Patent Publication Number: US-2022235692-A1

Title: Engine system

Description:
TECHNICAL FIELD 
     The disclosed technology relates to an engine system. 
     BACKGROUND OF THE DISCLOSURE 
     JP2016-128652A discloses a cooling device for an engine. This cooling device has a radiator path which circulates coolant between the engine and a radiator, and a radiator bypass path which bypasses the radiator and circulates the coolant. In the radiator bypass path, an ATF warmer which warms a heater core of an air-conditioner and lubrication oil of an automatic transmission is disposed. 
     The cooling device has a rotary flow rate control valve. The rotary flow rate control valve opens and closes the radiator path and the radiator bypass path according to a rotational position of a rotary valve body. Further, the rotary flow rate control valve has a radiator path connecting passage and a thermostat valve allocation passage. The radiator path connecting passage is connected to the radiator path. A thermostat valve is provided to the thermostat valve allocation passage. When opening the thermostat valve, the coolant flows into the radiator path from the thermostat valve allocation passage. 
     When the engine is warm with the coolant at a temperature above a given temperature, the rotary flow rate control valve rotates the rotary valve body to a rotational position where the coolant flows into each of the radiator bypass path and the thermostat valve allocation passage. Since the thermostat valve opens while the engine is warm, the coolant flows into the radiator path from the thermostat valve allocation passage. 
     When the temperature of the coolant further increases, the rotary flow rate control valve rotates the rotary valve body to a rotational position where the coolant flows to all of the radiator bypass path, the thermostat valve allocation passage, and the radiator path connecting passage. Further, the rotational position of the rotary valve body is adjusted so that a flow rate of the coolant to the radiator path increases as a temperature of the coolant, an engine load, and/or an engine speed increase. 
     The combustion chamber becomes high in the temperature after the engine has been fully warmed up. In order to cool the combustion chamber, a passage through which the coolant cooled by the radiator flows (a so-called “water jacket”) is provided to a part around the combustion chamber, such as a cylinder bore and a cylinder head, which constitute the engine body, which is also provided to the cooling device disclosed in JP2016-128652A. 
     Meanwhile, in the engine combustion control, the temperature inside the combustion chamber (in-cylinder temperature) is one of the important factors. The in-cylinder temperature requires a more precise control as the combustion control becomes more advanced. For example, in order to stably control compression ignition combustion, it is necessary to accurately control the in-cylinder temperature at a temperature higher than that of spark ignition combustion. In addition, since the heat generated inside the combustion chamber varies according to the engine load, the in-cylinder temperature also varies. 
     In the in-cylinder temperature control, a wall temperature of the combustion chamber is one of the important factors. It is demanded that the wall temperature of the combustion chamber is adjusted with good response to the change in the engine load. 
     The cooling device disclosed in JP2016-128652A lowers the temperature of the coolant by increasing the flow rate of the coolant which flows through the radiator path, when the temperature of the coolant becomes high. When the temperature of the coolant changes, the heat exchanging quantity between the coolant and the combustion chamber changes. If the heat exchanging quantity is changed according to the heat generated inside the combustion chamber, the wall temperature of the combustion chamber can be adjusted. 
     However, since the calorific capacity of the coolant is large, it requires a long period of time to change the temperature of the coolant. It is difficult for the temperature adjustment of the coolant to adjust the wall temperature of the combustion chamber with good response to the change in the engine load. 
     SUMMARY OF THE DISCLOSURE 
     The technology disclosed herein adjusts a wall temperature of a combustion chamber with high response according to a load of an engine. 
     The present inventers have completed the technology disclosed herein by paying attention to the adjustment of the wall temperature of the combustion chamber by changing a flow rate of coolant which flows through a water jacket to change a heat transfer coefficient between the coolant and the combustion chamber, without changing a temperature of the coolant. 
     According to one aspect of the present disclosure, an engine system is provided, which includes an engine having a water jacket formed around a combustion chamber, a circulation system that is attached to the engine and circulates coolant through the water jacket, and a controller configured to control the circulation system according to an operating state of the engine. The circulation system includes a radiator passage including a heat exchanger, a bypass passage bypassing the heat exchanger, a flow rate control device that adjusts a flow rate of coolant flowing through the water jacket by adjusting a flow rate of coolant flowing through each of the radiator passage and the bypass passage, and a thermally-actuated valve that is connected to the radiator passage and opens to allow the coolant to pass through the heat exchanger. The engine has a spark plug that forcibly ignites an air-fuel mixture, and switches between a first combustion in which the air-fuel mixture combusts without the forcible ignition of the spark plug, and a second combustion in which the air-fuel mixture combusts by the forcible ignition of the spark plug. The controller is electrically connected to the flow rate control device. When the engine performs the first combustion, the controller controls the flow rate control device to adjust the flow rate of the coolant flowing through the water jacket according to a load of the engine, by closing the radiator passage and adjusting the flow rate of the coolant flowing through the bypass passage. 
     According to this configuration, the coolant passing through the water jacket of the engine exchanges heat with the combustion chamber. The coolant circulates through the water jacket by the circulation system. 
     The circulation system includes the thermally-actuated valve which opens when the coolant reaches a given temperature. When the thermally-actuated valve opens, part of the coolant passes through the heat exchanger, and thus, a coolant temperature decreases. By the thermally-actuated valve, the coolant temperature is maintained at a specific temperature corresponding to a valve-opening temperature of the thermally-actuated valve. 
     When the engine performs the first combustion, the flow rate control device closes the radiator passage, and thus the coolant flows through the bypass passage. Further, the flow rate control device adjusts the flow rate of the coolant. Therefore, the flow rate of the coolant which flows through the water jacket changes. The flow rate of the coolant can be changed by the flow rate control device more promptly compared with the temperature of the coolant. Thus, the flow rate control device can adjust the flow rate of the coolant which flows through the water jacket with high response to the change of the load. 
     As the flow rate of the coolant which flows through the water jacket becomes lower, the heat transfer coefficient decreases, whereas, as the flow rate of the coolant which flows through the water jacket increases, the heat transfer coefficient increases. The heat generated inside the combustion chamber changes according to the engine load. Therefore, since the controller changes, through the flow rate control device, the flow rate of the coolant which flows through the water jacket according to the engine load, the engine system can adjust a wall temperature of the combustion chamber with high response. 
     When the engine performs the first combustion, the controller may increase the flow rate of the coolant flowing through the water jacket as the load increases. 
     As the engine load increases, the heat generated inside the combustion chamber also increases. As the load increases, the flow rate of the coolant flowing through the water jacket increases, and thus, the heat transfer coefficient increases. The wall temperature of the combustion chamber is maintained at the suitable temperature. 
     When the engine performs the second combustion, the controller may control the flow rate control device to allow the coolant to flow through each of the radiator passage and the bypass passage. 
     When the engine performs the second combustion (that is, when the air-fuel mixture combusts by the forcible ignition of the spark plug), the thermal efficiency drops compared with when performing the first combustion. The amount of heat released to the wall part of the combustion chamber increases. When the engine performs the second combustion, the controller allows the coolant to flow through each of the radiator passage and the bypass passage, through the flow rate control device. For example, by increasing the flow rate of the coolant flowing through the radiator passage, the coolant temperature is reduced. When the engine performs the second combustion, the wall temperature of the combustion chamber becomes suitable. 
     When the engine performs the second combustion, the controller may adjust the temperature of the coolant flowing through the water jacket according to the load by adjusting the flow rate of the coolant flowing through the bypass passage and the flow rate of the coolant flowing through the radiator passage. 
     When the flow rate of the coolant flowing through the radiator passage increases, the coolant temperature decreases. Although when the load becomes high, the heat generated inside the combustion chamber increases, by the temperature of the coolant flowing through the water jacket being adjusted according to the load, the wall temperature of the combustion chamber becomes suitable. 
     When the engine performs the second combustion, the controller may reduce the flow rate of the coolant flowing through the bypass passage and increase the flow rate of the coolant flowing through the radiator passage, as the load increases. 
     When the flow rate of the coolant flowing through the radiator passage increases, the coolant temperature decreases. By reducing the coolant temperature when the load is high and the heat generated inside the combustion chamber is also high, the wall temperature of the combustion chamber becomes suitable. On the other hand, when the flow rate of the coolant flowing through the radiator passage decreases, the coolant temperature increases. By increasing the coolant temperature when the load is low and the heat generated inside the combustion chamber is low, the wall temperature of the combustion chamber becomes suitable. 
     When the engine performs the second combustion, the controller may set the flow rate of the coolant flowing through the water jacket at a maximum flow rate. 
     When the engine performs the second combustion, the amount of heat released to the wall part of the combustion chamber increases. By making the flow rate of the coolant flowing through the water jacket the maximum flow rate, the wall temperature of the combustion chamber becomes suitable when the engine performs the second combustion. 
     Both when the engine performs the first combustion and when the engine performs the second combustion, the controller may maintain the wall temperature of the combustion chamber at a constant temperature. 
     The ideal wall temperature of the combustion chamber when the engine performs the first combustion, does not necessarily match with the ideal wall temperature of the combustion chamber when the engine performs the second combustion. When the engine performs the first combustion, since the air-fuel mixture combusts by self-ignition, the wall temperature of the combustion chamber is preferable to be high in view of stabilizing the ignition. On the other hand, when the engine performs the second combustion, if the wall temperature of the combustion chamber is excessively high, abnormal combustion, such as knocking, may occur. Therefore, changing the wall temperature of the combustion chamber according to the switching of the combustion mode is ideal. However, since the calorific capacity of the wall part of the combustion chamber is large, it is difficult to change the temperature of the wall part of the combustion chamber in a short period of time. 
     According to this configuration, both when the engine performs the first combustion and when the engine performs the second combustion, the wall temperature of the combustion chamber is maintained at a permissible specific temperature. More specifically, when the engine performs the first combustion, while maintaining the coolant temperature constant by using the thermally-actuated valve, the flow rate of the coolant which flows through the water jacket is adjusted according to the load, and therefore, the wall temperature of the combustion chamber can be maintained at the specific temperature. On the other hand, when the engine performs the second combustion, by adjusting the flow rate of the coolant which flows through the bypass passage and the flow rate of the coolant which flows through the radiator passage so that the temperature of the coolant which flows through the water jacket is adjusted according to the load, the wall temperature of the combustion chamber can be maintained at the same specific temperature. As a result, even when the combustion mode changes, the wall temperature of the combustion chamber becomes suitable. 
     When the engine performs the second combustion, the controller may lower the temperature of the coolant flowing through the water jacket below a valve-opening temperature of the thermally-actuated valve. 
     When the engine performs the second combustion, the amount of heat released to the wall part of the combustion chamber increases. By relatively lowering the temperature of the coolant flowing through the water jacket when the engine performs the second combustion, the wall temperature of the combustion chamber becomes suitable. 
     When the engine performs the first combustion, the amount of heat released to the wall part of the combustion chamber decreases. When the engine performs the first combustion, the coolant temperature is defined by the valve-opening temperature of the thermally-actuated valve as described above. By setting the valve-opening temperature of the thermally-actuated valve at the relatively high temperature, the temperature of the coolant flowing through the water jacket relatively increases, and thus, the wall temperature of the combustion chamber becomes suitable. 
     In a case where the engine performs the second combustion, when the load is below a given load, the controller may increase the flow rate of the coolant flowing through the radiator passage to lower the temperature of the coolant flowing through the water jacket as the load increases and, when the load is above the given load, the controller may increase the flow rate of the coolant flowing through the radiator passage to maintain the temperature of the coolant flowing through the water jacket constant with respect to the load increase. 
     When the load is lower than the given load, the temperature of the coolant flowing through the water jacket decreases as the load increases. The wall temperature of the combustion chamber can be maintained at a constant temperature with respect to the load increase. When the load is above the given load, the temperature of the coolant flowing through the water jacket becomes constant as the load increases. The wall temperature of the combustion chamber becomes suitable. 
     The controller may determine a combustion mode of the engine at least based on an accelerator opening detected, and control the circulation system according to the determined combustion mode. 
     The combustion mode of the engine may be determined according to at least the accelerator opening, in other words, according to the engine load. 
     The flow rate control device may be installed at a location branching into the bypass passage and the radiator passage, or a location where the bypass passage and the radiator passage are joined. The circulation system may further have a connecting passage connecting the bypass passage to the radiator passage. The thermally-actuated valve may open and close the connecting passage. 
     According to this configuration, while the radiator passage is closed, when the coolant temperature increases and the thermally-actuated valve opens, the coolant flows to the radiator passage from the bypass passage. Thus, the coolant temperature decreases. By the thermally-actuated valve, the coolant temperature can be maintained at the given temperature. 
     The flow rate control device may be installed at a location branching into the bypass passage and the radiator passage, or a location where the bypass passage and the radiator passage are joined. The circulation system may further have a connecting passage bypassing the flow rate control device and connecting the water jacket to the radiator passage. The thermally-actuated valve may open and close the connecting passage. 
     According to this configuration, while the radiator passage is closed by the flow rate control device, when the coolant temperature increases and the thermally-actuated valve opens, the coolant bypasses the flow rate control device and flows to the radiator passage. Thus, the coolant temperature decreases. Also in this case, by the thermally-actuated valve, the coolant temperature can be maintained at the given temperature. 
     The flow rate control device may include a housing provided with a first port that is connected to the bypass passage, a second port that is connected to the radiator passage, and a third port that communicates with each of the first port and the second port. The flow rate control device may include a rotary valve body rotatably accommodated in the housing, intervening between the first port, the second port and the third port, and having a first water flow opening that communicates with the first port and a second water flow opening that communicates with the second port. The flow rate control device may further include an actuator that rotates the rotary valve body to change openings of the first water flow opening and the second water flow opening so as to adjust the flow rate of the coolant which flows through each of the first port and the second port. 
     The flow rate control device having the rotary valve body can selectively close the bypass passage and/or the radiator passage, and can adjust the flow rate of the bypass passage and the flow rate of the radiator passage. The engine system provided with the flow rate control device can realize the flow rate adjustment of the water jacket described above with the simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an exemplary engine system. 
         FIG. 2  is a block diagram of the exemplary engine system. 
         FIG. 3  illustrates an exemplary control map of the engine system. 
         FIG. 4  illustrates an exemplary circulation system. 
         FIG. 5  illustrates an exemplary flow rate control device. 
         FIG. 6  illustrates an exemplary control of the circulation system. 
         FIG. 7  illustrates an exemplary control of the circulation system. 
         FIG. 8  illustrates an exemplary control procedure of the circulation system. 
         FIG. 9  illustrates an exemplary control procedure of the circulation system. 
         FIG. 10  illustrates an exemplary circulation system. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Hereinafter, one embodiment of an engine system is described with reference to the accompanying drawings. The engine system described herein is merely illustration. 
     (Example Configuration of Engine System) 
       FIGS. 1 and 2  illustrate one example of a configuration of an engine system  1 . The engine system  1  is mounted on an automobile. The engine system  1  is provided with an engine  10  which is an internal combustion engine. When the engine  10  operates, the automobile travels. Note that the automobile may be an automobile on which only the engine  10  is mounted as a propelling power source, or may be a hybrid vehicle on which the engine  10  and an electric motor are mounted. 
     The engine  10  is provided with a cylinder block  11  and a cylinder head  12 . A plurality of cylinders  13  are formed in the cylinder block  11 . The engine  10  is a multi-cylinder engine. 
     The plurality of cylinders  13  are lined up along a crankshaft  14  (also see  FIG. 4 ). A piston  15  is inserted in each cylinder  13 . The piston  15  is coupled to the crankshaft  14  via a connecting rod  151 . The piston  15 , the cylinder  13 , and the cylinder head  12  form a combustion chamber  16 . 
     An intake port  121  which communicates with each cylinder  13  is formed in the cylinder head  12 . An intake valve  122  disposed at the intake port  121  opens and closes the intake port  121 . An intake valve operating mechanism  123  (see  FIG. 2 ) opens and closes the intake valve  122  at a given timing. The intake valve operating mechanism  123  is a variable valve operating mechanism which can vary a valve timing and/or a valve lift. 
     An exhaust port  124  which communicates with each cylinder  13  is formed in the cylinder head  12 . An exhaust valve  125  disposed at the exhaust port  124  opens and closes the exhaust port  124 . An exhaust valve operating mechanism  126  opens and closes the exhaust valve  125  at a given timing. The exhaust valve operating mechanism  126  is a variable valve operating mechanism which can vary a valve timing and/or a valve lift. 
     An injector  131  is attached to the cylinder head  12  for every cylinder  13 . The injector  131  injects fuel directly into the cylinder  13 . A spark plug  132  is attached to the cylinder head  12  for every cylinder  13 . The spark plug  132  forcibly ignites an air-fuel mixture inside the cylinder  13 . 
     An intake passage  17  is connected to one side surface of the engine  10 . The intake passage  17  communicates with the intake port  121 . A throttle valve  171  is disposed at the intake passage  17 . The throttle valve  171  adjusts an introducing amount of air into the cylinder  13 . An exhaust passage  18  is connected to the other side surface of the engine  10 . The exhaust passage  18  communicates with the exhaust port  124 . 
     An exhaust gas recirculation (EGR) passage  19  is connected between the intake passage  17  and the exhaust passage  18 . The EGR passage  19  recirculates part of exhaust gas to the intake passage  17 . An EGR cooler  191  is disposed at the EGR passage  19 . The EGR cooler  191  cools the exhaust gas. An EGR valve  192  is disposed at the EGR passage  19 . The EGR valve  192  adjusts a flow rate of exhaust gas which flows through the EGR passage  19 . 
     The engine system  1  is provided with an ECU (Engine Control Unit)  100  for operating the engine  10 . The ECU  100  is a controller based on a well-known microcomputer, which includes a CPU (Central Processing Unit)  101 , memory  102 , and an I/F (interface) circuit  103 . The CPU  101  executes a program. The memory  102  is, for example, comprised of RAM (Random Access Memory) and/or ROM (Read Only Memory), and stores the program and data. The I/F circuit  103  inputs and outputs an electric signal. The ECU  100  is one example of a controller. 
     The ECU  100  is connected to various kinds of sensors SN 1 -SN 5 . The sensors SN 1 -SN 5  output signals to the ECU  100 . The sensors include the following sensors: 
     First water temperature sensor SN 1 : It outputs a signal corresponding to a temperature of coolant which flows into the engine  10 , in a circulation system  91  of the coolant (described later); 
     Second water temperature sensor SN 2 : It is attached to the engine  10 , and outputs a signal corresponding to a temperature of coolant which flows inside the engine  10 ; 
     In-cylinder pressure sensor SN 3 : It is attached to the cylinder head  12 , and outputs a signal corresponding to a pressure inside each cylinder  13 ; 
     Crank angle sensor SN 4 : It is attached to the engine  10 , and outputs a signal corresponding to a rotation angle of the crankshaft  14 ; and 
     Accelerator opening sensor SN 5 : It is attached to an accelerator pedal mechanism, and outputs a signal corresponding to an operating amount of the accelerator pedal. 
     The ECU  100  determines an operating state of the engine  10  based on the signals from the sensors SN 1 -SN 5 , and then calculates a controlled variable of each device according to control logic defined beforehand. The control logic is stored in the memory  102 . The control logic includes calculating targeted amounts and/or controlled variables by using a map stored in the memory  102 . The ECU  100  outputs electric signals according to the calculated controlled variables to the injector  131 , the spark plug  132 , the intake valve operating mechanism  123 , the exhaust valve operating mechanism  126 , the throttle valve  171 , the EGR valve  192 , and a coolant control valve  4  (described later). 
     In more detail, the ECU  100  has a load calculating module  104 , a combustion mode determining module  105 , a water temperature determining module  106 , and a CCV controlling module  107  executed by the CPU  101  to perform their respective functions. These modules are stored in the memory  102  as software modules. 
     The load calculating module  104  calculates a target load of the engine  10  based on the output signal of the accelerator opening sensor SN 5 . The combustion mode determining module  105  determines an operating range of the engine  10  in a base map  301  (described later, see  FIG. 3 ) based on the load of the engine  10  and the output signal of the crank angle sensor SN 4 , and determines a combustion mode corresponding to the operating range. The water temperature determining module  106  determines a temperature of coolant which flows through a water jacket  20  (see  FIG. 4 ) around the combustion chamber  16  based on the output signal of the second water temperature sensor SN 2 . The CCV controlling module  107  cools the engine  10  by controlling the coolant control valve  4  according to the operating state of the engine  10 . 
     (Engine Operation Control Map) 
       FIG. 3  illustrates the base map  301  according to the control of the engine  10 . The base map  301  is stored in the memory  102  of the ECU  100 . The illustrated base map  301  is for a case of the engine  10  being fully warmed up. 
     The base map  301  is defined by the load and engine speed of the engine  10 . The base map  301  is roughly divided into four ranges according to the load and the engine speed. In more detail, a first range  311  includes a range from the low load to high load at a high speed, and a range of the high load at a low speed and a middle speed. A second range  312  is a low-load range at the low speed and the middle speed. A third range  313  is a range from the low load to the middle load at the low speed and the middle speed. A fourth range  314  is a range from the middle load to the high load at the low speed and the middle speed. Note that the low-speed range, the middle-speed range, and the high-speed range may be a low-speed range, a middle-speed range, and a high-speed range when the entire operating range of the engine  10  is divided in the engine speed direction into three substantially equal ranges. 
     Next, operation of the engine  10  in each range is briefly described. The ECU  100  determines the operating range according to the target load for the engine  10  and the engine speed of the engine  10 , and the ECU  100  changes the open-and-close operation of the intake valve  122  and the exhaust valve  125 , the fuel injection timing, and the existence of the forcible ignition, according to the determined operating range. Therefore, the combustion mode of the engine  10  changes between SI (Spark Ignition) combustion, HCCI (Homogeneous Charge Compression Ignition) Combustion, MPCI (Multiple Premixed fuel injection Compression Ignition) combustion, and SPCCI (Spark Controlled Compression Ignition) combustion. 
     (SI Combustion) 
     When the operating state of the engine  10  is in the first range  311 , the ECU  100  carries out flame propagation combustion of the air-fuel mixture inside the cylinder  13 . The intake valve operating mechanism  123  opens the intake valve  122  at a given timing and/or by a given lift, and the exhaust valve operating mechanism  126  opens the exhaust valve  125  at a given timing and/or by a given lift. The injector  131  injects fuel into the cylinder  13  during an intake stroke and/or a compression stroke. The spark plug  132  ignites the air-fuel mixture near a compression top dead center. 
     (HCCI Combustion) 
     When the operating state of the engine  10  is in the second range  312 , the ECU  100  carries out compression ignition combustion of the air-fuel mixture inside the cylinder  13 . The intake valve operating mechanism  123  opens the intake valve  122  at a given timing and/or by a given lift, and the exhaust valve operating mechanism  126  opens the exhaust valve  125  at a given timing and/or by a given lift. The injector  131  injects fuel into the cylinder  13  during an intake stroke. The spark plug  132  does not ignite the air-fuel mixture. The air-fuel mixture carries out compression self-ignition and combusts near a compression top dead center. 
     (MPCI Combustion) 
     When the operating state of the engine  10  is in the third range  313 , the ECU  100  carries out compression ignition combustion of the air-fuel mixture inside the cylinder  13 . The intake valve operating mechanism  123  opens the intake valve  122  at a given timing and/or by a given lift, and the exhaust valve operating mechanism  126  opens the exhaust valve  125  at a given timing and/or by a given lift. The injector  131  injects fuel into the cylinder  13  during an intake stroke and a compression stroke. The injector  131  performs a divided injection. The spark plug  132  does not ignite the air-fuel mixture. The air-fuel mixture carries out compression self-ignition and combusts near a compression top dead center. 
     By the divided injection, the air-fuel mixture inside the cylinder  13  becomes heterogeneous. In this regard, the MPCI combustion differs from the HCCI combustion in which a homogeneous air-fuel mixture is formed. The MPCI combustion allows a control of a timing of the compression self-ignition when the load of the engine  10  is relatively high. 
     (SPCCI Combustion) 
     When the operating state of the engine  10  is in the fourth range  314 , the ECU  100  carries out flame propagation combustion of part of the air-fuel mixture inside the cylinder  13 , and carries out compression ignition combustion of the remaining air-fuel mixture. The intake valve operating mechanism  123  opens the intake valve  122  at a given timing and/or by a given lift, and the exhaust valve operating mechanism  126  opens the exhaust valve  125  at a given timing and/or by a given lift. The injector  131  injects fuel into the cylinder  13  during a compression stroke. The spark plug  132  ignites the air-fuel mixture near a compression top dead center. The air-fuel mixture starts flame propagation combustion. The temperature inside the cylinder  13  becomes high due to generation of combustion heat, and the pressure inside the cylinder  13  increases due to flame propagation. Accordingly, unburnt mixture gas carries out, for example, compression self-ignition after a compression top dead center to start combustion. The flame propagation combustion and the compression ignition combustion progress in parallel after the compression ignition combustion is started. 
     (Configuration of Circulation System) 
     Next, a configuration of the circulation system  91  which the engine system  1  has is described with reference to  FIG. 4 . The circulation system  91  is a device which is attached to the engine  10  and circulates the coolant through the water jacket  20 . 
     The water jacket  20  is formed inside the engine  10 . The water jacket  20  constitutes a circuit which is connected to the circulation system  91  and through which the coolant is circulated as well as the circulation system  91 . The water jacket  20  has an in-block jacket  21  and an in-head jacket  22 . The in-block jacket  21  is formed in the cylinder block  11  so that it spreads along the outer circumference of each cylinder  13 . 
     The in-head jacket  22  is formed in the cylinder head  12 . The in-head jacket  22  communicates with the in-block jacket  21  (see broken lines in  FIG. 4 ). The in-head jacket  22  has a first jacket  22   a  and a second jacket  22   b . The first jacket  22   a  and the second jacket  22   b  are independent from each other. 
     The first jacket  22   a  is formed so that it extends along an upper part of a plurality of lined-up combustion chambers  16 . The coolant which flows through the first jacket  22   a  mainly exchanges heat (mainly, cools) with the combustion chamber  16 . In detail, the coolant which flows through the first jacket  22   a  exchanges heat with the atmosphere inside the combustion chamber  16  via a wall surface of the combustion chamber  16 . 
     The second jacket  22   b  is formed so that it extends along a circumference part of the exhaust ports  124  of the plurality of lined-up cylinders  13 . The coolant which flows through the second jacket  22   b  mainly exchanges heat (mainly, cools) with the exhaust port  124  where hot exhaust gas flows. 
     A water pump  3  is installed in the cylinder block  11 , at an end of the engine  10  (inflow-side end part  10   a ). The water pump  3  constitutes a part of the circulation system  91 . 
     The water pump  3  is a mechanical pump in which a rotation shaft of the pump is connected with the crankshaft  14  of the engine  10  via a pulley, a belt, etc. The water pump  3  operates by a driving force of the engine  10 . Note that the water pump  3  may be an electric rotary pump which can operate independently from the engine  10 . 
     The in-block jacket  21  is connected with a discharge port  3   a  of the water pump  3  via a coolant introducing passage  23 . Therefore, the coolant discharged from the water pump  3  flows into the in-block jacket  21  through the coolant introducing passage  23 . The coolant which flowed into the in-block jacket  21  flows into the in-head jacket  22 . In detail, it dividedly flows into the first jacket  22   a  and the second jacket  22   b.    
     The coolant control valve (CCV)  4  (an example of a “flow rate control device” in the disclosed art) is installed in the cylinder head  12 , at an end (outflow-side end part  10   b ) opposite from the inflow-side end part  10   a  of the engine  10 . The coolant control valve  4  constitutes a part of the circulation system  91 . 
     A third port  65  (see  FIG. 5 ) of the coolant control valve  4  is connected with the first jacket  22   a  via a first coolant deriving passage  24 . Therefore, the coolant which flows through the first jacket  22   a  flows out of the engine  10  through the first coolant deriving passage  24 , and flows into the coolant control valve  4  (the details of the coolant control valve  4  will be described later). 
     A second coolant deriving passage  25  which communicates with the second jacket  22   b  is formed in a part of the outflow-side end part  10   b , on the exhaust side of the cylinder head  12 . Therefore, the coolant which flows through the second jacket  22   b  flows out of the engine  10  through the second coolant deriving passage  25 , and flows into a second circulation flow passage  31  (described later). 
     A third coolant deriving passage  26  which communicates with the in-block jacket  21  is formed in a part of the outflow-side end part  10   b , on the intake side of the cylinder block  11 . Therefore, part of the coolant which flows through the in-block jacket  21  flows out of the engine  10  through the third coolant deriving passage  26 , and flows into a third circulation flow passage  41  (described later). 
     The circulation system  91  includes, in addition to the water pump  3  and the coolant control valve  4  which are described above, a radiator  27  (an example of a “heat exchanger” in the disclosed art), and a thermally-actuated valve (thermostat valve)  28 . Further, the engine system  1  including the circulation system  91  roughly includes, as passages through which the coolant is circulated, a second circuit  30 , a third circuit  40 , and a first circuit  50 . 
     (Second Circuit) 
     The second circuit  30  has the second circulation flow passage  31  which is provided with a passage which branches into two (a first branch passage  31   a  and a second branch passage  31   b ). In the first branch passage  31   a , the EGR cooler  191  and a heater  71  are disposed. The heater  71  is built into an air-conditioner which adjusts air inside a vehicle cabin. In the second branch passage  31   b , the throttle valve (Electric Throttle Body: ETB)  171  and the EGR valve  192  are disposed. An upstream end of the second circulation flow passage  31  is connected to the second coolant deriving passage  25 . A downstream end of the second circulation flow passage  31  is connected to a suction port  3   b  of the water pump  3  in a state where it is joined to the first circuit  50  and the third circuit  40 . 
     Inside of the engine  10 , the in-block jacket  21 , the second jacket  22   b , and the second coolant deriving passage  25  constitute a passage of the second circuit  30 . Therefore, in the second circuit  30 , coolant which flowed through the in-block jacket  21  and the second jacket  22   b  among the coolant discharged from the water pump  3  dividedly flows into the first branch passage  31   a  and the second branch passage  31   b . Then, it returns to the water pump  3  after being joined. 
     The coolant which flows through the second circuit  30  exchanges heat with the engine  10  (mainly, with the exhaust port  124 ). Further, it also exchanges heat with the EGR cooler  191 , the heater  71 , the throttle valve  171 , and the EGR valve  192 . 
     (Third Circuit) 
     The third circuit  40  has the third circulation flow passage  41  in which an oil cooler  72  and an automatic transmission fluid (ATF) heat exchanger  73  are installed. The oil cooler  72  is installed in a system which circulates and supplies lubricating oil to the engine  10 . The ATF heat exchanger  73  is installed in a system which circulates and supplies hydraulic fluid of an automatic transmission. An upstream end of the third circulation flow passage  41  is connected to the third coolant deriving passage  26 . A downstream end of the third circulation flow passage  41  is connected to the suction port  3   b  of the water pump  3  in a state where it is joined to the first circuit  50  and the second circuit  30 . 
     Inside of the engine  10 , the in-block jacket  21  and the third coolant deriving passage  26  constitute a passage of the third circuit  40 . Therefore, in the third circuit  40 , among the coolant discharged from the water pump  3 , part of the coolant which flows through the in-block jacket  21  flows through the third circulation flow passage  41  and returns to the water pump  3 . The coolant which flows through the third circuit  40  exchanges heat with the oil cooler  72  and the ATF heat exchanger  73 . 
     (First Circuit) 
     The first circuit  50  has a bypass passage  51 , a connecting passage  52 , and a radiator passage  53 . Inside of the engine  10 , the in-block jacket  21 , the first jacket  22   a , and the first coolant deriving passage  24  constitute a passage of the first circuit  50 . 
     The passage of the first circuit  50  branches to the bypass passage  51  and the radiator passage  53  at the coolant control valve  4 . The downstream ends of the bypass passage  51  and the radiator passage  53  are connected to the suction port  3   b  of the water pump  3  in a state where they are joined to the second circuit  30  and the third circuit  40 . 
     The radiator  27  is provided to the radiator passage  53 . The radiator  27  is installed behind a front grille of the automobile. The coolant which flows through the radiator  27  exchanges heat mainly with outside air flow caused by the automobile traveling. The coolant radiates the heat and is cooled by flowing through the radiator passage  53 . 
     Therefore, the radiator passage  53  cools, by the radiator  27 , the coolant which is discharged from the water pump  3  and is heated by exchanging heat while flowing through the in-block jacket  21  and the first jacket  22   a , and recirculates it to the in-block jacket  21  and the first jacket  22   a.    
     The bypass passage  51  is a passage which bypasses the radiator passage  53 . The bypass passage  51  is shorter than the radiator passage  53 . Only the thermally-actuated valve  28  is provided to the bypass passage  51 . The thermally-actuated valve  28  is connected by the radiator passage  53  via the connecting passage  52  in a state where the upstream side and the downstream side of the bypass passage  51  always communicate with each other. 
     Therefore, the bypass passage  51  recirculates to the in-block jacket  21  and the first jacket  22   a  the coolant which was discharged from the water pump  3  and exchanged heat while flowing through the in-block jacket  21  and the first jacket  22   a , without cooling the coolant by the radiator  27 . 
     The thermally-actuated valve  28  is a known device which opens and closes at a high temperature set beforehand. The thermally-actuated valve  28  has a valve body which is biased in a closing direction by an elastic force of a spring. The thermally-actuated valve  28  opens and closes by the valve body being displaced according to an action of wax. The thermally-actuated valve  28  of the engine system  1  is set so that its valve-opening temperature is higher than a valve-opening temperature of a conventional thermally-actuated valve. 
     When the thermally-actuated valve  28  opens, the bypass passage  51  communicates the radiator passage  53  via the connecting passage  52 . Therefore, when the thermally-actuated valve  28  opens, part of the coolant which flows through the bypass passage  51  passes through the connecting passage  52 , and flows into the radiator passage  53 . 
     (Coolant Control Valve) 
       FIG. 5  illustrates the coolant control valve  4 . The coolant control valve  4  is a valve which can adjust a flow rate of the coolant, and is comprised of a housing  60 , a rotary valve body  61 , and an actuator  62 . 
     A cylindrical flow-dividing chamber  60   a  is provided inside the housing  60 . The cylindrical rotary valve body  61  is rotatably accommodated in the flow-dividing chamber  60   a . A first port  63  and a second port  64  are formed in the housing  60  so that they extend radially outward from a given position in an outer circumference of the flow-dividing chamber  60   a . The first port  63  is connected to the bypass passage  51 . The second port  64  is connected to the radiator passage  53 . 
     One end of the flow-dividing chamber  60   a  is opened. This opening constitutes the third port  65  through which the coolant flows into the flow-dividing chamber  60   a . Further, the housing  60  is attached to the cylinder head  12  so that the third port  65  is coaxially connected to the first coolant deriving passage  24 . Therefore, a circumferential wall of the rotary valve body  61  intervenes between the third port  65  and each of the first port  63  and the second port  64 . 
     A first water flow opening  61   a  and a second water flow opening  61   b  are formed at given positions of the circumferential wall of the rotary valve body  61 . The first water flow opening  61   a  has a length in the circumferential direction longer than the second water flow opening  61   b , and has a relatively large opening area. Depending on the rotational position of the rotary valve body  61 , the third port  65  communicates or does not communicate with the first port  63  and the second port  64  via the first water flow opening  61   a  and the second water flow opening  61   b , respectively. Further, when communicating with the ports, an opening between each of the first port  63  and the second port  64  and the third port  65  varies depending on the rotational position of the rotary valve body  61 . 
     The other end of the flow-dividing chamber  60   a  is sealed with a closure wall  66 . The actuator  62  is accommodated inside the housing  60 , on the opposite side of the flow-dividing chamber  60   a  with respect to the closure wall  66 . A rotation shaft  62   a  of the actuator  62  projects into the flow-dividing chamber  60   a  through a shaft hole which opens at the center of the closure wall  66 . The rotary valve body  61  is attached via support arms  62   b  to the rotation shaft  62   a  projected into the flow-dividing chamber  60   a . The ECU  100  outputs a control signal to the actuator  62 . By the ECU  100  controlling the actuator  62 , the rotary valve body  61  is rotated. 
     Returning to  FIG. 4 , the first water temperature sensor SN 1  is disposed at a passage where the first circuit  50 , the second circuit  30 , and the third circuit  40  join and flow into the water pump  3 . The second water temperature sensor SN 2  is disposed at the first jacket  22   a . The first water temperature sensor SN 1  measures a temperature of coolant which flows into the engine  10 . The second water temperature sensor SN 2  measures a temperature of coolant which flows into the water jacket  20  (more accurately, into the first jacket  22   a ). These sensors SN 1  and SN 2  are utilized for a coolant control and a combustion control. For example, when performing the advanced combustion control, the second water temperature sensor SN 2  is utilized for estimating the wall temperature of the combustion chamber  16 . The second water temperature sensor SN 2  is utilized for controlling the actuator  62 . 
     In this circulation system  91 , the ECU  100  controls the coolant control valve  4  based on the measurement of the second water temperature sensor SN 2 . This adjusts a flow rate of the coolant which flows through the first circuit  50  (i.e., the bypass passage  51  and the radiator passage  53 ). Note that the flow of the coolant in the connecting passage  52  is automatically adjusted by the thermally-actuated valve  28 . 
     The coolant which flows through the circulation system  91  is mainly cooled by the radiator  27  installed in the radiator passage  53 . The temperature of the coolant is adjusted. 
     That is, the main object of the circulation system  91  is the first circuit  50 . The flow rate and the temperature of the coolant in each of the second circuit  30  and the third circuit  40  change according to an adjustment of the flow rate and the temperature of the coolant in the first circuit  50 . In this circulation system  91 , although the first circuit  50  is essential, the second circuit  30  and the third circuit  40  are not essential. 
     (How Coolant Flows) 
     As described above, the coolant which flows through the first jacket  22   a  mainly exchanges heat with the wall part of the combustion chamber  16  to cool the wall part of the combustion chamber  16 . In this engine system  1 , a plurality of ways for the coolant to flow are set according to the temperature of the coolant which flows through the first jacket  22   a  (the measurement of the second water temperature sensor SN 2 ) in order to stably and efficiently perform the combustion control of the engine  10 .  FIG. 6  illustrates a flowing state of each circuit in the engine system  1  according to the temperature of the coolant. 
     In the coolant control valve  4 , the actuator  62  is controlled to adjust the flow rate of the coolant which flows through both the first port  63  and the second port  64 . That is, the opening of each of the first water flow opening  61   a  and the second water flow opening  61   b  is changed so that the rotary valve body  61  is at the given rotational position. 
     “Low Temperature” is a so-called state during “cold start,” such as immediately after the engine  10  is started. “Low Temperature” is a state where a temperature t of the coolant which flows through the first jacket  22   a  is below a first switching temperature t 11  (for example, 40° C.). “Full Warm-up” is a state where the engine  10  is warmed up to a temperature suitable for operation, and is a so-called state after “warmed up.” “Full Warm-up” is a state where the temperature t of the coolant which flows through the first jacket  22   a  is at or above a second switching temperature t 12  (for example, 80° C.). “Half Warm-up” is a state between “Low Temperature” and “Full Warm-up” (i.e., it is a transition state). “Half Warm-up” is a state where the temperature t of the coolant which flows through the first jacket  22   a  is at or above the first switching temperature t 11  and below the second switching temperature t 12 , and it is a state where the coolant temperature t is from 40° C. to 80° C. 
     As illustrated by a left state  81  in  FIG. 6 , the coolant neither flows into the bypass passage  51  nor the radiator passage  53  during “Low Temperature” (both the flow rates are zero). That is, in the first circuit  50 , the circulation of the coolant is not performed. At this time, in the coolant control valve  4 , the rotary valve body  61  is set at a rotational position where both the first port  63  and the second port  64  do not communicate with the third port  65 . 
     Since the coolant does not flow into the radiator passage  53 , the coolant will not be cooled by the radiator  27 . Therefore, the coolant rises promptly in the temperature. Further, the combustion chamber  16  is not cooled by the circulation of the coolant. The combustion chamber  16  can be promptly heated by the combustion heat. Since the engine  10  promptly rises to the temperature state suitable for combustion, fuel efficiency can be improved. At this time, the coolant discharged from the water pump  3  circulates through the second circuit  30  and the third circuit  40 . 
     As illustrated by a center state  82  in  FIG. 6 , during “Half Warm-up,” although the coolant flows into the bypass passage  51 , the coolant does not flow into the radiator passage  53  (the flow rate of the radiator passage  53  is zero). That is, in the first circuit  50 , the coolant only circulates through the bypass passage  51 . At this time, in the coolant control valve  4 , the rotary valve body  61  is set at a rotational position where only the first port  63  communicates with the third port  65 . The opening of the first water flow opening  61   a  is fully open, for example. 
     Since the coolant does not flow into the radiator passage  53 , the coolant promptly rises in the temperature. On the other hand, since the coolant flows into the bypass passage  51 , the coolant flows into the first jacket  22   a . The bypass passage  51  is short. Further, since the coolant control valve  4  is set to be fully opened, most of the coolant flows through the bypass passage  51  and the first jacket  22   a.    
     The combustion chamber  16  can be promptly heated by the circulating coolant. Since the coolant is circulated, the combustion chamber  16  and its circumference can be heated uniformly. Since the engine  10  promptly rises to the temperature state suitable for combustion, fuel efficiency can be improved. 
     Note that, at this time, the remainder of the coolant discharged from the water pump  3  circulates through the second circuit  30  and the third circuit  40  (similar during “Full Warm-up”). The temperature of the coolant during “Half Warm-up” is lower than the valve-opening temperature of the thermally-actuated valve  28 . Therefore, the thermally-actuated valve  28  is in a fully closed state. Part of the coolant will not flow into the radiator passage  53  from the bypass passage  51 . 
     During “Full Warm-up,” the engine  10  reaches the temperature state suitable for combustion. The engine  10  after fully warmed up changes the combustion mode according to the load and the engine speed, as described above. This engine system  1  controls the circulation system  91  so that the wall temperature of the combustion chamber  16  becomes a temperature suitable for the combustion mode. During “Full Warm-up,” the state  82  illustrated in the center of  FIG. 6  and a state  83  illustrated in the right of  FIG. 6  are switched according to the operating state of the engine  10 . The state  82  is a state where the bypass passage  51  is opened and the radiator passage  53  is closed, as described above. However, since the temperature of the coolant rises during “Full Warm-up,” the coolant may flow through the radiator passage  53  by the thermally-actuated valve  28  being opened, as will be described later. The state  83  is a state where the circulation of the coolant is performed using the entire first circuit  50  by opening both the bypass passage  51  and the radiator passage  53 . 
     In more detail, during “Full Warm-up,” as illustrated by the center state  82 , in the coolant control valve  4 , the rotary valve body  61  is set at a rotational position so that the first port  63  communicates with the third port  65 , and the second port  64  does not communicate with the third port  65 . Further, according to the load of the engine  10 , the flow rate of the coolant is adjusted at the first port  63  (bypass passage  51 ). 
     During “Full Warm-up,” as illustrated by the right state  83 , the coolant flows into both the bypass passage  51  and the radiator passage  53 . In that case, in the coolant control valve  4 , the rotary valve body  61  is set at a rotational position so that both the first port  63  and the second port  64  communicate with the third port  65 . Further, according to the load of the engine  10 , the flow rate of the coolant is adjusted at both the first port  63  (bypass passage  51 ) and the second port  64  (radiator passage  53 ). 
     (How Coolant Flows When Fully Warmed Up) 
       FIG. 7  illustrates a concrete example of how the coolant flows when fully warmed up. In  FIG. 7 , charts (A) to (D) illustrate changes in main properties according to the load of the engine  10 . 
     Chart (A) illustrates change G 1  in the flow rate of the coolant which passes through the coolant control valve  4 , and change G 2  in the flow rate of the coolant which passes through the radiator passage  53 . Chart (B) illustrates the details of the change in the flow rate of the coolant which flows through the first circuit  50 , that is, change G 3  in the flow rate of the coolant which flows into the bypass passage  51  from the coolant control valve  4 , change G 4  in the flow rate of the coolant which flows through the connecting passage  52 , and change G 5  in the flow rate of the coolant which flows into the radiator passage  53  from the coolant control valve  4 . 
     Chart (C) illustrates change G 6  in the temperature of the coolant which flows through the first jacket  22   a , and change G 7  in the temperature of the coolant which flows into the water pump  3 . In other words, changes in the measurements of the second water temperature sensor SN 2  and the first water temperature sensor SN 1  are illustrated. Chart (D) illustrates change G 8  in the wall temperature of the combustion chamber  16 . 
     The load range of the engine  10  is divided, in association with the control of the coolant, into three ranges comprised of a range below the first load L 1 , a range above the second load L 2 , and a range above the first load L 1  and below the second load L 2 . Each chart of  FIG. 7  corresponds to the case where the engine speed of the engine  10  is the low speed or the middle speed. The range below the first load L 1  is a range where the engine  10  performs HCCI combustion or MPCI combustion. The range above the second load L 2  is a range where the engine  10  performs SI combustion. The range above the first load L 1  and below the second load L 2  is a range where the engine  10  performs SPCCI combustion. 
     Further, in this engine system  1 , the flow rate control of the coolant is performed in the range where the engine  10  performs HCCI combustion or MPCI combustion, and the temperature control of the coolant is performed in the range where the engine  10  performs SPCCI combustion. The range where the engine  10  performs HCCI combustion or MPCI combustion is, in other words, a range where the air-fuel mixture combusts without forcible ignition of the spark plug  132 , and the range where the engine  10  performs SPCCI combustion is a range where the air-fuel mixture combusts by the forcible ignition of the spark plug  132 . 
     The engine system  1  maintain the wall temperature of the combustion chamber  16  at the specific constant temperature in the ranges where the load of the engine  10  is low and middle by switching between the flow rate control and the temperature control (see G 8 ). 
     That is, in order to realize the compression self-ignition combustion without forcible ignition, like HCCI combustion or MPCI combustion, it is necessary to accurately control the temperature inside the combustion chamber  16  (in-cylinder temperature) at a temperature higher than SI combustion. On the other hand, SPCCI combustion is combustion accompanied by forcible ignition though part of the air-fuel mixture combusts by compression ignition, and the temperature inside the combustion chamber  16  is permitted to be lower than that of HCCI combustion or MPCI combustion. On the contrary, if the temperature inside the combustion chamber  16  is too high, the air-fuel mixture may carry out self-ignition before forcible ignition is performed, or a rate of the self-ignition combustion may become too large in the SPCCI combustion where flame propagation combustion and self-ignition combustion are combined. That is, if the temperature inside the combustion chamber  16  is too high, stable SPCCI combustion will not be realized. 
     Therefore, it is ideal to change the wall temperature of the combustion chamber  16  according to the switching of the combustion mode. However, since the calorific capacity of the wall part of the combustion chamber  16  is large, it is difficult to change the wall temperature of the combustion chamber  16  with sufficient response to the switching of the combustion mode or the change in the load. Thus, in the range from the low load to the middle load, the engine system  1  maintains the wall temperature of the combustion chamber  16  at the specific constant temperature. This specific temperature is an intermediate temperature between an optimal temperature for HCCI combustion or MPCI combustion and an optimal temperature for SPCCI combustion, is a temperature permissible in the execution of HCCI combustion or MPCI combustion, and is a temperature also permissible in the execution of SPCCI combustion. Even if the combustion mode is switched or the load is changed, the wall temperature of the combustion chamber  16  becomes a suitable temperature by maintaining the wall temperature of the combustion chamber  16  at the constant temperature. 
     However, if the load of the engine  10  is low, the combustion heat increases in general, and if the load of the engine  10  increases, the combustion heat decreases in general. In order to maintain the constant wall temperature of the combustion chamber  16  regardless of the load of the engine  10 , it is necessary to adjust the heat exchanging quantity by the coolant with high response to the occurring combustion heat. 
     For example, in order to adjust the heat exchanging quantity, it is possible to adjust the temperature of the coolant according to the load of the engine  10 . However, since the calorific capacity of the coolant is large, it requires a long period of time to raise or lower the temperature of the coolant. It is difficult to adjust the temperature of the coolant with high response to the change in the load of the engine  10 . 
     Thus, this engine system  1  adjusts the flow rate of the coolant which flows through the first port  63  and the first jacket  22   a  by using the coolant control valve  4  according to the load of the engine  10 , while keeping the temperature of the coolant constant at a given temperature. Since the adjustment of the flow rate can be changed with high response, the heat transfer coefficient by the coolant can be adjusted with high response against the occurring combustion heat, and, as a result, the wall temperature of the combustion chamber  16  can be maintained constant. 
     (Range of HCCI Combustion or MPCI Combustion) 
     As illustrated in  FIG. 7 , in the range where HCCI combustion or MPCI combustion is performed, the coolant control valve  4  adjusts the flow rate of the coolant which flows through the bypass passage  51  without the coolant flowing to the radiator passage  53  (see G 3 , G 5 ). 
     Since the radiator passage  53  is closed, the temperature of the coolant is determined by a valve-opening temperature of the thermally-actuated valve  28 . The valve-opening temperature of the thermally-actuated valve  28  is set at a comparatively high temperature. The temperature of the coolant which flows through the first jacket  22   a  is constant at a first target temperature t 21 , regardless of the load (see G 6 ). The first target temperature t 21  is a temperature near the reliability limit temperature of the engine  10 . By setting the temperature of the coolant at the comparatively high temperature, in the range where HCCI combustion or MPCI combustion is performed, the wall temperature of the combustion chamber  16  can be maintained at the comparatively high temperature (that is, a target temperature tw). When the wall temperature of the combustion chamber  16  is high, it is advantageous to stabilize the compression self-ignition combustion without forcible ignition like HCCI combustion or MPCI combustion. Note that, in the example of the drawing, in the range below the first load L 1 , the temperature of the coolant which flows into the engine  10  gradually rises as the load of the engine  10  increases (see G 7 ). 
     In the range where HCCI combustion or MPCI combustion is performed, the coolant control valve  4  adjusts the flow rate so that the flow rate of the coolant which flows through the bypass passage  51  becomes less when the load of the engine  10  is low, and the flow rate of the coolant which flows through the bypass passage  51  becomes more when the load of the engine  10  is high. 
     At this time, in the coolant control valve  4 , the actuator  62  is controlled so that the rotary valve body  61  is located at a rotational position where the third port  65  does not communicate with the second port  64  and the third port  65  communicates with the first port  63 . Further, according to the load of the engine  10 , the opening between the third port  65  and the first port  63  is adjusted. 
     Note that, in the range where HCCI combustion or MPCI combustion is performed, the flow rate of the coolant which flows through the connecting passage  52  when the thermally-actuated valve  28  is opened changes corresponding to the change in the flow rate of the coolant which flows through the bypass passage  51  (see G 4 ). 
     Here, in the example of the drawing, although the load of the engine  10  and the flow rate of the coolant have a linear relationship, it is not limited to the linear relationship. 
     The flow rate of the coolant which flows through the first jacket  22   a  corresponds to the flow rate of the coolant which flows through the bypass passage  51 . Therefore, when the load of the engine  10  is low, the flow rate of the coolant which flows through the first jacket  22   a  is small, and when the load of the engine  10  is high, the flow rate of the coolant which flows through the first jacket  22   a  is large. In the example of  FIG. 7 , when the load of the engine  10  is the first load L 1 , the flow rate of the coolant which flows through the first jacket  22   a  becomes the maximum flow rate (see G 1 ). Note that when the load of the engine  10  is the first load L 1 , the flow rate of the coolant which flows through the first jacket  22   a  may be below the maximum flow rate. 
     When the flow rate of the coolant which flows through the first jacket  22   a  is small, the heat transfer coefficient with the combustion chamber  16  falls. Therefore, even if the combustion heat decreases, the wall temperature of the combustion chamber  16  can be adjusted to a high temperature. When the flow rate of the coolant which flows through the first jacket  22   a  is large, the heat transfer coefficient with the combustion chamber  16  increases. Therefore, even if the combustion heat increases, the wall temperature of the combustion chamber  16  can be adjusted to a low temperature. 
     In this way, while maintaining the temperature of the coolant constant by using the thermally-actuated valve  28  (see G 6 ), the flow rate of the coolant which flows through the first jacket  22   a  is fluctuated using the coolant control valve  4  with high response according to the load of the engine  10  (see G 1 , G 3 ). Therefore, the wall temperature of the combustion chamber  16  can be held constant at the target temperature tw (see G 8 ). 
     (Range of SPCCI Combustion) 
     The flow rate of the coolant which flows through the coolant control valve  4  (i.e., the flow rate of the coolant which flows through the first circuit  50 ) reaches an upper limit at the first load L 1  (see G 1 ). That is, the flow rate control cannot be performed at the load above the first load L 1 . Thus, in the range above the first load L 1  and below the second load L 2 , the temperature control of the coolant is performed. The wall temperature of the combustion chamber  16  is held at the target temperature tw by gradually allowing the coolant which flows through the bypass passage  51  to flow to the radiator passage  53  as the load of the engine  10  increases, to cool the coolant. 
     In detail, in a state where the flow rate of the coolant which flows through the first circuit  50  is held at the maximum flow rate, the coolant control valve  4  gradually increases the flow rate of the coolant which flows through the radiator passage  53 , while gradually reducing the flow rate of the coolant which flows through the bypass passage  51 , as the load of the engine  10  increases (see G 1 , G 2 , G 3 , G 5 ). In the range of SPCCI combustion, the coolant control valve  4  adjusts the temperature of the coolant which flows through the first jacket  22   a  by adjusting the flow rate of the coolant which flows through the radiator passage  53 . Note that if the load of the engine  10  is above the first load L 1 , the flow rate of the coolant which flows through the radiator passage  53  exceeds the flow rate of the coolant which flows through the bypass passage  51 . The load of the engine  10  at which the flow rate is reversed changes according to the operating environments of the engine  10  (for example, ambient temperature, wind quantity during the vehicle traveling, etc.). 
     The coolant control valve  4  controls the actuator  62  so that the rotary valve body  61  is located at a rotational position where the third port  65  communicates with both the first port  63  and the second port  64 . Further, according to the load of the engine  10 , the opening between the third port  65  and each of the first port  63  and the second port  64  is adjusted. 
     Thus, the temperature of the coolant which flows through the first jacket  22   a  and the temperature of the coolant which flows into the engine  10  become lower as the load of the engine  10  increases (see G 6 , G 7 ). When the load of the engine  10  increases to increase the combustion heat, since the temperature of the coolant which flows through the first jacket  22   a  is low even if the flow rate of the coolant is constant, the cooling quantity by the coolant which flows through the first jacket  22   a  can be maintained. Further, since the flow rate of the coolant which flows through the first circuit  50  is the maximum flow rate, it is advantageous to cool the combustion chamber  16 . As a result, also in the range of SPCCI combustion, the wall temperature of the combustion chamber  16  can be held at the target temperature tw (see G 8 ). 
     In order to suppress the excessive rise in the temperature of the combustion chamber  16 , in this cooling system, a second target temperature t 22  (for example, 88° C.) lower than the first target temperature t 21  is set as a target temperature of the coolant which flows through the first jacket  22   a . The temperature control is performed until the temperature of the coolant which flows through the first jacket  22   a  reaches the second target temperature t 22 . 
     Note that, as illustrated in G 5  of  FIG. 7 , when the temperature of the coolant reaches the second target temperature t 22 , the flow rate of the coolant which flows through the radiator passage  53  is below the maximum flow rate. If the flow rate of the coolant which flows through the radiator passage  53  is further increased, the temperature of the coolant can be further reduced. That is, even if the load of the engine  10  exceeds L 2 , it is possible to maintain the wall temperature of the combustion chamber  16  at the target temperature tw. 
     Thus, the engine system  1  can maintain the wall temperature of the combustion chamber  16  constant over the wide range from the low load to the middle load of the engine  10 , by the combination of the flow rate control and the temperature control. Since the wall temperature of the combustion chamber  16  is maintained at the suitable temperature even if the combustion mode is switched between HCCI combustion, MPCI combustion, and SPCCI combustion corresponding to the change in the load of the engine  10 , each combustion is stably performed. 
     The coolant control valve  4  having the rotary valve body  61  can selectively close the bypass passage  51  and/or the radiator passage  53 , and can adjust the flow rate of the bypass passage  51  and the flow rate of the radiator passage  53 . The engine system  1  provided with the coolant control valve  4  can realize the flow rate adjustment of the water jacket  20  described above with the simple configuration. 
     Note that, in the range of SPCCI combustion, the coolant which flows into the radiator passage  53  through the connecting passage  52  gradually decreases and will not flow as the load of the engine  10  increases (see G 4 ). In detail, the temperature of the coolant which flows into the bypass passage  51  from the coolant control valve  4  gradually decreases from the first target temperature t 21 . In connection with it, the temperature of the coolant which flows through the thermally-actuated valve  28  also decreases. Therefore, in the range of SPCCI combustion, the thermally-actuated valve  28  gradually closes, and it will become fully closed. Therefore, the coolant which flows into the radiator passage  53  through the connecting passage  52  gradually decreases, and will not flow. 
     Although in the example of  FIG. 7  a proportional relationship exists between the flow rate reduction of the coolant which flows through the bypass passage  51  and the flow rate increase of the coolant which flows through the radiator passage  53 , there is no necessity of being the proportional relationship. In the range of SPCCI combustion, the flow rate of the coolant which flows through the coolant control valve  4  may be below the upper limit. 
     (Range of SI Combustion) 
     In the range of SI combustion, the adjustment is performed in the coolant control valve  4  so that the temperature of the coolant which flows through the first jacket  22   a  is held at the second target temperature t 22 . In detail, the actuator  62  is controlled, and the adjustment is made so that the opening between the third port  65  and the second port  64  becomes large, and the opening between the third port  65  and the first port  63  becomes small, as the load of the engine  10  increases. Thus, the coolant which flows through the radiator passage  53  gradually increases, and the coolant which flows through the bypass passage  51  gradually decreases (see G 3 , G 5 ). By doing so, the temperature of the coolant which flows through the first jacket  22   a  can be held at the second target temperature t 22  (see G 6 ). 
     In the range where SI combustion is performed, it becomes possible to suppress abnormal combustion, such as knocking, by relatively lowering the temperature of the coolant. 
     In the range of SPCCI combustion (in other words, the range above the first load L 1  and below the second load L 2 ), in order to maintain the wall temperature of the combustion chamber  16  constant, the temperature of the coolant which flows through the first jacket  22   a  is positively lowered as the load of the engine  10  increases. Therefore, with respect to the increase in the load of the engine  10 , a degree of change in the flow rate of the coolant which flows into the bypass passage  51  from the coolant control valve  4 , and a degree of change in the flow rate of the coolant which flows into the radiator passage  53  from the coolant control valve  4  are relatively large. That is, slopes of G 3  and G 5  are larger. 
     On the other hand, in the range of SI combustion (in other words, the range above the second load L 2 ), in order to hold the temperature of the coolant at the second target temperature t 22 , with respect to the increase in the load of the engine  10 , a degree of change in the flow rate of the coolant which flows into the bypass passage  51  from the coolant control valve  4 , and a degree of change in the flow rate of the coolant which flows into the radiator passage  53  from the coolant control valve  4  are relatively small. That is, the slopes of G 3  and G 5  are small, and the slopes of G 3  and G 5  change at the second load L 2 . 
     Note that in the range of SI combustion (in other words, the range above the second load L 2 ), the proportional relationship between the flow rate reduction of the coolant which flows through the bypass passage  51  and the flow rate increase of the coolant which flows through the radiator passage  53  is not essential. In the range of SI combustion, the flow rate of the coolant which flows through the coolant control valve  4  may be below the upper limit. 
     In the range of SI combustion, the flow rate of the coolant which flows through the first jacket  22   a  is the maximum, and the temperature of the coolant is held at the second target temperature t 22 . Since the heat occurring inside the combustion chamber  16  increases as the load of the engine  10  increases, the wall temperature of the combustion chamber  16  gradually rises as the load of the engine  10  increases (see G 8 ). 
     Note that in the range of SI combustion, since the temperature of the coolant is maintained at the second target temperature t 22 , the thermally-actuated valve  28  is fully closed. The coolant does not flow into the connecting passage  52 . The bypass passage  51  and the radiator passage  53  constitute mutually independent passages. 
     Next, a control executed by the ECU  100  for cooling of the engine  10  is described with reference to  FIGS. 8 and 9 . 
       FIG. 8  is a flowchart for switching between a cold state, a half warmed up state, and a fully warmed up state of the engine  10 . First, at Step S 81  after the start, the ECU  100  acquires signal values outputted from various kinds of the sensors SN 1 -SN 5 . The subsequent Step S 82 , the ECU  100  determines whether the temperature t of the coolant is at or above the second switching temperature t 12  based on the signal from the second water temperature sensor SN 2 . If the temperature t of the coolant is at or above the second switching temperature t 12 , the process shifts from Step S 82  to Step S 83 . At Step S 83 , the ECU  100  executes a full warm-up control. The details of the full warm-up control is described with reference to  FIG. 9 . 
     If the temperature of the coolant is below the second switching temperature t 12 , the process shifts from Step S 82  to Step S 84 . At Step S 84 , the ECU  100  determines whether the temperature t of the coolant is at or above the first switching temperature t 11 . If the temperature t of the coolant is at or above the first switching temperature t 11 , the process shifts from Step S 84  to Step S 85 . At Step S 85 , the ECU  100  executes a half warm-up control. As described above, the ECU  100  opens the bypass passage  51  and closes the radiator passage  53 , through the coolant control valve  4 . 
     If the temperature t of the coolant is below the first switching temperature t 11 , the process shifts from Step S 84  to Step S 86 . At Step S 86 , the ECU  100  executes a low-temperature control. As described above, the ECU  100  closes the bypass passage  51  and closes the radiator passage  53 , through the coolant control valve  4 . 
       FIG. 9  illustrates a flow of the full warming-up control at Step S 83 . At Step S 91  after the start, the ECU  100  calculates a target load of the engine  10  based on the signal values outputted from the sensors SN 1 -SN 5 . At the subsequent Step S 92 , the ECU  100  determines whether the combustion mode is HCCI combustion or MPCI combustion based on the target load L. If the determination at Step S 92  is YES, the process shifts from Step S 92  to Step S 93 . At Step S 93 , the ECU  100  executes the flow rate control. That is, the ECU  100  closes the radiator passage  53  and adjusts the flow rate of the bypass passage  51  according to the load of the engine  10 , through the coolant control valve  4 . 
     If the combustion modes are not HCCI combustion and MPCI combustion, the process shifts from Step S 92  to Step S 94 . At Step S 94 , the ECU  100  determines whether the combustion mode is SPCCI combustion based on the target load L. If the determination at Step S 94  is YES, the process shifts from Step S 94  to Step S 95 . At Step S 95 , the ECU  100  executes the temperature control. That is, the ECU  100  adjusts the flow rates of the radiator passage  53  and the bypass passage  51  through the coolant control valve  4  according to the load of the engine  10  so that the wall temperature of the combustion chamber  16  becomes constant. 
     If the combustion mode is SI combustion, the process shifts from Step S 94  to Step S 96 . At Step S 96 , the ECU  100  adjusts the flow rates of the radiator passage  53  and the bypass passage  51  through the coolant control valve  4  according to the load of the engine  10  so that the temperature of the coolant becomes constant. 
     (Modification of Circulation System) 
       FIG. 10  illustrates a circulation system  92  according to a modification. This circulation system  92  differs from the circulation system  91  of  FIG. 4  in the position of the thermally-actuated valve  28 . 
     In detail, the thermally-actuated valve  28  is attached to the outflow-side end part  10   b  of the engine  10 , instead of the bypass passage  51 . A downstream end of the first jacket  22   a  provided to the cylinder head  12  branches into two. The coolant control valve  4  and the thermally-actuated valve  28  are connected to the first jacket  22   a.    
     The thermally-actuated valve  28  is connected by the radiator passage  53  via the connecting passage  52 . In more detail, the connecting passage  52  is connected to a part of the radiator passage  53  upstream of the radiator  27 . 
     Note that this circulation system  92  does not have the connecting passage which connects the bypass passage  51  to the radiator passage  53  in the circulation system  91  of  FIG. 4 . 
     How the coolant flows in the circulation system  92  is the same as the circulation system  91  of  FIG. 4 . That is, if the temperature t of the coolant is in “Low Temperature” state below the first switching temperature t 11 , the coolant neither flows into the bypass passage  51  nor the radiator passage  53  (both the flow rates are zero). At this time, in the coolant control valve  4 , the rotary valve body  61  is set at the rotational position where both the first port  63  and the second port  64  do not communicate with the third port  65 . Further, the thermally-actuated valve  28  is closed. Therefore, in the first circuit  50 , the circulation of the coolant is not performed. 
     If the temperature t of the coolant is in “Half Warm-up” state at or above the first switching temperature t 11  and below the second switching temperature t 12 , although the coolant flows to the bypass passage  51 , it does not flow to the radiator passage  53  (the flow rate of the radiator passage  53  is zero). At this time, in the coolant control valve  4 , the rotary valve body  61  is set at the rotational position where only the first port  63  communicates with the third port  65 . The opening of the first water flow opening  61   a  is fully open, for example. Further, since the temperature of the coolant is low, the thermally-actuated valve  28  is closed. In the first circuit  50 , the circulation of the coolant is performed only in the bypass passage  51 . 
     If the temperature t of the coolant is in “Ful Warm-up” state at or above the second switching temperature t 12 , the circulation system  92  is controlled according to the change of the combustion mode. 
     Concretely, when the operating state of the engine  10  is in the range of HCCI combustion or MPCI combustion, the flow rate control is performed. The temperature of the coolant is kept constant by the thermally-actuated valve  28 . The coolant control valve  4  opens the bypass passage  51  and closes the radiator passage  53 . Note that the coolant may pass through the radiator  27  by the thermally-actuated valve  28  being opened. The coolant control valve  4  adjusts the flow rate of the coolant which flows through the bypass passage  51  according to the load of the engine  10 . Therefore, the wall temperature of the combustion chamber  16  is maintained at the target temperature tw. 
     The temperature control is performed when the operating state of the engine  10  is in the range of SPCCI combustion and below the second load L 2 . The coolant control valve  4  opens both the bypass passage  51  and the radiator passage  53 . In more detail, the coolant control valve  4  reduces the flow rate of the coolant which flows through the bypass passage  51  and increases the flow rate of the coolant which flows through the radiator passage  53 , as the load of the engine  10  increases. Therefore, the wall temperature of the combustion chamber  16  is maintained at the target temperature tw. 
     When the operating state of the engine  10  is in the range of SI combustion, the coolant control valve  4  adjusts the flow rates of the coolant which flows through the bypass passage  51  and the radiator passage  53  so that the temperature t of the coolant becomes constant at the second target temperature t 22 . In more detail, the coolant control valve  4  reduces the flow rate of the coolant which flows through the bypass passage  51  and increases the flow rate of the coolant which flows through the radiator passage  53 , as the load of the engine  10  increases. The thermally-actuated valve  28  is closed. 
     Since the engine system  1  provided with the circulation system  92  performs the flow rate control in the range of HCCI combustion or MPCI combustion, it can change the flow rate of the coolant which flows through the first jacket  22   a  with high response to the load of the engine  10  changing, and can keep the wall temperature of the combustion chamber  16  constant. 
     Further, since the wall temperature of the combustion chamber  16  can be maintained at the target temperature tw by performing the temperature control in the range of SPCCI combustion, even if the combustion mode of the engine  10  is switched between HCCI combustion, MPCI combustion, and SPCCI combustion, the wall temperature of the combustion chamber  16  does not change. The HCCI combustion and MPCI combustion without forcible ignition can be performed stably, and the SPCCI combustion accompanied by forcible ignition can also be performed stably. 
     The circulation system  92  does not provide the thermally-actuated valve  28  to downstream of the coolant control valve  4 . The connecting passage  52  is a passage which bypasses the coolant control valve  4 . For this reason, even if the coolant control valve  4  has failed, such as valve adhesion, the thermally-actuated valve  28  can be opened to cool the coolant by the radiator  27  when the temperature of the coolant reaches the valve-opening temperature of the thermally-actuated valve  28 . Since the circulation system  92  can suppress that the temperature of the coolant becomes excessively high, it is advantageous to improve the reliability of the engine system  1 . 
     Other Embodiments 
     Note that in the circulation system  91  of  FIG. 4 , the position of the coolant control valve  4  may be changed. In detail, the coolant control valve  4  may be provided at a location where the bypass passage  51  and the radiator passage  53  join (a location surrounded by a one-dot chain line of  FIG. 4 ). In this configuration, the upstream end of the bypass passage  51  and the upstream end of the radiator passage  53  are connected mutually-independently to the first jacket  22   a . Further, the connecting passage  52  may connect the bypass passage  51  to a location of the radiator passage  53  downstream of the radiator  27 , and the thermally-actuated valve  28  may be provided so as to open and close the connecting passage  52 . 
     Similarly, in the circulation system  92  of  FIG. 10 , the position of the coolant control valve  4  may be changed. In detail, the coolant control valve  4  may be provided at a location where the bypass passage  51  and the radiator passage  53  join (a location surrounded by a one-dot chain line of  FIG. 10 ). In this configuration, the upstream end of the bypass passage  51  and the upstream end of the radiator passage  53  are connected mutually-independently to the first jacket  22   a . Further, the connecting passage  52  may connect a part of the radiator passage  53  downstream of the radiator  27  and a part upstream of the water pump  3  so as to bypass the coolant control valve  4 , and the thermally-actuated valve  28  may be provided so as to open and close the connecting passage  52 . 
     Further, the flow rate control device is not limited to be comprised of the coolant control valve  4  having the rotary valve body  61 . The flow rate control device may be comprised of a first flow rate control valve which adjusts the flow rate of the coolant which flows through the bypass passage  51 , and a second flow rate control valve which adjusts the flow rate of the coolant which flows through the radiator passage  53  and is independent from the first flow rate control valve. 
       FIG. 3  illustrates one example of the control of the engine system  1 . The switching of the combustion mode is not limited to the example of  FIG. 3 . 
     It should be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof, are therefore intended to be embraced by the claims. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
         
           
               1  Engine System 
               10  Engine 
               16  Combustion Chamber 
               100  ECU (Controller) 
               132  Spark Plug 
               22   a  First Jacket (Water Jacket) 
               27  Radiator (Heat Exchanger) 
               28  Thermally-actuated Valve 
               4  Coolant Control Valve (Flow Rate Control Device) 
               51  Bypass Passage 
               52  Connecting Passage 
               53  Radiator Passage 
               91  Circulation System 
               92  Circulation System