Abstract:
A method for operating a device at a desired temperature is described. In one example, current supplied to a heater that melts a wax medium or material controlling flow through a valve is adjusted to reduce valve opening and closing delay. The method may improve device temperature control, thereby reducing device emissions, enhancing device performance, and improving device durability.

Description:
BACKGROUND/SUMMARY 
     Temperature of a device may be regulated by a thermostat that controls flow of coolant from the device to a radiator or heat exchanger. Some example devices may include but are not limited to a fuel cell, engine, battery, motor, inverter, compressor, turbine, and amplifier. The thermostat is mechanically held closed when device temperature is less than a threshold temperature. The thermostat begins to open after device temperature is greater than the threshold temperature. Thermostat opening and closing is controlled via melting and solidification of wax material within the thermostat. Such systems may be sufficiently reliable; however, they may not regulate device temperature as tightly as is desired. 
     Another type of device temperature control system is described in U.S. Pat. No. 6,857,576 and it supplies heat to a wax material in a valve based on a temperature difference between an engine (e.g., the device) and a radiator. This system may improve engine temperature control as compared to a system that relies solely on melting of a wax material via engine coolant, but it also requires two temperature sensors and it may not respond as rapidly as is desired. Consequently, engine temperature control may not be as accurate as is desired. 
     The inventors herein have recognized the above-mentioned limitations and have developed a method for adjusting device temperature, comprising: adjusting an amount of electrical current supplied to a heater, the heater in thermal communication with a wax material in a valve, the amount of electrical current adjusted to one of two states, the amount of electrical current adjusted in response to a sign of a derivative of an output of a sole temperature sensor. 
     By adjusting heater current responsive to a derivative of device temperature, it may be possible to provide an accurate and fast response to changes in device temperature. In one example, heater current may switch between substantially zero current (e.g., less than 300 mA) and substantially rated heater current (e.g., within 500 mA of rated heater current) to control engine temperature. For example, when engine temperature is increasing and greater than a desired temperature, heater current can be rapidly increased to heat a wax medium that controls coolant flow through a valve. The heat causes the wax medium to change state and allows coolant to flow through the radiator from the engine, thereby providing cooling to the engine. Similarly, when engine temperature is decreasing and less than a desired temperature, heater current can be rapidly decreased to allow a wax medium that controls coolant flow through a valve to cool. The reduction in heat causes the wax medium to change state and limits coolant to flow through the radiator from the engine, thereby reducing cooling to the engine. In this way, a bang-bang controller that adjusts heater current to regulate coolant flow from an engine through a radiator may be provided to rapidly and accurately control engine temperature. 
     The present description may provide several advantages. In particular, the approach may improve engine temperature control. Further, the approach may reduce engine emissions, improve performance, and increase durability by more accurately controlling engine temperature. Additionally, the approach may provide improved temperature control while only relying on a single engine temperature sensor, if desired. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic depiction of an engine; 
         FIG. 2  shows an example engine operating sequence according to the method of  FIG. 4 ; 
         FIG. 3  shows an example device temperature control range and sub-range; 
         FIG. 4  shows an alternative device temperature control system; and 
         FIG. 5  shows an example method for controlling device temperature. 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to controlling a temperature of a device. In one example described herein the device is an engine as shown in  FIG. 1 . In the engine example, current supplied to a heater that changes a state of a wax medium in an engine coolant control valve is adjusted between a lower current limit and an upper current limit. The lower and upper current limits may be determined based on engine operating conditions.  FIG. 1  shows an example system that may be controlled as shown in the sequence of  FIG. 2  according to the method of  FIG. 5 . In one example, device temperature control ranges as shown in  FIG. 3  provide a basis for adjusting current supplied to the heater.  FIG. 4  shows an alternative system where the device is a fuel cell, battery, motor, inverter, compressor, turbine, or amplifier, etc. 
     Referring to  FIG. 1 , internal combustion engine  10 , comprising a plurality of cylinders, one cylinder of which is shown in  FIG. 1 , is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  40 . Combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Each intake and exhaust valve may be operated by an intake cam  51  and an exhaust cam  53 . The position of intake cam  51  may be determined by intake cam sensor  55 . The position of exhaust cam  53  may be determined by exhaust cam sensor  57 . 
     Fuel injector  66  is shown positioned to inject fuel directly into cylinder  30 , which is known to those skilled in the art as direct injection. Alternatively, fuel may be injected to an intake port, which is known to those skilled in the art as port injection. Fuel injector  66  delivers liquid fuel in proportion to a pulse width provided by controller  12 . Fuel is delivered to fuel injector  66  by a fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). 
     Intake manifold  44  is supplied air by compressor  162 . Exhaust gases rotate turbine  164  which is coupled to shaft  161 , thereby driving compressor  162 . In some examples, a bypass passage  77  is included so that exhaust gases may bypass turbine  164  during selected operating conditions. Flow through bypass passage  77  is regulated via waste gate  75 . Further, a compressor bypass passage  86  may be provided in some examples to limit pressure provided by compressor  162 . Flow through bypass passage  86  is regulated via valve  85 . In addition, intake manifold  44  is shown communicating with central throttle  62  which adjusts a position of throttle plate  64  to control air flow from engine air intake  42 . Central throttle  62  may be electrically operated. 
     Distributorless ignition system  88  provides an ignition spark to combustion chamber  30  for igniting an air-fuel mixture via spark plug  92  in response to controller  12 . In other examples, the engine may be a compression ignition engine without an ignition system, such as a diesel engine. Universal Exhaust Gas Oxygen (UEGO) sensor  126  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . Alternatively, a two-state exhaust gas oxygen sensor may be substituted for UEGO sensor  126 . 
     Converter  70  can include multiple catalyst bricks, in one example. In another example, multiple emission control devices, each with multiple bricks, can be used. Converter  70  can be a three-way type catalyst in one example. 
     Engine temperature is controlled via coolant circuit  90 . Coolant circuit  90  includes radiator or heat exchanger  91  which extracts excess heat from engine coolant. Additionally, coolant circuit  90  includes a coolant pump  92  and coolant control valve  94 . Wax medium  95  allows or limits coolant flowing to/from radiator  91  to engine  10  depending on the state of wax medium  95 . In one example, coolant flow through coolant control valve  94  is limited when wax medium  95  is less than a threshold temperature. Coolant flow through coolant control valve or thermostat  94  is allowed when wax medium  95  is greater than a threshold temperature. Thermostat heater  93  is in thermal communication with wax medium  95  and can supply heat to change the state of wax medium  95 , thereby permitting or restricting coolant flow through coolant valve  94 . 
     Controller  12  is shown in  FIG. 1  as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , read-only memory  106 , random access memory  108 , keep alive memory  110 , and a conventional data bus. Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114  or alternatively a cylinder head; a position sensor  134  coupled to an accelerator pedal  130  for sensing accelerator position adjusted by foot  132 ; a measurement of engine manifold pressure (MAP) from pressure sensor  122  coupled to intake manifold  44 ; an engine position sensor from a Hall effect sensor  118  sensing crankshaft  40  position; a measurement of air mass entering the engine from sensor  120  (e.g., a hot wire air flow meter); and a measurement of throttle position from sensor  58 . Barometric pressure may also be sensed (sensor not shown) for processing by controller  12 . Controller  12  also selectively provides current to thermostat heater  93 . In a preferred aspect of the present description, engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. 
     In some examples, the engine may be coupled to an electric motor/battery system in a hybrid vehicle. The hybrid vehicle may have a parallel configuration, series configuration, or variation or combinations thereof. Further, in some embodiments, other engine configurations or components which are not engines may be employed, for example a diesel engine, a fuel cell, a battery, a motor, an inverter, a compressor, etc. In these other examples, the aforementioned engine descriptions may not be applicable, but similar constructs can be envisioned by those skilled in the art. 
     During operation, each cylinder within engine  10  typically undergoes a four stroke cycle: the cycle includes the intake stroke, compression stroke, expansion stroke, and exhaust stroke. During the intake stroke, generally, the exhaust valve  54  closes and intake valve  52  opens. Air is introduced into combustion chamber  30  via intake manifold  44 , and piston  36  moves to the bottom of the cylinder so as to increase the volume within combustion chamber  30 . The position at which piston  36  is near the bottom of the cylinder and at the end of its stroke (e.g. when combustion chamber  30  is at its largest volume) is typically referred to by those of skill in the art as bottom dead center (BDC). During the compression stroke, intake valve  52  and exhaust valve  54  are closed. Piston  36  moves toward the cylinder head so as to compress the air within combustion chamber  30 . The point at which piston  36  is at the end of its stroke and closest to the cylinder head (e.g. when combustion chamber  30  is at its smallest volume) is typically referred to by those of skill in the art as top dead center (TDC). In a process hereinafter referred to as injection, fuel is introduced into the combustion chamber. In a process hereinafter referred to as ignition, the injected fuel is ignited by known ignition means such as spark plug  92 , resulting in combustion. During the expansion stroke, the expanding gases push piston  36  back to BDC. Crankshaft  40  converts piston movement into a rotational torque of the rotary shaft. Finally, during the exhaust stroke, the exhaust valve  54  opens to release the combusted air-fuel mixture to exhaust manifold  48  and the piston returns to TDC. Note that the above is described merely as an example, and that intake and exhaust valve opening and/or closing timings may vary, such as to provide positive or negative valve overlap, late intake valve closing, or various other examples. 
     Referring now to  FIG. 2 , an engine operating sequence in which engine temperature is controlled is shown. The operating sequence of  FIG. 2  may be provided via the system of  FIG. 1  executing instructions stored in non-transitory memory of the method of  FIG. 4 . Vertical markers at times T 0 -T 5  represent times of interest during the sequence. 
     The first plot from the top of  FIG. 2  represents engine speed versus time. The X axis represents time and time increases from the left side of the plot to the right side of the plot. The Y axis represents engine speed and engine speed increases in the direction of the Y axis arrow. 
     The second plot from the top of  FIG. 2  represents engine temperature versus time. The X axis represents time and time increases from the left side of the plot to the right side of the plot. The Y axis represents engine temperature and engine temperature increases in the direction of the Y axis arrow. Horizontal lines  202  and  203  represent bounds of an example desired engine temperature range. The desired temperature range is between horizontal lines  202  and  203 . 
     The third plot from the top of  FIG. 2  represents a thermostat heater command versus time. The X axis represents time and time increases from the left side of the plot to the right side of the plot. The Y axis represents a thermostat heater command where current supplied to the thermostat heater increases in the direction of the Y axis arrow as the thermostat heater command increases. Horizontal line  204  represents a thermostat upper threshold heater command that corresponds to an upper threshold heater current. 
     At time T 0 , the engine is stopped and engine temperature is low. The thermostat heater command is also low or zero. When the thermostat heater command is low, substantially no current is supplied to the thermostat heater which allows a wax medium in the thermostat valve to stay in or go to a state where coolant flow is not allowed from the engine to the radiator. 
     Between time T 0  and time T 1 , the engine is started in response to a driver request to start. Engine speed increases in response to starting the engine. The heater command and the engine temperature are low at the time of engine starting, but the engine temperature increases as the engine continues to operate for a longer period of time. A derivative of engine temperature is taken as engine temperature increases. The derivative has a positive sign indicating increasing engine temperature. The thermostat heater command remains at a low level while engine temperature is outside of the temperature range indicated by horizontal markers  202  and  203 . 
     At time T 1 , engine temperature reaches lower engine threshold temperature  203  and continues to rise. Consequently, in response to engine temperature above lower engine threshold temperature  203  and an engine temperature derivative with a positive sign, the thermostat heater command increases from substantially zero to a level  204  where the wax media changes state after being heated above a threshold temperature. In one example, the current is increased to a rated current of the thermostat heater. 
     Between time T 1  and time T 2 , the thermostat heater command and current is driven between substantially zero and a predetermined value  204 . The predetermined value may be adjusted for engine operating conditions. In some examples, the predetermined value is a rated heater current. Further, the heater current is driven between substantially zero and the predetermined value  204  based on a sign of a derivative of engine temperature. In particular, when the sign of the engine temperature derivative is positive, the thermostat heater current is increased to the predetermined level  204  from the substantially zero level. When the sign of the engine temperature derivative is negative, the thermostat heater current is decreased to substantially zero from the predetermined level  204 . Driving heater current between substantially zero and the predetermined level  204  provides for a quick response when engine temperature begins to increase or decrease. Thus, engine temperature may be controlled within a narrow range to improve engine temperature control. 
     At time T 2 , the engine is stopped and the thermostat heater command is reduced to zero so that substantially no current flows to the thermostat heater. The engine temperature is shown increasing in response to the engine being shut off. The engine temperature increases since heat is not driven out of the engine while the engine is not operating. The engine temperature increases to a level greater than the upper engine threshold temperature  202 . 
     At time T 3 , the engine is restarted in response to an engine start request from a driver. Engine temperature remains above upper engine threshold temperature  202 . Current supplied to the thermostat heater is increased after the engine start request to threshold level  204  in response to engine temperature being greater than threshold temperature  202 . 
     At time T 4 , engine temperature falls below upper engine threshold temperature  202  and reaches a steady value between upper engine threshold temperature  202  and lower engine threshold temperature  203 . The thermostat heater command is adjusted between two predetermined levels between the zero level and the predetermined level  203 . Engine speed remains relatively constant at idle speed. 
     At time T 5 , engine temperature increases toward upper engine temperature threshold  202  and the thermostat heater command and current are increased to predetermined level  204  to allow additional coolant to flow from the engine to the radiator. Further, engine speed increases in response to an operator torque command. The change in engine speed and output increases engine heat output; however, the change is counter acted via adjusting the thermostat heater command in response to a sign of a derivative of engine temperature. The thermostat heater command and current are driven between substantially zero and predetermined value  204  so that engine temperature stays between upper engine temperature threshold  202  and lower engine temperature threshold  203  for the remaining time shown in the sequence. 
     Referring now to  FIG. 3 , an example device temperature range versus time is shown. The X axis represents time and time increases from the left side of  FIG. 3  to the right side of  FIG. 3 . The Y axis represents device temperature and device temperature increases in the direction of the Y axis arrow. Several different device temperature ranges are shown. 
     Line  303  represents an upper device threshold temperature defining an upper boundary of a desired device temperature range that extends from line  303  down to line  309 . Area  302  is above line  303  and it represents a desired temperature zone where thermostat heater current is adjusted to an upper threshold current (e.g., rated heater current). Line  305  represents an upper sub-range desired threshold temperature defining an upper boundary of a desired temperature sub-range. Line  350  represents a desired device temperature. Line  307  represents a lower sub-range device threshold temperature defining a lower boundary of the device temperature sub-range. Line  309  represents a lower device threshold temperature defining a lower boundary of a desired device temperature range. 
     An upper threshold heater current may be applied to a thermostat heater when device temperature is in area  302  above upper engine threshold temperature  303 . A heater current based on a sign of a derivative of device temperature may be applied to the thermostat heater when device temperature is in areas  304  which are above and below device temperature sub-ranges  306 . Area  308 , below lower device threshold temperature  309 , is an area where substantially no current may be applied to the thermostat heater so that device temperature can increase toward desired device temperature  350 . 
     Thus, in one example, a device temperature range that includes a sub-range device temperature range may provide a basis for adjusting a thermostat heater command and current. The thermostat heater command and current may be controlled differently when device temperature is within different areas of the device temperature range. In this way, response and accuracy of device temperature control may be improved. 
     Referring now to  FIG. 4 , an alternative example system for controlling device temperature is shown. Components or elements in  FIG. 4  that have the same numerical tags as components in  FIG. 1  are the same devices and operate in the similar manner as described in  FIG. 1 . Device  402  may be comprised of a fuel cell, battery, motor, inverter, compressor, turbine, or amplifier. And, temperature of device  402  may be controlled according to the method of  FIG. 5  and similarly to as is described in  FIG. 2 . Device  402  is supplied coolant via pump  92 . Coolant flow to or from device  402  may be limited by coolant control valve  94  which includes wax medium  95 . Thermostat heater  93  supplies heat to change the state of wax medium  95 . Electrical current selectively flows from controller  12  to thermostat heater  93  depending on inputs to controller  12  and executable instructions within controller  12 . Coolant flows through device  402  and radiator or heat exchanger  91  when coolant control valve is in an open state. A sole temperature sensor  404  is a basis for supplying current to thermostat heater  93 , and it supplies an output to controller  12  that corresponds to a temperature of device  402 . Although temperature sensor  404  is shown as being located in device  402 , it may be located elsewhere, such as in the coolant lines either entering or exiting device  402 , or it may be located in such a way that the temperature of device  402  is inferred by controller  12  based on output from sensor  404 . 
     Referring now to  FIG. 5 , an example method for controlling or regulating device temperature is shown. The method of  FIG. 5  may be stored as executable instructions in non-transitory memory of controller  12 . Further, the method of  FIG. 5  may provide the operating sequence shown in  FIG. 2 . 
     At  502 , method  500  determines device operating conditions. Device operating conditions may include but are not limited to engine speed, engine load, amount of time since the engine was last stopped, engine temperature from within the engine, motor current, motor voltage, and desired engine torque level. Method  500  proceeds to  504  after device operating conditions are determined. 
     At  504 , method  500  determines a device temperature control range and sub-range (e.g., see  FIG. 3 ) in response to device operating conditions. In one example, engine speed and load are inputs to tables or functions that included empirically determined desired engine operating temperature ranges. For example, an upper engine threshold temperature and lower engine threshold temperature may be determined via indexing a table or function based on engine speed and load. Similarly, an engine sub-range temperature may be determined from upper and lower sub-range engine threshold temperatures that are retrieved from tables and/or functions using engine speed and load. In other examples, only a single device temperature range without a sub-range may be provided. In still other examples, the device temperature control range may simply be a single desired device temperature. Method  500  proceeds to  506  after the device temperature range is determined. 
     At  506 , method  500  stores device temperature to memory for subsequently determining a derivative of device temperature from the present device temperature and past device temperature. Method  500  proceeds to  508  after device temperature is stored to memory. 
     At  508 , method  500  judges whether or not device temperature is less than a lower threshold of a device temperature control range. For example, method  500  judges whether or not engine temperature is less than a temperature represented by line  309  of  FIG. 3 . If so, the answer is yes and method  500  proceeds to  522 . Otherwise, the answer is no and method  500  proceeds to  510 . 
     At  510 , method  500  judges whether or not device temperature is greater than an upper threshold of a device temperature control range. For example, method  500  judges whether or not engine temperature is greater than a temperature represented by line  303  of  FIG. 3 . If so, the answer is yes and method  500  proceeds to  520 . Otherwise, the answer is no and method  500  proceeds to  512 . 
     At  512 , method  500  determines a derivative of device temperature. In one example, the derivative is approximated via subtracting a device temperature sampled in the past from a present device temperature and dividing by the amount of time between the two device temperature samples. Additionally, in some examples derivatives of engine load, engine speed, or other device parameters may be determined at  512  in a similar manner as the device temperature derivative. Method  500  proceeds to  514  after derivatives are determined. 
     At  514 , method  500  judges whether or not device temperature is within a sub-range (e.g., between  305  and  307  as shown in  FIG. 3 ) of a desire device temperature control range. If so, the answer is yes and method  500  proceeds to  516 . Otherwise, the answer is no, and method  500  proceeds to  518 . 
     At  516 , method  500  adjusts a current level applied to the thermostat heater as a function of the derivative of device temperature. The current adjustment may be proportional to the device temperature derivative. Further, the current adjustment may be increased or decreased depending on device operating conditions. 
     In other examples, a current level applied to a thermostat heater may be adjusted between a lower sub-level and an upper sub-level (e.g., thermostat heater command between time T 4  and T 5  in  FIG. 2 ) that lie between the substantially zero current supplied at  522  and the upper threshold current level supplied at  520 . The lower sub-level and the upper sub-level of currents may be empirically determined and stored in tables or functions in controller memory. The tables or functions may be indexed via device parameters such as engine speed and load or other parameters which may be indicative of a forthcoming change in temperature. 
     Additionally, heater current may be adjusted in response to derivatives of other device parameters such as speed and/or engine load, voltage supplied to the device, and current supplied to the device. For example, if the derivative of engine speed or engine load is positive, heater current may be proportionately increased. Similarly, if the derivative of engine speed or engine load is negative, heater current may be proportionately decreased. In this way, an increase in device temperature may be predicted since increasing engine speed and/or load can increase thermal output of an engine so that heater current adjustments may begin before temperature sensor based heater current adjustments. Likewise, a decrease in device temperature may be predicted since decreasing engine speed and/or load can decrease thermal output of an engine so that heater current adjustments may begin before temperature sensor based heater current adjustments. This may be considered to be a form of feed forward control by those skilled in the art. Method  500  proceeds to exit after heater current adjustments are output. 
     At  518 , method  500  judges whether or not a sign of a derivative is positive. If so, the answer is yes and method  500  proceeds to  520 . Otherwise, the answer is no and method  500  proceeds to  522 . 
     At  520 , method  500  applies an upper threshold current to a heater supplying thermal energy to control a state of a valve regulating current flow into an engine. In one example, the upper threshold current is a rated current of the heater. In another example, the upper threshold current is a current level below the rated current of the heater. The upper threshold current may be adjusted based on device operating conditions. For example, the upper threshold current may be increased as engine temperature increases. Method  500  proceeds to exit after the upper threshold current is applied to the heater. 
     At  522 , method  500  ceases current flow or reduces is to substantially zero (e.g., less than 300 mA). Current flow to the heater may be reduced or increased via controlling an output of a transistor such as a field effect transistor. Method  500  proceeds to exit after heater current is reduced to substantially zero. 
     In this way, a controller can swing between upper and lower heater current limits to improve response time of a system that includes a valve that regulates coolant flow from an engine to a radiator. Additionally, by improving system response time, it may be possible to provide more accurate control of engine temperature. 
     Thus, the method of  FIG. 5  provides for adjusting device temperature, comprising: adjusting an amount of electrical current supplied to a heater, the heater in thermal communication with a wax material in a valve, the amount of electrical current adjusted to one of two states, the amount of electrical current adjusted in response to a sign of a derivative of an output of a sole device temperature sensor. In this way, a controller can drive current into or remove current quickly into a heating element that provides thermal energy to a wax medium that controls flow of coolant to a device. 
     The method includes where the sole device temperature sensor is a cylinder head temperature sensor. The method also includes where one of the two states includes substantially zero current flow. The method also includes where one of the two states includes rated current flow of the heater. The method further comprises increasing current flow through the heater from substantially zero current flow to substantially rated current flow of the heater in response to a positive sign of the device temperature derivative. The method further comprises decreasing current flow through the heater from substantially rated current flow of the heater to substantially zero current flow in response to a negative sign of the derivative. 
     In another example, the method of  FIG. 5  provides for adjusting device temperature, comprising: selecting a desired device temperature range in response to device operating conditions; and adjusting an amount of electrical current supplied to a heater responsive to the desired device temperature range, the heater in thermal communication with a wax material in a valve, the amount of electrical current adjusted to one of two states, the amount of electrical current adjusted in response to a sign of a derivative of an output of a sole device temperature sensor. In this way, heater current can be adjusted to control device temperature within a designate range. 
     In some examples, the method includes where a first of the two states is substantially zero current, and further comprising adjusting the amount of electrical current to the first state in response to an engine temperature less than a lower threshold of the desired device temperature range. The method also includes where a second of the two states is substantially rated heater current, and further comprising adjusting the amount of electrical current to the second state in response to a device temperature greater than an upper threshold of the desired device temperature range. The method includes where the two states are less than rated heater current. 
     In one example, the method further comprises adjusting the amount of electrical current in response to engine speed. The method further comprises adjusting the amount of electrical current in response to engine load. Additionally, the method includes where the two states are adjusted in response to operating conditions. 
     The method of  FIG. 5  also provides for adjusting device temperature, comprising: selecting a desired device temperature range and a desired device sub-temperature range, the desired device sub-temperature range within the desired device temperature range, in response to device operating conditions; and adjusting an amount of electrical current supplied to a heater to one of only two states in response to device temperature being in the desired device temperature range and not in the desired device sub-temperature range, and adjusting the amount of electrical current supplied to the heater as a function of a derivative of a sole device temperature in response to device temperature being within the desired device sub-temperature range, the heater being in thermal communication with a wax material controlling flow through a valve. 
     The method also includes where a first of the only two states is substantially zero current and where a second of the only two states is a rated heater current. The method includes where the amount of electrical current is adjusted proportionately with the derivative of the sole device temperature. The method includes where the desired temperature range varies as device operating conditions vary. The method also includes where a first of the only two states is substantially zero current and where a second of the only two states is less than a rated heater current. The method further comprises adjusting the amount of electrical current in response to engine speed. The method further comprises adjusting the amount of electrical current in response to engine load. 
     As will be appreciated by one of ordinary skill in the art, the method described in  FIG. 5  may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the objects, features, and advantages described herein, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used. 
     This concludes the description. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the description. For example, single cylinder, I2, I3, I4, I5, V6, V8, V10, V12 and V16 engines operating on natural gas, gasoline, diesel, or alternative fuel configurations could use the present description to advantage.