Low temperature cooling device for internal combustion engine

A low temperature cooling device applied to an internal combustion engine includes an EGR device, a low temperature coolant circuit, a prediction unit predicting whether an EGR cooler falls into a state where a cooling performance falls short according to at least one of an operating state of an internal combustion engine and an outside air environment while a control that dehumidifies an EGR gas by cooling the EGR gas in the EGR cooler is performed, and a control unit performing at least one of a first increase control that increases a flow rate of a coolant flowing into the EGR cooler, a second increase control that increases an air rate of a radiator fan cooling a radiator, and an inhibition control that inhibits the EGR gas from flowing back when the prediction unit predicts that the EGR cooler falls into the state where the cooling performance falls short.

CROSS REFERENCE TO RELATED APPLICATION

This application is the U.S. national phase of International Application No. PCT/JP2016/002028 filed on Apr. 14, 2016 which designated the U.S. and claims priority to Japanese Patent Application No. 2015-94786 filed on May 7, 2015, the entire contents of each of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a low temperature cooling device applied to an internal combustion engine which includes a low temperature coolant circuit circulating a coolant through an intercooler, an EGR cooler, and a radiator.

BACKGROUND ART

An internal combustion engine installed to a vehicle is equipped with an EGR device which returns a part of an exhaust gas to an intake passage as an EGR gas with an aim of enhancing fuel efficiency and reducing knocking and an emission of an exhaust gas. However, when an EGR gas with a high water content is returned to the intake passage, condensate water may be produced when an intake gas, which is a mixture of the EGR gas and intake air (fresh air), is cooled in an intercooler. The condensate water possibly gives rise to a corrosion of a metal part.

A technique of restricting production of condensate water in the intercooler is described in, for example, Patent Literature 1. According to the disclosed technique, a coolant circuit circulating a coolant through an intercooler and an EGR cooler is provided, and condensate water is forcedly produced by cooling an EGR gas in the EGR cooler. The condensate water is collected into a trap portion to dehumidify the EGR gas. The EGR gas is then heated in an EGR heater to lower a relative humidity and returned to an intake passage.

PRIOR ART LITERATURES

Patent Literature

SUMMARY OF INVENTION

Inventors of the present disclosure discovered a new problem as follows while conducting a study on a system including a low temperature coolant circuit circulating a coolant through an intercooler, an EGR cooler, and a radiator.

That is, when an internal combustion engine is decelerating, a received heat amount of the coolant in the EGR cooler decreases as a flow rate of an EGR gas decreases, and a released heat amount of the coolant in the radiator decreases as a vehicle speed decreases. However, the coolant which has passed through the EGR cooler arrives the radiator after a delay comparable to a volume from the EGR cooler to the radiator. Hence, the coolant which passes through the radiator after a released heat amount in the radiator has decreased is the relatively warm coolant which had passed through the EGR cooler before a received heat amount in the EGR cooler has decreased. Accordingly, a temperature of the coolant which has passed through the radiator rises temporarily after a deceleration is started, and then a temperature of the coolant flowing into the EGR cooler rises temporarily, too. The EGR cooler thus may possibly fall into a state where cooling performance temporarily falls short. Hence, an EGR gas may not be cooled sufficiently in the EGR cooler during a transient operation to accelerate the once-decelerated internal combustion engine, in which case the EGR gas may not be dehumidified sufficiently and condensate water may possibly be produced when an intake gas is cooled in the intercooler.

The present disclosure has an object to provide a low temperature cooling device applied to an internal combustion engine which restricts production of condensate water during a transient operation.

According to an aspect of the present disclosure, the low temperature cooling device applied to the internal combustion engine includes an EGR device returning a part of an exhaust gas of an internal combustion engine to an intake passage as an EGR gas, a low temperature coolant circuit circulating a coolant through an intercooler cooling an intake gas of the internal combustion engine, an EGR cooler cooling the EGR gas, and a radiator, a prediction unit predicting whether the EGR cooler falls into a state where a cooling performance falls short according to at least one of an operating state of the internal combustion engine and an outside air environment while a control that dehumidifies the EGR gas by cooling the EGR gas in the EGR cooler is performed, and a control unit performing at least one of a first increase control that increases a flow rate of the coolant flowing into the EGR cooler, a second increase control that increases an air rate of a radiator fan cooling the radiator, and an inhibition control that inhibits the EGR gas from flowing back when the prediction unit predicts that the EGR cooler falls into the state where the cooling performance falls short.

In the above description, a delay of the coolant caused after passing through the EGR cooler and before arriving the radiator can be shorter by performing the first increase control to increase the flow rate of the coolant flowing into the EGR cooler is performed when the EGR cooler is predicted to fall into the state where the cooling performance falls short while a control that dehumidifies the EGR gas by cooling the EGR gas in the EGR cooler in a decelerating operation of the internal combustion engine. Owing to the configuration as above, a rise in inflow coolant temperature of the EGR cooler can be restricted by restricting a rise in outflow coolant temperature of the radiator after a deceleration is started. Hence, an inconvenience that the EGR cooler falls into the state where the cooling performance falls short can be avoided. Thus, the EGR gas can be sufficiently dehumidified by cooling the EGR gas sufficiently in the EGR cooler during the transient operation where the engine is accelerated after being decelerated. Consequently, production of condensate water in the intercooler can be restricted.

Further, a decrease in released heat amount in the radiator can be restricted by performing the second increase control to increase the air rate of the radiator fan when the EGR cooler is predicted to fall into the state where the cooling performance falls short while a control that dehumidifies the EGR gas by cooling the EGR gas in the EGR cooler is performed. Owing to the configuration as above, a rise in inflow coolant temperature of the EGR cooler can be restricted by restricting a rise in outflow coolant temperature of the radiator after a deceleration is started. Hence, an inconvenience that the EGR cooler falls into the state where the cooling performance falls short can be avoided. Thus, the EGR gas can be sufficiently dehumidified by cooling the EGR gas sufficiently in the EGR cooler during the transient operation where the engine is accelerated after being decelerated. Consequently, production of condensate water in the intercooler can be restricted.

Furthermore, the inhibition control that inhibits the EGR gas from flowing back is performed when the EGR cooler is predicted to fall into the state where the cooling performance falls short in the decelerating operation of the internal combustion engine. Consequently, even when the EGR cooler falls into the state where the cooling performance falls short during the transient operation where the engine is accelerated after being decelerated, production of condensate water in an intercooler can be restricted.

DESCRIPTION OF EMBODIMENTS

Hereinafter, concrete embodiments for carrying out the present disclosure will be described.

First Embodiment

A first embodiment of the present disclosure will be described according toFIG. 1throughFIG. 6.

A schematic configuration of an engine control system will be described first according toFIG. 1.

An air cleaner13is provided uppermost-stream of an intake pipe12(intake passage) of an internal combustion engine11(hereinafter, referred to simply as an engine11). An air flow meter14detecting an amount of intake air is provided downstream of the air cleaner13. Meanwhile, a catalyst16, such as a three-way catalyst purifying CO, HC, and NOx in an exhaust gas, is provided to an exhaust pipe15of the engine11.

The engine11is equipped with a supercharger17supercharging an intake gas into the engine11. The supercharger17is an exhaust turbine driving type. The intake gas can be intake air (fresh air) alone or a mixed gas of intake air and an EGR gas. The supercharger17includes an exhaust turbine18provided upstream of the catalyst16in the exhaust pipe15, and a compressor19provided downstream of the air flow meter14in the intake pipe12. The exhaust turbine18and the compressor19are coupled to rotate as one unit. Hence, the supercharger17supercharges the intake gas into the engine11using the compressor19which is rotationally driven by rotationally driving the exhaust turbine18with kinetic energy of an exhaust gas.

A throttle valve20is provided downstream of the compressor19in the intake pipe12and an opening degree of the throttle valve20is regulated by a motor (not shown). An intercooler21cooling the intake gas and a surge tank (not shown) are integrally provided downstream of the throttle valve20. The intercooler21is a water cooling type. The intercooler21uses a coolant and cools the intake gas which has been supercharged by the supercharger17and therefore become hot. Consequently, in-cylinder charging efficiency of the intake gas can be increased, which can in turn enhance an output of the engine11.

A fuel injection valve (not shown) performing in-cylinder injection or intake port injection is attached to each cylinder of the engine11. Sparking plugs (not shown) for respective cylinders are attached to a cylinder head of the engine11to ignite an air-fuel mixture in the respective cylinders with a spark discharge by the corresponding sparking plugs.

An EGR device22that is an LPL (Low Pressure Loop) type and returns a part of an exhaust gas from the exhaust pipe15to the intake pipe12as an EGR gas is equipped to the engine11. The EGR device22includes an EGR pipe23connected between a downstream side of the exhaust turbine18in the exhaust pipe15(for example, downstream of the catalyst16) and an upstream side of the compressor19in the intake pipe12. An EGR valve24regulating a flow rate of the EGR gas is provided to the EGR pipe23. A flow rate of the EGR gas referred to herein means a flow rate of the EGR gas which has passed through the EGR pipe23(EGR device22). An EGR cooler25cooling the EGR gas, a separator26separating and collecting condensate water in the EGR gas which has passed through the EGR cooler25, and an EGR heater27heating the EGR gas which has passed through the separator26are also provided to the EGR pipe23. The EGR cooler25is a water cooling type.

The EGR cooler25forcedly produces condensate water by cooling the EGR gas with the coolant in a low water temperature system as the coolant of the intercooler21. The separator26separates and collects the condensate water in the EGR gas. The condensate water collected at the separator26is discharged to the exhaust pipe15through a pipe28. The EGR heater27heats the EGR gas with the coolant in a high water temperature system as a coolant of the engine11to lower a relative humidity of the EGR gas.

An outside air temperature sensor29detecting an outside air temperature (To) and an outside air humidity sensor30detecting an outside air humidity are provided to a place less susceptible to heat of the engine11, such as upstream of the intake pipe12or an outside of the intake pipe12. An intake gas temperature sensor31detecting a temperature of the intake gas which has passed through the intercooler21is provided downstream of the intercooler21(for example, the surge tank or an intake manifold). An EGR gas temperature sensor32detecting a temperature of the EGR gas which has passed through the EGR cooler25is provided downstream of the EGR cooler25(for example, between the EGR cooler25and the separator26or between the separator26and the EGR heater27).

Outputs of the foregoing sensors are inputted into an electronic control unit (ECU)33. The ECU33is chiefly formed of a micro-computer and controls an amount of fuel injection, ignition timing, a throttle opening degree (amount of intake air), and so on according to an engine operating state by running various engine control programs pre-stored in an internal ROM (storage medium).

The ECU33calculates a target EGR ratio according to an engine operating state (for example, an engine speed and an engine load), and controls an opening degree of the EGR valve24to reach the target EGR ratio.

A schematic configuration of a low temperature cooling system will now be described according toFIG. 2.

An intercooler channel37to circulate the coolant through the intercooler21and an EGR cooler channel38to circulate the coolant through the EGR cooler25are connected in parallel between an inlet channel35connected to an inlet port of a radiator (low water temperature radiator)34and an outlet channel36connected to an outlet port of the low water temperature radiator34. A low temperature coolant circuit39cooling the coolant in the low water temperature radiator34circulate through the intercooler21and the EGR cooler25is thus formed. In the present embodiment, a low temperature cooling device for the engine11has the EGR device22, the low temperature coolant circuit39, and the ECU33.

The low temperature coolant circuit39includes a water pump40provided to the outlet channel36, and a flow rate control valve41located at a branch point of the intercooler channel37and the EGR cooler channel38. The water pump40is an electric driving type. The flow rate control valve41is driven on a motor or the like and regulates a flow rate ratio between the coolant flowing to the intercooler21and the coolant flowing into the EGR cooler25according to an operating position of a valve body. The flow rate control valve41has a self-return function by which the valve body is pushed in a direction to an initial position (a position at which a flow rate proportion of the coolant flowing into the intercooler21reaches a maximum) to return the valve body to the initial position when energization is stopped for the flow rate proportion of the coolant flowing into the intercooler21to reach a maximum (for example, 100%).

The intercooler channel37is provided with a first coolant temperature sensor42detecting an outflow coolant temperature Toic of the intercooler21(a temperature of the coolant which has passed through the intercooler21). The EGR cooler channel38is provided with a second coolant temperature sensor43detecting an outflow coolant temperature Toec of the EGR cooler25(a temperature of the coolant which has passed through the EGR cooler25). An electric radiator fan44is provided near the low water temperature radiator34to cool the radiator34.

In a case where the EGR gas with a high water content is returned to the intake pipe12, condensate water may be produced when the intake gas, which as a mixture of the EGR gas and intake air (fresh air), is cooled in the intercooler21. The condensate water possibly gives rise to a corrosion of a metal part.

In order to eliminate such an inconvenience, the EGR gas is dehumidified by forcedly producing condensate water by cooling the EGR gas in the EGR cooler25and separating and collecting the condensate water in the EGR gas by the separator26. The EGR gas is then heated in the EGR heater27to lower a relative humidity and returned to the intake pipe12.

As is shown inFIG. 3, when the engine11is decelerating, a received heat amount (Qiec) of the coolant in the EGR cooler25decreases as a flow rate of the EGR gas decreases, and a released heat amount (Qor) of the coolant in the radiator34decreases as a vehicle speed (V) decreases. However, the coolant which has passed through the EGR cooler25arrives the radiator34after a delay comparable to a volume from the EGR cooler25to the radiator34. Hence, the coolant which passes through the radiator34after a released heat amount in the radiator34has decreased is the relatively warm coolant which had passed through the EGR cooler25before a received heat amount in the EGR cooler25has decreased. Accordingly, unless any preventive measure is taken, an outflow coolant temperature (Tor) of the radiator34(a temperature of the coolant which has passed through the radiator34) rises temporarily after a deceleration is started, and then an inflow coolant temperature (Tiec) of the EGR cooler25(a temperature of the coolant flowing into the EGR cooler25) rises temporarily, too. The EGR cooler25may thus possibly fall into a state where cooling performance temporarily falls short. Hence, the EGR gas may not be cooled sufficiently in the EGR cooler25during a transient operation where the engine11is accelerated after being decelerated, in which case the EGR gas may not be dehumidified sufficiently and condensate water may possibly be produced when the intake gas is cooled in the intercooler21. InFIG. 3, Qir represents a received heat amount of the coolant in the radiator34.

To eliminate such an inconvenience, a control as follows is performed in the first embodiment by performing a condensate water restriction control routine ofFIG. 5by using the ECU33. While a control that dehumidifies the EGR gas by cooling the EGR gas in the EGR cooler25is performed, whether the EGR cooler25falls into a state where cooling performance falls short is predicted according to at least one of an engine operating state and an outside air environment. When the EGR cooler25is predicted to fall into a state where cooling performance falls short, a first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and a second increase control that increases an air rate of a radiator fan44are performed.

By performing the first increase control to increase a flow rate of the coolant flowing into the EGR cooler25according to a prediction that the EGR cooler25falls into a state where cooling performance falls short when the engine11is decelerating, a delay of the coolant caused after passing through the EGR cooler25and before arriving the radiator34can be shorter. Owing to the configuration as above, as is shown inFIG. 4, an amount of the relatively warm coolant which has passed through the EGR cooler25before a received heat amount in the EGR cooler25decreases and passes through the radiator34after a released heat amount in the radiator34decreases can be reduced. Hence, a rise in inflow coolant temperature of the EGR cooler25can be restricted by restricting a rise in outflow coolant temperature of the radiator34after a deceleration is started. Consequently, an inconvenience that the EGR cooler25falls into a state where cooling performance falls short can be avoided.

By performing the second increase control to increase the air rate of the radiator fan44according to a prediction that the EGR cooler25falls into a state where cooling performance falls short when the engine11is decelerating, a decrease in released heat amount in the radiator34can be restricted. Owing to the configuration as above, too, a rise in inflow coolant temperature of the EGR cooler25can be restricted by restricting a rise in outflow coolant temperature of the radiator34after a deceleration is started. Consequently, an inconvenience that the EGR cooler25falls into a state where cooling performance falls short can be avoided.

The following will describe a processing content of the condensate water restriction control routine ofFIG. 5performed by the ECU33in the first embodiment.

The condensate water restriction control routine depicted inFIG. 5is performed repetitively in predetermined cycles while a control that dehumidifies the EGR gas by cooling the EGR gas in the EGR cooler25is performed. When the routine is started, an engine operating state (for example, an engine load, an engine speed, a flow rate of the EGR gas, and an outflow coolant temperature of the EGR cooler25) and an outside air environment (for example, an outside air temperature and an outside air humidity) are obtained first in101.

Subsequently, advancement is made to102, in which whether the EGR cooler25falls into a state where cooling performance falls short is predicted by predicting whether the inflow coolant temperature of the EGR cooler25exceeds an allowable upper limit value (Luti). Processing in102functions as a prediction unit.

Herein, the inflow coolant temperature of the EGR cooler25is predicted according to, for example, one or two or more of an engine load, an engine speed, a flow rate of the EGR gas, and an outflow coolant temperature of the EGR cooler25. The allowable upper limit value (for example, an upper limit value of the inflow coolant temperature of the EGR cooler25necessary to ensure required cooling performance of the EGR cooler25) is set according to an outside air temperature and an outside air humidity. Whether the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value is predicted according to whether a prediction value of the inflow coolant temperature of the EGR cooler25is greater than the allowable upper limit value.

When it is predicted in102that the inflow coolant temperature of the EGR cooler25does not exceed the allowable upper limit value, the EGR cooler25is predicted not to fall into a state where cooling performance falls short. Hence, the routine is ended without performing processing in103.

Meanwhile, when it is predicted in102that the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value, the EGR cooler25is predicted to fall into a state where cooling performance falls short. Hence, advancement is made to103, in which the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44are performed. The processing in103functions as a control unit.

In the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25, a flow rate of the coolant flowing into the EGR cooler25is increased by controlling the flow rate control valve41to increase a flow rate proportion of the coolant flowing into the EGR cooler25or by controlling the water pump40to increase a discharge amount of the water pump40. A flow rate of the coolant flowing into the EGR cooler25may be controlled according to an engine operating state and an outside air environment.

In the second increase control that increases the air rate of the radiator fan44, the air rate of the radiator fan44is increased by switching the radiator fan44at rest to a rotation state or controlling the radiator fan44to rotate at a higher speed. The air rate of the radiator fan44may be controlled according to an engine operating state and an outside air environment.

In the first embodiment described above, the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44are performed when the EGR cooler25is predicted to fall into a state where cooling performance falls short while a control that dehumidifies the EGR gas by cooling the EGR gas in the EGR cooler25is performed. Owing to the configuration as above, a rise in inflow coolant temperature of the EGR cooler25can be restricted by restricting a rise in outflow coolant temperature of the radiator34after a deceleration is started. Hence, an inconvenience that the EGR cooler25falls into a state where cooling performance falls short can be avoided. Thus, the EGR gas can be sufficiently dehumidified by cooling the EGR gas sufficiently in the EGR cooler25during the transient operation where the engine11is accelerated after being decelerated. Consequently, production of condensate water in the intercooler21can be restricted.

In the first embodiment, the EGR cooler25is predicted to fall into a state where cooling performance falls short when it is predicted that the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value. Hence, whether the EGR cooler25falls into a state where cooling performance falls short can be predicted with accuracy.

In the first embodiment, the inflow coolant temperature of the EGR cooler25is predicted according to operating state parameters, such as an engine load, an engine speed, a flow rate of the EGR gas, and an outflow coolant temperature of the EGR cooler25. Because the inflow coolant temperature of the EGR cooler25varies with the foregoing operating state parameters, the inflow coolant temperature of the EGR cooler25can be predicted with accuracy by using the foregoing operating state parameters. Also, the allowable upper limit value is set according to an outside air temperature and an outside air humidity. The allowable upper limit value of the inflow coolant temperature of the EGR cooler25varies with required cooling performance of the EGR cooler25, which varies with an outside air temperature and an outside air humidity. Hence, by using an outside air temperature and an outside air humidity, the allowable upper limit value can be set appropriately.

In the first embodiment, the separator26separating and collecting condensate water in the EGR gas which has passed through the EGR cooler25, and the EGR heater27heating the EGR gas which has passed through the separator26are provided. Hence, an effect of restricting production of condensate water in the intercooler21can be enhanced.

In the first embodiment, both of the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44are performed when it is predicted that the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value (that is, when the EGR cooler25is predicted to fall into a state where cooling performance falls short). However, the present disclosure is not limited to the configuration as above and only either one of the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44may be performed.

Second Embodiment

A second embodiment of the present disclosure will now be described usingFIG. 6. A description will be omitted or given simply for portions substantially same as counterparts of the first embodiment above and the following will chiefly describe a portion different from the first embodiment above.

In the second embodiment, an EGR cooler25is predicted to fall into a state where cooling performance falls short when an engine11reaches a predetermined decelerating operation by performing a condensate water restriction control routine ofFIG. 6by using the ECU33.

In the condensate water restriction control routine ofFIG. 6, an engine operating state and an outside air environment are obtained first in201.

Subsequently, advancement is made to202, in which whether the EGR cooler25falls into a state where cooling performance falls short (that is, whether the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value) is predicted by determining whether the engine11is in the predetermined decelerating operation. Whether the engine11is in the predetermined decelerating operation is determined according to, for example, whether an amount of decrease in engine load or engine speed per predetermined time is at or above a predetermined value.

When it is determined in202that the engine11is not in the predetermined decelerating operation, the EGR cooler25is predicted not to fall into a state where cooling performance falls short. Hence, the routine is ended without performing processing in203.

Meanwhile, when it is determined in202that the engine11is in the predetermined decelerating operation, the EGR cooler25is predicted to fall into a state where cooling performance falls short. Hence, advancement is made to203, in which the first increase control that increases a flow rate of a coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44are performed. Processing in202functions as a prediction unit and processing in203functions as a control unit.

In the second embodiment described above, the EGR cooler25is predicted to fall into a state where cooling performance falls short when the engine11reaches the predetermined decelerating operation. Hence, whether the EGR cooler25falls into a state where cooling performance falls short can be predicted easily.

In the second embodiment, both of the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44are performed when the engine11reaches the predetermined decelerating operation (that is, when the EGR cooler25is predicted to fall into a state where cooling performance falls short). However, the present disclosure is not limited to the configuration as above and only either one of the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44may be performed.

Third Embodiment

A third embodiment of the present disclosure will now be described usingFIG. 7. A description will be omitted or given simply for portions substantially same as counterparts of the first embodiment above and the following will chiefly describe a portion different from the first embodiment above.

In the third embodiment, an inhibition control that inhibits an EGR gas from flowing back is performed by performing a condensate water restriction control routine ofFIG. 7by using the ECU33when neither the first increase control that increases a flow rate of a coolant flowing into the EGR cooler25nor the second increase control that increases the air rate of the radiator fan44is enabled.

In the condensate water restriction control routine ofFIG. 7, an engine operating state and an outside air environment are obtained first in301.

Subsequently, advancement is made to302, in which whether the EGR cooler25falls into a state where cooling performance falls short is predicted by predicting whether the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value.

When it is predicted in302that the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value, the EGR cooler25is predicted to fall into a state where cooling performance falls short. Hence, advancement is made to303, in which whether the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25is enabled is determined according to, for example, a state of a water pump40, a state of a flow rate control valve41, and a state of a battery (not shown). Processing in302functions as a prediction unit.

When it is determined in303that the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25is enabled, advancement is made to304, in which the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25is performed. Processing in304functions as a control unit.

Subsequently, advancement is made to305, in which whether the second increase control that increases the air rate of the radiator fan44is enabled is determined according to, for example, a state of the radiator fan44and a state of the battery (not shown).

When it is determined in305that the second increase control that increases the air rate of the radiator fan44is enabled, advancement is made to307, in which the second increase control that increases the air rate of the radiator fan44is performed. That is, both of the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44are performed.

By contrast, when it is determined in305that the second increase control that increases the air rate of the radiator fan44is disabled, the routine is ended without performing the second increase control to increase the air rate of the radiator fan44. That is, only the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25is performed.

Meanwhile, when it is determined in303that the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25is disabled, advancement is made to306, in which whether the second increase control that increases the air rate of the radiator fan44is enabled is determined.

When it is determined in306that the second increase control that increases the air rate of the radiator fan44is enabled, advancement is made to307, in which the second increase control that increases the air rate of the radiator fan44is performed. That is, only the second increase control that increases the air rate of the radiator fan44is performed. Processing in307functions as the control unit.

By contrast, when it is determined in306that the second increase control that increases the air rate of the radiator fan44is disabled, that is, when neither the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25nor the second increase control that increases the air rate of the radiator fan44is enabled, advancement is made to308, in which the inhibition control that inhibits the EGR gas from flowing back is performed. In the inhibition control that inhibits the EGR gas from flowing back, a flow rate of the EGR gas returned to an intake pipe12is reduced to zero by keeping an EGR valve24closed. Herein, the EGR gas is inhibited from flowing back until a predetermined time (for example, a time required for the inflow coolant temperature of the EGR cooler25to fall to or below the allowable upper limit value) elapses. Alternatively, the EGR gas may be inhibited from flowing back until a period during which the inflow coolant temperature of the EGR cooler25is at or above a predetermined value (for example, the allowable upper limit value or a temperature at a value slightly less than the allowable upper limit value) is completed. Processing in308functions as the control unit.

In the third embodiment described above, the inhibition control that inhibits the EGR gas from flowing back is performed when the EGR cooler25is predicted to fall into a state where cooling performance falls short. Owing to the configuration as above, the EGR gas can be prevented from flowing back into the intake pipe12by performing the inhibition control to inhibit the EGR gas from flowing back according to a prediction that the EGR cooler falls into a state where cooling performance falls short when the engine11is decelerating. Consequently, even when the EGR cooler falls into a state where cooling performance falls short during the transient operation where the engine11is accelerated after being decelerated, production of condensate water in an intercooler21can be restricted.

Moreover, in the third embodiment, the inhibition control that inhibits the EGR gas from flowing back is performed when neither the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25nor the second increase control that increases the air rate of the radiator fan44is enabled. Owing to the configuration as above, when the EGR cooler25is predicted to fall into a state where cooling performance falls short, the EGR gas is inhibited from flowing back at as low a frequency as possible.

In the third embodiment, the inhibition control that inhibits the EGR gas from flowing back is performed by inhibiting the EGR gas from flowing back until a predetermined time elapses or a predetermined period during which a temperature of the coolant flowing into the EGR cooler is at or above a predetermined value is completed. When configured in the manner as above, an inconvenience that the EGR gas is inhibited from flowing back longer than necessary can be avoided.

In the third embodiment, whether the EGR cooler25falls into a state where cooling performance falls short is predicted by predicting whether the inflow coolant temperature of the EGR cooler25exceeds the allowable upper limit value. However, the present disclosure is not limited to the configuration as above, and whether the EGR cooler25falls into a state where cooling performance falls short may be predicted by determining whether the engine11is in the predetermined decelerating operation.

In the third embodiment, the inhabitation control that inhibits the EGR gas from flowing back is performed according to a prediction that the EGR cooler25falls into a state where cooling performance falls short only when neither the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25nor the second increase control that increases the air rate of the radiator fan44is enabled. However, the present disclosure is not limited to the configuration as above, and the inhabitation control that inhibits the EGR gas from flowing back may be performed whenever the EGR cooler25is predicted to fall into a state where cooling performance falls short without determining whether the first increase control that increases a flow rate of the coolant flowing into the EGR cooler25and the second increase control that increases the air rate of the radiator fan44are enabled.

In the respective first through third embodiments above, the flow rate control valve41is provided at a branch point of the intercooler channel37and the EGR cooler channel38. However, the present disclosure is not limited to the configuration as above. For example, the flow rate control valve41may be provided to the intercooler channel37to regulate a flow rate ratio between the coolant flowing into the intercooler21and the coolant flowing into the EGR cooler25by regulating a flow rate of the coolant flowing into the intercooler21by the flow rate control valve41. Conversely, the flow rate control valve41may be provided to the EGR cooler channel38to regulate a flow rate ratio between the coolant flowing into the intercooler21and the coolant flowing into the EGR cooler25by regulating a flow rate of the coolant flowing into the EGR cooler25by the flow rate control valve41.

In the respective first through third embodiments above, functions performed by the ECU33(for example, a function as the control unit and a function as the fail-safe control unit), either in part or whole, may be formed of hardware using one or more than one IC or the like.