Cooling system and control method for a medium / heavy duty engine

Abstract

The application provides a cooling system and control method of a medium and heavy engine. The thermostat comprises a wax thermostat and an electric control rotary valve thermostat which are arranged in parallel on a cooling water path of the engine. The engine is provided with a first cooling water path and a second cooling water path. The second cooling water path is connected to a rear of a hydraulic retarder and is merged into a water path with the first cooling water path to connect to an inlet of the thermostat. The electric control thermostat and the wax thermostat are arranged in parallel. The engine cooling liquid working temperature is improved, the engine fuel consumption is reduced, the water return failure of the traditional engine cooling system is reduced, the water return failure of the hydraulic retarder is greatly reduced, and the temperature, temperature rising rate and working condition or load can be flexibly controlled.

Description

Technical Field

[0001] The present invention belongs to the technical field of engines and relates to a cooling system and a control method for a medium- and heavy-duty engine. Background Art

[0002] With the increase in logistics volume and the improvement in transportation efficiency, heavy-duty vehicles now require diesel engines with increasingly greater power and torque. To ensure safe driving, heavy-duty vehicles are generally equipped with retarders as high-speed auxiliary braking components. The retarder generates a lot of heat when in operation, which needs to be dissipated by the engine's radiator cooling system. In order to reduce engine friction work, the maximum torque operating speed of medium and heavy-duty diesel engines tends to develop at low speeds. The thermostat of the cooling system is mostly a wax thermostat. When the water temperature rises to a certain temperature (set temperature), the thermostat opens. When the water temperature of the engine is lowered, the thermostat closes. Whether it is an ordinary thermostat, an electronically controlled rotary valve (ball valve), or an electrically heated thermostat, it is controlled in this way.

[0003] The conventional engine cooling system uses an electrically controlled ball valve or rotary cylinder (barrel or can) connected in parallel with a wax-type thermostat. When the outlet water temperature is low or the load is low, the thermostat closes. When the outlet water temperature of the engine's internal circulation rises and reaches the set thermostat opening temperature, the wax-type thermostat opens. If the engine is heavily loaded, the water temperature rises rapidly, reaching or exceeding the set opening temperature of the wax-type thermostat, and the wax-type thermostat opens. However, this solution still has drawbacks: when the vehicle is traveling downhill, the engine load decreases. Due to the vehicle being in a higher gear (12th gear) and a speed of 60 km / h to 70 km / h, the engine speed is low (1000 rpm), resulting in low engine flow. At this time, the wax-type thermostat is either closed or at a small opening. If a hydraulic retarder is used, the high heat generated by the hydraulic retarder and the slow response of the wax-type thermostat make it difficult to meet cooling requirements, which can easily lead to engine overheating and backflow.

[0004] There are also technical solutions in the existing technology to prevent water backflow in the engine cooling system. For example, the engine cooling system disclosed in Chinese patent CN215633286U prevents water from flowing back by regulating air pressure. However, the disadvantages are that the water temperature channel is small, the resistance is large, the backflow control has a certain hysteresis, and the cooling effect is poor. It cannot meet the heat dissipation requirements of vehicles with retarders as high-speed auxiliary braking components. Summary of the Invention

[0005] The present invention provides a cooling system and control method for medium and heavy-duty engines. The parallel structure of an electronically controlled thermostat and a wax-type thermostat increases the operating temperature of the engine coolant and reduces the engine's fuel consumption. At the same time, the malfunction of traditional engine cooling systems prone to water backflow is reduced, and the malfunction of using a hydraulic retarder prone to water backflow is greatly reduced. The system can be flexibly controlled according to temperature, temperature rise rate, working conditions or load.

[0006] To achieve the above-mentioned objectives, the present invention provides a cooling system for medium and heavy-duty engines, which is used for the coordinated cooling of the engine and the hydraulic retarder. The system comprises a water pump, an engine, a thermostat, and a radiator water tank which are connected end to end in sequence to form a water circulation. The system also comprises a transmission and a hydraulic retarder which are arranged in sequence at the output end of the engine. The thermostat comprises a wax thermostat and an electronically controlled rotary valve thermostat which are arranged in parallel on the cooling water path of the engine. The engine is provided with a first cooling water path and a second cooling water path. After the second cooling water path is connected to the hydraulic retarder, it merges with the first cooling water path to form a water path which is connected to the inlet of the thermostat.

[0007] The main cooling water route of the engine flows from the water pump to the engine, the secondary cooling water route flows from the engine to the hydraulic retarder, and then returns to the thermostat. At the same time, a part of the first cooling water route is throttled through the throttle plate and directly goes to the thermostat.

[0008] If the thermostat is open, the water will flow to the water tank radiator after passing through the thermostat, and then return to the water pump inlet after passing through the radiator water tank. At this time, it is in a large circulation condition.

[0009] If the thermostat is in the closed state, water flows through the small circulation hole of the thermostat to the water pump inlet, and is now in the small circulation working condition.

[0010] The thermostats used in the cooling systems of medium and heavy-duty vehicles are mostly wax-type. When the water temperature rises to the set point (generally 83°C to 85°C), the thermostat opens (usually by 0.1mm). When the temperature continues to rise to 95±2°C, it reaches maximum lift (usually 10±2mm). When the engine water temperature drops, the thermostat closes.

[0011] Preferably, the present invention further comprises a temperature sensor, which is arranged on the water path at the inlet of the thermostat.

[0012] Preferably, the system further includes an engine controller, to which the temperature sensor and the engine are electrically connected. The engine controller issues commands to control the opening of the electronically controlled rotary valve thermostat based on the engine outlet water temperature and engine operating condition data. The engine controller can identify the engine operating conditions and the engine outlet water temperature to precisely control the opening of the electronically controlled rotary valve thermostat, achieving rapid control speed and avoiding the slow response of wax-type thermostats.

[0013] Preferably, the present invention further includes an expansion water tank, which is provided with two inlets, one of which is connected to the position where the first cooling water circuit and the second cooling water circuit merge, and the other inlet is connected to the expansion water tank; the outlet of the expansion water tank is connected to the inlet of the water pump. The expansion water tank of the present invention is used for degassing, and the degassing water circuit has two branches. One branch flows from the engine to the expansion water tank, and then returns to the water inlet of the water pump from the expansion water tank. The other branch flows from the heat dissipation water tank to the expansion water tank, and then returns to the water inlet of the water pump from the expansion water tank. The use of two degassing branches ensures that there are no bubbles in the cooling water circuit, ensuring the cooling effect.

[0014] The present invention also provides a method for controlling a cooling system of a medium-to-heavy-duty engine, which uses the above-mentioned cooling system of the medium-to-heavy-duty engine and includes the following steps:

[0015] S1: Identify the vehicle operating condition and detect the engine water outlet temperature. If the vehicle is in a normal driving condition, proceed to step S2; if the vehicle is in a long downhill condition, proceed to step S3; if the vehicle is in a climbing condition, proceed to step S4;

[0016] S2: When the engine outlet water temperature reaches the cut-off temperature of the wax thermostat, the wax thermostat opens and the electronically controlled rotary valve thermostat closes;

[0017] S3: The vehicle is in high gear, the hydraulic retarder is engaged, and the engine outlet water temperature is between 80 and 87°C. The engine controller commands the electronically controlled rotary valve thermostat to open 25%. As the vehicle speed increases, the driver engages the hydraulic retarder. Due to the downhill slope, the engine load is low, and the outlet water temperature is between 80 and 87°C. The hydraulic retarder triggers the electronically controlled rotary valve thermostat to open 25%. This allows cooling water to flow through the electronically controlled rotary valve thermostat and into the radiator, reducing the risk of backflow caused by the high heat dissipation generated by the hydraulic retarder. In contrast, in vehicles without a rotary valve thermostat, the engine speed is low, and engine flow is low. At this time, the wax thermostat is open at a small angle. Due to the high heat dissipation generated by the hydraulic retarder, the wax thermostat responds slowly, failing to meet cooling requirements and potentially causing backflow.

[0018] S4: When the engine outlet water temperature reaches the range of 92-98℃, the wax thermostat and the electric control rotary valve thermostat are opened at the same time, and the wax thermostat and the electric control rotary valve thermostat work in parallel.

[0019] In step S4, the wax thermostat and the electric control rotary valve thermostat work in parallel. The reason is:

[0020] If only an ordinary wax thermostat is used, if the thermostat opening is designed to be high, such as 85℃ for initial opening, it will not be able to respond, the water temperature will easily increase at the opening speed, and water backflow will occur; if the opening temperature is set lower, such as 78℃ for initial opening, the coolant will take away more heat, resulting in higher fuel consumption.

[0021] If only an electric rotary valve thermostat is used, a relatively large flow rate electric rotary valve thermostat is required. Although the system cooling performance can meet the requirements, the cost is relatively high. The use of wax thermostats working in parallel can ensure sufficient cooling flow and reduce costs.

[0022] Preferably, the initial opening temperature of the wax thermostat is set to 87°C. In step S4, considering the opening work of the electronically controlled rotary valve thermostat, a wax thermostat with a high initial opening temperature can be used, because the higher the opening temperature of the wax thermostat, the more fuel-efficient it is, which can reduce fuel consumption and take into account both fuel consumption and cost.

[0023] Preferably, in step S4, the engine controller controls the opening of the electronically controlled rotary valve thermostat based on the engine outlet water temperature. When the engine outlet water temperature is 88°C, the opening of the electronically controlled rotary valve thermostat is 25%, when the engine outlet water temperature is 90°C, the opening of the electronically controlled rotary valve thermostat is 50%, when the engine outlet water temperature is 92°C, the opening of the electronically controlled rotary valve thermostat is 75%, and when the engine outlet water temperature is 94°C, the opening of the electronically controlled rotary valve thermostat is 100%. This configuration reduces the flow resistance through the wax thermostat, reduces the rate of water temperature increase, and reduces the occurrence of water backflow failures.

[0024] In the present invention, the electronically controlled rotary valve thermostat can be flexibly controlled by the engine. The engine outlet water temperature is not the only factor affecting the electronically controlled rotary valve thermostat, and the thermostat can be flexibly controlled according to the temperature, temperature rise rate, working conditions or load.

[0025] Specifically, in step S3 of the present invention, the long downhill operating condition of the vehicle is: the gear is 12 gear, the vehicle speed is 60-70 km / h, the engine speed is 900-1100 rpm, and the hydraulic retarder is working. At this time, the engine load becomes smaller and the wax thermostat may be in a closed state. At this time, if hydraulic retarding is used, since the hydraulic retarding generates relatively large heat, and the engine is in an internal engine circulation, the engine's antifreeze capacity is small and the heat capacity is also small, resulting in a sharp increase in the engine antifreeze temperature. The wax thermostat has a slow response time, and the engine overheats and backflows. If the cooling system of the present invention is used, the electronically controlled rotary valve thermostat can be triggered to forcibly open by using the hydraulic retarder. Since the electronically controlled rotary valve thermostat is opened, the antifreeze enters a large circulation, avoiding the occurrence of engine backflow.

[0026] Specifically, during normal driving conditions, the hydraulic retarder is not operating, and the engine outlet water temperature is between 80°C and 95°C. The wax thermostat's opening in step S2 is controlled based on the engine outlet water temperature. During normal driving conditions, the hydraulic retarder is not in use, and the engine outlet water temperature is between 80°C and 90°C. The standard wax thermostat controls the opening based on the aforementioned operating temperature. At this point, the engine ECU is not issuing any commands, and the electronically controlled rotary valve thermostat is closed.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The parallel structure of the electronically controlled rotary valve thermostat and the wax-type thermostat increases the operating temperature of the engine coolant, reduces the engine's fuel consumption, and reduces the fault of water backflow in the traditional engine cooling system;

[0029] 2. The parallel structure of the electronically controlled rotary valve thermostat and the wax-type thermostat greatly reduces the fault of water backflow when using the hydraulic retarder.

[0030] 3. The electronically controlled rotary valve thermostat can be flexibly controlled by the ECU, with a fast reflection speed. Water temperature is not the only factor affecting the electronically controlled rotary valve thermostat, and it can be flexibly controlled according to temperature, temperature rise rate, working conditions or load. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a structural schematic diagram of a cooling system for a medium and heavy-duty engine according to the present invention.

[0032] Figure 2 It is a structural diagram of the thermostat described in the present invention.

[0033] Figure 3 It is a structural schematic diagram of the wax thermostat described in the present invention.

[0034] Figure 4 It is a structural schematic diagram of the electronically controlled rotary valve thermostat of the present invention.

[0035] Figure 5 This is a coolant flow diagram of a cooling system of a medium- and heavy-duty engine described in the present invention when the vehicle is in normal driving conditions.

[0036] Figure 6 The present invention provides a coolant flow diagram for a cooling system of a medium- to heavy-duty engine when the vehicle is operating in a long downhill slope.

[0037] Figure 7 The present invention provides a coolant flow diagram of a cooling system for a medium- to heavy-duty engine when the vehicle is in a climbing condition.

[0038] Figure 8The present invention discloses a coolant flow diagram of a cooling system of a medium- to heavy-duty engine in a degassing working state.

[0039] In the figure: 1-water pump, 2-engine, 3-thermostat, 31-wax thermostat, 32-electrically controlled rotary valve thermostat, 4-radiator tank, 5-transmission, 6-hydraulic retarder, 7-temperature sensor, 8-engine controller, 9-expansion tank. DETAILED DESCRIPTION

[0040] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The exemplary embodiments of the present invention and their description are used to explain the present invention and do not constitute improper limitations on the present invention.

[0041] In the description of the present invention, it should be understood that the terms "left", "right", "up", "down", "front", "back", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or structure referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0042] Example 1:.

[0043] like Figures 1-2 As shown, this embodiment is applied to a vehicle equipped with a hydraulic retarder 6, which serves as a high-speed auxiliary brake component. The retarder generates significant heat during operation, which requires the engine 2's radiator cooling system to dissipate the heat. This provides for the coordinated cooling of the engine 2 and the hydraulic retarder 6. The cooling system for a medium- and heavy-duty engine described in this embodiment includes a water pump 1, an engine 2, a thermostat 3, and a radiator tank 4, all connected end-to-end to form a water circulation system. Furthermore, the system also includes a transmission 5 and a hydraulic retarder 6, which are sequentially arranged at the output end of the engine 2.

[0044] like Figure 2 As shown, in this embodiment, the thermostat 3 includes a wax thermostat 31 and an electronically controlled rotary valve thermostat 32 arranged in parallel on the cooling water path of the engine 2. The engine 2 is provided with a first cooling water path and a second cooling water path. The second cooling water path is connected to the hydraulic retarder 6 and then merges with the first cooling water path to form a water path connected to the inlet of the thermostat 3.

[0045] The structure of wax thermostat 31 is as follows Figure 3 As shown, the structure of the electronically controlled rotary valve thermostat 32 is as follows Figure 4 shown.

[0046] The main cooling water route of engine 2 flows from water pump 1 to engine 2, and the secondary cooling water route flows from engine 2 to hydraulic retarder 6, and then returns to thermostat 3;

[0047] At the same time, the first cooling water channel is throttled by the throttle plate and directly reaches the thermostat 3.

[0048] If the thermostat 3 is open, the water flows through the thermostat 3 to the water tank radiator, passes through the radiator water tank and then returns to the water inlet of the water pump 1. At this time, it is in a large circulation working condition.

[0049] If the thermostat 3 is in the closed state, the water flows through the small circulation hole of the thermostat 3 to the water inlet of the water pump 1, and is now in the small circulation working state.

[0050] The thermostat 3 used in the cooling system of medium- and heavy-duty vehicles is mostly a wax-type thermostat 31. When the water temperature rises to the set point (generally 83°C to 85°C), the thermostat 3 opens, typically with a lift of 0.1mm. When the temperature continues to rise to 95±2°C, it reaches maximum lift (generally 10±2mm). When the water temperature of the engine 2 drops, the thermostat 3 closes.

[0051] This embodiment further includes a temperature sensor 7, which is disposed in the water path between the inlet and outlet of the thermostat 3. The temperature sensor 7 is used to obtain the engine outlet water temperature and transmit it to the engine controller 8.

[0052] This embodiment also includes an engine controller 8, to which the temperature sensor 7 and the engine 2 are electrically connected. The engine controller 8 issues commands to control the opening of the electronically controlled rotary valve thermostat 32 based on the engine's outlet water temperature and the engine's operating condition data. The engine controller 8 can identify the engine's operating conditions and the engine's outlet water temperature, precisely controlling the opening of the electronically controlled rotary valve thermostat 32 with high speed, thus avoiding the slow response of the wax-type thermostat 31.

[0053] like Figure 8 As shown, this embodiment further includes an expansion water tank 9, which is provided with two inlets, one of which is connected to the position where the first cooling water circuit and the second cooling water circuit merge, and the other inlet is connected to the expansion water tank 9; the outlet of the expansion water tank 9 is connected to the inlet of the water pump 1. Figure 8 As shown, the expansion tank 9 of this embodiment is used for degassing. The degassing water path has two branches. One branch flows from the engine 2 to the expansion tank 9, and then returns from the expansion tank 9 to the water inlet of the water pump 1. The other branch flows from the radiator tank 4 to the expansion tank 9, and then returns from the expansion tank 9 to the water inlet of the water pump 1.

[0054] In this embodiment, the cooling water circuit of the cooling system of the engine 2 with a hydraulic retarder 6 using the electronically controlled rotary valve thermostat 32 is substantially the same as that of the cooling system of the engine with a hydraulic retarder 6, except that an additional electronically controlled rotary valve thermostat 32 is connected in parallel. The electronically controlled rotary valve thermostat 32 and the wax-type thermostat 31 can operate independently or in parallel. In addition to the same temperature control principle, they can also be flexibly controlled to be opened and closed according to the operating conditions of the engine 2.

[0055] Example 2:

[0056] This embodiment is a method for controlling a cooling system of a medium-to-heavy engine, which is used to control the cooling system of the medium-to-heavy engine in the above embodiment, and includes the following steps:

[0057] S1: Identify the vehicle operating condition and detect the engine water outlet temperature. If the vehicle is in a normal driving condition, proceed to step S2; if the vehicle is in a long downhill condition, proceed to step S3; if the vehicle is in a climbing condition, proceed to step S4;

[0058] S2: If Figure 5 As shown, when the engine outlet water temperature reaches the disconnection temperature of the wax thermostat 31, the wax thermostat 31 opens and the electronically controlled rotary valve thermostat 32 closes; the coolant flow path is: engine 2, first cooling water circuit, wax thermostat 31, radiator water tank 4, water pump 1.

[0059] S3: If Figure 6 As shown, the vehicle is in the high-speed range, the hydraulic retarder 6 is used, the engine outlet water temperature is between 80 and 87°C, and the engine controller 8 issues a command to open the electronically controlled rotary valve thermostat 32 to 25%; there are two coolant flow paths, the first one is: engine 2, first cooling water path, electronically controlled rotary valve thermostat 32, radiator water tank 4, water pump 1, and the second one is: engine 2, second cooling water path, hydraulic retarder 6, electronically controlled rotary valve thermostat 32, radiator water tank 4, water pump 1.

[0060] S4: As Figure 7 As shown, when the engine outlet water temperature reaches the range of 92-98°C, the wax thermostat 31 and the electronically controlled rotary valve thermostat 32 open simultaneously, operating in parallel. The coolant flows through two pathways: the first: engine 2, first cooling water path, electronically controlled rotary valve thermostat 32 and wax thermostat 31, radiator tank 4, and water pump; the second: engine 2, second cooling water path, hydraulic retarder 6, electronically controlled rotary valve thermostat 32 and wax thermostat 31, radiator tank 4, and water pump 1.

[0061] In this embodiment, the initial opening temperature of the wax thermostat 31 is set to 87°C.

[0062] In this embodiment, in step S4, the engine controller 8 controls the opening of the electronically controlled rotary valve thermostat 32 according to the engine outlet water temperature. When the engine outlet water temperature is 88°C, the opening of the electronically controlled rotary valve thermostat 32 is 25%. When the engine outlet water temperature is 90°C, the opening of the electronically controlled rotary valve thermostat 32 is 50%. When the engine outlet water temperature is 92°C, the opening of the electronically controlled rotary valve thermostat 32 is 75%. When the engine outlet water temperature is 94°C, the opening of the electronically controlled rotary valve thermostat 32 is 100%.

[0063] In this embodiment, in step S3, the long downhill operating condition of the vehicle is: the gear is 12, the vehicle speed is 60k~70km / h, the speed of the engine 2 is 900~1100rpm, and the hydraulic retarder 6 is working.

[0064] In this embodiment, during normal driving conditions, the hydraulic retarder 6 does not work, the engine outlet water temperature is between 80° C. and 95° C., and the opening of the wax thermostat 31 in step S2 is controlled according to the engine outlet water temperature.

Claims

1. A cooling system for a medium or heavy-duty engine, characterized by: The invention comprises a water pump (1), an engine (2), a thermostat (3), and a radiator water tank (4) which are connected end to end to form a water circulation, and also comprises a transmission (5) and a hydraulic retarder (6) which are arranged in sequence at the output end of the engine (2), the thermostat (3) comprises a wax thermostat (31) and an electric control rotary valve thermostat (32) which are arranged in parallel on the cooling water path of the engine (2), the engine (2) is provided with a first cooling water path and a second cooling water path, the second cooling water path is connected to the hydraulic retarder (6) and then merged with the first cooling water path to form a water path which is connected to the inlet of the thermostat (3); the invention also comprises a temperature sensor (7), the temperature sensor The thermostat (3) is provided with a temperature sensor (7) on the water path at the inlet of the thermostat (7); the engine controller (8) is also included, the temperature sensor (7) and the engine (2) are electrically connected to the engine controller (8), and the engine controller (8) issues instructions to control the opening of the electronically controlled rotary valve thermostat (32) according to the engine outlet water temperature and the engine (2) working condition data; the expansion water tank (9) is also included, and the expansion water tank (9) is provided with two inlets, one inlet is connected to the position after the first cooling water path and the second cooling water path merge, and the other inlet is connected to the expansion water tank (9); the outlet of the expansion water tank (9) is connected to the inlet of the water pump (1).

2. A method for controlling the cooling system of a medium and heavy-duty engine, characterized in that A cooling system for a medium- and heavy-duty engine according to claim 1 comprises the following steps: S1: Identify the vehicle operating condition and detect the engine water outlet temperature. If the vehicle is in a normal driving condition, proceed to step S2; if the vehicle is in a long downhill condition, proceed to step S3; if the vehicle is in a climbing condition, proceed to step S4; S2: When the engine outlet water temperature reaches the cut-off temperature of the wax thermostat (31), the wax thermostat (31) opens and the electric control rotary valve thermostat (32) closes; S3: The vehicle is in the high gear range, the hydraulic retarder (6) is activated, the engine outlet water temperature is between 80 and 87°C, and the engine controller (8) issues a command to open the electronically controlled rotary valve thermostat (32) to 25%; S4: When the engine outlet water temperature reaches the range of 92-98°C, the wax thermostat (31) and the electric control rotary valve thermostat (32) are opened at the same time, and the wax thermostat (31) and the electric control rotary valve thermostat (32) work in parallel.

3. The method for controlling a cooling system of a medium- to heavy-duty engine according to claim 2, characterized in that: The initial opening temperature of the wax thermostat (31) is set to 87°C.

4. The method for controlling a cooling system of a medium- to heavy-duty engine according to claim 2, wherein: In step S4, the engine controller (8) controls the opening of the electronically controlled rotary valve thermostat (32) according to the engine outlet water temperature. When the engine outlet water temperature is 88°C, the opening of the electronically controlled rotary valve thermostat (32) is 25%. When the engine outlet water temperature is 90°C, the opening of the electronically controlled rotary valve thermostat (32) is 50%. When the engine outlet water temperature is 92°C, the opening of the electronically controlled rotary valve thermostat (32) is 75%. When the engine outlet water temperature is 94°C, the opening of the electronically controlled rotary valve thermostat (32) is 100%.

5. The method for controlling the cooling system of a medium-to-heavy engine according to claim 2, characterized in that In step S3, the long downhill operating condition of the vehicle is: the gear position is 12, the vehicle speed is 60-70 km / h, the engine (2) speed is 900-1100 rpm, and the hydraulic retarder (6) is working.

6. The method for controlling a cooling system of a medium- to heavy-duty engine according to claim 2, characterized in that: In normal driving conditions, the hydraulic retarder (6) does not work, the engine outlet water temperature is between 80°C and 95°C, and the opening of the wax thermostat (31) in step S2 is controlled according to the engine outlet water temperature.

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