Diesel engine exhaust purifying device

Details Diesel engine exhaust purifying device Diesel engine exhaust purifying device

2003-01-23

2004-11-30

Tran, Binh Q. (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C060S274000, C060S284000, C060S295000, C123S090150, C123S090190, C123S090230

Reexamination Certificate

active

06823661

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a device for purifying exhaust gas of diesel engines for removing particulate matters from the exhaust gas.
BACKGROUND ART
Regulations against the exhaust gases of diesel engines mounted on vehicles are becoming stringent year after year. In particular, it is becoming an urgent necessity to decrease the particulate matters (hereinafter abbreviated as PMs) which consists of carbon as a main component. As a device for removing the PMs from the exhaust gas, there has been known a diesel particulate filter (hereinafter abbreviated as DPF), and a trend toward obligatorily furnishing the vehicles mounting diesel engines with the DPF becomes also serious.
The DPF with which the diesel engine mounted on the vehicle is furnished must be regenerated by burning the PMs that have been trapped because the PMs deposit on the DPF due to repetitive operation of the engine. As a means for regeneration of the DPF, there is a method to burn the PMs by heating them with an electric heater, a burner or the like. According to this method, a system that a plurality of DPFs are arranged in parallel in the exhaust gas passage to alternately conduct the trapping and the burning is constituted. As another means for regenerating the DPF, there has also been studied a so-called continuous regeneration type DPF according to which an oxidizing catalyst is disposed in the exhaust gas passage on the upstream side of the DPF, NO in the exhaust gas is oxidized into NO
2
by the oxidizing catalyst, and the PMs are continuously burned with NO
2
while trapping the PMs as disclosed in, for example, Japanese Patent No. 3012249. Further, as another continuous regeneration type DPF, there has been known a method in which a NOx occluding/reducing catalyst is carried on the DPF and the PMs trapped are continuously burned by using active oxygen that generates when the NOx is occluded and reduced, as disclosed in Japanese Patent No. 2600492. In both of these continuous regeneration type DPFs, the PMs burn in a low temperature region of 250 to 400° C. (which may shift up and down to some extent depending on the material of the catalyst) without requiring any particular heating means such as electric heater, burner or the like, giving such an advantage that the devices as a whole can be constructed simply and in a compact size.
Although it has been thus made possible to burn the PMs in a range of a so-called active temperature region of the catalyst, which can be easily accomplished by the exhaust gas temperature of the engine as described above, there may occur an operation condition that does not lie in this temperature range depending on the operation conditions of the engine. When the engine operates under a low-load condition, in particular, there occurs a case where the temperature of the exhaust gas does not rise and often fails to reach 250° C., while during a high-load operation condition, too, there is a case where the exhaust gas temperature often exceeds the active temperature region and the PMs cannot often be burned continuously.
In the above cases, when according to the operation conditions, the temperature of the exhaust gas does not come into the active temperature region of the catalyst and the PMs trapped by the DPF do not burn, the PMs that do not burn remain trapped by the DPF and accumulate thereon. Then, when the engine operation condition changes and the temperature of the exhaust gas comes into the active temperature region of the catalyst, the PMs accumulated burn due to the action described above. At this moment, if the accumulated PMs start burning all at once, the temperature of combustion of PMs becomes as high as 2000° C. causing such problems as melt-damage or the like to the filter body.
As described above, the continuous regeneration type DPF continuously conducts the regeneration when the exhaust gas is emitted while the PMs being trapped. Therefore, if the DPF can be continuously regenerated at all times whenever the engine is in operation, then, the temperature does not reach so high a temperature as to cause melt-damage. Because of this reason, it is important to maintain the temperature of the exhaust gas to lie within the active temperature region of the catalyst at all times.
By taking the above problems into consideration, the present applicant has proposed Japanese Patent Applications Nos. 2000-185897 and 2001-79266 in an attempt to maintain the temperature of the exhaust gas in the active temperature region of the catalyst.
The inventions of the above two applications were proposed based on a knowledge that the temperature of the exhaust gas is greatly affected by the amount of the air taken in the cylinder of the engine, and it becomes high as the excess air ratio (&lgr;), which it the ratio of the excess air to the fuel, approaches &lgr;=1 from a large state and as the temperature of the air taken in the cylinder becomes high (the temperature of the exhaust gas becomes low if they are reversed).
That is, Japanese Patent Application No. 2000-185897 discloses that there are arranged an intake throttle valve and a variable supercharger as air amount adjusting means, and the temperature of the exhaust gas is controlled by squeezing the intake throttle valve to decrease the amount of the intake air and by controlling the variable supercharger to increase the amount of the intake air.
Further, Japanese Patent Application No. 2001-79266 discloses that there are provided a so-called EGR passage communicating the exhaust gas passage of the engine with the intake air passage and an EGR valve for controlling the passage area of the EGR passage, and the temperature of the exhaust gas is controlled by controlling the flow rate of the EGR gas refluxed from the exhaust gas reflux (EGR) passage to the intake air passage side and by further controlling the amount of the intake air by the intake air shutter in the intake air passage and by the exhaust gas shutter in the exhaust gas passage.
In a diesel engine, the amount of the intake air is not usually controlled, and the excess air ratio (&lgr;) becomes large as the load is low, i.e., when the fuel is injected in small amounts, and becomes &lgr;=10 or larger during the idling operation. To bring the excess air ratio (&lgr;) close to 1, therefore, the amount of the intake air must be squeezed to a considerable degree.
Meanwhile, as is well known, the diesel engine is a combustion system based on the self-ignition by the compression. That is, the intake air is compressed based on the compression ratio of the cylinder defined by a bore of the cylinder and the stroke of the piston, the fuel is injected into the cylinder in which the temperature has been raised due to the compression and hence, the temperature of the fuel itself is elevated, so that the fuel is self-ignited while being vaporized. Therefore, the above method of decreasing the amount of the intake air hinders the elevation of the temperature of the intake air by the compression and produces a condition where the self-ignition does not easily take place. Namely, the above method arouses a new problem inducing incomplete combustion in the cylinder and emitting unburned HC (hydrocarbons).
FIG. 9
is a graph illustrating the effect produced as a result of squeezing the intake air shutter, in which the ordinate represents the exhaust gas temperature (° C.) and HC (×100 ppm) and the abscissa represents the amount of the intake air [kg/h]. The amount of the intake air is limited by squeezing the intake air shutter stepwise. Solid lines in the graph represent a change in the exhaust gas temperature and the HC contained in the exhaust gas in the prior art, that were measured by operating the intake air shutter only. The data of
FIG. 9
were measured by using a 4-cylinder 3-liter diesel engine while driving it at a rotational speed (Ne) of 1000 rpm, injecting the fuel at a rate (Q) of 10 mm
3
/st and measuring the exhaust gas temperature at the outlet of the exhaust manifold.
As will be obvious from
FIG. 9
, the

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