Patent Publication Number: US-2012037606-A1

Title: Vehicular fluid heater

Description:
The invention refers to a vehicular fluid heater, the invention refers in particular to an automotive water heater, comprising at least one heat exchanger, at least one electrically operated heating unit and at least one control unit for controlling power supply to the heating unit, the heat exchanger comprising at least one thermally conductive body defining at least one fluid channel for the fluid to be heated, the heating unit being attached to a heat conductive surface of the heat exchanger. 
     An automotive water heater of the above-referred kind is for instance disclosed in US 2008/0138052 A1. This US patent publication refers to an automotive water heater having application to a windshield of an automobile which is able to produce hot water than can be sprayed onto the windshield of a motor car to melt accumulated snow and frost. The automotive water heater according to the prior art comprises an aluminum heat exchanger defining at least one fluid path through which water to be heated can flow. Heat conductive surfaces of the heat exchanger are provided with electrically operated heating units. The heating units comprise laminated heating strips joint to plate electrodes. Moreover, the heating units utilize PTC stones (ceramic resistance members with Positive Temperature Coefficient) as electro-thermal material. 
     Once electrical power is applied to the heating units, the ceramic resistors will heat up and transfer their heat to the thermal conductive heat exchanger through which water or another fluid to be heated can flow. 
     Automotive water heaters of this type are designed to deliver heated screen wash on demand at a pre-programmed target temperature of between 60° C. to 70° C. The flow channel or flow path defined by the heat exchangers defines a certain liquid volume, usually in the order of 60 to 80 cc, which for instance on ignition of the car will be heated up to the target temperature of 60° C. to 70° C. Once the screen wash fluid has reached the target temperature, a washing fluid pump of the car&#39;s screen wash cleaning device dispenses a series of heated shots of screen wash fluid onto the windshield of the car. 
     Generally, it is desirable that the target temperature within the heat exchanger is reached within a relatively short time period after activation of the system. Resistive heating elements usually draw high current to generate an electrical heating power to achieve the specified thermal performances. 
     In order to control performance and heat dissipation of the heating units, usually electronic control means, for instance control boards or circuit boards, are required. Usually some electronic components on control boards need to be cooled. In particular high performance semiconductor elements require cooling due to the fact that these elements produce a considerably amount of loss heat. In order to dissipate the loss heat normally heat sinks are required. Since such heat sinks normally have an enlarged surface for heat dissipation, such heat sinks require a huge amount of space. This is particularly disadvantageous if the vehicular fluid heater is to be designed as an integrated unit. 
     It is therefore an object of the present invention to provide a vehicular fluid heater with effective cooling for the electronic controls which is also simple and inexpensive. 
     This and other objects are achieved by a vehicular fluid heater, in particular by an automotive water heater, comprising at least one heat exchanger, at least one electrically operated heating unit and at least one control unit for controlling power supply to the heating unit, the heat exchanger comprising at least one thermally conductive body defining at least one fluid channel for the fluid to be heated, the heating unit being attached to a heat conductive surface of the heat exchanger, the vehicular fluid heater being characterized in that the control unit is thermally connected to the heat exchanger. 
     Briefly summarized, the vehicular fluid heater according to the invention utilizes the heat exchanger for the fluid at the same time as heat-dissipating means for the control unit. Accordingly, additional cooling means are not required. Moreover, efficiency of the heating unit is enhanced and less energy will be required a predetermined amount of fluid. 
     In one advantageous embodiment, the control unit is connected to the heat exchanger by a heat sink. The heat sink may be very small and simple due to the fact that the heat exchanger also functions as a heat sink. 
     Accordingly, the heat sink may be in the form of a thermally conducting metal strip. Such metal strip could be for instance designed as a copper or aluminum strip. 
     The control unit may be arranged on a control board. Alternatively, the control unit may be directly attached to the heat exchanger and/or to the heating unit. In this event an intermediate layer of an electrically insolating material can be provided between the control unit and the heat exchanger. 
     In one embodiment of the invention, the heat exchanger, at least one associated heating unit and the control unit may be encapsulated by a common housing. 
     The control unit may comprise a switching unit, preferably a transistor, and more preferably a metal oxide semiconductor field effect transistor (MOSFET) which is thermally connected to the heat sink. Said high performance transistor in operation draws extremely high current and accordingly heats itself up very quickly. The MOSFET can be directly positioned on the heat sink which in turn may be adhered to a heat conductive surface of the heat exchanger. 
    
    
     
       The invention is hereinafter described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  shows an automotive screen wash device, 
         FIG. 2  shows a perspective view of the vehicular fluid heater according to the invention, 
         FIG. 3  shows a perspective view of the heat exchanger in sealed position, 
         FIG. 4   a  shows a perspective view of the heat exchanger without the sealing covers, 
         FIG. 4   b  shows a perspective view of the heat exchanger according to another embodiment of the invention, 
         FIG. 5  shows a perspective view of a heating unit, 
         FIG. 6   a  shows a cross-sectional view through the vehicular fluid heater in the longitudinal direction, 
         FIG. 6   b  shows a sectional elevation of the vehicular fluid heater, 
         FIG. 7  shows an enlarged cross-sectional view of the right hand side of the vehicular fluid heater in  FIG. 6 , 
         FIG. 8  shows an enlarged cross-sectional view of the left hand side of the vehicular fluid heater as shown in  FIG. 6 , 
         FIG. 9   a  shows another enlarged cross-sectional view of the vehicular fluid heater showing the connection of the circuit board of the electrical control to the heat exchanger, 
         FIG. 9   b  shows another enlarged cross-sectional view of the vehicular fluid heater according to the embodiment shown in  FIG. 4   b,    
         FIG. 10  shows an exploded view of the vehicular fluid heater according to the invention, 
         FIG. 11  shows a functional diagram of the heating element in combination with a control assembly, 
         FIG. 12  shows a circuit diagram of a measurement circuitry to measure the voltage at a sampling resistor, 
       Graph  1  shows the resistance of a PTC stone versus the actual temperature of the PTC stone, 
       Graph  2  shows the current flowing through a PTC stone versus the actual temperature of the PTC stone for a constant voltage, 
       Graph  3  shows the actual temperature of the PTC stone versus time in case a voltage is applied to the PTC stone, and 
       Graph  4  shows an exemplary rectangular shaped control signal. 
     
    
    
       FIG. 1  shows a schematic view of a windshield screen wash device for a vehicle comprising a washing fluid reservoir  1 , a washing fluid pump  2 , a vehicular fluid heater  3  and screen wash nozzles  4  associated with a windshield of a car which is not shown. During normal screen wash operation, cleaning fluid is drawn from the cleaning fluid reservoir  1  by an electrically operated pump  2  towards the windshield of a vehicle. It is to be understood that the cleaning fluid can also be delivered to headlamps, rear lamps or other screens to be cleaned. The cleaning fluid enters the vehicular fluid heater  3  via inlet port  5  and will be discharged via outlet port  6 . As this can be seen from  FIG. 1 , the inlet port  5  is connected to the washing fluid pump  2  by a flexible hose  7 . In the same way, the outlet port  6  is connected to the washing fluid nozzles  4  by another flexible hose  7 .  FIG. 1  shows the screen wash device only by way of example and very simplified. 
     The washing fluid reservoir normally contains washing fluid at ambient temperatures which can be in the order from −40 to 40° C. The vehicular fluid heater  3 , as this will be described in detail hereinafter, may contain a fluid volume between 60 and 70 cc. The vehicular fluid heater  3  is designed to deliver heated screen wash fluid on demand at a pre-programmed target temperature of between 50 to 70° C., preferably at a temperature below the evaporation temperature of alcohol which is normally to be found in all winter mixtures of cleaning fluid. On turning the ignition of the vehicle, the vehicular fluid heater is designed to heat up to its target temperature. This can be visualized by an LED in the cabin of the vehicle. Either the user can defrost on demand or the defrost mode may be started automatically. When a defrost switch in the cabin of the vehicle is momentarily depressed, the heater module sends a signal to the wiper control unit which in turn signals the washing fluid pump  2  to dispense a series of heated shots of heated screen wash fluid, typically  4  to  6  shots. The wiper may also be operated at this time to help with the cleaning process. 
     The vehicular fluid heater comprises a heat exchanger  8 , electrically operated heating units  9  and an electrical control board  10 , all parts enclosed by a common housing  11 . The housing  11  comprises three parts, namely a main body  11   a , a first end cap  11   b  and a second end cap  11   c . The first and second end caps  11   b, c  are connected to the main body  11   a  via snap-fit connectors  12 . 
     The housing may consist of thermoplastic material and may be for instance made by injection-molding. 
     As this can be taken in particular from  FIG. 2 , the second end cap  11   c  is provided with nippels  13  from which one communicates with the inlet port  5  and the other one communicates with the outlet port  6 . The first end cap  11   a  is provided with terminal connectors  14  which establish the electrical connection of the vehicular fluid heater  3 . 
     As this can be seen from  FIGS. 3 ,  4   a  and  4   b , a central part of the vehicular fluid heater is the heat exchanger  8  which consists of an extruded aluminum profile defining a fluid channel  15  allowing the fluid to flow into the heat exchanger  8  sequentially by help of sealing covers  16   a  and  16   b  sealingly closing the front and rear end of the heat exchanger  8 . 
     The side of the heat exchanger  8  shown in  FIG. 3  facing the reader for sake of simplicity is in the further description designated the front end, whereas the opposite end of the heat exchanger  8  will be addressed as the rear end. The sealing covers  16  fulfill the sealing function for the front and rear end of the heat exchanger and for sealing the side-by-side sections of the fluid channel  15 . 
     As this can be taken from  FIG. 6   b , the fluid channel  15   a  is at the front end of the heat exchanger  8  sealed by sealing cover  16   a  relative to fluid channel  15   b , whereas at the rear end of the heat exchanger  8  the sealing cover  16   b  establishes fluid connection between fluid channel  15   a  and fluid channel  15   b . Moreover, at the front end of the heat exchanger  8 , fluid channel  15   b  communicates with fluid channel  15   c  whereas fluid channel  15   c  is sealed relative to fluid channel  15   d.    
     Furthermore, the sealing cover  16   a  comprises an inlet opening  17   a  and an outlet opening  17   b.    
     The sealing covers  16   a  and  16   b  are made from an elastically deformable material such as natural or synthetic rubber and function as a kind of diaphragm or membrane in order to compensate the volume change of the cleaning fluid in the frozen state as this has been described before. The sealing covers  16   a, b  are in the described embodiment loosely fit onto the front and rear ends of the heat exchanger and are held in place by the housing  11 , such as it is hereinafter described in more detail. 
     In order to define a continuously extending fluid channel  15   a ,  15   b ,  15   c ,  15   d  within the heat exchanger  8  which is made from an extruded aluminum profile, the sealing covers  16   a  and  16   b  comprise diaphragm type bridging members  50   a  and  50   b , the sealing cover  16   a  comprising one bridging member  50   a  connecting the fluid channels  15   b  and  15   c  with each other, whereas sealing cover  16   b  comprises two bridging members  50   b , one connecting the fluid channels  15   a  and  15   b , the other one connecting the fluid channels  15   c  and  15   d . Each of the diaphragm type bridging members  50   a ,  50   b  is surrounded by a circumferential sealing rim  51 . 
     As this can be seen in more detail from  FIG. 6   b  in cross section the sealing rim  51  defines an outer groove  52  and an inner groove  53 . The inner groove  53  sealingly receives the peripheral walls of the fluid channels  15   a ,  15   b ,  15   c  and  15   d , whereas the outer groove  52  receives locating webs  54  of the first and second end caps  11   b  and  11   c  of the main body  11   a  when mounted. In the event the bridging members  50   a  and  50   b  should flex due to freezing cleaning fluid, the sealing rim  51  is properly held in place by the locating webs  54  of the end caps  11   b  and  11   c , thus allowing fluid expansion/contraction without significantly effecting the sealing function of the sealing covers  16   a  and  16   b.    
     As mentioned before, the heat exchanger  8  is made from a thermally conductive material such as aluminum. At the side surfaces of the heat exchanger  8 , heating units  9  are provided. The electrically operating heating units  9  are adhered to the heat exchanger by a heat curable silicon glue. Those heating units  9  utilize a laminated structure. Although in a preferred embodiment the heating units  8  utilize a ceramic resistor with a positive temperature coefficient of resistivity (PTCR), it is to be understood that the heating units  9  can be in form of heating strips with a polymer-resistant material with thermal electrical properties or an heating wire, encapsulated or not, having thermal-electrical properties. 
     In one preferred embodiment, the heating unit ( FIG. 5 ) comprises a laminated frame  19  supporting ceramic elements  20 , a cathode contact plate  21  and an anode contact plate  22  insulated relative to the cathode contact plate  21 . 
     Within the frame  19  there is a void  23  the function of which will be explained later. 
     The heating unit  9  comprises one or more positive temperature coefficient ceramic resistor heating elements  20 , afterwards referred to as PTC stones  20 , the cathode contact plate  21  and the anode contact plate  22  for conduction of electricity, for example 13 V, to the PTC stones  20 . The anode contact plate  22 /anode terminal is in direct contact with the heat exchanger  8  and the contact plate portion covers the anode sides of the PTC stones  20  which is fixed in position by the position frame  19 . The cathode terminal/contact plate  21  is on top of the cathode sides of the PTC stones  20  thereby parallel connecting all PTC stones  20 . 
     Due to this design the heat exchanger  8  is connected to ground (GND) so that any static charge build up in the fluid may be deflected. 
     PTC stones  20  are semi-conductors having conductivity inversely proportional to their overall temperature. Thus, while the heating unit  9  is cold, the conductivity of the PTC stones  20  is high, and high current will flow through the PTC stones  20 ; thereby generating a great amount of thermal energy. On the other hand, if PTC stones  20  rise in temperature the conductivity of the PTC stones  20  drop dramatically resulting in the generation of only a small amount of heat. As a result, since a PTC stone  20  is capable of maintaining its own target temperature (thermally self-regulating), a heating unit  9  using PTC stones  20  as heating elements does not require protection by thermostats or thermofuses. PTC stones  20  are available with different target temperatures, for example 65° C. or 135° C. 
     Graph  1  shows the resistance (R) of the PTC stone  20  versus the actual temperature (T HE ) of the PTC stone  20 . As mentioned above, in case the PTC stone  20  is cold, its resistance (R) is low. The resulting high current flowing through the PTC stone  20  generates a great amount of thermal energy which heats up the PTC stone  20 . As can be seen from graph  1 , the resistance (R) of the PTC stone  20  increases with an increase of its actual temperature (T HE ). In case the actual temperature (T HE ) of the PTC stone  20  equals the maximum temperature, the resistance (R) of the PTC stone  20  starts to decrease in accordance to a decrease in the actual temperature (T HE ) of the PTC stone  20 . This results in a higher current through the PTC stone  20  which again heats up the PTC stone  20 , resulting in an increase of the resistance (R) of PTC stone  20 . Correspondingly, as shown in graph  2 , the current (I) flowing through the PTC stone  20  decreases with an increase of its actual temperature (T HE ). Hence, less thermal energy is generated. Using this mechanism, the PTC stone  20  limits its maximum temperature to a specific target temperature. 
     In a heating application the PTC stone  20  can reach an equilibrium state where the current consumption is equal to the thermal dissipation rate of the PTC stone  20  in a constant ambient condition. 
     PTC stones  20  will adopt their current consumption to reach an equilibrium state with the ambient condition, e.g. a greater thermal dissipation (cooling)will lead to a higher current consumption of the PTC stones  20  in the equilibrium state. 
     Once power is applied to the PTC stones  20  they immediately try to reach their target temperature. In the beginning the temperature increases rapidly, but with an increase of the actual temperature (T HE ) of the PTC stone  20 , the increase rate slows down. This relationship between the actual temperature (T HE ) of the PTC stone  20  and the time is shown in graph  3 . 
     In one preferred embodiment the heating unit  9  is designed to heat up the screen wash fluid to a target temperature of for example 65° C. This could be accomplished by using PTC stones  20  with a target temperature of 65° C. This would require a relatively long time needed to heat up the PTC stones  20  to their target temperature and hence to heat up the screen wash fluid to this target temperature. The heated screen wash fluid is used to remove the accumulated snow/frost and to improve the cleaning effectiveness during warmer seasons. 
     According to another embodiment, PTC stones  20  with a target temperature of 135° C. are used to shorten the time needed to heat up the PTC stones  20 . This shortens the time needed to heat up the PTC stones  20  to the target temperature of 65° C. because the PTC stones  20  operate in the range where the increase rate of the temperature is high. A functional diagram of a PTC stone  20  with an control assembly  10  is shown in  FIG. 11 . 
     The control assembly  10  comprises a control unit  31  and a switching unit  32 . In a first step the actual resistance of the PTC stone  20  is measured. This can be accomplished by a resistance measurement of the PTC stone  20  or a voltage/current measurement at a sampling resistor  34 , as will be explained later. The control unit  31 , preferably a microprocessor, maps the result of this measurement to an actual temperature of the PTC stone  20  by means of a comparison chart or an algorithm. The actual temperature of the PTC stone  20  afterwards will be compared to an adjustable target temperature, which in this embodiment is 65° C. In the next step, the control unit  31  produces a control signal  33  with an adjustable pulsewidth. The pulsewidth of the control signal  33  depends on the actual temperature of the PTC stone  20 . The control signal  33  controls the switching unit  32  which controls the conductivity of electricity to the PTC stone  20 . 
     In this embodiment the switching unit  32  consists of a MOSFET. During the on cycle of the control signal  33  the switching unit  33  supplies power to PTC stone  20 , so that the PTC stone  20  further heats up. During the off cycle of the control signal  33  no power is supplied to the PTC stone  20  by the switching unit  32 . Hence, the PTC stone  20  does not further heat up. The control unit  31  reduces the on cycle of the control signal  33  in case the temperature of the PTC stone  20  rises. Using this mechanism, the actual temperature of the PTC stone  20  is limited to for example 65° C. 
     Graph  4  shows an exemplary control signal  33  with an adjustable pulsewidth. As can be seen, the control signal  33  consists of retangular impulses. In the beginning, during the initial heating of the PTC stone  20 , the control signal  33  only consists of an on cycle and no off cycle. As the PTC stone  20  reaches the adjustable target temperature of 65° C., the control unit  31  reduces the pulsewidth of the control signal  33  in order to lower the heating of the PTC stone  20 . In the event the PTC stone  20  exceeds the adjustable temperature of 65° C., the control unit  20  produces a control signal  33  only consisting of an off cycle, so that the PTC stone  20  is not further heated up. In case the temperature of the PTC stone  20  drops below 65° C., the control unit  31  again increases the pulsewidth of the control signal  33  to heat up the PTC stone  20 . 
     As mentioned above, in a first step the actual resistance of the PTC stone  20  is measured. This can be accomplished by means of a voltage measurement at a sampling resistor  34  which in this embodiment has a resistance of 13 mΩ (see  FIG. 12 ). This sampling resistor  34  is connected into series with the PTC stone  20 . As the input voltage to the serial connection of PTC stone  20  and sampling resistor  34  is fixed, the voltage drop at the sampling resistor  34  is directly proportional to the resistance of the PTC stone  20 . The voltage drop at the sampling resistor  34  is amplified by operational amplifier  35 . As known by a person skilled in the art, the rate of amplification is defined by resistors  36 ,  37 ,  38 . The measured and amplified voltage drop at sampling resistor  35  is passed to the control unit  31 . The control unit  31  maps this amplified voltage drop at sampling resistor  35  to an actual temperature of the PTC stone  20  by means of a comparison chart or an algorithm. 
     With reference to  FIG. 6   a , it can be seen that the housing  11  has a heat exchanger compartment  24  and a control board compartment  25 , the control board  10  as well as the heat exchanger  8  being completely encapsulated by the housing  11 . The heat exchanger compartment  25  of the housing  11  thereby defining a front cavity  26  and a rear cavity  27  in which the elastically deformable sealing covers  16   a ,  16   b  which are loosely fitted to the heat exchanger  8  may flex upon phase change of the washing fluid which might happen for instance when the defrosting agent concentration within the cleaning fluid is not high enough. 
     It is to be understood that, due to the diaphragm type properties of the sealing covers  16   a, b , optimal freeze protection is guaranteed. 
     As this can be seen from  FIGS. 7 and 8 , the sealing covers  16   a  and  16   b  abut against the housing  11  such that the sealing covers  16   a  and  16   b  are held in place by the housing  11 . 
     As an alternative solution, the sealing covers  16   a ,  16   b  may be glued or otherwise adhered to the heat exchanger  8 . In this event it is not necessary to provide a housing. 
     As this can be seen from  FIGS. 6   a  and  6   b , the rearwardly facing sealing cover  16   b  abuts against a part of the housing within the heat exchanger compartment  24  whereas the sealing cover  16   a  at the front end of the heat exchanger abuts against the second end cap  11   c  of the housing  11 . Within the front end rear cavities  26  and  27  a foam-backing member  28  is arranged. This foam-backing members  28  are made of a resilient closed cell foam. 
     As also can be seen from  FIG. 6   a , the first end cap comprises thermal connectors  14  for the electrical connection of the vehicular fluid heater. Electrical power is supplied via a MOSFET (Metal Oxide Semiconductor Field Effect Transistor)  29  arranged on the control board  10 . Moreover, on the control board a microcontroller not designated by any reference numeral is arranged. 
     Utilization of a MOSFET  29  has been proven to be advantageous for the power control of the ceramic elements  20 . 
     According to the invention, the heat dissipated by the MOSFET  29  during operation is conducted to the exterior surface of the heat exchanger. In one embodiment ( FIG. 9   a ) the heat dissipated by the MOSFET  29  during operation is conducted via heat sink to the exterior surface of the heat exchanger. The heat sink in the embodiment according to  FIG. 9   a  is designed as a resilient conductive metal strip  30 . The metal strip  30  can for instance be made of copper or another thermally conductive material. The metal strip  30  is directly adhered to one side face  18  of the heat exchanger  8 . For this purpose, a void  23  is provided in one frame  19  of one heating unit  9 . 
     In the embodiment shown in  FIGS. 4   b  and  9   b  the MOSFET  29  is directly attached to the heat exchanger  8  so that the heat dissipated by the MOSFET  29  will be directly transferred into the heat exchanger and/or into the heating unit and thus utilized for heating the cleaning fluid. The MOSFET  29  is preferably electrically insolated on the conductive body of the heat exchanger  8 , for instance by an intermediate layer with high dielectric values (for instance AL 2 O 3 ) between the MOSFET  29  and the heat exchanger  8 . MOSFET  29  may be joined to the PTC as well. Electrical connection to the circuit board  10  may be established by terminal connector  55 . 
     REFERENCE NUMERALS 
       1  Washing fluid reservoir 
       2  Washing fluid pump 
       3  Vehicular fluid heater 
       4  Nozzles 
       5  Inlet port 
       6  Outlet port 
       7  Hose 
       8  Heat exchanger 
       9  Heating unit 
       10  Control board 
       11  Housing 
       11   a  Main body 
       11   b  First end cap 
       11   c  Second end cap 
       12  Snap-fit connectors 
       13  Nippels 
       14  Terminal connectors 
       15 ,  15   a,b,c,d  Fluid channel 
       16   a, b  Sealing cover 
       17   a  Inlet opening 
       17   b  Outlet opening 
       18  Side faces 
       19  Frame 
       20  Ceramic elements 
       21  Cathode contact plate 
       22  Anode contact plate 
       23  Void 
       24  Heat exchanger compartment 
       25  Control board compartment 
       26  Front cavity 
       27  Rear cavity 
       28  Backing members 
       29  MOSFET 
       30  Metal strip 
       31  Control unit 
       32  Switching unit 
       33  Control signal 
       34  Sampling resistor 
       35  Operational amplifier 
       36  Resistor 
       37  Resistor 
       38  Resistor 
       50   a ,  50   b  Bridging member 
       51  Sealing rim 
       52  Outer groove 
       53  Inner groove 
       54  Locating webs 
       55  Terminal connectors