Exhaust gas cleaning apparatus having particulate collector for use in automotive vehicle

An exhaust gas cleaning apparatus (e.g., for a diesel engine) uses a collector for collecting particulates contained in the exhaust gas and is disposed in an exhaust pipe of the engine. Damages in the collector are detected based on a pressure difference between an inlet and an outlet of the collector. In order to correctly detect the damages based on the pressure difference, an exhaust gas volume flowing through the collector is increased to a target volume by raising exhaust gas temperature. The exhaust gas temperature is raised by injecting fuel into the engine at a time when an engine output is not increased by such fuel injection. The pressure difference, based on which damages in the collector are detected, is measured after the exhaust gas volume has reached the target volume. It is determined that damages in the collector have occurred if the pressure difference is lower than a predetermined value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims benefit of priority of Japanese Patent Application No. 2006-72635 filed on Mar. 16, 2006, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust gas cleaning apparatus having a particulate collector for use in an automotive vehicle.

2. Description of Related Art

Recently, an exhaust gas cleaning apparatus having a collector for collecting particulates exhausted from a diesel engine has been used. The collector is made of ceramics having plural small passages through which exhaust gas from a diesel engine flows. Particulates contained in the exhaust gas adhere to walls separating the small passages, and thereby the particulates are trapped in the collector. As an amount of particulates trapped in the collector becomes large, a pressure loss in the collector increases. The amount of accumulated particulates is estimated based on a pressure difference measured at upstream and downstream ends of the collector.

When the estimated amount of accumulated particulates reaches a predetermined level, the accumulated particulates are burnt to regenerate the collector. More particularly, fuel (unburned hydrocarbon) is supplied to an oxidizing catalyst contained in the collector by means of a post injection (a fuel injection performed after a main injection, not to contribute to generation of power). By oxidizing the hydrocarbon, a temperature in the collector is increased to thereby burn the accumulated particulates.

On the other hand, if the collector is damaged due to dropping-off or melting-down of a downstream portion of the collector, the pressure difference between the upstream end and the downstream end of the collector becomes smaller than a normal pressure difference. Accordingly, such damages in the collector are detected based on the pressure difference. An example of this kind of damage detection device is disclosed in JP-A-2003-155920. In this device, however, it is difficult to correctly detect the damages in the collector based on the pressure difference because changes in the pressure difference are not large enough when an amount of exhaust gas is small.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved exhaust gas cleaner, in which damages in the particulate collector are more surely detected without fail.

An exhaust gas cleaning apparatus is disposed in an exhaust pipe of an internal combustion engine. The exhaust gas cleaning apparatus includes a collector for collecting particulates contained in exhaust gas from an internal combustion engine such as a diesel engine. If a pressure difference between an inlet and an outlet of the collector becomes lower than a predetermined value, it is determined that damages, such as dropping-off or melting-down of a downstream portion of the collector, have occurred. It is difficult, however, to surely detect a decrease in the pressure difference when an exhaust gas volume flowing through the collector is small.

In order to more surely detect changes in the pressure difference, the exhaust gas volume is increased to a target volume. A temperature rise in the exhaust gas, which is necessary to increase the exhaust gas volume to the target volume, is first calculated, and fuel for realizing such temperature rise is injected into the engine at a timing when such fuel injection does not increase an output of the engine (referred to as a post injection).

The pressure difference between the inlet and the outlet of the collector is measured after the exhaust gas volume flowing through the collector has reached the target volume. If the pressure difference is smaller than a predetermined value, it is determined that damages in the collector occurred. Preferably, if the temperature in the exhaust gas is expected to be raised by the post injection beyond a predetermined temperature limit, the post injection is not performed to prevent a catalyst disposed in the collector from being deteriorated.

According to an exemplary embodiment of the present invention, the damages in the collector are more surely detected without fail. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described with reference to accompanying drawings. As shown inFIG. 1, an intake air passage2and an exhaust pipe3through which exhaust gas flows are connected to a diesel engine1. In the exhaust pipe3, a collector4for collecting particulates contained in the exhaust gas is connected. A diesel particulate filter (referred to as a DPF) is contained in the collector4. The DPF is a porous honeycomb filter made of cordierite or silicon carbide. The particulates in the exhaust gas accumulate on surfaces of the DPF, on which oxidizing catalyst made of materials such as platinum or a palladium is held. The oxidizing catalyst help the particulates burn under a predetermined condition at a proper temperature.

A first temperature sensor51is disposed at an upstream end of the collector4to measure a temperature of exhaust gas flowing into the DPF (an inlet temperature), and a second temperature sensor52is disposed at a downstream end of the collector4to measure a temperature of exhaust gas flowing out of the DPF (an outlet temperature). A first branch31branching out from the exhaust pipe3is connected to the upstream end of the collector4, and a second branch32branching out from the exhaust pipe3is connected to the downstream end of the collector4.

A pressure sensor53for detecting a pressure difference between the upstream end and the downstream end of the collector4(simply referred to as a pressure difference Pd) is disposed between the first branch31and the second branch32. An airflow meter54is disposed in the intake air passage2to detect an amount of intake air sucked into the engine1. An accelerator sensor55for measuring a pressing-down amount of an accelerator pedal (not shown) is connected to the accelerator pedal. A rotational speed of the engine1is measured by an engine speed sensor56.

Output signals from the airflow meter54, the engine speed sensor56and the accelerator sensor55are fed to an electronic control unit6(referred to as an ECU). The ECU6is a known microcomputer including a CPU, a ROM, a RAM and an EEPROM. The ECU6performs various functions, such as control of an amount of fuel injected into the engine1, regeneration of the collector4and detection of damages in the collector4, according to programs stored therein.

The damages in the collector4, such as dropping-off or melting-down of a downstream portion of the collector, are detected in the following manner. As shown inFIG. 2A, a pressure difference measured by the pressure sensor53increases as an amount of exhaust gas increases. The amount of exhaust gas is always expressed as an amount in volume throughout this specification. Line (a) inFIG. 2Ashows a lower limit of the pressure difference when the collector4is normal, line (b) shows a permissible lower limit of the pressure difference when the collector4is damaged. Line (c) shows a damage-detection level (Pdd) of the pressure difference. Namely, if the pressure difference (pd) is lower than the damage-detection level (Pdd), it is determined that the collector4is damaged. Though the permissible lower limit is shown by line (b), the damage-detection level (Pdd) is set to a little higher level than the permissible level in a region for detecting damages, considering a safety margin.

In order to accurately detect the damages in the collector4, it is necessary to measure the pressure difference under a condition where a flow amount of the exhaust gas is higher than a certain level. Such a level is referred to as a target flow amount of the exhaust gas VEXtrg (liter/minute). If a present flow amount VEXi (liter/minute) is lower than VEXtrg, the flow amount is forcibly increased by ΔVEX (=VEXtrg−VEXi) to increase the flow amount to the level of VEXtrg. The flow amount is increased by increasing a temperature of the exhaust gas to a temperature level corresponding to the target flow amount VEXtrg.

The target flow amount VEXtrg is calculated according to formula (1) shown inFIG. 2B. In the formula (1), Ga (gram/sec) is a flow amount of the intake air in quantity; Q (gram/sec) is an amount of fuel consumption; T(° C.) is a present temperature of the exhaust gas in the collector4; ΔT(° C.) is an amount of temperature rise in the exhaust gas, required for increasing the flow amount of the exhaust gas by ΔVEX; and P(kPa) is a pressure at an upstream end of the collector4. In the formula (1), the term Ga×22.4/28.8 is a term for converting the flow amount of the intake air Ga in quantity to a flow amount of the intake air in volume; the term Q×0.45×22.4/13.8 is a term for converting the fuel consumption Q in quantity to fuel consumption in flow volume; the term [(T+ΔT)+273]/273 is a temperature adjustment term for the flow amount in volume; the term 101.325/(P+101.325) is a temperature adjustment term for the flow amount in quantity; and 60 at the last is a number for converting a flow amount per second to a flow amount per minute. Formula (2) for calculating the exhaust gas temperature rise ΔT(° C.) shown inFIG. 2Bis derived from formula (1).

The process of detecting damages in the collector4will be further described with reference to a flowchart shown inFIG. 3. At step S101, whether the engine is normally operated is determined. That is, when both of an amount of change per unit time in the flow amount of the exhaust gas and an amount of change per unit time in the pressure difference between the upstream end and the downstream end of the collector4are small, it is determined that the engine is normally or stably operated.

If the engine is normally operated, the process proceeds to step S102, where the present flow amount of the exhaust gas VEXi is calculated. VEXi is calculated by converting the flow amount of the intake air Ga in quantity based on a present temperature T in the collector4and a pressure P at the upstream end of the collector4. The present temperature Tin the collector4is calculated by adding a temperature rise in the collector4due to reaction heat in the collector4to the temperature at the upstream end of the collector4measured by the first temperature sensor51. The temperature rise in the collector4due to reaction heat is proportional to an amount of hydrocarbon fed to the collector4. Accordingly, the temperature rise in the collector can be calculated based on the amount of hydrocarbon fed to the collector, which is in turn estimated from an amount of post injection.

The pressure P at the upstream end of the collector4is calculated by adding the pressure difference detected by the pressure sensor53to the pressure at the downstream end of the collector4. The pressure at the downstream end of the collector4is obtained from a map stored in the ROM contained in the ECU. The map shows the pressure at the downstream end of the collector4corresponding to rotational speed of the engine and the amount of intake air.

Then, at step S103, whether the present amount of exhaust gas VEXi is smaller than the target amount VEXtrg stored in the ROM is determined. If the VEXi is smaller than VEXtrg (i.e., the present amount of the exhaust gas is not large enough for correctly detecting damages in the collector4), the process proceeds to step S104, where the temperature rise ΔT in the collector which is necessary for increasing the present amount of exhaust gas VEXi to the target amount VEXtrg is calculated according to the formula (2) shown inFIG. 2B. The formula (2) is stored in the ROM. Then, at step S105, whether the temperature in the collector is lower than a temperature limit Tlim even after the temperature in the collector is increased by ΔT. In other words, whether (T+ΔT) is lower than the Tlim is determined, i.e., whether the collector temperature does not exceed the temperature limit that is set for protecting the catalyst in the collector). The temperature limit Tlim is set to a temperature at which the catalyst deteriorates or a temperature somewhat lower than that.

If the expected temperature (T+ΔT) is lower than the temperature limit Tlim, the process proceeds to step S106, where an amount of the post injection Qpost that is necessary to increase the collector temperature by ΔT is calculated. The amount of the post injection Qpost is easily calculated from the required temperature rise ΔT because Qpost is proportional to ΔT as shown inFIG. 4. Then, at step S107, the amount of fuel Qpost is injected into the engine by performing the post injection. The post injection is performed when it does not contribute to generation of the engine power, i.e., the fuel is injected during an exhaust stroke of the engine. The temperature in the collector4is increased by the post injection, and thereby the amount of exhaust gas in volume increases to the target volume VEXtrg.

Then, at step S108, whether the pressure difference Pd between the upstream end and the downstream end of the collector4is lower than a damage-detection pressure level Pdd shown inFIG. 2A. The damage-detection pressure level Pdd is stored in the ROM. If Pd is lower than Pdd, the process proceeds to step S109, where a warning lamp mounted on an instrument panel is lit, and a flag for prohibiting regeneration of the collector4is set in the ECU. Then, the process comes to the end.

On the other hand, if it is determined that the engine is not normally operated at step S101, the process stays there until the engine operation becomes normal. This is because the damage-detection is not correctly performed when the engine is not normally operated. If it is determined that the present amount of exhaust gas VEXi is higher than the target amount VEXtrg, the process directly proceeds to step S108because it is not necessary to increase the exhaust gas amount in this situation. If it is determined that the expected temperature (T+ΔT) is higher than the temperature limit Tlim at step S105, the process returns to step S101without performing the post injection to protect the catalyst. If it is determined that Pd is higher than Pdd, i.e., damages are not detected in the collector, the process returned to S101.

With reference to time charts shownFIG. 5, the process of detecting damages in the collector4will be further explained. An example shown inFIG. 5shows the detection process which is performed for a four-cylinder diesel engine having 2000 CC displacement driven at 2200 rpm, driving a vehicle at 60 km/h. The target amount of exhaust gas VEXtrg is set to 4500 liter/mm, which is necessary for correctly detecting that an ability of the collector4for collecting diesel particulates has decreased to a level of 25 percent of an original ability. The damages in the collector4usually occur when a downstream portion of the collector4drops off or melts down by heat.

At time t1, the vehicle is driven at 60 km/h, a flow amount of exhaust gas is 2000 l/m, and a exhaust gas temperature is 200° C. By performing a post injection, the temperature in the collector4is increased by 450-500° C. to reach 650-700° C. A temperature rise of 40-50° C. is attained by the post injection in an amount of 1 mm3/stroke. This means that the exhaust gas temperature rises to 650-700° C. by performing the post injection in an amount of 10-13 mm3/stroke. According to the temperature increase in the collector4, the flow amount of the exhaust gas in volume increases by 2500-2800 l/m, thereby reaching 4500-4800 l/m. In this manner, the damages in the collector4are accurately detected.

Advantages attained in the present invention are summarized below. The temperature rise ΔT required for attaining the target exhaust gas volume VEXtrg flowing through the collector is calculated. Then, the post injection is performed to raise the temperature by ΔT, and the exhaust gas volume reaches the target volume VEXtrg which is necessary to correctly detect the damages in the collector4. If the exhaust gas temperature were raised to a target temperature by simply performing the post injection, the exhaust gas volume attained by the temperature rise would vary according to an initial temperature of the exhaust gas. The exhaust gas volume would be less or more than the amount required for correctly detecting the damages. This means that the post injection would be performed in an excessive amount, resulting in waste of fuel, or in a too small amount, resulting in inability of correct detection of the damages. In addition, the post injection is not performed in the present invention in the case where the expected temperature (T+ΔT) exceeds the temperature limit Tlim. Therefore, the catalyst in the collector is prevented from deteriorating by excessive heat.

The present invention is not limited to the embodiment described above, but it may be variously modified. For example, instead of performing the post injection, injection timing may be delayed to decrease efficiency of the engine and to raise the exhaust gas temperature. It may be possible to inject fuel at an upstream portion of the collector to raise the exhaust gas temperature. Further, instead of performing the post injection, the exhaust gas volume may be increased by various methods, such as by squeezing an EGR (exhaust gas recirculation) valve, by increasing an opening degree of an intake air orifice, by squeezing an opening of a variable turbo nozzle, or by decreasing a ratio of a transmission. The exhaust gas amount can be increased by performing one or more of the above-exemplified methods.