Patent Description:
In a diesel engine exhaust gas purification system, an NOx occlusion reduction catalyst is used in order to reduce nitrogen oxides (NOx) contained in exhaust gas, unlike a gasoline engine (see <CIT>, for example). This is because since the exhaust gas of the diesel engine is a lean atmosphere in terms of the air-fuel ratio, it is impossible to directly employ a three-way catalyst used in a stoichiometric atmosphere as in the case of the gasoline engine.

This diesel engine exhaust gas purification system performs a regeneration operation in which an NOx occlusion material (an alkali metal or alkali earth metal such as K and Ba) is allowed to temporarily occlude NOx within the exhaust gas when it is in the lean state, and the state thereof is periodically changed into rich so as to release the occluded NOx to reduce it by virtue of the three-way function.

In order to change the state of the exhaust gas of the diesel engine into rich, it is necessary to throttle the amount of air-intake, and/or supply fuel into the exhaust gas by post-injection or exhaust injection. However, the former reduction of the amount of air-intake might lead to deterioration in drivability of a vehicle, and the latter fuel supply might lead to deterioration in fuel consumption. <CIT> refers to a diesel engine operating method. The method involves using an exhaust gas feedback device between the exhaust and induction systems. The feedback device is actuated by a control element activated by an auxiliary force-actuated drive depending on signals from an electronic controller. An engine controller enables weakness/richness control of the diesel engine depending on its operating parameters. A storage catalyser in the exhaust system adsorbs, desorbs and reduces nitrogen oxides. A sensor downstream of the catalyser detects the NOx conc. in the exhaust flow. When a NOx threshold value varying with speed and load is reached the operating mode is changed over from a lambda value of greater than unity to one less than unity. <CIT> is directed to use of an on-board diagnosis apparatus for monitoring nitrogen oxide absorption catalyst regeneration, including examination of reliability-critical components on detection of anomalies. The catalyst loading with nitrogen oxides (NOx) is determined, and compared with a maximum limit. An on-board diagnosis apparatus monitors catalyst activity. On exceeding the limit or detecting irregularity in catalyst activity, in a third stage, reliability of NOx regeneration is examined. Correct function of components relevant to reliability is examined, and/or current operation is examined through observation of predetermined parameters. It is examined whether, meeting predetermined regeneration parameters, an NOx regeneration can be undertaken, if necessary adjusting the required NOx regeneration parameters, and introducing the NOx regeneration given permissibility of such a measure, otherwise repeating the third stage and/or if appropriate signaling a detected functional disturbance of a component relevant to reliability. NOx regeneration is carried out until a predetermined NOx regeneration level is achieved. Normal operational conditions are set and reversion to the start, or premature discontinuation or interruption of the NOx regeneration, in the case that the actual results of the reliability examination of the third stage, if necessary returning to the start or the third stage, and/or if appropriate signaling detection of a functional defect of one of the components relevant to reliability. <CIT> refers to a method of regenerating a NOx purification catalyst of an engine mounted on a vehicle when the vehicle is decelerating. As a result of this exhaust gas purification method, when the vehicle is decelerating, the fuel system rich control can be performed in the regeneration control by in-cylinder fuel injection methods such as multistage injection and post injection without worsening drive comfort, since it is not necessary for the internal combustion engine to generate torque during the deceleration. Consequently, the worsening of drive comfort during the regeneration control period that occurs in the conventional art when performing the regeneration control during the vehicle deceleration is prevented.

An object of the present invention is to provide a diesel engine exhaust gas purification method and an exhaust gas purification system which can purify exhaust gas while suppressing deteriorations in drivability and fuel consumption.

The diesel engine exhaust gas purification method to achieve the above-described object is an exhaust gas purification method for purifying exhaust gas of a diesel engine mounted on a vehicle using an NOx occlusion reduction catalyst, and includes the steps of: when the exhaust gas enters a rich reduction waiting state, opening an EGR valve and closing an intake throttle if a temperature of the NOx occlusion reduction catalyst is at or above a predetermined temperature and the vehicle is decelerating from a speed at or above a predetermined speed; and then when a predefined condition is satisfied closing an exhaust throttle provided downstream of the NOx occlusion reduction catalyst, and supplying a reductant to the NOx occlusion reduction catalyst, wherein the predetermined temperature is <NUM> and the predetermined speed is <NUM>/h.

In the above-described diesel engine exhaust gas purification method, the predefined condition is satisfied when a predetermined time has elapsed after opening of the EGR valve and closing of the intake throttle. Alternatively, the predefined condition is satisfied when a mass air flow value of intake air of the diesel engine becomes equal to or smaller than a reference value after opening of the EGR valve and closing of the intake throttle.

Further, according to the invention the predetermined temperature is <NUM> and the predetermined speed is <NUM>/h.

The exhaust gas purification system of the diesel engine to achieve the above-described object is an exhaust gas purification system including an intake throttle assembled to an intake passage of a diesel engine mounted on a vehicle, an NOx occlusion reduction catalyst installed in an exhaust passage, an EGR valve assembled to an EGR passage communicating from the intake passage to the exhaust passage, and a reductant supply unit configured to supply a reductant to the NOx occlusion reduction catalyst, and includes: an exhaust throttle provided to the exhaust passage on a downstream side of the NOx occlusion reduction catalyst, and a control unit configured to control the intake throttle, EGR valve and exhaust throttle, wherein the control unit, when exhaust gas flowing through the exhaust passage enters a rich reduction waiting state, opens the EGR valve and closes the intake throttle if a temperature of the NOx occlusion reduction catalyst is at or above a predetermined temperature, and the vehicle is decelerating from a speed at or above a predetermined speed, and then when a predefined condition is satisfied closes the exhaust throttle and activates the reductant supply unit, wherein the predetermined temperature is <NUM> and the predetermined speed is <NUM>/h.

According to the diesel engine exhaust gas purification method and the exhaust gas purification system of the present invention, the state of the exhaust gas is changed into rich when the vehicle is decelerating from a speed at or above the predetermined speed, thus suppressing deterioration in drivability. Further, since the amount of the exhaust gas is throttled when the state of the exhaust gas is changed into rich, less fuel is needed to be supplied to change the state of the exhaust gas into rich, thus suppressing deterioration in fuel consumption.

Hereinafter, a description will be given of an embodiment of the present invention with reference to the drawings.

<FIG> shows a diesel engine exhaust gas purification system according to an embodiment of the present invention. In a diesel engine <NUM> equipped with this diesel engine exhaust gas purification system, air A taken from an inlet port <NUM> into an intake passage <NUM> sequentially passes through an air cleaner <NUM> and a mass air flow sensor (MAF sensor) <NUM>, and after the amount of air-intake is regulated with an intake throttle <NUM>, is supplied into each cylinder from an intake manifold <NUM>. Then, after fuel injected through a common rail injection system <NUM> is burned, the air is discharged from an exhaust manifold <NUM> into an exhaust passage <NUM> as exhaust gas G. Then, it passes through an NOx occlusion reduction catalyst <NUM> and is discharged from an exhaust port <NUM> as purified exhaust gas Gc. Further, part of exhaust gas G is diverted into an EGR passage <NUM> as EGR gas Ge, and after being cooled down by an EGR cooler <NUM>, is recirculated to the intake manifold <NUM> via an EGR valve <NUM>.

The NOx occlusion reduction catalyst <NUM> is constructed by a catalyst metal and an NOx occlusion material being supported on a surface of a monolithic honeycomb-cell support formed of γ-alumina, etc. Pt or Pd is used as a catalyst metal. Further, any one of alkali metals such as K, Na, Li, Cs and alkali earth metals such as Ba, Ca, or a plurality of them in combination is/are used as an NOx occlusion material.

In this NOx occlusion reduction catalyst <NUM>, when the exhaust gas G is in the lean state, NO within the exhaust gas G is oxidized into NO<NUM> by the oxidation catalyst, diffused into the catalyst in the form of NO<NUM>-and absorbed by the NOx occlusion material in the form of a nitrate. Then, when the state of the exhaust gas G is changed into rich, NO<NUM>- is released from the NOx occlusion material in the form of NO<NUM>. This released NO<NUM> is reduced to N<NUM> by the action of the oxidation catalyst with the aid of a reductant such as unburned HC contained in the exhaust gas G.

In order to change the state the exhaust gas G into rich, a reductant such as Diesel oil fuel is supplied into the exhaust gas G by a reductant supply unit. Examples of this reductant supply unit include post-injection in fuel injection into the cylinder, a fuel injection nozzle (not shown) provided in the exhaust passage <NUM>, etc. Note that using the latter fuel injection nozzle has the advantage that the problem of fuel dilution of engine oil, which arises in the case of the post-injection, can be avoided. Regulating the amount of fuel supply is performed by regulating the injection amount and injection timing in these reductant supply units.

A catalyst inlet exhaust concentration sensor <NUM> and a catalyst inlet temperature sensor <NUM> are installed on the inlet of the NOx occlusion reduction catalyst <NUM> (near the upstream side), and a catalyst outlet exhaust concentration sensor <NUM> and a catalyst outlet temperature sensor <NUM> are installed on the outlet (near the downstream side). These exhaust concentration sensors <NUM> and <NUM> measure the excess air ratio λ and the NOx concentration of the exhaust gases G and Gc.

The diesel engine exhaust gas purification system of the present invention is configured such that an exhaust throttle <NUM> is provided to the exhaust passage <NUM> on the downstream side of the NOx occlusion reduction catalyst <NUM>, with an ECU (Engine Control Unit) <NUM> serving as a control unit controlling this exhaust throttle <NUM>, the intake throttle <NUM> and the EGR valve <NUM>. Note that the ECU <NUM> also controls the MAF sensor <NUM>, the common rail injection system <NUM>, the exhaust concentration sensors <NUM> and <NUM>, and the catalyst temperature sensors <NUM> and <NUM>, and gathers measurement data thereof. Note that dashed lines shown in <FIG> represent signal transmission paths.

Hereinafter, a description will be given of an exhaust gas purification method using the diesel engine exhaust gas purification system having the above configuration, based on a flow diagram shown in <FIG>.

Initially, the ECU <NUM> determines whether the NOx occlusion reduction catalyst <NUM> is in a rich reduction waiting state (S10). The rich reduction waiting state corresponds to a state in which the absorptive capacity of the NOx occlusion material is nearly saturated. Exemplary methods for the determination include a method which compares an NOx occlusion amount (increase from the previous reduction treatment) or an NOx purification rate calculated from measured values of the exhaust concentration sensors <NUM> and <NUM>, with a predefined threshold, etc..

When it is determined that the rich reduction waiting state has been reached, it is checked whether the temperature of the NOx occlusion reduction catalyst <NUM> is at or above a predetermined temperature, and a vehicle mounting the diesel engine <NUM> is decelerating from a speed at or above a predetermined speed (S20). The temperature of the NOx occlusion reduction catalyst <NUM> can be determined, for example, by calculating it from measured values of the catalyst temperature sensors <NUM> and <NUM>, by measuring it with a thermocouple (not shown) installed near the catalyst, and so on. Further, the predetermined temperature is preferably <NUM>. If the temperature of the NOx occlusion reduction catalyst <NUM> is below <NUM>, reduction action by the reductant significantly comes down.

The vehicle's speed can be calculated from an engine rotation speed, etc. Further, the predetermined speed is preferably <NUM>/h. If the vehicle's speed is below <NUM>/h, there is only a short time from deceleration to stopping, giving insufficient time for reduction reaction after closure of the exhaust throttle <NUM> described below. Further, whether the vehicle is in a decelerating state is determined from the amount of fuel injection of the common rail injection system <NUM> (e.g., injection amount = <NUM>), the accelerator opening (e.g., opening = <NUM>°), etc..

Then, if the temperature of the NOx occlusion reduction catalyst <NUM> is at or above the predetermined temperature, and the vehicle is decelerating from a speed at or above the predetermined speed, the EGR valve <NUM> is opened and the intake throttle <NUM> is closed (S30).

Next, when a predefined condition for closure of the exhaust throttle <NUM> is satisfied (S40), the exhaust throttle <NUM> is closed (S50), and the reductant supply unit is activated to supply the fuel into the exhaust gas G so as to change the state thereof into rich (S60). This reductant injection amount is set such that the excess air ratio λ calculated from the exhaust concentration sensors <NUM> and <NUM> equals to a target value calculated from the air-intake amount. More precisely, the target value of the excess air ratio λ is set by predefining a target value on a trial basis, and referring to a three-dimensional map of "a target value with respect to an engine rotation speed and a fuel flow rate" created from the target value, at the time of control. Then, the amount of injection is determined from the difference between the amount of in-cylinder injection by the common rail injection system <NUM> (an indicated value, or a calculated value from the measured values of the exhaust concentration sensors <NUM> and <NUM> and the measured value of the MAF sensor <NUM>), and a required fuel flow rate calculated from the target value of the excess air ratio λ and the measured value of the MAF sensor <NUM>.

Finally, after a certain period of time has elapsed from the fuel supply, or after the deceleration of the vehicle has ended, the reductant supply unit is stopped and the exhaust throttle <NUM> is opened.

As described above, the state of the exhaust gas G is changed into rich when the vehicle is decelerating from a speed at or above the predetermined speed, thus suppressing deterioration in drivability. Further, because when the state of the exhaust gas G is changed into rich, the EGR valve <NUM> is opened, and the intake throttle <NUM> and the exhaust throttle <NUM> are closed to throttle the exhaust gas amount, less fuel is needed to be supplied from the reductant supply unit, thus suppressing deterioration in fuel consumption.

Moreover, since the temperature of the exhaust gas G is increased and the flow rate thereof is decreased, the time period required for reduction is extended, thus improving the reduction efficiency. In order to ensure that this time period required for reduction be sufficient, the vehicle should be in the course of deceleration from a speed at or above the predetermined speed, as described above. Further, from a mechanical viewpoint, since only the exhaust throttle has to be provided to a conventional engine configuration, the exhaust gas purification system can be realized at low cost.

The predefined condition (S40) for closure of the exhaust throttle <NUM> maybe defined such that it is closed after the elapse of the predetermined time period (such as <NUM> to <NUM> seconds, for example) determined in advance, as well as at the time point when the measured value of the MAF sensor <NUM> exceeds a reference value. This reference value for the MAF sensor <NUM> is set such that the peak value of the exhaust pressure when the exhaust throttle <NUM> is closed falls within a range in which the NOx occlusion reduction catalyst <NUM> itself, the sensors <NUM> to <NUM> in the vicinity thereof and so on will not be damaged. The reference value may be about <NUM> to <NUM> kPa for a common heavy vehicle, although it cannot be uniformly set because it varies depending on the size, specification, etc. of the diesel engine <NUM>.

Note that, if a time lag is provided from the opening of the EGR valve <NUM> and the closing of the intake throttle <NUM>, until the closing of the exhaust throttle <NUM>, rapid increase of the exhaust pressure is suppressed, allowing the exhaust throttle <NUM> to be operable even under a high engine rotation speed, which facilitates implementation of the diesel engine exhaust gas purification method.

Although, in <FIG>, the rich reduction waiting state is initially determined, the step S10 regarding such determination may be omitted. In this case, the processes of the steps S20 to S60 are performed regardless of the rich reduction waiting state whenever the temperature of the NOx occlusion reduction catalyst <NUM> is at or above the predetermined temperature and the vehicle is decelerating from a speed at or above the predetermined speed, thereby preventing the temperature drop of the NOx occlusion reduction catalyst <NUM>, which further improves the reduction efficiency.

During actual driving, acceleration may sometimes be started immediately on deceleration of the vehicle. In such a case, since sufficient time period required for reduction cannot be ensured, it is preferred to open the exhaust throttle <NUM> giving priority on acceleration performance.

In <FIG>, the result of purifications of the exhaust gas of the diesel engine <NUM> is shown, which were respectively performed using the diesel engine exhaust gas purification system (practical example) configured as shown in <FIG>, and an exhaust gas purification system (comparative example) in which the exhaust throttle <NUM> has been removed from the configuration of <FIG>. Note that it is assumed that the NOx occlusion reduction catalyst <NUM> is at or above the predetermined temperature. Temporal changes of these exhaust gas purification systems are described as follows.

Claim 1:
An exhaust gas purification method for purifying exhaust gas of a diesel engine (<NUM>) mounted on a vehicle using a NOx occlusion reduction catalyst (<NUM>), comprising the steps of:
when a NOx occlusion reduction catalyst (<NUM>) has reached a rich reduction waiting state,
opening an EGR valve (<NUM>) and closing an intake throttle (<NUM>) if a temperature of the NOx occlusion reduction catalyst (<NUM>) is at or above a predetermined temperature and the vehicle is decelerating from a speed at or above a predetermined speed; and then when a predefined condition is satisfied by determining whether a predetermined time has elapsed after opening of the EGR valve (<NUM>) and closing of the intake throttle (<NUM>), or by determining whether a mass air flow value of intake air of the diesel engine (<NUM>), which is obtained after opening of the EGR valve (<NUM>) and closing of the intake throttle (<NUM>), has become equal to or smaller than a reference value after opening of the EGR valve (<NUM>) and closing of the intake throttle (<NUM>),
and when it is determined that the predetermined time has elapsed or the mass air flow value has become smaller than reference value,
closing an exhaust throttle (<NUM>) provided downstream of the NOx occlusion reduction catalyst (<NUM>), and supplying a reductant to the NOx occlusion reduction catalyst (<NUM>), wherein the predetermined temperature is <NUM> and the predetermined speed is <NUM>/h.