Engine cooling system

Abstract

The present invention addresses the problem of providing an engine cooling system in which the control accuracy of the flow rate of cooling water passing through an engine water pump is improved. An engine cooling system is provided with a cooling circuit and a control device that calculates a target rotational speed of an engine water pump on the basis of a target flow rate of cooling water flowing through the engine water pump, an opening degree of a thermostat, and a rotational speed of a heater water pump, and that controls the rotational speed of the engine water pump to the target rotational speed. The target rotational speed is calculated to be higher as the target flow rate is higher, the opening degree of the thermostat is smaller, the target rotational speed is calculated to be higher, the target rotational speed is reduced by a predetermined reduced rotational speed according to the rotational speed of the heater water pump and the opening degree of the thermostat, and the reduced rotational speed is calculated to be higher as the rotational speed of the heater water pump is higher, and the opening degree of the thermostat is smaller. And the reduced rotating speed is calculated to be larger.

Description

TECHNICAL FIELD

[0001] The present application relates to an engine cooling system. BACKGROUND

[0002] There is an engine cooling system provided with a cooling circuit that circulates cooling water between an engine, a heater core for heating, and a radiator by a water pump (for example, refer to Patent Literature 1).

[0003] [Patent Literature]

[0004] [Patent Literature]

[0005] [Patent Literature 1] WO2016 / 059791 SUMMARY

[0006] [Problems to be Solved by the Invention]

[0007] Sometimes, an engine water pump and a heater water pump that make cooling water flow through the engine and the heater core, respectively, are provided. Also, sometimes, a thermostat is provided between the engine and the radiator. In such a case, it is considered that the flow rate of the cooling water through the engine water pump is controlled to a target flow rate by controlling the rotation speed of the engine water pump to a target rotation speed. However, due to the influence of the rotation speed of the heater water pump and the opening degree of the thermostat, the control accuracy of the flow rate of the cooling water through the engine water pump can be reduced.

[0008] Therefore, an object of the present application is to provide an engine cooling system that improves the control accuracy of the flow rate of the cooling water through the engine water pump.

[0009] [Means for Solving the Problems]

[0010] The above object can be achieved by an engine cooling system that achieves circulation of cooling water between an engine and a heater core in a thermostat fully closed state and recirculation of cooling water that has passed through the engine into the heater core and again into the engine and recirculation of cooling water that has passed through the engine into a radiator and again into the engine in a thermostat fully open state by an engine water pump that circulates cooling water to the engine and a heater water pump that circulates cooling water to the heater core, and a control device that calculates a target rotation speed of the engine water pump based on a target flow rate of cooling water that circulates in the engine water pump, an opening degree of the thermostat, and a rotation speed of the heater water pump, controls the rotation speed of the engine water pump to the target rotation speed, and is configured to calculate the target rotation speed higher as the target flow rate is larger, calculate the target rotation speed higher as the opening degree of the thermostat is smaller, and lower the target rotation speed by a prescribed lowering rotation speed according to the rotation speed of the heater water pump and the opening degree of the thermostat, and calculate the lowering rotation speed larger as the rotation speed of the heater water pump is higher, and calculate the lowering rotation speed larger as the opening degree of the thermostat is smaller.

[0011] Also, the control device can calculate the lowering rotation speed to zero regardless of the rotation speed of the heater water pump in the thermostat fully open state.

[0012] Also, the cooling circuit can include an on-off valve that adjusts an opening rate of a path through which cooling water that has passed through the engine is recirculated through the engine without passing through the radiator, and the control device can calculate the target rotation speed higher as the opening rate is smaller.

[0013] [Effects of Invention]

[0014] An engine cooling system that improves control accuracy of a flow rate of cooling water that passes through an engine water pump can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is an explanatory diagram of an engine cooling system.

[0016] Figure 2 In, (A) to (F) are explanatory diagrams of communication states of a four-way valve.

[0017] Figure 3 In, (A) is an explanatory diagram of a flow path of cooling water in a thermostat fully closed state, and (B) is an explanatory diagram of a flow path of cooling water in a thermostat fully open state.

[0018] Figure 4is a flowchart illustrating flow control of EWP executed by the ECU.

[0019] Figure 5 In the meantime, (A) to (C) are exemplary graphs that define a map of a target rotational speed of the EWP corresponding to a target flow rate of the cooling water through the EWP.

[0020] Figure 6 is an exemplary graph that defines a map of a target rotational speed of the EWP corresponding to a target flow rate of the cooling water through the EWP.

[0021] [REFERENCE NUMERALS]

[0022] 1 engine cooling system

[0023] 2 engine cooling circuit

[0024] 10 engine

[0025] 12 engine water pump

[0026] 14 radiator

[0027] 20 heater core

[0028] 22 heater water pump

[0029] 30 thermostat

[0030] 40 four-way valve (on-off valve) DETAILED DESCRIPTION

[0031] [Engine cooling system]

[0032] Figure 1 is an explanatory diagram of the engine cooling system 1. The engine cooling system 1 is mounted on a vehicle, for example. The engine cooling system 1 has an engine cooling circuit 2 and an ECU (Electronic Control Unit) 100. The engine cooling circuit 2 includes an engine 10, an engine water pump (hereinafter referred to as an EWP) 12, a radiator 14, a water reservoir 16, a temperature sensor 18, a heater core 20, a heater water pump (hereinafter referred to as an HWP) 22, a heater 24, a thermostat 30, a four-way valve 40, and a heat exchanger 50.

[0033] The engine 10 is a motive power source of the vehicle. The EWP 12 is an electric water pump that circulates the cooling water to the engine 10 by pressurizing the cooling water toward the engine 10 in the direction of the arrow. Figure 1 The radiator 14 cools the cooling water by exchanging heat between the cooling water and the air outside the vehicle. The water reservoir 16 stores the remaining cooling water. The heater core 20 heats the vehicle cabin using the heat of the cooling water. The HWP 22 is an electric water pump that circulates the cooling water to the heater core 20 by pressurizing the cooling water toward the heater core 20 in the direction of the arrow. Figure 1The electric water pump 12 is an electrically driven water pump that draws cooling water from the heater core 20 in the direction of the arrow and causes the cooling water to flow to the heater core 20. The heater 24 heats the cooling water in a case where the temperature of the cooling water is insufficient for heating the vehicle cabin with respect to the heater core 20.

[0034] When the temperature of the cooling water flowing into the thermostat 30 is lower than the first temperature, the thermostat 30 becomes a fully closed state, and when the temperature of the cooling water flowing into the thermostat 30 is equal to or higher than the second temperature that is higher than the first temperature, the thermostat 30 becomes a fully open state. In addition, when the temperature of the cooling water flowing into the thermostat 30 is equal to or higher than the first temperature and lower than the second temperature, the higher the temperature of the cooling water, the greater the opening degree of the thermostat 30. For example, before the warm-up of the engine 10 is completed, the temperature of the cooling water flowing into the thermostat 30 is lower than the first temperature, and after the warm-up is completed, the temperature of the cooling water flowing into the thermostat 30 becomes equal to or higher than the second temperature. The flow path of the cooling water in the fully closed state and the fully open state of the thermostat 30 will be described later in detail.

[0035] The four-way valve 40 causes the cooling water to flow in a prescribed path that will be described later by switching the communication states of the four paths. The four-way valve 40 has a rotor housed in a housing in a rotatable manner and an actuator that drives the rotor. The communication states of the four paths are switched according to the rotational position of the rotor controlled by the actuator.

[0036] The heat exchanger 50 causes the cooling water flowing in the engine cooling circuit 2 to exchange heat with the cooling water flowing in a battery cooling circuit for cooling a battery not shown.

[0037] The ECU 100 is an electronic control unit that has an arithmetic processing circuit that performs various arithmetic processes related to the travel control of the vehicle and a memory that stores programs and data for control. The ECU 100 acquires the temperature of the cooling water based on the temperature sensor 18. The ECU 100 controls the rotational speed of each of the EWP 12 and the HWP 22, the energization amount of the heater 24, and the communication state of the four-way valve 40 according to the operating state of the engine 10, the heating demand, and the cooling demand of the battery. The ECU 100 calculates the target rotational speed of the EWP 12 based on the target flow rate of the cooling water flowing in the EWP 12 and controls the rotational speed of the EWP 12 to the target rotational speed, as will be described later in detail.

[0038] Path 61 has a thermostat 30 at its upstream end and connects to a four-way valve 40 at its downstream end, with an EWP 12 and an engine 10 arranged along its middle. Path 62 has a four-way valve 40 at its upstream end and a thermostat 30 at its downstream end, with a heater core 20, an HWP 22, and a heater 24 arranged along its middle. Path 63 connects the heater 24 and thermostat 30 of path 62 at its upstream end and connects to the four-way valve 40 at its downstream end. Path 64 connects the engine 10 and four-way valve 40 of path 61 at its upstream end and connects to the heater 24 and thermostat 30 of path 62 at its downstream end. A temperature sensor 18 is located at the connection point between paths 61 and 64. Path 65 connects the four-way valve 40 at its upstream end and connects the heater core 20 and HWP 22 of path 62 at its downstream end, with a heat exchanger 50 arranged along its middle. The upstream end of path 66 is connected between the engine 10 and the temperature sensor 18 of path 61, and the downstream end of path 66 is connected to the thermostat 30. A radiator 14 and a water tank 16 are arranged in the middle of path 66.

[0039] Four-way valve

[0040] Next, the four-way valve 40 will be explained. Figure 2 Diagrams (A) to (F) illustrate the connected states of the four-way valve 40. As the rotor of the four-way valve 40 rotates in one direction, according to... Figure 2 The connection states are switched sequentially from (A) to (F). Figure 2 In (A), path 63 is connected to path 62, while paths 61 and 65 are cut off. In this state, EWP12 stops, and HWP22 is activated. Cooling water circulates in heater core 20 and heater 24 via a portion of path 62 and path 63. The opening ratio of the opening connecting path 63 and path 62 of the four-way valve 40 is maximized. Figure 2 In (A), the rotor of the four-way valve 40 is in the initial position. By rotating the rotor from the initial position in one direction, the opening ratio of the opening connecting path 63 and path 62 is reduced, such as... Figure 2 As shown in (B), the opening ratio of the opening connecting path 63 and path 65 is increased.

[0041] exist Figure 2 In (B), path 63 connects with paths 62 and 65, while path 61 is cut off. In this state, EWP12 stops, and HWP22 is driven. Cooling water circulates in heater core 20 and heater 24 via a portion of path 62 and path 63, and circulates in heat exchanger 50 via path 65. The cooling water flows through the rotor from... Figure 2 (B) rotates in one direction, such as Figure 2(C) of FIG. 6, the opening ratio of the opening that communicates the passage 63 with the passage 62 is reduced to zero, and the opening ratio of the opening that communicates the passage 63 with the passage 65 is increased to the maximum.

[0042] In Figure 2 (C) of FIG. 6, the passage 63 communicates with the passage 65, and the passages 61 and 62 are cut off. In this state, the EWP 12 is stopped, and the HWP 22 is driven. The cooling water circulates in the heat exchanger 50 and the heater 24 via a part of the passage 62, the passage 63, and the passage 65. The opening ratio of the opening that communicates the passage 63 with the passage 65 of the four-way valve 40 is the maximum. According to Figure 2 (C) of FIG. 6, the rotor is rotated in one direction, as shown in Figure 2 (D) of FIG. 6, the opening ratio of the opening that communicates the passage 63 with the passage 65 is reduced, and the opening ratio of the opening that communicates the passage 61 with the passage 62 is increased.

[0043] In Figure 2 (D) of FIG. 6, the passage 63 communicates with the passage 65, and the passage 61 communicates with the passage 62. In addition, the passages 63 and 65 do not communicate with the passages 61 and 62. In this state, the EWP 12 and the HWP 22 are driven. The cooling water circulates in the heat exchanger 50 and the heater 24 via a part of the passage 62, the passage 63, and the passage 65, and circulates in the engine 10, the heater core 20, and the heater 24 via the passages 61 and 62. Therefore, the passages 61 and 62 correspond to the passages through which the cooling water that has passed through the engine 10 passes again through the engine 10 without passing through the radiator 14. When the thermostat 30 is in the fully open state, the cooling water also circulates to the radiator 14 and the water reservoir 16 via the passage 66. According to Figure 2 (D) of FIG. 6, the rotor is rotated in one direction, as shown in Figure 2 (E) of FIG. 6, the opening ratio of the opening that communicates the passage 63 with the passage 65 is reduced to zero, and the opening ratio of the opening that communicates the passage 61 with the passage 62 is increased to the maximum.

[0044] In Figure 2 (E) of FIG. 6, the passage 61 communicates with the passage 62, and the passages 63 and 65 are cut off. In this state, at least the EWP 12 is driven. The cooling water circulates in the engine 10, the heater core 20, and the heater 24 via the passages 61 and 62. The opening ratio of the opening that communicates the passage 61 with the passage 62 of the four-way valve 40 is the maximum. When the thermostat 30 is in the fully open state, the cooling water also circulates to the radiator 14 and the water reservoir 16 via the passage 66. According to Figure 2 (E) of FIG. 6, the rotor is rotated in one direction, as shown in Figure 2 (F) of FIG. 6, the opening ratio of the opening that communicates the passage 61 with the passage 62 is reduced, and the opening ratio of the opening that communicates the passage 61 with the passage 65 is increased.

[0045] exist Figure 2 In (F), path 61 connects with paths 62 and 65, while path 63 is disconnected. Coolant circulates through the engine 10, heater core 20, heater 24, and heat exchanger 50 via paths 61, 62, and 65, at least through EWP12. Therefore, paths 61, 62, and 65 are equivalent to coolant passing through the engine 10 without passing through the radiator 14 and then passing through the engine 10 again. When the thermostat 30 is fully open, coolant also circulates to the radiator 14 and reservoir 16 via path 66.

[0046] Figure 2 The sum of the aperture ratios of the openings connecting path 61 and path 62 and the opening ratios of the openings connecting path 61 and path 65 in (F) is less than Figure 2 The maximum opening ratio of the opening connecting path 61 and path 62 in (E). For example, Figure 2 In (E), the opening ratio of the opening connecting path 61 and path 62 is set to 100%. Figure 2 The opening ratio of the opening connecting path 61 and path 62 in (F) is 40%, and the opening ratio of the opening connecting path 61 and path 65 is also 40%. Therefore, Figure 2 The total opening ratio of path 61 in (F) is 80%, which is higher than... Figure 2 The opening ratio in (E) is 100% low.

[0047] Therefore, Figure 2 The aperture ratio of the opening connecting path 61 and path 62 in (C) increases from zero to become Figure 3 The state of (D). Next, the opening ratio of the opening connecting path 61 and path 62 becomes the maximum, becoming Figure 3 The state of (E). Next, the opening ratio of the opening connecting path 61 with paths 62 and 65 gradually decreases, becoming... Figure 3 The state of (F). Thus, the four-way valve 40 is an example of an opening valve that adjusts the opening ratio of the path through which the coolant that has passed through the engine 10 passes through the radiator 14 and then passes through the engine 10 again. The ECU 100 obtains the opening ratio of such a path by referring to a mapping specified according to the target rotational position of the rotor.

[0048] [Thermostat]

[0049] Next, the fully closed and fully open states of the thermostat 30 will be explained. Figure 3 (A) is an illustration of the flow path of cooling water when the thermostat 30 is in the fully closed state. Figure 3(A) indicates that the four-way valve 40 connects paths 61 and 62 and disconnects paths 63 and 65. Coolant flows in the following order: EWP12, engine 10, four-way valve 40, heater core 20, HWP22, heater 24, and thermostat 30. In path 64, coolant flows from temperature sensor 18 towards path 62. Because the thermostat 30 is fully closed, coolant does not flow to radiator 14, and engine 10 is preheated.

[0050] Figure 4 (B) is an illustration of the cooling water flow path when the thermostat 30 is fully open. Figures 5-6 (B) is also with Figures 5-6 Similarly, (A) shows the state where the four-way valve 40 connects path 61 and path 62 and disconnects paths 63 and 65. A portion of the coolant passing through the engine 10 flows to the heater core 20 via path 62, and the remaining portion flows to the radiator 14 via path 66. Additionally, coolant passing through the heater core 20, HWP 22, and heater 24 via path 62 flows into the thermostat 30. Furthermore, coolant passing through the radiator 14 and the reservoir 16 also flows into the thermostat 30 via path 66. Thus, coolant flowing into the thermostat 30 from both directions circulates in the EWP 12 and the engine 10 via path 61. In this way, a portion of the coolant also flows to the radiator 14, thus suppressing the overheating of the coolant.

[0051] [EWP Flow Control]

[0052] Figure 5 This is a flowchart illustrating the flow control of EWP12 performed by ECU100. ECU100 obtains the target flow rate of coolant flowing in EWP12, the opening degree of thermostat 30, the rotational speed of HWP22, and the opening ratio of path 61 based on four-way valve 40 (step S1). The target flow rate of coolant flowing in EWP12 is calculated by engine 100 primarily based on the operating state of engine 10. The opening degree of thermostat 30 is estimated by ECU100, for example, based on the temperature of coolant detected by temperature sensor 18. ECU100 obtains, for example, the target rotational speed of HWP22 calculated according to heating requirements as the rotational speed of HWP22. ECU100 obtains the opening ratio of path 61, for example, by referring to a mapping that specifies the opening ratio of path 61 based on the target rotational position of the rotor of four-way valve 40 as described above.

[0053] Next, the ECU 100 calculates the target rotational speed of the EWP 12 for achieving the target flow rate based on the acquired information (step S2). The method of calculating the target rotational speed will be described later in detail. Next, the ECU 100 controls the duty ratio of the voltage applied to the EWP 12 in such a manner that the rotational speed of the EWP 12 becomes the target rotational speed (step S3). Thereby, the flow rate through the EWP 12 can be controlled to the target flow rate.

[0054] [Method of calculating target rotational speed of EWP]

[0055] The ECU 100 calculates the target rotational speed of the EWP 12 with reference to the map exemplified in Figure 5 Figure 3 is an example of a map that defines the target rotational speed of the EWP 12 corresponding to the target flow rate of the cooling water through the EWP 12. Figure 5 (A) of is an example of a map that defines the target rotational speed of the EWP 12 corresponding to the target flow rate of the cooling water through the EWP 12 and the opening degree of the thermostat 30. The greater the target flow rate of the cooling water through the EWP 12, the higher the value of the target rotational speed of the EWP 12 is defined.

[0056] In addition, in the map of (A) of Figure 5 the smaller the opening degree of the thermostat 30, the higher the value of the target rotational speed of the EWP 12 is defined. This is because the smaller the opening degree of the thermostat 30, the lower the flow rate of the cooling water flowing through the radiator 14 and the water reservoir 16 shown in (B) of and to the EWP 12 and the engine 10 via the thermostat 30, even if the rotational speed of the EWP 12 is constant. Figure 5

[0057] In addition, the target rotational speed of the EWP 12 is defined to decrease the prescribed decrease rotational speed according to the rotational speed of the HWP 22 and the opening degree of the thermostat 30. Figure 5 (B) of and Figure 5 (C) of is an explanatory view of the decrease rotational speed of the target rotational speed according to the rotational speed of the HWP 22 and the opening degree of the thermostat 30.

[0058] Figure 5 (B) of is Figure 6 ​​The diagram illustrates the reduction speed of EWP12 when the thermostat 30 is fully closed in the mapping of (A). When the speed of HWP22 is 0, the reduction speed of EWP12 is calculated as zero. When the speed of HWP22 is speed R1, the reduction speed of EWP12 is calculated as speed D1. When the speed of HWP22 is speed R2, which is higher than speed R1, the reduction speed of EWP12 is calculated as speed D2, which is higher than speed D1. That is, the higher the speed of HWP22, the larger the reduction speed of EWP12 is calculated. This is because the higher the speed of HWP22, the greater the flow rate of cooling water from heater core 20, HWP22, and heater 24 through thermostat 30 to EWP12 and engine 10. Therefore, the higher the speed of HWP22, the larger the reduction speed of EWP12 is calculated, and thus the smaller the target speed of EWP12 is calculated.

[0059] Figure 6 (C) is in Figures 5-6 The diagram in (A) illustrates the reduction in engine speed when the thermostat 30 is in a partially open state. Similar to the fully closed state, when the speed of HWP22 is 0, the reduction in engine speed of EWP12 is calculated as zero. When the speed of HWP22 is speed R1, the reduction in engine speed of EWP12 is calculated as speed d1. When the speed of HWP22 is speed R2, the reduction in engine speed of EWP12 is calculated as speed d2, which is greater than speed d1. Here, speed d1 is lower than speed D1. Speed ​​d2 is lower than speed D2. That is, even when the speed of HWP22 is the same, when the thermostat 30 is in a partially open state, the reduction in engine speed of EWP12 is lower than when the thermostat 30 is in a fully closed state. When the thermostat 30 is in a partially open state, unlike when the thermostat 30 is in a fully closed state, cooling water from the radiator 14 and the water tank 16 flows into EWP12 and engine 10 via the thermostat 30. This is because the flow rate of cooling water flowing from the radiator 14 side into the EWP12 is less affected by the rotational speed of the HWP22.

[0060] When the thermostat 30 is fully open, the speed reduction of EWP12 is calculated as zero, regardless of the speed of HWP22. This is because, when the thermostat 30 is fully open, compared to when it is partially open or fully closed, the proportion of cooling water flowing from the radiator 14 side to EWP12 is higher than the proportion of cooling water flowing from the HWP22 side to EWP12. Therefore, the speed of HWP22 has a smaller impact on the flow of cooling water through EWP12. Thus, the smaller the opening of the thermostat 30, the larger the calculated speed reduction of EWP12.

[0061] ​is an example of a map that defines a target rotational speed of the EWP 12 corresponding to a target flow rate of the cooling water passing through the EWP 12 and an opening rate of the passage 61 of the four-way valve 40. ​ A map when the thermostat 30 is in the half-open state is exemplified. The smaller the opening rate of the passage 61, the higher value the target rotational speed of the EWP 12 is defined as. This is because the smaller the opening rate of the passage 61, the lower the flow rate of the cooling water passing through the EWP 12 and the engine 10. Irrespective of the open / close state of the thermostat 30, the smaller the opening rate of the passage 61, the higher value the target rotational speed of the EWP 12 is defined as. The ECU 100 calculates the target rotational speed of the EWP 12 with reference to a plurality of maps like the above. Thereby, the flow rate of the cooling water passing through the EWP 12 can be controlled with high precision to the target flow rate.

[0062] A case where the target rotational speed of the EWP 12 is calculated based on the map as shown in ​ , is exemplified, but is not limited thereto. For example, the target rotational speed of the EWP 12 can be calculated by an arithmetic expression with the target flow rate of the cooling water circulating in the EWP 12, the opening degree of the thermostat 30, the rotational speed of the HWP 22, and the opening rate of the passage 61 of the four-way valve 40 as arguments.

[0063] A case where the lower rotational speed is set to zero irrespective of the rotational speed of the HWP 22 in the fully-open state of the thermostat 30 is exemplified, but is not limited thereto. For example, if the smaller the opening degree of the thermostat 30, the larger value the lower rotational speed is calculated as, it can also be that even when the thermostat 30 is in the fully-open state, the larger the rotational speed of the HWP 22, the larger value the lower rotational speed is calculated as.

[0064] The above describes an embodiment of the present application, but the present application is not limited to this particular embodiment, and various modifications and changes can be made within the scope of the gist of the present application recited in the claims.

Claims

1. An engine cooling system, comprising: a cooling circuit that circulates cooling water between an engine and a heater core in a thermostat fully closed state by an engine water pump that circulates the cooling water to the engine and a heater water pump that circulates the cooling water to the heater core, and that circulates a part of the cooling water that has passed through the engine to the heater core and then to the engine and a part of the cooling water that has passed through the engine to a radiator and then to the engine in a thermostat fully open state; and a control device that calculates a target rotation speed of the engine water pump based on a target flow rate of the cooling water that circulates in the engine water pump, an opening degree of the thermostat, and a rotation speed of the heater water pump, and controls the rotation speed of the engine water pump to the target rotation speed, wherein the control device is configured to calculate the target rotation speed higher as the target flow rate is larger, to calculate the target rotation speed higher as the opening degree of the thermostat is smaller, to lower the target rotation speed by a prescribed lowering rotation speed in accordance with the rotation speed of the heater water pump and the opening degree of the thermostat, to calculate the lowering rotation speed larger as the rotation speed of the heater water pump is higher, and to calculate the lowering rotation speed larger as the opening degree of the thermostat is smaller.

2. The engine cooling system according to claim 1, wherein the control device calculates the lowering rotation speed as zero regardless of the rotation speed of the heater water pump in the thermostat fully open state.

3. The engine cooling system according to claim 2, wherein the cooling circuit includes an on-off valve that adjusts an opening rate of a path through which the cooling water that has passed through the engine is circulated to the engine without passing through the radiator, and the control device calculates the target rotation speed higher as the opening rate is smaller. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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