Exhaust emission control system

An exhaust emission control system for a vehicle including a primary engine and a secondary engine having a displacement smaller than that of the primary engine is provided with an exhaust passage having a junction portion at which exhaust gas discharged from the primary engine and exhaust gas discharged from the secondary engine join together, and an exhaust emission purifying device that purifies the exhaust gas joined at the junction portion in the exhaust passage.

INCORPORATION BY REFERENCE

The disclosures of Japanese Patent Applications No. 2003-182952 filed on Jun. 26, 2003, and No. 2003-15370 filed on Jan. 23, 2003 including the specifications, drawings and abstracts are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to an exhaust emission control system in a vehicle including a plurality of engines.

2. Description of Related Art

JP-A-6-93855 (pp.3–4,FIG. 1) discloses a vehicle including two engines, that is, a primary engine and a secondary engine, one or both of which may be operated for driving in accordance with the running state. In the aforementioned vehicle of the publication, exhaust gas discharged from the primary engine is supplied to a turbine of a turbocharger via an exhaust manifold that extends to the rear of the primary engine, and is further supplied to a case that contains an exhaust catalyst via an exhaust pipe. The exhaust gas then flows into a silencer via an exhaust pipe provided downstream of the catalyst for noise muffling, and is finally discharged into atmosphere. The exhaust gas discharged from the secondary engine is supplied to an exhaust pipe provided downstream of the exhaust catalyst via an exhaust manifold that extends to the rear of the secondary engine so as to join the exhaust gas discharged from the primary engine.

In the aforementioned type of the vehicle, however, the exhaust gas from the secondary engine joins the exhaust gas from the primary engine at a point downstream of the exhaust catalyst, and then discharged into atmosphere. That is, the exhaust gas from the secondary engine is discharged into atmosphere without having its harmful components removed.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an exhaust emission control system that removes harmful components of the exhaust gas discharged from an auxiliary engine so as to prevent deterioration in the exhaust emission.

An exhaust emission control system for a vehicle including a primary engine and a secondary engine having a displacement smaller than that of the primary engine includes an exhaust passage having a junction portion at which exhaust gas discharged from the primary engine and exhaust gas discharged from the secondary engine join together, and an exhaust emission purifying device that purifies the exhaust gas joined at the junction portion in the exhaust passage. In this case, the exhaust gas from the primary engine joins the exhaust gas from the secondary engine at a junction portion formed on an exhaust passage so as to be purified in the exhaust gas purifying unit. Accordingly, each exhaust gas from the primary and the secondary engines can be purified only by a single exhaust gas purifying unit.

In this case, it is preferable that the secondary engine is operated to drive the accessories of the vehicle. Therefore, those accessories can be driven in an efficient operation range without being influenced by the operation state of the primary engine.

An exhaust emission control system for a vehicle including a primary engine and a secondary engine having a displacement smaller than that of the primary engine includes an exhaust emission purifying device that purifies exhaust gas discharged from the secondary engine. The exhaust emission purifying device is warmed under heat of exhaust gas discharged from the primary engine. According to an embodiment of the invention, the exhaust gas purifying unit for purifying the exhaust gas discharged from the secondary engine is warmed under the heat of the exhaust gas discharged from the primary engine. Accordingly the exhaust gas purifying unit can be kept in an activated state in spite of a low frequency of operating the secondary engine. This makes it possible to purify the exhaust gas discharged from the secondary engine.

The aforementioned system allows the secondary engine to be operated for driving accessories of the vehicle. In this case, accessories for the vehicle are driven by the secondary engine. Therefore, those accessories can be driven in an efficient operation range without being influenced by the operation state of the primary engine.

The aforementioned system further includes a primary engine exhaust emission purifying device provided between the primary engine and the junction portion so as to purify the exhaust gas discharged from the primary engine. In the aforementioned structure, the primary engine exhaust gas purifying unit provided upstream of the junction portion serves to purify the exhaust gas discharged from the primary engine. This makes it possible to reduce the flow rate of the exhaust gas to be purified by the exhaust gas purifying unit downstream of the junction portion, resulting in reduction in size of the exhaust gas purifying unit.

The aforementioned system further includes a primary engine air/furl ratio detection unit provided between the primary engine exhaust emission purifying device and the junction portion so as to detect an air/fuel ratio of the exhaust gas discharged from the primary engine. In this case, the primary engine air/fuel ratio detection unit is provided upstream of the junction portion. Therefore, the air/fuel ratio of the exhaust gas from the primary engine can be detected under no influence of the exhaust gas from the secondary engine. This makes it possible to detect the air/fuel ratio of the exhaust gas from the primary engine accurately.

In the aforementioned system, the exhaust passage is branched into a plurality of passages between the primary engine air/fuel ratio detection unit and the junction portion, and preferably, at least one of the plurality of passages is connected with the junction portion. The aforementioned structure may reduce the exhaust gas flowing from the primary engine into the exhaust gas purifying unit, restraining decrease in the temperature of the exhaust gas purifying unit owing to the exhaust gas at a low temperature discharged from the primary engine. This makes it possible to prevent decrease in the ratio of purifying the exhaust gas from the secondary engine.

The aforementioned system further includes a first air/fuel ratio detection unit provided between the primary engine and the junction portion for detecting an air/fuel ratio of exhaust gas, a second air/fuel ratio detection unit provided downstream of the exhaust emission purifying device for detecting an air/fuel ratio of the exhaust gas. An air/fuel ratio of air/fuel mixture each admitted into the primary engine and the secondary engine is controlled based on the air/fuel ratio detected by the first air/fuel ratio detection unit and the air/fuel ratio detected by the second air/fuel ratio detection unit, respectively. In this case, the air/fuel ratio of the air/fuel mixture to be admitted by the primary engine and the secondary engine, respectively is determined based on the air/fuel ratio of the exhaust gas detected by the first air/fuel ratio detection unit between the primary engine and the junction portion, and the air/fuel ratio of the exhaust gas detected by the second air/fuel ratio detection unit downstream of the exhaust gas purification unit, respectively. This makes it possible to control the air/fuel ratio of the exhaust gas from the secondary engine requiring no direct detection thereof.

The aforementioned system further includes a third air/fuel ratio detection unit provided between the primary engine and the junction portion for detecting an air/fuel ratio of exhaust gas, a fourth air/fuel ratio detection unit provided between the secondary engine and the junction portion for detecting an air/fuel ratio of exhaust gas. An air/fuel ratio of air/fuel mixture admitted into the primary engine is controlled based on the air/fuel ratio detected by the third air/fuel ratio detection unit, and an air/fuel ratio of air/fuel mixture admitted into the secondary engine is controlled based on the air/fuel ratio detected by the fourth air/fuel ratio detection unit. In this case, the third air/fuel ratio detection unit is provided between the primary engine and the junction portion, and the fourth air/fuel ratio detection unit is provided between the secondary engine and the junction portion. Therefore, the air/fuel ratio of the exhaust gas from the primary engine can be detected by the third air/fuel ratio detection unit, and the air/fuel ratio of the exhaust gas from the secondary engine can be detected by the fourth air/fuel ratio detection unit under no influence thereof. This makes it possible to control each air/fuel ratio of the exhaust gas from the primary and the secondary engine, respectively with high accuracy.

The aforementioned system further includes a fifth air/fuel ratio detection unit provided between the primary engine and the junction portion for detecting an air/fuel ratio of exhaust gas, a sixth air/fuel ratio detection unit provided between the junction portion and the exhaust emission purifying device for detecting an air/fuel ratio of exhaust gas. An air/fuel ratio of air/fuel mixture admitted into the primary engine is controlled based on the air/fuel ratio detected by the fifth air/fuel ratio detection unit, and an air/fuel ratio of air/fuel mixture admitted into the secondary engine is controlled based on the air/fuel ratio detected by the sixth air/fuel ratio detection unit. In this case, the fifth air/fuel ratio detection unit is provided between the primary engine and the junction portion, and the sixth air/fuel ratio detection unit is provided between the junction portion and the exhaust gas purification unit. As a result, the exhaust gas from the secondary engine serves to activate the sixth air/fuel ratio detection unit at an earlier stage. This makes it possible to start the air/fuel ratio control at an earlier stage.

In the aforementioned system, an activated state of the exhaust emission purifying device is determined, and the secondary engine is started when it is determined that the exhaust emission purifying device is not in the activated state. If it is determined that the exhaust gas purification unit is not in an activated state, the secondary engine is started. Then the exhaust gas from the secondary engine serves to raise the temperature of the exhaust gas purification unit so as to be activated.

Preferably the aforementioned system further includes a temperature detection unit that detects a temperature of an catalyst of the exhaust emission purifying device, wherein the secondary engine is stopped when the detected temperature of the catalyst is equal to or higher than a predetermined value. When the temperature of the exhaust catalyst is equal to or higher than a predetermined value, the secondary engine is stopped by cutting the fuel supply thereto. The intake air into the secondary engine is supplied to the exhaust catalyst under pressure. The aforementioned intake air serves to cool the exhaust catalyst, preventing the exhaust catalyst from being excessively heated. This makes it possible to prevent deterioration in the purification capability of the exhaust catalyst.

In the aforementioned system, the exhaust emission purifying device is formed as an NOxabsorbing type catalyst, and an air/fuel ratio of air/fuel mixture admitted into the secondary engine is controlled into a rich state with respect to a theoretical air/fuel ratio when quantity of NOxabsorbed in the NOxabsorbing type catalyst becomes equal to or larger than a predetermined value. When the quantity of NOxabsorbed by the NOxabsorbing catalyst becomes equal to or larger than a predetermined value, the air/fuel ratio of the exhaust gas from the secondary engine that is not related to the operation for driving the vehicle is brought into a fuel rich state. This makes it possible to perform reduction in the NOxabsorbing catalyst, thus reducing the exhaust emission without deteriorating drivability of the vehicle.

The aforementioned system further includes a first valve position detection unit that detects an exhaust valve position of the primary engine, a second valve position detection unit that detects an intake valve position of the primary engine, a third valve position detection unit that detects an exhaust valve position of the secondary engine, and a fourth valve position detection unit that detects an intake valve position of the secondary engine. In this system, an operation for stopping a drive of the primary engine is inhibited when it is determined that the intake valve and the exhaust valve of the primary engine are opened based on output values detected by the first and the second valve position detection units. An operation for stopping a drive of the secondary engine is inhibited when it is determined that the intake valve and the exhaust valve of the secondary engine are opened based on output values detected by the third and the fourth valve position detection units. If it is determined that the intake valve and the exhaust valve of the primary engine are opened, stop of operation of the primary engine is prohibited. If it is determined that the intake valve and the exhaust valve of the secondary engine are opened, stop of operation of the secondary engine is prohibited. This makes it possible to prevent the exhaust gas from the engine in operation from reverse flowing into the intake pipe of the stopped engine.

Preferably the aforementioned system further includes a first intake air quantity detection unit that detects a flow rate of intake air admitted into the primary engine, and a second intake air quantity detection unit that detects a flow rate of intake air admitted into the secondary engine. In this system, quantity of fuel injected into the primary engine is controlled in accordance with the flow rate of intake air detected by the first intake air quantity detection unit, and quantity of fuel injected into the secondary engine is controlled in accordance with the flow rate of intake air detected by the second intake air quantity detection unit. Each intake air quantity admitted by the primary engine and the secondary engine is independently detected, resulting in highly accurate detection of the intake air quantity. Each of the fuel injection quantity supplied to the primary engine and the secondary engine is controlled in accordance with the detected intake air quantity. This makes it possible to control the air/fuel ratio of the exhaust gas from the primary engine and the secondary engine, respectively with high accuracy.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention will be described hereinafter referring to the drawings. The same elements in the respective embodiments are designated as the same reference numerals, and each description of those elements, thus, will be omitted.

First Embodiment

A whole structure of an exhaust emission control system1A according to a first embodiment will be described referring toFIG. 1.

The exhaust emission control system1A includes a main engine5and a sub-engine6having lower displacement than that of the main engine5.

The exhaust emission control system1A further includes an exhaust catalyst4that purifies exhaust gas discharged from both the main engine5and the sub-engine6, air/fuel ratio sensors45a,45bprovided upstream and downstream of the exhaust catalyst4respectively, and an electronic control unit9that controls each air/fuel ratio of the air/fuel mixture in the main engine5and the sub-engine6based on the respective output values of the air/fuel ratio sensor45a,45b.

A main starter20is provided at a joint portion where the main engine5is connected with a transmission10. A crankshaft (drive shaft)5aof the main engine5is provided with an electromagnetic clutch30that connects/disconnects transfer of the driving force of the main engine5. The driving force output from the main engine5is transferred to a main crank pulley5bvia the electromagnetic clutch30.

The sub-engine6functions as a gasoline engine of 100 to 150 cc displacement, having thermal efficiency improved by elongating the stroke or increasing the expansion ratio, for example. The sub-engine6is provided with a sub starter22via a gear (not shown). A crankshaft (drive shaft)6bof the sub-engine6is connected with a planetary gear unit7that amplifies the driving force of the sub-engine6via a one-way clutch60.

The one-way clutch60serves to transfer the driving force output from the sub-engine6to the planetary gear unit7while cutting the transfer of the driving force of the main engine5via a belt B1and the planetary gear unit7.

The planetary gear unit7is formed of a sun gear7a, a planetary gear7bprovided around the sun gear7a, a ring gear7cprovided on the outer periphery of the planetary gear7b, and a planetary carrier that holds the planetary gear7b.

The crank shaft6bis connected with the sun gear7a. The driving force of the sub-engine6that has been transferred to the sun gear7ais amplified in accordance with a gear ratio (reduction gear ratio) of the planetary gear unit7, and output from the planetary carrier7d.

The gear ratio of the planetary gear unit7can be expressed as a following equation (1).

gear⁢⁢ratio=1+ρρ(1)
where the number of teeth of the sun gear7ais represented as Zs, the number of teeth of the ring gear7cis represented as Zi, and

In this embodiment, the gear ratio is set to 6. Therefore, the planetary gear unit7amplifies the driving force of the sub-engine6six times, and reduces the rotational speed to ⅙. The aforementioned gear ratio is determined based on the driving force of the sub-engine6, the maximum starting torque of the main engine5, and the like.

The driving force output from the planetary carrier7dis transferred to a sub crank pulley6cformed as a double pulley provided with a large-diameter pulley6dand a small-diameter pulley6ehaving the smaller diameter than that of the large-diameter pulley6din this embodiment.

The belt B1is set around the small diameter pulley6eand the main crank pulley5bso as to transfer the driving force therebetween.

In this embodiment, although the pulley ratio between the small diameter pulley6eand the main crank pulley5bis set to 2.5, it is not limited thereto. The aforementioned pulley ratio makes it possible to further amplify the driving force to be transferred to the main engine5by 2.5 times.

Accordingly, the driving force of the sub-engine6may be amplified by 15 times and the rotational speed thereof may be reduced to 1/15 resulting from the combination of the planetary gear unit7with the pulley ratio between the main crank pulley5band the small diameter pulley6e.

A belt B2is set around a pulley8aattached to accessories (for a vehicle) such as a water pump, an alternator, a power steering pump, and a compressor for air conditioning unit. As the large diameter pulley6drotates, the pulley8aattached to the accessories is rotated, thus driving those accessories.

In the main engine5, intake air flow from an air cleaner200is throttled by an electronically controlled throttle valve230provided in an intake pipe220. The intake air then flows through an intake manifold240so as to be admitted into each cylinder of the main engine5. The flow rate of the admitted air from the air cleaner200is detected by an air flow meter210provided between the air cleaner200and the throttle valve230. The air flow meter210serves as a first intake air quantity detecting unit. The air flow meter210in this embodiment may be formed as a hot wire air flow sensor.

The intake manifold240is provided with an injector244through which the fuel supplied under pressure thereinto is injected. In each of the cylinders, the intake air and the fuel are mixed and burned. The resultant exhaust gas is then discharged into an exhaust manifold250.

The exhaust manifold250is connected with an exhaust pipe260, downstream of which is provided with the exhaust catalyst4. The exhaust catalyst4is provided with a catalytic temperature sensor51for detecting the catalytic temperature of the exhaust catalyst4. That is, the catalytic temperature sensor51serves as the temperature detection unit.

The exhaust catalyst4is a three-way catalyst that oxidizes hydrocarbon (HC) and carbon monoxide (CO) contained in the exhaust gas and reduces nitrogen oxides (NOX) at the same time such that the harmful gas component of the exhaust gas is purified into harmless carbon dioxide (CO2), steam (H2O), and nitrogen (N2). That is, the exhaust catalyst4serves as the exhaust emission purifying device.

The exhaust catalyst4becomes effective when its temperature is increased to be equal to or higher than a specific activated temperature. Therefore, the function of the exhaust catalyst4for removing the harmful components cannot be fully performed until the catalytic temperature reaches the specific activated temperature.

In the sub-engine6, the intake air flow from an air cleaner202is throttled by an electronically controlled throttle valve232provided in an intake pipe222, and then is admitted into each cylinder of the sub-engine6. The flow rate of the intake air from the air cleaner202is detected by an air flow meter212provided between the air cleaner202and the throttle valve232. That is, the air flow meter212serves as the second intake air quantity detection unit. The air flow meter212in this embodiment may be formed as a hot wire air flow sensor.

An intake pipe222is provided with an injector246through which the fuel supplied under pressure thereinto is injected. In each cylinder of the sub-engine6, the intake air and the fuel are mixed and burned. The resultant exhaust gas is discharged into an exhaust pipe262.

The exhaust pipe262is joined to the exhaust pipe260of the main engine5at a joint portion264upstream of the exhaust catalyst4. The exhaust gas discharged from the sub-engine6to the exhaust pipe262joins the exhaust gas of the main engine5, which will be purified by the exhaust catalyst4.

The electronic control unit (ECU)9includes an ECU for controlling the main engine5(hereinafter referred to as a main ECU)40and an ECU for controlling the sub-engine6(hereinafter referred to as a sub ECU)50.

The main ECU40is connected with a crank position sensor41that detects a crank position of the main engine5, an accelerator opening sensor42that detects an opening an accelerator pedal, an intake air temperature sensor43that detects a temperature of intake air admitted into the main engine5, a water temperature sensor44that detects a cooling water temperature, an air flow meter210that detects a flow rate of intake air admitted into the main engine5, and an intake air pressure sensor45that detects a pressure of the intake air.

Each position of the intake valve and the exhaust valve of the main engine5can be obtained based on the crank position of the main engine5detected by the crank position sensor41. That is, the crank position sensor41serves as both a first and a second valve position detection units.

The main ECU40includes a microprocessor that executes various computations, a ROM that stores programs to allow the microprocessor to executes various types of processing, a RAM that stores various data such as computed results, and a back-up RAM in which the stored data are maintained by a battery of 12 V (not shown).

The main ECU40contains an air/fuel ratio control section40afor controlling the air/fuel ratio of the air/fuel mixture by adjusting the quantity of the fuel injected through the injector244. The main ECU40controls the fuel injection quantity and the air/fuel ratio.

The main ECU40also contains an ignition timing calculating section40bthat calculates the timing for igniting the air/fuel mixture using a spark plug, an electromagnetic clutch control section40cthat controls engagement/disengagement of the electromagnetic clutch30, a load control section40dthat adjusts the load of the accessory8, and an engine stop control section40e that inhibits the main engine5or the sub-engine6from stopping. The main ECU40serves to calculate the ignition timing, and control an operation of the electromagnetic clutch, load, and stop operation of the engine.

The sub ECU50is connected with the accelerator opening sensor that detects the opening degree of the accelerator pedal, a catalytic temperature sensor (temperature detecting unit)51that detects a catalytic temperature of the exhaust catalyst4, the crank position sensor52that detects the crank position of the sub-engine6, the air flow meter212that detects the flow rate of air admitted into the sub-engine6, and the like.

Each position of the intake valve and the exhaust valve of the sub-engine6is obtained based on the crank position of the sub-engine6that has been detected by the crank position sensor52. The crank position sensor52, therefore, serves as both the third and the fourth valve position detection units.

The sub ECU50is formed of the same components (microprocessor and the like) as those of the main ECU40.

The sub ECU50contains a sub-engine starting section50athat stops or re-starts the sub-engine6based on the output value from the catalytic temperature sensor51. The sub ECU50serves to sop and start the secondary engine. The sub ECU50calculates an optimum value of the fuel injection quantity or the ignition timing based on the output value from various sensors such that operations of the sub-engine6are controlled based on the calculated optimum values. The sub ECU50serves to control the fuel injection quantity and the air/fuel ratio.

The main ECU40and the sub ECU50are connected via a communication line48such that data are exchanged therebetween.

The air/fuel ratio feedback control for the main engine5will be briefly described referring toFIGS. 2A to 2C. The air/fuel ratio feedback control for the sub-engine6is identical to that for the main engine5. The description of such control, therefore, will be omitted.

Under the air/fuel ratio feedback control, the air/fuel ratio of the air/fuel mixture is adjusted into a theoretical air/fuel ratio so as to reduce the exhaust emission. The aforementioned control is performed by executing the program stored in the ROM of the main ECU40based on the detection results from the various sensors connected with the main ECU40.

Each of the air/fuel ratio sensors45a,45bis formed as an O2sensor (hereinafter referred to as the O2sensor45a, and O2sensor45b). The O2sensor45ais interposed between the main engine5and the joint portion264, and the O2sensor45bis provided downstream of the exhaust catalyst4. Each of those O2sensors45a,45bis characterized in that the output value changes in accordance with the concentration of oxygen contained in the exhaust gas. This makes it possible to determine whether the air/fuel ratio of the air/fuel mixture burned in the main engine5is in the fuel rich state or in the fuel lean state with respect to the theoretical air/fuel ratio based on the output value from the O2sensors45a,45bin terms of ON/OFF. A whole-area air/fuel ratio sensor (linear air/fuel ratio sensor) that is capable of linearly detecting the air/fuel ratio of the air/fuel mixture burned in the main engine5may be employed in place of the O2sensors45a,45b.

The ECU40executes the air/fuel ratio feedback control or the correction control to be described below based on the output values from the aforementioned various sensors so as to determine the fuel injection quantity TAU. Then the fuel is injected from the injector244by the quantity as the determined value TAU.

The fuel injection quantity TAU for operating the main engine5is determined using the following equation:
TAU=βTAUP×EFTOTAL+(1)

The TAUP represents the basic fuel injection quantity defined by the intake air quantity and the engine speed. The obtained TAUP is corrected in accordance with the engine operation state so as to determine the final value representing the fuel injection quantity TAU. The basic fuel injection quantity TAUP may be obtained based on the intake air pressure and the engine speed, or based on the accelerator opening degree and the engine speed.

The term EFTOTAL represents the total value that reflects the air/fuel ratio. The EFTOTAL serves to correct the basic fuel injection quantity TAU so as to adjust the air/fuel ratio into the target air/fuel ratio, such as the intake air temperature correction value based on the intake air temperature. The air/fuel ratio feedback control is executed using the EFTOTAL. The terms α and β serves as other correction elements like the warm-up fuel increase correction value immediately after starting, or the acceleration increase correction value upon acceleration. The EFTOTAL may be obtained by adding the air/fuel ratio feedback correction coefficient FAF to an air/fuel ratio learned value KG. In the aforementioned case, the above equation (1) may be expressed in the following equation (2).
TAU=α×TAUP×(FAF+KG)+β  (2)

The air/fuel ratio feedback correction coefficient FAF is used for correcting the feedback correction so as to control the air/fuel ratio derived from the oxygen concentration within the exhaust gas detected by the O2sensors45a,45binto the target value. For example, referring toFIGS. 2A and 2B, if the air/fuel ratio detected by the O2sensors45a,45brepresents the fuel rich state with respect to the theoretical air/fuel ratio, the feedback correction coefficient FAF is set to the value to gradually decrease the fuel injection quantity. If the air/fuel ratio detected by the O2sensors45a,45brepresentative of the fuel rich state has been changed to represent the fuel lean state, the feedback correction coefficient FAF is set to the value to increase the fuel injection quantity for improving the response in a skip manner.

Meanwhile if the air/fuel ratio detected by the O2sensors45a,45brepresents the fuel lean state with respect to the theoretical air/fuel ratio, the feedback correction coefficient FAF is set to the value to gradually increase the fuel injection quantity. If the air/fuel ratio detected by the O2sensors45a,45brepresentative of the fuel lean state has been changed to represent the fuel rich state, the feedback correction coefficient FAF is set to the value to decrease the fuel injection quantity for improving the response in a skip manner. In this way, the air/fuel ratio feedback correction coefficient FAF serves to keep the air/fuel ratio always at the theoretical air/fuel ratio value.

The delay time DT1or DT2may be considered for the air/fuel ratio feedback correction coefficient FAF so as to cope with the delay in detecting operation performed by the O2sensor45aor45bas shown inFIG. 2C.

The air/fuel ratio learned value KG is a correction value that serves to reflect the individual difference or the change over time in the respective injector244and O2sensors45a,45bon the air/fuel ratio. The aforementioned air/fuel ratio learned value KG is derived from the air/fuel ratio feedback correction coefficient FAFAV that has been obtained by averaging the air/fuel ratio feedback correction coefficients FAF.

If the average value of the air/fuel ratio feedback correction coefficients FAFAV takes a value that represents the fuel lean state, the air/fuel ratio learned value KG is increased so as to increase the fuel injection quantity TAU. Meanwhile if the average value of the FAFAV takes a value that represents the fuel rich state, the air/fuel ratio learned value KG is decreased so as to decrease the fuel injection quantity TAU.

Under the air/fuel ratio feedback control using the air/fuel ratio feedback correction coefficient FAF, the air/fuel ratio can be corrected into the theoretical air/fuel ratio without using the aforementioned air/fuel ratio learned value KG. However, the air/fuel ratio learned value KG makes it possible to correct the air/fuel ratio into the theoretical value at an earlier stage with higher accuracy not only in the open loop control but also the feedback control by considering the individual difference or the change over time in the respective elements of the engines. The air/fuel ratio learned value KG is stored in the back-up RAM of the main ECU40after learning and then read when it is required.

The aforementioned air/fuel ratio feedback correction coefficient FAF or the air/fuel ratio learned value KG are calculated and updated by the program stored in the ROM of the main ECU40. Based on those values FAF and KG calculated in the main ECU40, the fuel injection quantity TAU is determined. The fuel is then injected by the determined quantity TAU.

Referring to the flowchart ofFIGS. 3A and 3B, operations of the exhaust emission control system1A will be described. The flowchart represents the air/fuel ratio feedback control executed in the exhaust emission control system1A.

First in step S100, a flow rate of intake air admitted into the main engine5is detected by the air flow meter210, and the detected intake air quantity is sent to the main ECU40. A flow rate of intake air admitted into the sub-engine6is detected by the air flow meter212, and the detected intake air quantity is sent to the sub ECU50.

In step S102, each fuel injection quantity TAU for the main engine5and the sub-engine6is calculated using the above-described equation (2) based on the detected flow rate of intake air admitted into the main engine5and the sub-engine6, respectively, which have been read in step S100.

In step S104, the injectors244and246are opened for a period to be taken for injecting the fuel by the fuel injection quantity TAU for the main engine5and the sub-engine6, respectively, which have been calculated in step S102. Then the intake air and the injected fuel are mixed and admitted into the respective cylinders of both engines so as to be burned therein. The resultant exhaust gas is discharged into the exhaust pipes260,262.

In step S106, an output signal corresponding to the air/fuel ratio of the air/fuel mixture determined based on the oxygen concentration of the exhaust gas is read from the O2sensor45aprovided upstream of the exhaust catalyst4and the O2sensor45bprovided downstream of the exhaust catalyst4, respectively.

In step S108, it is determined whether the output of the O2sensor45aupstream of the catalyst represents that the exhaust gas is in the fuel rich state. If NO is obtained in step S108, that is, it is determined that the output from the O2sensor45adoes not indicate the fuel rich state, the process proceeds to step S110. Meanwhile, if YES is obtained in step S108, that is, it is determined that the output from the O2sensor45aindicates the fuel rich state, the process proceeds to step S118.

In step S110, it is further determined whether the air/fuel ratio learned value KG obtained based on the output signal of the O2sensor45bdownstream of the catalyst indicates deviation of the air/fuel ratio to the fuel rich state. If NO is obtained in step S110, the process proceeds to step S112where the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel rich side. Then the routine ends. If YES is obtained in step S110, the process proceeds to step S114.

In step S114, the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel lean side.

In step S116, the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the main engine5into the fuel rich side. Then the routine ends.

If YES is obtained in step S108, the process proceeds to step S118where it is determined whether the air/fuel ratio learned value KG indicates deviation of the air/fuel ratio to the fuel lean side based on the output signal of the O2sensor45bdownstream of the catalyst. If NO is obtained in step S118, the process proceeds to step S124where the air/fuel ratio feedback coefficient FAF is set to the value to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel lean side. The routine then ends. Meanwhile, if YES is obtained in step S118, the process proceeds to step S120.

In step S120, the air/fuel ratio feedback correction coefficient TAF is set to the value to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel rich side.

Then in step S122, the air/fuel ratio feedback correction coefficient FAF is set to the value to gradually decrease the fuel injection quantity for controlling the fuel injection quantity for the main engine5into the fuel lean side. The routine then ends.

In this embodiment, the O2sensors45a,45bare provided upstream and downstream of the exhaust catalyst4so as to execute the air/fuel ratio feedback control for the main engine5and the sub-engine6, respectively. The aforementioned air/fuel ratio control of the main engine5and the sub-engine6makes it possible to reduce the exhaust emission.

Second Embodiment

Referring toFIG. 4, the structure of an exhaust emission control system1B according to a second embodiment will be described. The same elements of the exhaust emission control system1B as those of the exhaust emission control system1A according to the first embodiment will be designated with the same reference numerals.

The exhaust emission control system1B of this embodiment is the same as the exhaust emission control system1A of the first embodiment except that an O2sensor45cis provided between the sub-engine6and the junction portion264in addition to the O2sensor45abetween the main engine5and the junction portion264, and the O2sensor45bdownstream of the exhaust catalyst4.

The air/fuel ratio feedback control executed in the exhaust emission control system1B will be described referring to a flowchart ofFIGS. 5A to 5C.

In step S200, the intake air quantity admitted into the main engine5is read from the air flow meter210to the main ECU40, and the intake air quantity admitted into the sub-engine6is read from the air flow meter212to the sub ECU50, respectively.

In step S202, each of the fuel injection quantity TAU for the main engine5and the sub-engine6is calculated using the equation (2) based on each value of the intake air quantity admitted by the main engine5and the sub-engine6, which has been read in step S200.

In step S204, the injectors244and246are opened for a period to be taken for injecting the fuel by the fuel injection quantity TAU for the main engine5and the sub-engine6, which have been calculated in step S202. Then the fuel is injected into the main engine5and the sub-engine6, respectively. The intake air is mixed with the injected fuel, and the resultant air/fuel mixture is burned in the respective cylinders. The exhaust gas is then discharged into the exhaust pipes260and262.

In step S206, signals output from the O2sensor45aprovided in the exhaust pipe260, the O2sensor45cprovided in the exhaust pipe262, and the O2sensor45bprovided downstream of the exhaust catalyst4each corresponding to the air/fuel ratio of the air/fuel mixture based on the oxygen concentration of the exhaust gas output signals are read.

In step S208, it is determined whether the output of the O2sensor45aprovided in the exhaust pipe260represents the fuel rich state. If NO is obtained, that is, it is determined that the output does not represent the fuel-rich state, the process proceeds to step S210. Meanwhile if NO is obtained in step S208, that is, it is determined that the output represents the fuel rich state, the process proceeds to step S218.

In step S210, it is determined whether the output of the O2sensor45cprovided in the exhaust pipe262represents the fuel rich state. If NO is obtained in step S210, the process proceeds to step S212where the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel rich side. The process then proceeds to step S226. Meanwhile, if YES is obtained in step S210, the process proceeds to step S214.

In step S214, the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel lean side.

The process proceeds to step S216where the air/fuel ratio correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the main engine5into the fuel rich side. The process then proceeds to step S226.

If YES is obtained in step S208, the process proceeds to step S218where it is determined whether the output of the O2sensor45cprovided in the exhaust pipe262represents the fuel lean state. If NO is obtained in step S218, the process proceeds to step S224where the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel lean side. The process then proceeds to step S226. Meanwhile, if YES is obtained in step S218, the process proceeds to step S220.

In step S220, the air/fuel ratio correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel rich state.

In step S222, the air/fuel ratio correction coefficient FAF is set to the value so as to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the main engine5into the fuel lean side. The process then proceeds to step S226.

In step S226, the air/fuel ratio learned value KG is obtained based on the output signal of the O2sensor45bdownstream of the catalyst.

In step S228, it is determined whether the air/fuel ratio learned value KG obtained in step S226represents deviation of the air/fuel ratio into the fuel-lean side. If NO is obtained in step S228, the process proceeds to step S230where the lean skip amount of the air/fuel ratio feedback correction coefficient FAF for the sub-engine6is increased. The routine then ends. Meanwhile if YES is obtained in step S228, the process proceeds to step S232.

In step S232, the rich skip amount of the air/fuel ratio feedback correction coefficient FAF for the sub-engine6is increased. The routine then ends.

In this embodiment, the O2sensor45adetects the air/fuel ratio of the air/fuel mixture in the main engine5, and the O2sensor45c detects the air/fuel ratio of the air/fuel mixture in the sub-engine6independently. This makes it possible to control each air/fuel ratio of the air/fuel mixture in the main engine5and the sub-engine6with higher accuracy.

Third Embodiment

The structure of an exhaust emission control system1C according to a third embodiment will be described referring toFIG. 6. InFIG. 6, the same elements of the exhaust emission control system1C as those of the exhaust emission control system1A according to the first embodiment will be designated with the same reference numerals.

The third embodiment is the same as the first embodiment except that an O2sensor45dis provided between the junction portion264and the exhaust catalyst4in place of the O2sensor45bprovided downstream of the exhaust catalyst4.

The air/fuel ratio feedback control executed in the exhaust gas emission control system1C will be described referring to the flowchart ofFIGS. 7A and 7B.

In step S300, a flow rate of intake air admitted into the main engine5is detected by the air flow meter210, and the detected result is read by the main ECU40. A flow rate of intake air admitted into the sub-engine6is detected by the air flow meter212, and the detected result is read by the sub ECU50.

In step S302, each fuel injection quantity TAU for the main engine5and the sub-engine6is calculated using the equation (2) based on each quantity of the intake air admitted into the main engine5and the sub-engine6, which has been read in step S300.

Then in step S304, the injectors244and246are opened for a period to be taken for injecting the fuel injection quantity TAU for the main engine5and the sub-engine6, which have been calculated in step S302. Then the fuel is injected into the main engine5and the sub-engine6, respectively. The intake air is mixed with the injected fuel, and the resultant air/fuel mixture is burned in the respective cylinders. The exhaust gas is then discharged into the exhaust pipes260and262.

In step S306, each output signal from the O2sensor45aupstream of the junction portion264, and the O2sensor45ddownstream of the junction portion264is read. The output signal corresponds to the air/fuel ratio of the air/fuel mixture based on the oxygen concentration of the exhaust gas.

In step S308, it is determined whether the output of the O2sensor45aupstream of the junction portion264represents the fuel-rich state. If NO is obtained in step S308, that is, it is determined that the output of the O2sensor45adoes not represent the fuel rich state, the process proceeds to step S310. If YES is obtained in step S308, that is, it is determined that the output of the O2sensor45arepresents the fuel rich state, the process proceeds to step S318.

In step S310, it is determined whether the output of the O2sensor45ddownstream of the junction portion264represents the fuel rich state. If NO is obtained in step S310, the process proceeds to step S312where the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel rich state. The routine then ends. If YES is obtained in step S310, the process proceeds to step S314.

In step S314, the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel lean side.

Then in step S316, the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the main engine5into the fuel rich side. The routine then ends.

Meanwhile if YES is obtained in step S308, the process proceeds to step S318where it is determined whether the output of the O2sensor45ddownstream of the junction portion264represents the fuel lean state. If NO is obtained in step S318, the process proceeds to step S324where the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel lean side. The routine then ends. If YES is obtained in step S318, the process proceeds to step S320.

In step S320, the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually increase the fuel injection quantity for controlling the fuel injection quantity TAU for the sub-engine6into the fuel rich side.

In step S322, the air/fuel ratio feedback correction coefficient FAF is set to the value so as to gradually decrease the fuel injection quantity for controlling the fuel injection quantity TAU for the main engine5into the fuel lean side. The routine then ends. In this embodiment, the exhaust gas discharged from the sub-engine6serves to heat the O2sensor45dto reach the activated temperature at an earlier stage so as to start the air/fuel ratio control at an earlier timing. This makes it possible to reduce the exhaust emission.

Fourth Embodiment

A structure of an exhaust emission control system1D according to a fourth embodiment will be described referring toFIG. 8. InFIG. 8, the same elements as those of the exhaust emission control system1A according to the first embodiment will be designated with the same reference numerals.

The fourth embodiment is the same as the first embodiment except that the crankshaft of the sub-engine6is not provided with the one-way clutch60.

A routine for activating the catalyst in the exhaust emission control system1D by operating the sub-engine6will be described referring toFIG. 9.

First in step S400, it is determined whether the main engine5is required to be re-started. For example, the requirement for re-starting the main engine5is determined upon detection of the depression force applied to the accelerator pedal.

If NO is obtained in step S400, that is, it is determined that re-start of the main engine5is not required, this routine ends. Meanwhile, if YES is obtained in step S400, that is, re-start of the main engine5is required, the process proceeds to step S402.

In step S402, the sub-engine6is cranked by a sub starter motor22, and supplied with the fuel and ignited for starting.

In step S404, the exhaust catalyst4is heated in the exhaust gas discharged from the sub-engine6. For this, the ignition timing in the sub-engine6is retarded so as to be operated in the fuel lean state at the air/fuel ratio of 16, for example.

In step S406, the main engine5is cranked by a main starter motor20, and supplied with the fuel and ignited for starting.

In step S408, it is determined whether the temperature of the exhaust catalyst4is equal to or higher than a predetermined temperature (for example, 250° C.). If NO is obtained, that is, the temperature of the catalyst4is lower than the predetermined temperature, the routine ends without changing the operation state of the sub-engine6. Meanwhile if YES is obtained, that is, the temperature of the catalyst4is equal to or higher than the predetermined temperature, the process proceeds to step S410.

In step S410, the air/fuel ratio in the sub-engine6is controlled to the theoretical air/fuel ratio, and the ignition timing that has been retarded is returned into the normal state.

The air/fuel ratio in the sub-engine6that is not operated for driving the vehicle is brought into the fuel lean state, and the ignition timing for the sub-engine6is retarded so as to activate the exhaust catalyst4at an earlier stage without deteriorating drivability. In the above-described structure, the exhaust catalyst4may be heated by increasing the quantity of the intake air to the sub-engine6so as to increase the quantity of the exhaust gas discharged from the sub-engine6.

In steps402and404, the sub-engine6is operated while the main engine5is stopped. If the intake valve and the exhaust valve are opened in the aforementioned state, the exhaust gas from the sub-engine6may flow in the reverse direction into the intake pipe of the main engine5. This may cause fluctuation in the combustion upon re-start of the main engine5to the greater degree.

If it is determined that the intake valve and the exhaust valve of the main engine5are opened, the stopping operation of the main engine5is inhibited. In the aforementioned structure, as at least one of the intake valve and the exhaust valve is closed while stop of the main engine5, the exhaust gas discharged from the sub-engine6does not flow in reverse into the main engine5.

The structure in which the sub-engine6is stopped is the same as the one as described above.

In this embodiment, if it is determined that both the intake and exhaust valves of the main engine5and the sub-engine6are opened, each stopping operation of the main engine5and the sub-engine6is inhibited. Alternatively if the main engine5is stopped and the sub-engine6is operated, or the main engine5is operated and the sub-engine6is stopped, the electronically controlled throttle valve230may be fully closed so as to prevent reverse flow of the exhaust gas.

The control routine for preventing excessive heating of the catalyst executed in the exhaust emission control system1D will be described referring toFIG. 10.

First in step S500, it is determined whether the temperature of the exhaust catalyst4is equal to or higher than a predetermined temperature of 650° C., for example. If NO is obtained in step S500, that is, the catalytic temperature is lower than the predetermined temperature, the routine ends. If YES is obtained in step S500, that is, it is determined that the catalytic temperature is equal to or higher than the predetermined temperature, the process proceeds to step S502.

In step S502, the electromagnetic clutch30for the main engine5is engaged such that the power generated in the main engine5is transferred to the sub-engine6via the belt B1. The sub-engine6is then cranked.

In step S504, the fuel supply to the sub-engine6is interrupted so as to be stopped. Accordingly, the sub-engine6is cranked by the power generated by the main engine5in the state where the fuel supply thereto is interrupted. In the sub-engine6, the intake air is supplied to the exhaust pipe262under pressure. The air sent by the sub-engine6under pressure serves to decrease the temperature of the exhaust catalyst4.

In step S506, it is determined whether the temperature of the exhaust catalyst4is equal to or lower than a predetermined temperature of 500° C., for example. If NO is obtained in step S506, that is, it is determined that the catalytic temperature is higher than the predetermined temperature, the routine ends. If YES is obtained in step S506, that is, it is determined that the catalytic temperature is equal to or lower than the predetermined temperature, the process proceeds to step S508.

In step S508, the sub-engine6is supplied with the fuel and ignited for start-up.

In step S510, the electromagnetic clutch30for the main engine5is disengaged such that the power transfer from the main engine5to the sub-engine6is interrupted. The routine for preventing excessive heating of the catalyst ends.

When the temperature of the exhaust catalyst4reaches the predetermined temperature or higher, the sub-engine6serves to send air under pressure to the exhaust catalyst4in order to prevent excessive heating of the catalyst.

In this embodiment, the lean burn engine capable of burning the fuel in a fuel lean state (at lean air/fuel ratio) is used as the main engine5, and a lean NOXcatalyst is used as the exhaust catalyst4. The lean NOXcatalyst may be formed as a lean NOXabsorption type catalyst (LNC) which absorbs NOXto be discharged in the fuel lean state, and reduces the absorbed NOXin the fuel rich state or at the stoichiometric air/fuel ratio.

Because of limitation in the capacity of the lean NOXabsorption type catalyst, the engine operation in the fuel lean state cannot be continued for an extended period. Then the air/fuel ratio is temporarily increased into the fuel rich state to release the absorbed NOXso as to be reduced.

The routine for reducing the exhaust catalyst (hereinafter referred to as the lean NOXcatalyst)4executed in the exhaust emission control system1D will be described referring toFIG. 11.

In step S600, it is determined whether the temperature of the lean NOXcatalyst4is within a predetermined range between 250° C. and 450° C., for example. If NO is obtained in step S600, that is, it is determined that the catalytic temperature is not within the predetermined range, the routine ends. If YES is obtained in step S600, that is, the catalytic temperature is within the predetermined range, the process proceeds to step S602.

In step S602, it is determined whether the condition for the request of reducing the lean NOXcatalyst4is established. For example, it is determined whether the total value of the fuel injection quantity is equal to or greater than a predetermined value. If NO is obtained in step S602, that is, it is determined that the condition is not established, the routine ends. If YES is obtained in step S602, that is, it is determined that the condition is established, the process proceeds to step S604.

In step S604, the air/fuel ratio of the sub-engine6is set to the value so as to be brought into the fuel rich state at, for example, 11.

Then in step S606, it is determined whether the process for reducing the lean NOXcatalyst4has been completed. The determination is made based on the total value of the time period taken for operating the engine in a fuel rich state with respect to the predetermined value. If NO is obtained in step S606, that is, it is determined that the reducing process has not been completed, the routine ends. If YES is obtained in step S606, that is, it is determined that the reducing process has been completed, the process further proceeds to step S608.

In step S608, the air/fuel ratio of the sub-engine6is returned from the fuel rich state to the predetermined value of 14.7, for example. The routine then ends.

As aforementioned, the air/fuel ratio of the sub-engine6that is not related to the operation for driving the vehicle is brought into the fuel rich state so as to reduce the NOXcatalyst4. This makes it possible to reduce the exhaust emission without deteriorating the drivability.

Fifth Embodiment

A structure of an exhaust emission control system1E according to a fifth embodiment will be described referring toFIG. 12. InFIG. 12, the same elements as those of the exhaust emission control system1B according to the second embodiment will be designated with the same reference numerals.

This embodiment is the same as the second embodiment except that an exhaust catalyst for the main engine (main engine exhaust emission purifying device)4band a main engine O2sensor for the main engine45e(main engine air/fuel ratio detection unit) are provided upstream of the junction portion264, that is, between the main engine5and the junction portion264. In this embodiment, the O2sensor45bas shown in the second embodiment is not provided downstream of the exhaust catalyst4.

In the aforementioned case, the exhaust gas discharged from the main engine5is purified in the exhaust catalyst4b. The air/fuel ratio of the air/fuel mixture in the main engine5is controlled based on the air/fuel ratio of the exhaust gas detected by the O2sensor45aand the main engine O2sensor45e.

The control routine of the air/fuel ratio of the sub-engine6executed in the exhaust emission control system1E will be described referring to the flowchart ofFIG. 13.

In step S700, it is determined whether the temperature of the exhaust catalyst4is equal to or higher than a predetermined temperature. If YES is obtained in step S700, that is, it is determined that the catalytic temperature is equal to or higher than the predetermined temperature, the process proceeds to step S704.

If NO is obtained in step S700, that is, it is determined that the catalytic temperature is lower than the predetermined temperature, the process proceeds to step S702. In step S702, the air/fuel ratio in the sub-engine6is controlled into the fuel lean state, and the ignition timing is retarded. This may allow the sub-engine6to increase the temperature of the exhaust catalyst4under the heat of the exhaust gas discharged therefrom. The air/fuel ratio control into the fuel lean state and retard of the ignition timing are continued until the temperature of the exhaust catalyst4is increased to reach the predetermined temperature or higher. The temperature of the exhaust catalyst4may be increased by increasing quantity of the intake air into the sub-engine6so as to increase the flow rate of the exhaust gas discharged therefrom.

If YES is obtained in step S700, that is, it is determined that the catalytic temperature exceeds the predetermined temperature, the process proceeds to step S704. In step S704, it is determined whether an engine load of the sub-engine6is lower than a predetermined value. If YES is obtained in step S704, that is, it is determined that the engine load is lower than the predetermined value, the process proceeds to step S706where the air/fuel ratio in the sub-engine6is controlled into the fuel lean state. Meanwhile if NO is obtained in step S704, that is, it is determined that the engine load of the sub-engine6is equal to or higher than the predetermined value, the process proceeds to step S708where the air/fuel ratio in the sub-engine6is controlled into the theoretical or stoichiometric air/fuel ratio.

As aforementioned, the sub-engine6is operated in the fuel lean state at the low load. This makes it possible to improve the specific fuel consumption of the sub-engine6.

In this embodiment, as the O2sensor45aand the main engine O2sensor45eare provided upstream of the junction portion264, the air/fuel ratio of the exhaust gas discharged from the main engine5can be detected under no influence of the exhaust gas discharged from the sub-engine6. Even if the sub-engine6is operated at the air/fuel ratio (fuel lean state, for example) which is different from the one at which the main engine5is operated, the air/fuel ratio in the main engine5can be independently detected. This makes it possible to accurately control the air/fuel ratio in the main engine5independently.

According to this embodiment, the exhaust gas discharged from the main engine5can be purified by the main engine exhaust catalyst4b, which is provided upstream of the junction portion264. Accordingly the flow rate of the exhaust gas to be purified by the exhaust catalyst4downstream of the junction portion264can be reduced. This may reduce the size of the exhaust catalyst4.

Sixth Embodiment

A structure of an exhaust emission control system1F according to a sixth embodiment will be described referring toFIG. 14. InFIG. 14, the same elements as those of the exhaust emission control system1E according to the fifth embodiment will be designated with the same reference numerals.

A main engine5A of this embodiment is formed as a V-type engine including an exhaust manifolds250R and250L provided with its right bank5R and the left bank5L, respectively. The exhaust manifolds250R and250L are connected with exhaust pipes260R and260L, respectively. The exhaust pipes260R and260L join together at a joint portion265. The joined exhaust pipe261is further branched into exhaust pipes263R and263L at a branch portion266downstream of the joint portion265.

The exhaust pipe260R is provided with the main engine exhaust catalyst (main engine exhaust gas purifying unit)4b. The O2sensor45ais provided upstream of the main engine exhaust catalyst4b. The exhaust pipe260L is provided with the main engine exhaust catalyst4b, upstream of which is provided with the O2sensor45a.

The exhaust pipe261includes the main engine exhaust catalyst4b. The main engine O2sensor45eis provided downstream of the main engine exhaust catalyst4b.

The exhaust gas discharged from the main engine5A is purified by the main engine exhaust catalysts4bprovided in the exhaust pipes206R,260L, and261, respectively. The air/fuel ratio of the right bank5R is controlled based on the air/fuel ratio of the exhaust gas to be detected by the O2sensor45aand the main engine O2sensor45eboth provided in the exhaust pipe260R. The air/fuel ratio of the left bank5L is controlled based on the air/fuel ratio of the exhaust gas to be detected by the O2sensor45aand the main engine O2sensor45eboth provided in the exhaust pipe260L.

The exhaust pipe262provided in the sub-engine6is connected with the exhaust pipe263L of the main engine5A at the joint portion264. The exhaust gas discharged from the sub-engine6to the exhaust pipe262is merged with a part of the exhaust gas from the main engine5A at the junction portion264. The exhaust gas is then purified by the exhaust catalyst4downstream of the junction portion. Other features or characteristics are the same as those described in the fifth embodiment, and the description will be omitted.

In this embodiment, the exhaust gas discharged from the main engine5A is branched at the branch portion266into the exhaust pipes263R and263L. Accordingly the flow rate of the exhaust gas flowing from the main engine5A into the exhaust catalyst4is reduced. This may prevent the exhaust gas at a low temperature discharged from the main engine5A from decreasing the temperature of the exhaust catalyst4, avoiding decrease in the ratio of purifying the exhaust gas discharged from the sub-engine6.

Although various embodiments of the invention have been described, it is to be understood that the invention is not limited to the aforementioned embodiments and can be modified into various forms without departing from the sprit of the invention. For example, the number of the O2sensors or arrangement thereof is not limited to the aforementioned embodiments.

According to the invention, the exhaust emission control system includes an exhaust passage having a junction portion at which each exhaust gas discharged from the main engine and the sub-engine join together, and an exhaust emission purifying unit that purifies the exhaust gas joined at the junction portion provided in the exhaust passage. The aforementioned structure allows the harmful component of the exhaust gas discharged from the sub-engine to be removed, thus restraining deterioration in the exhaust emission.