A thermoactuator is known in which a shaft is moved back and forth by expansion and contraction of wax contained in a case induced by changes in temperature.  Moreover, conventionally, using this thermoactuator as a drive source for an automobile radiator shutter has been proposed.  For example, in Patent Document 1 a ventilation shutter device equipped with a thermoactuator and capable of responding immediately to sudden changes in engine load is disclosed.  FIG.  8 is a cross-sectional view of the ventilation shutter device disclosed in Patent Document 1. 
The ventilation shutter device 200 illustrated in FIG.  8 is one that opens and closes a shutter (not illustrated) connected to the tip of a shaft 201 (on the left in the drawing) as the shaft moves back and forth.  The shutter opens a radiator ventilation duct when cooling the engine and at all other times keeps the ventilation duct closed, and so is provided so as to be openably closable within the ventilation duct. 
As illustrated in the drawing, the ventilation shutter device 200 is constructed so that the rear end of the shaft 201 (the end on the right in the drawing) is connected to a diaphragm 203 disposed within a case 202, and is biased rearward by a spring 204.  In front of the diaphragm 203 an empty chamber 205 is formed that is connected to the engine intake manifold (not illustrated) by a pipe 206.  In back of the diaphragm 203 another empty chamber 207 is formed that also is connected to the engine intake manifold through a valve 208 and another pipe 209. 
The valve 208 is constructed so that a substantially conical valve stem 210 is pressed against a valve seat 212 formed on the case 202 by a spring 211.  When a thermo-element 213 mounted on the rear end of the case 202 and disposed inside a radiator 215 is heated, a thermal expansion material such as wax housed within the thermo-element 213 expands and pushes out a piston 214, and this piston 214 pushes against the valve stem 208 and pushes the valve stem away from the valve seat 212 to open the valve.  It should be noted that a thermoactuator 216 is configured as a mechanism that contains the piston 214 and the thermo-element 213 that causes the piston to move back and forth. 
In the ventilation shutter device 200 configured as described above, when the engine load is small and the radiator 215 coolant temperature is low, the valve 208 closes and negative pressure of the intake manifold reduces the pressure inside the chamber 205, drawing the diaphragm 203 toward it, so that the shaft 201 protrudes outward (the shutter is in a closed state).  By contrast, when the engine load suddenly increases, although the temperature of the coolant in the radiator 215 is still low, with the engine throttle wide open the negative pressure of the intake manifold decreases.  As a result, the pressure of the chamber 205 increases, the force of the spring 204 pushes the shaft 201 back in as it pushes the diaphragm 203 into the chamber 207, and the shutter opens. 
As the radiator 215 coolant temperature rises, the piston 214 of the thermoactuator 216 is pushed out, the valve 208 opens, the pressure becomes the same in both chambers 205, 207, and the diaphragm 203 stops acting.  Then, the force of the spring 204 causes the piston 201 to retract, opening the shutter.  In other words, in the engine ventilation shutter device 200 is constructed in such a way that the shutter opens when the coolant temperature rises and when the coolant temperature is low, that is, when the intake manifold negative pressure decreases.  In the engine ventilation shutter device 200 disclosed in Patent Document 1 in this manner, the opening and closing of the shutter for the purpose of controlling the ventilation is implemented using the thermoactuator 216.