Power plants – Internal combustion engine with treatment or handling of... – Having auxiliary device mechanically driven by exhaust gas
Reexamination Certificate
2002-12-05
2004-11-30
Nguyen, Tu M. (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Having auxiliary device mechanically driven by exhaust gas
C060S288000, C060S295000, C060S297000, C060S300000, C060S324000
Reexamination Certificate
active
06823660
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission purification system for a diesel engine which cleans up particulate matter (hereinafter, referred to as PM) and NOx (nitrogen oxide) in exhaust gas from an engine such as a diesel engine.
As regards discharge of PM to be discharged from the diesel engine, a restriction has been enhanced together with NOx, CO (carbon monoxide), unburned hydrocarbon and the like year by year, and there has been developed a technique by which this PM is collected by a filter called “Diesel Particulate Filters” (hereinafter, referred to as DPF) to reduce an amount of PM to be discharged outwardly.
For DPF for directly collecting this PM, there are a wall flow type monolithic honeycomb filter made of ceramic, a fiber type filter obtained by making ceramic or metal into a fibrous shape, and the like. An exhaust emission purification device using these DPFs is arranged in an exhaust pipe of an engine to clean up exhaust gas to be generated by the engine.
In this DPF, however, clogging develops as PM is collected and exhaust gas pressure (exhaust pressure) is increased, and therefore, it is necessary to remove the PM collected by this DPF, and several methods and systems have been developed.
Among them, there are a system for burning and removing PM by heating the filter with an electric heater or a burner, and a system for inverse-washing by flowing air in the opposite direction. In the case of these systems, however, since energy for heating is supplied from the outside to burn PM, there are a problem for worsening fuel consumption and a problem that it is difficult to control regeneration.
Also, when any of these systems is adopted, in the majority of cases, the structure is arranged such that there are provided two lines of exhaust passages each having a filter, and collection of PM and regeneration of the filter are alternately repeated in each exhaust passage. For this reason, the system becomes large and the cost is also prone to be increased.
In order to cope with these problems, there has been proposed a continuously regenerating type DPF system in which regeneration temperature of the filter is lowered to reduce an amount of energy supplied from the outside, and exhaust heat from the engine is utilized to regenerate the filter. In this system, a wall flow type filter and a catalyst are combined. This filter is constructed such that a multiplicity of exhaust passages (cells) whose periphery is enclosed with a porous wall surface are formed and an inlet side and an outlet side of these exhaust passages are sealed in a zigzag shape, respectively.
In this system, since regeneration of the filter and PM collection are basically performed continuously and an exhaust passage becomes one line of system, control of regeneration is also simplified. This system has the following three types.
A first type is a nitrogen dioxide regenerating type DPF system, and is composed of an oxidation catalyst on the upstream side and a wall flow type filter on the downstream side. Through the use of this oxidation catalyst such as platinum, NO (nitrogen monoxide) in the exhaust gas is oxidized. Through the use of NO
2
(nitrogen dioxide) generated by this oxidization, PM collected by the filter is oxidized into CO
2
(carbon dioxide) to remove the PM. The oxidation of PM due to this NO
2
has lower energy barrier than the oxidation of PM due to O
2
(oxygen) and is performed at, low temperature. For this reason, thermal energy in the exhaust gas is utilized, whereby PM can be oxidized and removed while the PM is continuously being collected to regenerate the filter.
Also, a second type is an integrated model nitrogen dioxide regenerating DPF system, and is an improvement of the first system. This system is formed by coating a wall surface of the wall flow type filter with the oxidation catalyst. This wall surface performs both oxidation of NO in the exhaust gas and oxidation of PM due to NO
2
. Thereby, the system is simplified.
Thus, a third DPF system with a PM oxidation catalyst is formed by a precious metal oxidation catalyst such as platinum and a wall flow type filter with PM oxidation catalyst obtained by coating the wall surface with PM oxidation catalyst This wall surface oxidizes PM at lower temperature. This PM oxidation catalyst is a catalyst for directly oxidizing PM by activating O
2
in the exhaust gas, and is formed of cerium dioxide or the like.
In this third system, in a low-temperature oxidation region (350° C. to about 450° C.), PM is oxidized with NO
2
through the use of a reaction in which NO of oxidation catalyst is oxidized into NO
2
. In a medium-temperature oxidation region (400° C. to about 600° C.), through the use of PM oxidation catalyst, O
2
in the exhaust gas is activated to oxidize PM through the use of a reaction in which PM is directly oxidized. In a high-temperature oxidation region (about 600° C. or higher) higher than temperature at which PM burns with O
2
in the exhaust gas, PM is oxidized with O
2
in the exhaust gas.
In these continuously regenerating type DPF systems, through the use of the oxidation reaction of PM due to the catalyst and NO
2
, the temperature at which PM can be oxidized is lowered.
On the other hand, in exhaust gas from the diesel engine, the exhaust gas temperature varies as shown in
FIG. 8
depending upon a load and a number of revolutions of the engine. For the reason, the DPF is not always in an optimum temperature state. When the exhaust gas temperature is in the low-temperature region, the activity of the catalyst decreases, and the PM cannot be sufficiently oxidized. Accordingly, there is a problem that it is difficult to have an excellent PM cleanup performance over the entire engine operating region.
As one of measures against this problem, there is an exhaust emission purification device for a diesel engine proposed in Japanese Patent Application No. 155894/2001 by the present inventor.
As shown in
FIG. 5
, this exhaust emission purification device is provided with: a first continuously regenerating type DPF
12
A in an exhaust passage
9
of the engine; a bypass passage
101
on the upstream side of this first DPF
12
A; a second continuously regenerating type DPF
13
A provided in this bypass passage
101
; and exhaust gas temperature rise means. This exhaust gas temperature rise means uses an intake throttle valve (intake shutter)
22
, an exhaust gas introduction mechanism and an exhaust throttle valve (exhaust shutter)
23
. In an engine operating state during idling or the like in which exhaust gas temperature is low, the exhaust gas temperature is raised to 300° C. or higher, and the exhaust gas is caused to flow into a bypass passage
101
to treat the exhaust gas through the use of a second continuously regenerating type DPF
13
A.
In this exhaust emission purification device, a second continuously regenerating type DPF
13
A consisting of the oxidation catalyst
131
and DPF
132
shown in
FIG. 6
is, as shown in
FIG. 5
, arranged at a position closest to an exhaust manifold
4
. This arrangement causes exhaust gas whose temperature has been raised by the exhaust gas temperature rise means to pass through the second continuously regenerating type DPF
13
A before its temperature is not lowered even in the low exhaust gas temperature operating state.
With control of a change-over valve
102
, when the exhaust gas temperature is within a predetermined low-temperature region, the exhaust gas is caused to pass through a second continuously regenerating type DPF
13
A in the vicinity of the exhaust manifold, and when the exhaust gas temperature is within a predetermined high-temperature region, the exhaust gas is caused to flow through the first continuously regenerating type DPF
12
A. Thereby, in the entire operating state region of the engine, the temperature of the exhaust gas passing through each DPF
122
,
132
is set to 300° C. or higher.
The temperature of exhaust gas passing through each of these DPFs
12
A and
13
A is maintained at high tempe
Isuzu Motors Limited
Nguyen Tu M.
Staas & Halsey , LLP
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