JP2946893B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus provided with a NOx purifying catalyst in an exhaust passage of an internal combustion engine, and to an exhaust gas purifying apparatus for an internal combustion engine capable of sufficiently suppressing the emission of HC even during warm-up after durability. .

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No. 1-139145 discloses that a NOx catalyst in which Cu is ion-exchanged and supported on zeolite is installed in an exhaust passage of an internal combustion engine capable of lean combustion so that NOx is exhausted even in the lean air-fuel ratio region. Can be purified, and a three-way catalyst or an oxidation catalyst is further disposed downstream of the H
An exhaust gas purification device capable of oxidizing C and CO has been proposed.

[0003] In addition, even in the exhaust gas in the lean air-fuel ratio range, NOx
In addition to the Cu / zeolite catalyst, there is a catalyst in which Pt is supported on alumina or zeolite. The Pt / alumina catalyst is slightly inferior in NOx purification rate as compared with the Cu / zeolite catalyst, but is superior in heat resistance.

[0004]

SUMMARY OF THE INVENTION In a conventional exhaust gas purifier having a serial arrangement of a Cu / zeolite catalyst and a three-way catalyst or an oxidation catalyst, the durability of the exhaust gas purifier is improved by replacing the Cu / zeolite catalyst with a Pt / alumina catalyst. When improving heat resistance, there are the following problems.

That is, as shown in FIG. 5, the Pt / alumina catalyst has a temperature window showing a good NOx purification rate as in the case of the Cu / zeolite catalyst, and shows the highest NOx purification rate of the Pt / alumina catalyst. Temperature is about 300
° C. Therefore, when a Pt / alumina catalyst is mounted on a vehicle, the Pt / alumina catalyst is located below the vehicle floor where the exhaust gas temperature is relatively low apart from the engine, and the three-way catalyst or the oxidation catalyst downstream therefrom is lower than about 300 ° C. Temperature. Since the activation start temperature of the three-way catalyst and the oxidation catalyst is slightly below 300 ° C. in the initial stage,
Even during normal operation, the temperature of the three-way catalyst and the oxidation catalyst is close to the activation start temperature, and the activity becomes a problem,
The purification of HC and CO becomes a problem.

In addition, after the endurance, the activation start temperature shifts to the high temperature side due to the deterioration of the three-way catalyst and the oxidation catalyst. Therefore, the purification of HC and CO after the endurance is more problematic than in the early stage of use.

An object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine provided with a NOx catalyst and a three-way catalyst or an oxidation catalyst downstream thereof so as to sufficiently exhibit the purifying ability of HC and CO.

[0008]

According to the present invention, the above object is achieved by the following apparatus for purifying exhaust gas of an internal combustion engine. That is, an internal combustion engine capable of lean combustion and its exhaust passage, and provided in the exhaust passage, without heating means,
Active at low temperature range composed of Pt / alumina catalyst
That the NOx catalyst, the exhaust passage, disposed downstream of the NOx catalyst, dated heaters, than the NOx catalyst
A catalyst with a heater having an active oxidizing ability at a high temperature ,
An exhaust gas purification device for an internal combustion engine, comprising:

[0009]

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the heater is turned on when the temperature of the oxidizing catalyst downstream of the NOx catalyst is lower than the activation start temperature or in the operating condition that may be lowered. By raising the temperature of the catalyst having oxidizing ability, and regardless of the installation location or after endurance, the catalyst having oxidizing ability can be used at or above its activation start temperature, so that HC,
The emission of CO to the outside can always be suppressed.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings. Figure 1,
As shown in FIG. 2, the exhaust passage 4 of a lean burn internal combustion engine capable of 2, supporting Pt on alumina, without the heating means, capable of reducing NOx in the exhaust gas of lean air-fuel ratio range, NOx catalyst 6 are arranged. A catalyst 8 having a heater and having an oxidizing ability is provided in a portion of the exhaust passage 4 of the internal combustion engine downstream of the NOx catalyst 6. The catalyst 8 having an oxidizing ability comprises a three-way catalyst or an oxidation catalyst such as a Pt / alumina catalyst.

The heater-equipped catalyst 8 is formed, for example, by coating a metal plate that generates resistance heat when energized with a carrier such as alumina, and carrying Pt in the case of a Pt / alumina catalyst, and Pt and Rh in the case of a three-way catalyst. Consists of In the case of a three-way catalyst, by carrying Rh, the NOx purification rate in exhaust gas having an air-fuel ratio richer than the stoichiometric ratio is increased as compared with a catalyst carrying only Pt. The heater of the heater-equipped catalyst may be provided immediately upstream of the catalyst 8 which is separated from the oxidizing catalyst 8 but downstream of the NOx catalyst 6.

Since the NOx catalyst 6 exhibits the highest NOx purification rate near about 300 ° C., the NOx catalyst 6 is separated from the internal combustion engine 2 at a position where the exhaust gas temperature becomes close to 300 ° C. by natural heat radiation.
Therefore, it is installed under the floor of the vehicle. Also, since the temperature is not set to be high, the heat when the heater 8a of the catalyst with heater 8 is turned on is not transmitted to the NOx catalyst 6 by heat conduction.
It is desirable to provide a space between the catalysts 6 and 8.

The activation start temperature of the heater-equipped catalyst 8 comprising a Pt / alumina catalyst and a three-way catalyst is 3 at the beginning of use.
Slightly lower than 00 ° C, 300 ° after endurance
May be a little over C. If the NOx catalyst 6 is arranged at a position where the exhaust gas becomes close to 300 ° C., the temperature of the heater-equipped catalyst 8 downstream therefrom may become 300 ° C. or less. When the temperature becomes lower than the activation start temperature or when there is a possibility that the temperature becomes lower than the activation start temperature, a heater control means 10 for controlling energization to the heater 8a is provided so as to make the temperature higher than the activation start temperature.

Various sensors and a control device 12 are provided so that this control can be performed. As the sensor, a catalyst temperature sensor 14 (a temperature representing the catalyst bed temperature, for example, the catalyst 8) for detecting the catalyst bed temperature of the heater-equipped catalyst 8 is used.
May be a sensor for detecting the temperature of the gas discharged from the engine), an intake pressure sensor 16 for detecting the intake pressure PM for detecting the load state of the engine, and a throttle for detecting the throttle opening (correlated with the intake air amount QN). An opening degree sensor 18, a crank angle sensor 20 built in the distributor for detecting the engine rotation speed NE to know the operating state of the engine, and the like are provided.

The control unit (ECU) 12 is composed of a microcomputer, and, like a general microcomputer, a central processor unit (C) for executing an operation.
PU), a read-only memory (ROM) which is a read-only memory, a random access memory (RAM) for temporary storage, an analog / digital converter for converting an analog input amount into a digital amount, an input interface, and an output interface. The digital input amount from the crank angle sensor 20 is input to the input interface,
Analog quantities of other sensors are converted to digital quantities by an analog / digital converter and input to an input interface. The command calculated by the CPU is sent from the output interface of the control device 12 to the heater control switch 24 to control the energization of the heater 8a. In addition, 22 is a battery.

The ROM of the control device 12 stores the control programs shown in FIGS. 3 and 4.
The data is read out to the PU and a predetermined operation is executed. The program in FIG. 3 constitutes the heater control means 10, and the program in FIG. 4 is a normal fuel injection control program.

The fuel injection control routine of FIG. 4 is interrupted at predetermined time intervals or at predetermined crank angles.
First, in step 102, an input value for knowing the operating state of the engine, that is, the engine speed NE (can be calculated from the output of the crank angle sensor 20) and the intake pressure PM (the output of the intake pressure sensor 16) are read. Subsequently, step 104
, A basic fuel injection amount is obtained from the NE and PM using a two-dimensional map, and various corrections, such as a water temperature correction (a correction for increasing the fuel injection amount immediately after starting when the engine cooling water temperature is low, etc.), an intake air temperature The fuel injection amount TAU is obtained by performing correction, EGR correction, and the like. Subsequently, at step 106, as in the case of the fuel injection amount TAU, the ignition timing SA is obtained from the NE and PM using a two-dimensional map.
The fuel injection and ignition are executed according to the obtained fuel injection amount TAU and ignition timing SA. Then return,
The routine of FIG. 3 is entered.

The routine of the heater control means 10 shown in FIG. 3 will be described. At step 202, the catalyst bed temperature Tcat (output of the catalyst temperature sensor 12) of the catalyst with heater 8 is read, and the routine proceeds to step 204, where the catalyst bed temperature Tcat is lower than the endurance catalyst activation start temperature Tcat0, for example, 320 ° C. It is determined whether or not. If Tcat <Tcat0, the heater-equipped catalyst 8 may be used at a temperature lower than the activation start temperature, and HC and CO may hardly be purified. Therefore, the process proceeds to step 206, where the heater 8a is turned on (the heater control switch 24 is turned on). Then, the heater-equipped catalyst 8 is heated.

In step 204, the catalyst bed temperature Tcat is set to Tc.
If it is determined that the value is at0 or more, the process proceeds to step 208. In step 208, the engine load is obtained from the intake pressure PM (output of the intake pressure sensor 16), the slot opening obtained from the output of the throttle opening sensor 18, or the intake air amount QN. Subsequently, the routine proceeds to step 210, where it is determined whether or not the light load state (a state in which the rotational speed is higher than the idling rotational speed but the throttle opening is close to the fully closed state) has continued for a predetermined time or more. This is because the temperature of the exhaust gas is also low at light load, and if the state continues for a certain period of time or more, the catalyst bed temperature of the heater-equipped catalyst 8 falls and finally falls below the activation start temperature. is there. If it is determined in step 210 that the light load has continued for a predetermined time or more, the routine proceeds to step 206, where the heater 8a is turned on, and the temperature of the heater-equipped catalyst 8 is raised. If it is determined in step 210 that the light load has not been continued for a predetermined time or more, the process proceeds to step 212.

In step 212, it is determined whether or not the current engine operating state is an idle state. This can be determined when the throttle opening is fully closed and the engine speed is equal to or lower than a predetermined speed (for example, 800 rpm). The reason for determining whether or not the engine is in the idle state is to prevent the exhaust temperature from being lower in the idle state than in the light load state and the temperature of the heater-equipped catalyst 8 gradually falling below the activation start temperature. is there. If it is determined in step 212 that the current engine operation state is the idle state, the routine proceeds to step 206, where the heater 8a is turned on to raise the temperature of the heater-equipped catalyst 8. If it is determined in step 212 that the vehicle is not in the idle state, the process proceeds to step 214.

In step 214, the power supply to the heater 8a of the catalyst with heater 8 is turned off. Proceeding to step 214 means that the temperature of the heater-equipped catalyst 8 is equal to or higher than the predetermined temperature, and is not at the time of light load or idling, so that the catalyst temperature of the heater-equipped catalyst 8 should not drop extremely. When it is determined that the temperature is not lower than the starting temperature, the heater 8a
Is turned off. The process proceeds from step 206 or step 214 to the next step, and the calculation of the interrupt cycle is completed.

The heater control means 10 shown in FIG. 3 includes a catalyst bed temperature determination means of steps 202 and 204,
8, 210 and 212, the heater control means 10 includes at least one of these determination means or a determination means equivalent to the one determination means (for example, catalyst bed temperature). Is determined based on the catalyst outlet gas temperature).

Next, the operation will be described. During normal operation, the temperature of the exhaust gas discharged from the internal combustion engine 2 drops due to natural heat dissipation while flowing to the NOx catalyst 6, and the temperature at the NOx catalyst 6 becomes approximately 300 ° C., as shown in FIG. NOx catalyst 6 composed of Pt / alumina catalyst has the highest NOx
The purification rate can be indicated, and the emission of NOx is suppressed.

However, the heater-equipped catalyst 8 further downstream than the NOx catalyst 6 is lower than the NOx catalyst 6 by an amount corresponding to a temperature decrease due to natural heat release while the exhaust gas flows from the NOx catalyst 6 to the heater-equipped catalyst 8. Since the temperature is low, the temperature of the heater-equipped catalyst 8 may be lower than 300 ° C. when the heater is off, and may be lower than the activation temperature of the heater-equipped catalyst. If the temperature falls below the activation start temperature, HC, CO
As shown in FIG. 5, the purification rate of the heater-equipped catalyst 8 decreases as shown in FIG. 5, so that HC and CO are released to the outside air.

However, according to the present invention, when the temperature of the heater-equipped catalyst 8 becomes lower than or equal to its activation start temperature (for example, when the light load operation continues for a predetermined time or more,
When the idling operation is continued), the heater 8a is turned on to heat the heater-equipped catalyst 8, so that the heater-equipped catalyst 8 is always kept at the catalyst activation start temperature or higher, and the emission of HC and CO is suppressed.

Although a Pt / alumina catalyst or a three-way catalyst is deteriorated in durability, the heater 8a is turned on by turning on the heater 8a even when the catalyst activation start temperature becomes 300 ° C. or higher due to the durability deterioration. The catalyst 8 can be kept at or above the activation start temperature during normal operation.

[0027]

According to the present invention, Pt without heating means
/ Since the catalyst provided downstream of the NOx catalyst composed of the alumina catalyst is provided with a heater, the temperature of the catalyst with a heater can be raised by turning on the power supply to the heater,
During normal operation, always keep the temperature above the activation start temperature,
CO can be purified. Further, since the NOx catalyst can be made without a heating means, the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an arrangement of a NOx catalyst and a heater-equipped catalyst in an exhaust gas purifying apparatus for an internal combustion engine according to one embodiment of the present invention.

FIG. 2 is an overall system diagram of an exhaust gas purification device for an internal combustion engine according to one embodiment of the present invention.

FIG. 3 is a flowchart showing a control routine of a heater control means in the exhaust gas purifying apparatus for an internal combustion engine according to one embodiment of the present invention.

FIG. 4 is a flowchart of a general fuel injection control routine for an internal combustion engine.

FIG. 5 is a graph showing NOx, HC, and CO purification rates of a Pt / alumina catalyst versus temperature.

[Explanation of symbols]

 2 Internal combustion engine 4 Exhaust system 6 NOx catalyst 8 Catalyst with heater 8a Heater 10 Heater control means 12 Controller 14 Temperature sensor for detecting catalyst bed temperature 16 Intake pressure sensor 18 Throttle opening degree sensor 20 Crank angle sensor

Continuation of the front page (72) Inventor Tetsu Iguchi 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-1-139145 (JP, A) JP-A-5-31359 (JP, A) JP-A-48-27124 (JP, A) JP-A-4-129829 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) F01N 3/20 F01N 3/24 F01N 3 / 28 301

Claims (1)

(57) [Claims] 1. An internal combustion engine capable of lean combustion and an exhaust passage thereof, and a Pt / A without heating means provided in the exhaust passage.
NOx active in low temperature range composed of lumina catalyst
A catalyst, which is provided at a downstream side of the NOx catalyst in the exhaust passage and has a heater and is activated at a higher temperature than the NOx catalyst.
Exhaust purifying apparatus for an internal combustion engine characterized by comprising a catalyst with a heater, a having an oxidizing ability that.