing exhaust gas is lean and releases the stored NOXwhen the air fuel ratio of the inflowing exhaust gas becomes a stoichiometric air fuel ratio or rich and arranging in the engine exhaust passage upstream of this NOXstorage catalyst a compact three-way catalyst (see for example Japanese Patent Publication (A) No. 2004-108176). In this internal combustion engine, if the NOXstorage ability of the NOXstorage catalyst approaches saturation, the air fuel ratio of the exhaust gas is temporarily made rich whereby NOXis released from the NOXstorage catalyst and reduced.

However, there is a problem in that, in this internal combustion engine, when making the NOXstorage catalyst release NOXby feeding mist fuel upstream of the NOXstorage catalyst, the NOXreleased from the NOXstorage catalyst cannot be properly reduced.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purification device of an internal combustion engine able to reduce the NOXreleased from an NOXstorage catalyst well.

According to the present invention, there is provided an exhaust purification device of an internal combustion engine arranging an NOXselective reducing catalyst in an engine exhaust passage, arranging an NOXstorage catalyst able to store NOXcontained in the exhaust gas in the engine exhaust passage upstream of the NOXselective reducing catalyst, arranging a fuel feed valve in the engine exhaust passage upstream of the NOXstorage catalyst to feed a mist fuel from the fuel feed valve to the NOXstorage catalyst, reacting NOXstored in the NOXstorage catalyst and fed fuel on the NOXstorage catalyst to produce an intermediate product comprising bonded molecules of NH and a hydrocarbon molecule more than an equivalent ratio with respect to one NOXmolecule and making the intermediate product produced in the NOXstorage catalyst be adsorbed at the NOXselective reducing catalyst so as to reduce NOXin the exhaust gas by the adsorbed intermediate product.

That is, in the present invention, by feeding mist fuel from the fuel feed valve to make the NOXstorage catalyst release NOXand reducing the released NOXto, NH2, the NOXis purified.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1is an overview of a compression ignition type internal combustion engine.

Referring toFIG. 1,1indicates an engine body,2a combustion chamber of each cylinder,3an electronically controlled fuel injector injecting fuel into each combustion chamber2,4an intake manifold, and5an exhaust manifold. The intake manifold4is connected through an intake duct6to a compressor7aof an exhaust turbocharger7, while the inlet of the compressor7ais connected through an intake air amount detector8to an air cleaner9. Inside the intake duct6, a throttle valve10driven by the step motor is arranged. Further, around the intake duct6, a cooling device11for cooling the intake air flowing through the intake duct6is arranged. In the embodiment shown inFIG. 1, the engine cooling water is led into the cooling device11where the engine cooling water is used to cool the intake air.

On the other hand, the exhaust manifold5is connected to an inlet of an exhaust turbine7bof the exhaust turbocharger7. The outlet of the exhaust turbine7bis connected to the inlet of a NOXstorage catalyst12able to store the NOXcontained in the exhaust gas. The cutlet of the NOXstorage catalyst12is connected to a NOXselective reducing catalyst14via an exhaust pipe13. Further, a fuel feed valve15for feeding a fuel into the exhaust gas flowing within the exhaust manifold5is attached to the exhaust manifold5.

The exhaust manifold5and intake manifold4are connected to each other through an exhaust gas recirculation (hereinafter referred to as “EGR”) passage16. Inside the EGR passage16, an electronic control type EGR control valve17is arranged. Further, around the EGR passage16, a cooling device18for cooling the EGR gas flowing through the EGR passage16is arranged. In the embodiment shown inFIG. 1, engine cooling water is led to the cooling device18where the engine cooling water cools the EGR gas. On the other hand, each fuel injector3is connected through a fuel tube19to a common rail20. This common rail20is fed with fuel from an electronically controlled variable discharge fuel pump21. The fuel fed into the common rail20is fed through each fuel tube19into the fuel injectors3.

Initially, the NOXstorage catalyst12will be explained. This NOXstorage catalyst12is comprised of a substrate on which for example a catalyst carrier comprised of alumina is carried.FIG. 2(A), (B) illustrates the cross-section of the surface part of this catalyst carrier30. As shown inFIG. 2(A), (B), the catalyst carrier30carries a precious metal catalyst31diffused on the surface. Further, the catalyst carrier30is formed with a layer of an NOXabsorbent32on its surface.

In the embodiment according to the present invention, as the precious metal catalyst31, platinum Pt is used. As the ingredient forming the NOXabsorbent32, for example, at least one element selected from potassium K, sodium Na, cesium Cs, and other such alkali metals, barium Ba, calcium Ca, and other such alkali earths, lanthanum La, yttrium Y, and other rare earths is used.

If the ratio of the air and fuel (hydrocarbons) fed into the engine intake passage, combustion chamber2, and exhaust passage upstream of the NOXstorage catalyst12is called the “air-fuel ratio of the exhaust gas”, an NOXabsorption and release action such that the NOXabsorbent32absorbs the NOXwhen the air-fuel ratio of the exhaust gas is lean and releases the absorbed NOXwhen the oxygen concentration in the exhaust gas falls is performed.

That is, explaining this taking as an example the case of using barium Ba as the ingredient forming the NOXabsorbent32, when the air-fuel ratio of the exhaust gas is lean, that is, the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas, as shown inFIG. 2(A), is oxidized on the platinum Pt31to become NO2, next is absorbed in the NOXabsorbent32and bonds with the barium oxide BaO to diffuse in the form of nitrate ions NO3−into the NOXabsorbent32. In this way, NOXis absorbed in the NOXabsorbent32. So long as the oxygen concentration in the exhaust gas is high, NO2is formed on the platinum Pt31. So long as the NOXabsorbent32is not saturated in NOXabsorption ability, NO2is absorbed in the NOXabsorbent32and nitrate ions NO3−are formed.

As opposed to this, for example if the exhaust gas is made a rich air-fuel ratio or stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the reverse direction (NO3−→NO2), therefore the nitrate ions NO3−in the NOXabsorbent32are released in the form of NO2from the NOXabsorbent32.

On the other hand, the NOXselective reducing catalyst14is comprised of an ammonia adsorption type Fe zeolite or a titania/vanadium based-catalyst having no ammonia adsorption function, which are capable of selectively reducing the NOXin the exhaust gas by ammonia when the air-fuel ratio of the exhaust gas is lean. In the embodiment shown inFIG. 1, the NOXselective reducing catalyst14is comprised of an ammonia adsorption type Fe zeolite.

Now, when the air-fuel ratio of the exhaust gas is lean as mentioned above, that is, when combustion is performed under a lean air-fuel ratio, the NOXin the exhaust gas is absorbed in the NOXabsorbent32. However, if combustion is continued under a lean air-fuel ratio, the NOXabsorption ability of the NOXabsorbent32will end up becoming saturated and therefore the NOXabsorbent32will end up unable to absorb NOX. Here, in an embodiment of the present invention, before the absorption ability of the NOXabsorbent32becomes saturated, fuel is fed from the fuel feed valve15to make the NOXstorage catalyst12release NOX. This will be explained in the following.

In this embodiment of the present invention, diesel fuel or a heavy fuel having diesel fuel as a main ingredient is fed in a mist state, that is, in the form of particulates, from the fuel feed valve15. Part of the fed fuel is oxidized, but the majority, as shown inFIG. 2(B), adheres to the surface of platinum Pt31and the surface of the NOXabsorbent32. If the fed fuel adheres to the surface of the platinum Pt31, the oxygen concentration on the surface of the platinum Pt31will fall, causing the NO3−of the NOXabsorbent32, as shown inFIG. 2(B), to be released in the form of NO2.

If a large amount of fuel of an extent whereby the air-fuel ratio of the exhaust gas becomes considerably rich is fed from the fuel feed valve15, that is, if the reducing agent for reducing the NOX is fed in a large amount, the released NO2, as shown inFIG. 2(B), will be reduced to NO and then to NH2. Next, this NH2 immediately reacts with the hydrocarbons HC adhering to the platinum31, whereby, as shown inFIG. 2(B), an intermediate product33comprising the bonded molecules of the hydrocarbons HC and NH2 is produced. Note that the number of carbon atoms of the hydrocarbons HC in the fed fuel is considerably large, accordingly, in the NOX storage catalyst12, the stored NOX and the fed fuel produce an intermediate product comprising bonded molecules of NH2 and a hydrocarbon molecule more than an equivalent ratio=1 with respect to one NOX molecule.

Thereby, the NOXstored in the NOXstorage catalyst12is released from the NOXstorage catalyst12by the fed fuel, and the released NOXis reduced.

Next, the intermediate product produced in the NOXstorage catalyst12is fed into the NOXselective reducing catalyst14and is adsorbed at the NOXselective reducing catalyst14. The intermediate product adsorbed at the NOXselective reducing catalyst14is broken down to hydrocarbons HC and ammonia NH3in the NOXselective reducing catalyst14if the temperature of the NOXselective reducing catalyst14rises. The hydrocarbons HC are oxidized by the oxygen contained in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, whereby the NOXcontained in the exhaust gas is reduced by the ammonia NH3adsorbed at the NOXselective reducing catalyst14.

In this way, the NOXstored in the NOXstorage catalyst12is transferred in the form of amine NH2to the NOXselective reducing catalyst14, converted to ammonia NH3in the NOXselective reducing catalyst14, and used for the purification of NOX.

LIST OF REFERENCES