Engine cooling system

An engine cooling system includes a flow control valve and an electronic control unit (ECU) for controlling the flow control valve. The flow control valve regulates the flow rate of coolant flowing through a radiator in a coolant circuit. The ECU feedback controls the opening size of the flow control valve such that the temperature of coolant at an engine outlet seeks a predetermined target value. During the feedback control, the ECU controls the flow control valve such that the opening size of the flow control valve remains above a predetermined lowest value. As a result, the flow control valve is prevented from falling in a small opening size range in which in which it is difficult to cause the engine outlet coolant temperature to seek the target value, and the engine outlet coolant temperature is favorably adjusted.

BACKGROUND OF THE INVENTION

The present invention relates to engine cooling systems.

Generally, a water cooling type engine of a vehicle includes a cooling system provided with a radiator and a flow control valve. The radiator is located in an engine coolant circuit for cooling the coolant. The flow control valve regulates the flow of the coolant that passes through the radiator. The flow control valve is controlled to change the coolant flow in the radiator (hereafter, “the radiator flow”). This adjusts the temperature of the coolant, which cools the engine.

For example, Japanese Laid-Open Patent No. 10-317965 describes a known control procedure of the flow control valve. According to the procedure, the flow control valve is fully closed to minimize the radiator flow when the coolant temperature is relatively low. In contrast, when the coolant temperature is relatively high, the flow control valve is fully opened to maximize the radiator flow. Otherwise, a feedback control procedure is performed to vary the opening size of the flow control valve (the radiator flow) depending on the coolant temperature, such that the coolant temperature seeks a predetermined target value.

Thus, when the coolant temperature is relatively low, such as, if the engine has been started immediately before, the flow control valve is held in a fully closed state to warm up the engine quickly. Afterwards, when the coolant temperature rises to a relatively high level, feedback controlling is started such that the coolant temperature seeks the target value.

During the feedback controlling, if the opening size of the flow control valve falls in a range close to the fully closed state, or a relatively low opening size range, under a certain condition, the opening size of the flow control valve is adjusted in this range such that the coolant temperature seeks the target value. However, when the flow control valve is in the relatively low opening size range, the coolant temperature may change excessively with respect to the opening size adjustment of the flow control valve. This causes hunting in the coolant temperature, thus reducing the reliability of the feedback controlling of the flow control valve for adjusting the coolant temperature to the target value.

Also, as long as the opening size of the flow control valve remains in the relatively low range, changing of the radiator flow in response to the opening size adjustment of the flow control valve may become insufficient, depending on the flow characteristics of the flow control valve. For example, if the opening size of the flow control valve is decreased in the relatively low range by the feedback controlling to raise the coolant temperature to the target value, the coolant temperature does not rise sufficiently quickly. The opening size of the flow control valve is thus excessively reduced by the feedback controlling. In this case, if the engine operational state changes later such that the radiator flow, or the opening size of the flow control valve, must be increased, increasing of the opening size of the flow control valve is delayed. This causes overshooting of the coolant temperature, thus decreasing the reliability of the feedback controlling of the flow control valve for adjusting the coolant temperature to the target value. By contrast, if the opening size of the flow control valve is increased in the relatively low range by the feedback controlling to lower the coolant temperature to the target value, the coolant temperature does not drop sufficiently quickly. The opening size of the flow control valve is thus excessively increased by the feedback controlling. In this case, if the engine operational state changes later such that the radiator flow, or the opening size of the flow control valve, must be reduced, decreasing of the opening size of the flow control valve is delayed. This causes undershooting of the coolant temperature, thus decreasing the reliability of the feedback controlling of the flow control valve for adjusting the coolant temperature to the target value.

Further, in the feedback controlling of the flow control valve, delay is caused in the response of the radiator flow, or the coolant temperature, with respect to the adjustment of the opening size of the flow control valve. Such a delay decreases the efficiency for adjusting the coolant temperature to the target value by the feedback controlling. The controlling reliability of the coolant temperature with respect to the target value is thus decreased.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide an engine cooling system that maintains reliability of feedback controlling of a flow control valve for adjusting the coolant temperature to a target value.

To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides an engine cooling system that includes a coolant circuit, which extends through an engine, a radiator, which is provided in the coolant circuit and cools coolant passing through the coolant circuit, a flow control valve, which regulates the flow rate of coolant flowing through the radiator, and a controller. The controller feedback controls the opening size of the flow control valve such that an engine coolant temperature, which is the temperature of coolant passing through the engine, seeks a predetermined target value.

In one aspect of the present invention, during the feedback control, the controller controls the flow control valve such that the opening size of the flow control valve remains above a predetermined lowest value.

In another aspect of the present invention, when the engine coolant temperature shifts from increasing to decreasing during the feedback control, the controller decreases the opening size of the flow control valve by a predetermined amount from the current opening size. When the engine coolant temperature shifts from decreasing to increasing during the feedback control, the controller increases the opening size of the flow control valve by a predetermined amount from the current opening size.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention applied to an automobile engine will now be described with reference toFIGS. 1to7.

With reference toFIG. 1, a cooling system of an engine1includes a coolant circuit2for circulating coolant such that the coolant passes through the engine1. The coolant circuit2includes a water pump3, which is driven by the engine1. When the water pump3is activated, the coolant flows in the coolant circuit2in a rightward rotational direction, as viewed in the drawing. The coolant thus passes through a cylinder block and a cylinder head (neither is illustrated) of the engine1. This transmits heat from the engine1to the coolant, thus cooling the engine1.

The coolant circuit2has two branches downstream of the engine1, which are merged into a single flow at a position upstream of the water pump3. One of the branches forms a radiator line5, and the other a bypass6. The radiator line5sends coolant to a radiator4and recirculates the coolant to the engine1after the coolant is cooled by the radiator4. The bypass6sends coolant to the engine1without passing the coolant through the radiator4. A flow control valve7is formed at a position at which the radiator line5and the bypass6are merged into the single flow. The flow control valve7regulates the flow of the coolant in the radiator line5and the flow of the coolant in the bypass6. The flow control valve7is configured to gradually increase the coolant flow in the radiator line5as the opening size of the flow control valve7becomes larger.

More specifically, the flow control valve7adjusts the coolant flow in the radiator line5to control the temperature of the coolant for cooling the engine1. In other words, if the coolant flow in the radiator5is increased, the proportion of the coolant cooled by the radiator4is raised, with respect to the total flow of the coolant that flows to the engine1in the coolant circuit2. This lowers the temperature of the coolant that cools the engine1. In contrast, if the coolant flow in the radiator5is decreased, the proportion of the coolant cooled by the radiator4is lowered, with respect to the total flow of the coolant that flows to the engine1in the coolant circuit2. This raises the temperature of the coolant that cools the engine1.

An electronic control unit (ECU)8, which is installed in the vehicle, drives and controls the flow control valve7. The electronic control unit8receives detection signals from the following sensors:

A radiator coolant temperature sensor9for detecting the coolant temperature downstream of the radiator4in the radiator line5;

An engine coolant temperature sensor10for detecting the coolant temperature at an outlet of the coolant circuit2from the engine1;

An accelerator position sensor12for detecting the depression amount of an accelerator pedal11(the accelerator depression amount), which is depressed by the vehicle's driver;

A throttle position sensor15for detecting the opening size of a throttle valve14(the throttle opening size), which is located in an intake passage13of the engine1;

A vacuum sensor16for detecting the pressure downstream of the throttle position sensor15in the intake passage13(the intake pressure): and

A crank position sensor17for outputting a signal reflecting rotation of a crankshaft1a,or an output shaft of the engine1.

The electronic control unit8fully closes the flow control valve7to warm up the engine1, if, for example, the engine1has been started immediately before and is not yet completely warmed up. When the engine1is completely warmed up, or, for example, the coolant temperature at an outlet of the coolant circuit2from the engine1(hereafter, engine outlet coolant temperature) becomes higher than or equal to 80 degrees Celsius, feedback controlling of the flow control valve7is performed in accordance with the engine outlet coolant temperature, such that the engine outlet coolant temperature seeks a predetermined target value. The engine outlet coolant temperature is obtained in accordance with a detection signal generated by the engine coolant temperature sensor10.

The feedback controlling is performed by adjusting the opening size of the flow control valve7based on an instructed opening size Afin, which is obtained depending on, for example, the engine outlet coolant temperature. The computing procedure for the instructed opening size Afin will hereafter be explained with reference to the flowchart ofFIG. 2, which indicates the corresponding routine. The instructed opening size computing procedure ofFIG. 2is periodically conducted by the electronic control unit8with interruption at predetermined time intervals.

If the engine outlet coolant temperature is higher than or equal to 80 degrees Celsius, the condition for starting the feedback controlling is satisfied (S101: YES). In this case, a basic instructed opening size Abse, a feedback correction value h1, and an adjusting speed correction value h2, which are used for computing the instructed opening size Afin, are obtained in this order (in steps S102, S103, and S104). The instructed opening size Afin is computed by the following equation (1), using the basic instructed opening size Abse, the feedback correction value h1, and the adjustment correction value h2(in step S105):
Afin=Abse+h1+h2(1)

Afin: Instructed opening size

Abse: Basic instructed opening size

h1: Feedback correction value

h2: Adjustment speed correction value

In the equation (1), the basic instructed opening size Abse is computed in relation to the coolant temperature at an outlet of the coolant circuit2from the radiator4(hereafter, the radiator outlet coolant temperature), the engine speed, and the engine load. More specifically, the basic instructed opening size Abse is a theoretical opening size of the flow control valve7that is needed for cooling the engine1in accordance with the current operation state of the engine1.

The radiator outlet coolant temperature is obtained in accordance with a detection signal generated by the radiator coolant temperature sensor9. The engine speed is determined in accordance with a detection signal generated by the crank position sensor17. The engine load is determined in relation to a parameter that is varied depending on the engine speed and the air intake of the engine1. The parameter may be the accelerator depression amount based on a detection signal of the accelerator position sensor12, the throttle opening size based on a detection signal of the throttle position sensor15, or the intake pressure based on a detection signal of the vacuum sensor16.

The feedback correction value h1is variable with respect to “0” depending on the difference between the engine outlet coolant temperature and its target value, such that the engine outlet coolant temperature becomes the target value. More specifically, if the engine outlet coolant temperature is lower than the target value, the feedback correction value h1is gradually decreased by predetermined amounts x at predetermined time intervals to reduce the instructed opening size Afin. In contrast, if the engine outlet coolant temperature is higher than the target value, the feedback correction value h1is gradually increased by the amounts x at predetermined time intervals to increase the instructed opening size Afin.

The adjusting speed correction value h2is determined for improving the efficiency for adjusting the engine outlet coolant temperature to the target value. The adjusting speed correction value h2is obtained by an adjusting speed correction value computing routine ofFIGS. 5 and 6, which will be later described.

The opening size of the flow control valve7is controlled based on the instructed opening size Afin, which is obtained as described above, such that the engine outlet coolant temperature seeks the target value. However, during the feedback controlling, the opening size of the flow control valve may be decreased to a value close to the fully closed state under a certain condition, for example, when the radiator outlet coolant temperature is relatively low or the engine1is in an operation state in which heat generation is relatively low. If the opening size of the flow control valve7is adjusted in a relatively low range close to the fully closed state, the engine outlet coolant temperature does not change appropriately in response to the adjustment of the opening size of the flow control valve7.

This problem is caused depending on the flow characteristics of the flow control valve7, or the changing characteristics of the coolant flow in the radiator line5in response to the opening size adjustment of the flow control valve7, and the changing characteristics of the engine outlet coolant temperature in response to the opening size adjustment of the flow control valve7. The factors that cause the aforementioned problem will hereafter be explained with reference toFIGS. 3 and 4. The graphs ofFIGS. 3 and 4respectively indicate the changing characteristics of the coolant flow in the radiator line5and the changing characteristics of the engine outlet coolant temperature, in response to the opening size adjustment of the flow control valve7, when the operation state of the engine1remains constant.

With reference toFIG. 3, the flow control valve7of the first embodiment indicates the flow characteristics that the coolant flow in the radiator line5is gradually increased at a constant rate as the opening size of the flow control valve7becomes greater. The engine outlet coolant temperature is changed in response to increasing of the opening size of the flow control valve7, as indicated in FIG.4. More specifically, when the opening size of the flow control valve7is adjusted in a relatively low range close to the fully closed state (the range A of FIG.4), the engine outlet coolant temperature changes excessively in response to the opening size adjustment of the flow control valve7. This may be one of the factors that cause the aforementioned problem.

Further, when the opening size of the flow control valve7remains in the relatively low range close to the fully closed state, the coolant flow in the radiator line5is nullified or significantly reduced. Thus, only the coolant near the radiator coolant temperature sensor9and the engine coolant temperature sensor10is warmed by heat generated by, for example, an exhaust pipe of the engine1. In this case, the detection signals of the coolant temperature sensors9,10become inappropriate and the feedback controlling, which is performed depending on these detection signals, also becomes inappropriate. This may also be one of the factors that cause the aforementioned problem.

Accordingly, in the instructed opening size computing routine ofFIG. 2of the first embodiment, a minimum value of the instructed opening size Afin is restricted for preventing the instructed opening size Afin computed in step S105from falling in the range A, or the relatively low opening size range. In other words, if the instructed opening size Afin obtained in step S105falls in the range A, the instructed opening size Afin is set to a predetermined minimum value that is larger than the range A (the relatively low opening size range) in step S106. The feedback controlling of the flow control valve7is conducted based on the corrected instructed opening size Afin. Thus, the opening size adjustment of the flow control valve7in the range A, which is close to the fully closed state, is avoided.

Next, step S104, or the computing procedure of the adjusting speed correction value h2, will be explained with reference toFIGS. 5,6, and7.FIGS. 5,6are flowcharts indicating the adjusting speed correction value computing routine.FIG. 7is a graph indicating variation of the engine outlet coolant temperature as time elapses. The computing routine ofFIGS. 5 and 6is conducted by the electronic control unit8, every time step S104of the instructed opening size computing routine (FIG. 2) is performed.

In the adjusting speed correction value computing routine, it is judged whether or not the adjusting efficiency of the engine outlet coolant temperature with respect to the target value need be improved. In step S201, a flag F1indicates whether or not the judgment is currently being carried out. If the flag F1is “0”, it is indicated that the judgment is not currently being carried out (S201: YES). In this case, it is judged whether or not the difference between the engine outlet coolant temperature and its target value is greater or equal to a predetermined value α (in step S202). If the judgment of S202is negative (S202: NO), the adjusting speed correction value h2is set at “0” in step S211, and the instructed opening size computing routine ofFIG. 2is resumed. In this case, the adjusting efficiency of the engine outlet coolant temperature remains unchanged.

In contrast, if the judgment of S202is positive, or it is judged that the difference between the engine outlet coolant temperature and its target value is greater than or equal to the value α (at timing T1of FIG.7), the current engine outlet coolant temperature is stored as a coolant temperature THW1(in step S203). Further, in step S204, the flag F1is set at “1”. Subsequently, when a predetermined time t elapses after the setting of the flag F1to “1” (at timing T2of FIG.7), the judgment of step S205turns positive. The current engine outlet coolant temperature is then stored as a coolant temperature THW2in step S206.

Subsequently, in step S207, the flag F1is set to “0”, which indicates that the judgment is not currently being carried out. Afterwards, in steps S208and S209ofFIG. 6, whether the adjusting efficiency of the engine outlet coolant temperature with respect to the target value need be improved or not is judged depending on the coolant temperatures THW1and THW2. More specifically, the judgments of steps S208and S209are based on the following points:

In step S208, it is judged whether or not the difference between the coolant temperature THW2and the target value is more than or equal to the difference between the coolant temperature THW1and the target value, indicating that the adjustment of the engine outlet coolant temperature to the target value cannot be achieved under the current conditions; and

In step S209, it is judged whether or not the difference between the coolant temperatures THW1and THW2(the change of the engine outlet coolant temperature during the time t) is less than a predetermined value ΔT, indicating that the adjusting speed of the engine outlet coolant temperature with respect to the target value is excessively slow.

If the judgments of steps S208and S209are both negative, it is indicated that the adjusting efficiency of the engine outlet coolant temperature with respect to the target value is currently maintained at a relatively high level. Thus, it is judged that the adjusting efficiency of the engine outlet coolant temperature need not be further improved. In this case, the adjusting speed correction value h2is maintained at “0”, and the instructed opening size computing routine ofFIG. 2is resumed.

By contrast, if one of the judgments of steps S208and S209is positive, it is indicated that the adjusting efficiency of the engine outlet coolant temperature with respect to the target value is currently low. Thus, it is judged that the adjusting efficiency of the engine outlet coolant temperature need be improved. In this case, the adjusting speed correction value h2is computed based on the difference between the current engine outlet coolant temperature and the target value (step S210).

More specifically, if the engine outlet coolant temperature is higher than the target value, the adjusting speed correction value h2is gradually increased with respect to “0” (to increase the instructed opening size Afin) as the difference between the engine outlet coolant temperature and the target value becomes larger. In contrast, if the engine outlet coolant temperature is lower than the target value, the adjusting speed correction value h2is gradually decreased with respect to “0” (to decrease the instructed opening size Afin) as the difference between the engine outlet coolant temperature and the target value becomes larger.

When the computation of the adjusting speed correction value h2is completed, the instructed opening size computing routine ofFIG. 2is resumed. In the routine, the instructed opening size Afin is determined using the adjusting speed correction value h2. The opening size of the flow control valve7is controlled based on the obtained, instructed opening size Afin, thus improving the adjusting efficiency of the engine outlet coolant temperature with respect to the target value. Accordingly, for example, following the timing T2ofFIG. 7, the engine outlet coolant temperature is quickly adjusted to the target value, as indicated by the solid line in the timing chart.

The first embodiment has the following effects.

(1) In the first embodiment, the minimum value of the instructed opening size Afin is restricted such that the instructed opening size Afin does not fall in the relatively low range close to the fully closed state, or the range A ofFIG. 4, in which the engine outlet coolant temperature changes excessively in response to the opening size adjustment of the flow control valve7. The opening size adjustment of the flow control valve7is thus prevented from being performed in the range A during the feedback controlling. This suppresses hunting of the engine outlet coolant temperature, and therefore improves the reliability of the feedback controlling for adjusting the engine outlet coolant temperature to the target value.

(2) In the feedback controlling, the adjusting speed correction value h2is increased or decreased with respect to “0”, if the adjusting speed of the engine outlet coolant temperature is excessively slow or the adjustment of the engine outlet coolant temperature cannot be achieved. The opening size of the flow control valve7(the instructed opening size Afin) is thus corrected such that the engine outlet coolant temperature is adjusted to a value close to the target value. Accordingly, the engine outlet coolant temperature quickly seeks the target value.

(3) The adjusting speed correction value h2, which serves for improving the adjusting efficiency of the engine outlet coolant temperature with respect to the target value, is varied in relation to the difference between the current engine outlet coolant temperature and the target value. The opening size adjustment of the flow control valve7based on the adjusting speed correction value h2is thus appropriately conducted. Accordingly, the engine outlet coolant temperature seeks the target value further quickly.

Next, a second embodiment of the present invention will be described with reference toFIGS. 8 and 9.

The flow control valve7of the second embodiment has flow characteristics that are different from those of the flow control valve7of the first embodiment. In the second embodiment, the minimum value of the instructed opening size Afin is set in a different manner from that of the first embodiment.

The graphs ofFIGS. 8 and 9respectively indicate the changing characteristics of the coolant flow in the radiator line5and the changing characteristics of the engine outlet coolant temperature, in response to the opening size adjustment of the flow control valve7of the second embodiment, when the operation state of the engine1is constant.

With reference toFIG. 8, the flow control valve7has the flow characteristics as follows. That is, the flow control valve7of the second embodiment is configured to gradually increase the coolant flow in the radiator line5as the opening size of the flow control valve7becomes larger. However, when the opening size of the flow control valve7falls in a part of a relatively low range, or a part of a range B ofFIG. 8, increasing of the coolant flow in the radiator line5in response to the opening size adjustment of the flow control valve7is almost completely suppressed. Further, the engine outlet coolant temperature is varied in response to the opening size adjustment of the flow control valve7, as indicated by FIG.9. More specifically, changing of the engine outlet coolant temperature in response to the opening size adjustment of the flow control valve7occurs excessively slowly (or is almost completely suppressed), when the opening size of the flow control valve7is in the portion of the relatively low range (the range B).

Thus, when the opening size of the flow control valve7is adjusted by the feedback controlling in the portion of the range B such that the engine outlet coolant temperature seeks the target value, the changing amount of the engine outlet coolant temperature in response to the opening size adjustment of the flow control valve7becomes excessively small. The opening size of the flow control valve7is thus excessively changed. In this case, when the engine1is operated in a different operation state later and the opening size of the flow control valve7needs to be further adjusted, the opening size adjustment of the flow control valve7cannot be achieved quickly. This causes overshooting or undershooting in the engine outlet coolant temperature, thus reducing the reliability of the feedback controlling for adjusting the engine outlet coolant temperature to the target value.

Accordingly, in the second embodiment, the minimum value of the instructed opening size Afin is restricted such that the instructed opening size Afin dose not fall in the range B. The flow control valve7is controlled in accordance with the instructed opening size Afin that is set to the restricted minimum value. This prevents the opening size adjustment of the flow control valve7from being performed in the range B for adjusting the engine outlet coolant temperature to the target value.

The second embodiment has the following effect.

(4) In the second embodiment, the minimum value of the instructed opening size Afin is restricted such that the instructed opening size Afin does not fall in the relatively low range close to the fully closed state, or the range B ofFIG. 9, in which the changing amount of the engine outlet coolant temperature in response to the opening size adjustment of the flow control valve7becomes excessively small. The opening size adjustment of the flow control valve7is thus prevented from being performed in the range B during the feedback controlling. This suppresses excessive adjustment of the opening size of the flow control valve7, and therefore improves the reliability of the feedback controlling for adjusting the engine outlet coolant temperature to the target value.

A third embodiment of the present invention will hereafter be described with reference toFIGS. 10to14.

In the feedback controlling of the first or second embodiment, delay is caused in the response of the engine outlet coolant temperature with respect to the adjustment of the opening size of the flow control valve7based on the instructed opening size Afin. This reduces the adjusting efficiency of the engine outlet coolant temperature with respect to the target value. Thus, in the third embodiment, when variation of the engine outlet coolant temperature is shifted between increasing and decreasing during the feedback controlling, the opening size of the flow control valve7is changed in accordance with a skipping amount S, which will be later described, such that the engine outlet temperature quickly seeks the target value.

As described, the opening size of the flow control valve7is adjusted in accordance with the instructed opening size Afin. In the third embodiment, the instructed opening size Afin is computed using the skipping amount S, in addition to the basic opening size Abse, the feedback correction value h1, and the adjusting speed correction value h2. More specifically, the instructed opening size Afin of the third embodiment is obtained by the following equation (2):
Afin=Abse+h1+h2+S(2)

Afin: Instructed opening size

Abse: Basic instructed opening size

h1: Feedback correction value

h2: Adjustment speed correction value

The initial value of the skipping amount S is, for example, “0”. The skipping amount S is computed as a negative value when the variation of the engine outlet coolant temperature is shifted from increasing to decreasing. In contrast, the skipping amount S is computed as a positive value when the variation of the engine outlet coolant temperature is shifted from decreasing to increasing. The instructed opening size Afin is obtained in accordance with the skipping amount S, which is determined as described. In other words, when the variation of the engine outlet coolant temperature is shifted between increasing and decreasing, the opening size of the flow control valve7is changed in accordance with the skipping amount S.

The opening size adjustment of the flow control valve7in accordance with the skipping amount S will hereafter be explained with reference to the timing chart of FIG.10.FIG. 10indicates the variation of the opening size of the flow control valve7as time elapses, and the variation of the engine outlet coolant temperature as time elapses.

With reference to the timing chart, after the engine outlet coolant temperature becomes higher than the target value, the variation of the engine outlet coolant temperature is shifted from increasing to decreasing (at timing T3). The opening size of the flow control valve7is then reduced in accordance with the skipping amount S. The flow control valve7is fixed at the reduced opening size until after the engine outlet coolant temperature reaches the target value (at timing T4). The engine outlet coolant temperature thus decreases rapidly from the level higher than the target value to the target value. After the engine outlet coolant temperature reaches the target value (at timing T4), the fixing of the opening size of the flow control valve7is stopped. That is, the opening size adjustment of the flow control valve7is resumed such that the engine outlet coolant temperature seeks the target value.

Afterwards, when the engine outlet coolant temperature becomes lower than the target value, the variation of the engine outlet coolant temperature is shifted from decreasing to increasing (at timing T5). The opening size of the flow control valve7is then increased in accordance with the skipping amount S. The flow opening valve7is fixed at the increased opening size until after the engine outlet coolant temperature reaches the target value (at timing T6), in the same manner as above. The engine outlet coolant temperature thus increases rapidly from the level lower than the target value to the target value. After the engine outlet coolant temperature reaches the target value (at timing T6), the fixing of the opening size of the flow control valve7is stopped. That is, the opening size adjustment of the flow control valve7is resumed such that the engine outlet coolant temperature seeks the target value.

Next, a procedure of computing the instructed opening size Afin will be described with reference to the flowcharts ofFIGS. 11 and 12, which indicate the corresponding routine. In the routine, the portion corresponding to steps S301and S303to S305is identical to the portion corresponding to steps S101to S104of the routine ofFIG. 2according to the first embodiment.

In the instructed opening size computing routine of the third embodiment, it is first judged whether or not the conditions for the feedback controlling are satisfied in step S301. If the judgment is positive (S301: YES), it is judged whether or not a flag F2is “0” in step S302. More specifically, the flag F2indicates whether or not the flow control valve7is fixed at the opening size changed in accordance with the skipping amount S. If the flag f2is “0”, it is indicated that the opening size of the flow control valve7is currently non-fixed.

If the judgment of S302is positive, the basic opening size Abse, the feedback correction value h1, and the adjusting speed correction value h2are computed in this order in steps S303, S304, and S305. Subsequently, in steps S306to S309ofFIG. 12, the skipping amount S is computed.

More specifically, in step S306, it is judged whether or not the variation of the engine outlet coolant temperature has been shifted from increasing to decreasing. If the judgment is positive (S306: YES), the skipping amount S is computed as a negative value in relation to the engine speed in step S307. With reference toFIG. 13, the skipping amount S is gradually increased with respect to “0” as the engine speed becomes greater such that the coolant displacement of the water pump3, or the coolant flow in the coolant circuit2, gradually increases. That is, as the coolant flow in the coolant circuit2becomes greater, the increasing amount of the engine outlet coolant temperature, with the opening size of the flow control valve7reduced in accordance with the skipping amount S, becomes greater. It is thus preferred that the skipping amount S is varied in relation to the engine speed, as described, for enabling the engine outlet coolant speed to quickly seek the target value.

Further, if the judgment of S306is negative, it is judged whether or not the variation of the engine outlet coolant temperature has been shifted from decreasing to increasing in step S308. If the judgment is positive (S308: YES), the skipping amount S is computed as a positive value in relation to the engine speed in step S309. With reference toFIG. 14, the skipping amount S is gradually decreased with respect to “0” as the engine speed becomes greater such that the coolant displacement of the water pump3, or the coolant flow in the coolant circuit2, increases. That is, as the coolant flow in the coolant circuit2becomes greater, the decreasing amount of the engine outlet coolant temperature, with the opening size of the flow control valve7increased in accordance with the skipping amount S, becomes greater. It is thus preferred that the skipping amount S is varied in relation to the engine speed for enabling the engine outlet coolant speed to quickly seek the target value.

If the judgments of S308and S309are both negative, the skipping amount S is maintained at a previously computed value.

After the skipping amount S is computed in steps S307or S309, the flag F2is set to “1”, indicating that the flow control valve7is fixed at the changed opening size, in step S310. More specifically, as long as the flag F2is held at “1”, the judgment of S302(FIG. 11) remains negative, and steps of S303to S310are not performed. In other words, the computation of the basic instructed opening size Abse, the feedback correction value h1, the adjusting speed correction value h2, or the skipping amount S is not performed. Thus, the flow control valve7is maintained at the opening size changed in accordance with the skipping amount S as long as the flag F2remains “1”.

Once the engine outlet coolant temperature reaches the target value with the flow control valve7fixed at the opening size changed in accordance with the skipping amount S (S311: YES), the flag F2is reset to “0”, indicating that the opening size of the flow control valve7is currently non-fixed. Afterwards, the instructed opening size Afin is computed in step S313.

The third embodiment has the following effects, in addition to the items (2) and (3), which have been described about the first embodiment.

(5) In the third embodiment, when the variation of the engine outlet coolant temperature is shifted between increasing and decreasing, the opening size of the flow control valve7is changed in accordance with the skipping amount S such that the engine outlet coolant temperature quickly seeks the target value. Thus, even if changing of the engine outlet coolant temperature in response to the opening size adjustment of the flow control valve7is delayed, the control reliability of the engine outlet coolant temperature with respect to the target value is prevented from being lowered due to the delayed response.

(6) When the opening size of the flow control valve7is changed in accordance with the skipping amount S after the variation of the engine outlet coolant temperature is shifted between increasing and decreasing, the flow control valve7is fixed at the changed opening size until the engine outlet coolant temperature reaches the target value. The engine outlet coolant temperature thus quickly seeks the target value.

(7) The skipping amount S is varied in relation to the engine speed, which is a parameter associated with the coolant flow in the coolant circuit2. Thus, even if the changing amount of the engine outlet coolant temperature in response to the opening size adjustment of the flow control valve7is varied depending on the coolant flow in the coolant circuit2, the opening size of the flow control valve7is appropriately adjusted based on the skipping amount S, regardless of the coolant flow in the coolant circuit2. Accordingly, the engine outlet coolant temperature quickly seeks the target value.

The illustrated embodiments may be modified as follows.

In the third embodiment, the engine speed is used as the parameter associated with the coolant flow in the coolant circuit2. However, instead of using the parameter, a flow sensor or the like may directly detect the coolant flow in the coolant circuit2. In this case, the skipping amount S is computed as a variable value based on the detection value.

Further, the skipping amount S does not necessarily have to be variable but may be fixed.

In the third embodiment, the flow control valve7is fixed at the opening size changed in accordance with the skipping amount S until the engine outlet coolant temperature reaches the target value. However, the opening size of the flow control valve7may be fixed only for a predetermined time. Further, the time for fixing the opening size of the flow control valve7may be varied depending on the difference between the engine outlet coolant temperature and the target value when the variation of the engine outlet coolant temperature is shifted between increasing and decreasing.

In each of the illustrated embodiments, the adjusting speed correction value h2does not necessarily have to be varied in relation to the difference between the engine outlet coolant temperature and the target value. Instead, the adjusting speed correction value h2may be a fixed value.

Also, the opening size adjustment of the flow control valve7in accordance with the adjusting speed correction value h2dose not necessarily have to be conducted.

In the first embodiment, the minimum opening size of the flow control valve7is restricted such that the opening size adjustment of the flow control valve7is not performed in the range A, or the relatively low opening size range close to the fully closed state. The range A may be reduced to a smaller range or enlarged to a larger range, as necessary.