Patent Publication Number: US-2006013704-A1

Title: Liquid aeration delivery apparatus

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a liquid aeration delivery apparatus in which a liquid such as urea water used for purposes of exhaust gas purification is mixed with air and then delivered.  
      Urea water (a urea aqueous solution) is widely used as a reducing agent in the purification of exhaust gas from diesel engines and the like. As disclosed in JP H7-279650 A, JP 2000-8833 A, JP 2003-232215 A and U.S. Pat. No. 3874822, for instance, urea water is injected through an injection nozzle into a discharge pipe located further toward the exhaust gas upstream side relative to the reduction catalyst. The injected urea water becomes hydrolyzed with the heat from the exhaust gas, thereby generating ammonia, and NO x  in the exhaust gas is reduced by the ammonia thus generated on the catalyst. Namely, the NO x  is converted to harmless substances, i.e., nitrogen (N 2 ) and water (H 2 O).  
      The urea water used as the reducing agent in the process described above is supplied by a pump, is mixed with air in a mixing chamber located halfway through the supply path and reaches the nozzle through which it is injected into the discharge pipe in an aerated and atomized state.  
      Urea water used in the application described above has a disadvantage in that an orifice located at a position immediately preceding the mixing chamber becomes closed off by urea which has become deposited from the solution and has become crystallized during an operation us well as when the pump is in a stopped state. In addition, if an electromagnetic pump which is caused to make reciprocal movement by a pulse current is utilized as the pump, the supply pressure with which the urea water is output pulsates synchronously with the number of pulses. This is the natural outcome of the pulse-driven electromagnetic pump making the reciprocal movement. The pulsating supply pressure may become lower than the pressure of the air supplied into the mixing chamber to be mixed with the urea water, and in such a case, the air is allowed to flow in the reverse direction toward the pump, if only temporarily, which affects the injection quantity at the nozzle to lead to destabilization of the injection quantity. This gives rise to a problem such that the stability and reproducibility of the injection quantity are compromised.  
     SUMMARY OF THE INVENTION  
      Accordingly, an object of the present invention is to prevent the nozzle from becoming clogged even when a solute of the solution becomes deposited and to prevent the air which is mixed with the liquid in the mixing chamber from flowing backward to the metering pump that supplies the liquid.  
      Other objects of the present invention are to prevent the liquid from freezing and to prevent the internal pressure from rising to an abnormally high level.  
      A liquid aeration delivery apparatus according to the present invention comprises at least a metering pump which can control an output volume; an outlet flow passage provided on an outlet side of said metering pump; a mixing chamber provided at an end of said outlet flow passage, in which a liquid supplied from the metering pump is mixed with air; an orifice through which the liquid is supplied into the mixing chamber; an electromagnetic valve for opening/closing the out flow passage; and a needle inserted at the office and moving in cooperation with opening/closing movement of the electromagnetic valve.  
      Since the orifice is constantly cleaned by moving the needle with the electromagnetic valve for opening/closing the outlet flow passage, the substance contained in the liquid (urea water) force-fed from the metering pump, which has become deposited and crystallized, is not allowed to clog the orifice.  
      The liquid aeration delivery apparatus further comprises a means for preventing backward flow which prevents backward flow of air from the mixing chamber to the metering pump.  
      In the structure described above, the orifice is constantly cleaned by moving the needle via the electromagnetic valve for opening/closing the outlet passage to prevent a substance contained the liquid, having become deposited and crystallized, from clogging the orifice. In addition, since it has the means for preventing backward flow, the backward flow from of air from the mixing chamber is prevented, so that injection quantity can be stabilized.  
      The means for preventing backward flow is an air control valve which is provided in an air flow passage for supplying air to said mixing chamber; said air control valve closing said air flow passage in non-operating state, a drive pulse of said metering pump applying to said air control valve in operating state to be driven synchronously with said metering pump.  
      Accordingly, the air control valve can be controlled synchronously with a drive pulse of the metering pump, so that air&#39;s discharge to the mixing chamber can be stopped synchronously to prevent the air backward flow.  
      It is preferred that the means for preventing backward flow is to make said electromagnetic valve opening/closing movement synchronously with a drive pulse of said metering pump. Accordingly, the outlet flow passage is closed synchronously by operating the electromagnetic valve synchronously with the drive pulse of the metering pump to prevent the air backward flow.  
      The metering pump includes an electromagnetic coil to which a pulse current is applied, a plunger which is caused to move reciprocally by the electromagnetic coil, and an intake valve and an outlet valve that in conjunction with the plunger, achieve a pump function. The metering pump also includes a stopper that comes into contact with the plunger pressed by a resilient spring provided at one side of the plunger and a magnetic pole which attracts the plunger toward the spring at the plunger. As a result, an advantage is achieved in that the plunger is allowed to start moving away from the stopper any time by applying a pulse, which in turn, allows the metering pump to vary its output volume over a wide application frequency range.  
      A pressure sensor that also functions as an accumulator may be provided at the outlet flow passage extending from the metering pump and the mixing chamber so as to use the output of the pressure sensor as an indicator to monitor the operation of the aeration atomizing apparatus. In this case, the operating state can be ascertained based upon the output of the pressure sensor. In addition, at the pressure sensor, the pressure inside the outlet flow passage is received via a diaphragm, a piston having a magnet is disposed on the side of the diaphragm opposite from the side where the pressure is received and any displacement of the piston is detected with a magnetic sensor.  
      A temperature sensor may be provided within the outlet flow passage extending from the metering pump to the mixing chamber or in the vicinity of the outlet flow passage. By adopting this structure, it becomes possible to detect freezing of the urea water inside the pump caused by a decrease in the outside air temperature or any abnormal heat generation.  
      A liquid aeration delivery apparatus according to the present invention further comprises a means such that heat is generated by applying a DC current to the electromagnetic coil if the temperature sensor detects a temperature level equal to or lower than a predetermined level in a non-operating state thereof and the current applied to the electromagnetic coil is turned on/off based upon the output from the temperature sensor. Accordingly, the temperature of the liquid inside the pump is monitored by the temperature sensor, and the DC current is supplied to the electromagnetic coil at the metering pump if the liquid temperature is lowered to the freezing level to generate heat and thus prevent freezing. It is to be noted that the power is turned on as the liquid temperature becomes lower than −7° C. and is turned off once the liquid temperature reaches  0 C.  
      Furthermore, a liquid aeration delivery apparatus according to the present invention further comprises a means for preventing an inner pressure from rising to an excessively high level such that the electromagnetic valve controlling opening/closing of the outlet flow passage is opened if the pressure sensor detects that the pressure in the metering pump and in the outlet flow passage has risen to a level equal to or higher than a predetermined level in an non-operating state thereof Since it is possible to release the pressure to the outside by opening the electromagnetic valve when, for instance, the volume of the liquid in the pump has increased due to freezing by adopting this structure, the pump does not become ruptured. It is to be noted that when the liquid temperature is lowered to the freezing level, the temperature sensor described earlier also functions in conjunction with the pressure sensor to keep the pressure from rising.  
      As described above, according to the present invention, the displacement of the electromagnetic valve for opening/closing the outlet flow passage causes the needle to move to constantly clean the orifice and, as a result, a substance contained in the liquid (e.g. urea water) being force-fed, having become deposited and crystallized, does not clog the orifice.  
      Furthermore, the means for preventing backward flow for preventing air backward flow from the mixing chamber stops supplying air or closes the outlet passage even if an output pressure of the liquid from the metering pump is in a low level, so that the backward flow can be prevented. Accordingly, stabilization of the injection quantity is achieved.  
      The air supplied for mixing is supplied into the mixing chamber synchronously with the drive pulse of the metering pump by the air control valve, so that the backward flow can be prevented.  
      Also, since the electromagnetic valve closes the outlet passage synchronously with an output pulsation of the liquid from the metering pump when an output pressure of the liquid from the metering pump is in a low level, the air backward flow is prevented to achieve stabilization of injection quantity. Accordingly, in this case, the air control valve can be omitted to distribute to minimization of a device.  
      The plunger is allowed to start moving away from the stopper any time by applying a pulse, which in turn, allows the metering pump to vary the output volume over a wide application frequency range.  
      The pressure sensor is utilized as an indicator for operational monitoring as well as a pressure gauge. Accordingly, it becomes possible to infer the proper function of the metering pump.  
      The pressure sensor disclosed in the invention is a simpler structure.  
      Temperature management in the apparatus may become possible by the temperature sensor according to the present invention.  
      Furthermore, according to the present invention, if the temperature sensor detects a freezing temperature level in a non-operating state, a DC current is supplied to the electromagnetic coil at the metering pump to generate heat and the current applied to the electromagnetic coil is controlled based upon the temperature detected at by the temperature sensor.  
      In addition, according to the present invention, a rupture is prevented by opening the electromagnetic valve for opening/closing the outlet flow passage and thus releasing the pressure to the outside if the pressure sensor detects that the pressure has risen to a dangerously high level in a non-operating state.  
      Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view of a liquid aeration delivery apparatus according to a first embodiment of the present invention;  
       FIG. 2  is a sectional view of the metering pump which is a component of the liquid aeration delivery apparatus according to the first embodiment;  
       FIG. 3  is a sectional view of the mixing device which is a component of the liquid aeration delivery apparatus according to the first embodiment;  
       FIG. 4  is a sectional view of the air control valve which is a component of the liquid aeration delivery apparatus according to the first embodiment;  
       FIG. 5  is a sectional view of the pressure sensor which is a component of a liquid aeration delivery apparatus according to the first embodiment;  
       FIG. 6  is a control characteristic flowchart diagram of the first embodiment of the present invention;  
       FIG. 7  is a flowchart presenting an example of control implemented to prevent freezing based upon the output from the temperature sensor according to the first embodiment of the present invention;  
       FIG. 8  is a sectional view of a liquid aeration delivery apparatus according to a second embodiment of the present invention;  
       FIG. 9  is a sectional view of the metering pump which is a component of the liquid aeration delivery apparatus according to the second embodiment;  
       FIG. 10  is a sectional view of the mixing device which is a component of the liquid aeration delivery apparatus according to the second embodiment;  
       FIG. 11  is a sectional view of the pressure sensor which is a component of the liquid aeration delivery apparatus according to the second embodiment;  
       FIG. 12  is a flowchart presenting an example of control implemented to prevent freezing based upon the output from the temperature sensor according to the second embodiment of the present invention; and  
       FIG. 13  is a control characteristic flowchart diagram of the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1  shows a liquid aeration delivery apparatus  1  according to a first embodiment of the present invention. A metering pump  2  in the liquid aeration delivery apparatus  1  is now explained in reference to  FIGS. 1 and 2 . The metering pump  2  includes a case  4  constituted of a magnetic material such as iron and mounted at an apparatus main unit  5  at an open end thereof, and also an electromagnetic coil  6  disposed inside the case  4 , to which a pulse current is applied from a control unit (not shown).  
      At the electromagnetic coil  6 , which is formed by winding an electric wire around a resin bobbin  3 , a non-magnetic guide pipe  9  is fitted at a through hole  8  passing through the center of the bobbin  3 . A right plate  10  and a left plate  11  are provided at the right end and the left end of the bobbin  3  respectively, to constitute a magnetic circuit together with the case  4 .  
      To the right of the guide pipe  9 , a magnetic rod  13  to constitute a magnetic pole is disposed, whereas a stopper  14  is fitted at the left end of the guide pipe  9 . The magnetic rod  13  is constituted of a magnetic material such as iron, with substantially half of the magnetic rod  13  on left side inserted at the guide pipe  9  via an O-ring  15  and the remaining half, i.e., the right half, inserted at a barrel portion  19  of an intake coupling  17  to be detailed later via an O-ring  16 . In addition, a communicating hole  18  passing through along the lateral direction is formed inside the magnetic rod  13 , and the communicating hole  18  is connected to a urea water tank (not shown). Reference numeral  24  indicates a filter provided at the communicating hole  18 .  
      In a communicating hole  20  formed at the magnetic rod  13 , a check valve (intake valve)  21  constituted of rubber, resin or the like is disposed, and the check valve  21  made to sit at a valve seat  23  provided at the communicating hole  20  with a pressing force imparted by a spring  22 .  
      An electromagnetic plunger operation chamber in which an electromagnetic plunger  27  constituted of a magnetic material such as iron is disposed is formed inside the guide pipe  9 . The electromagnetic plunger  27  includes a large diameter portion  27   a  and a small diameter portion  27   b  continuous to the large diameter portion  27   a  and projecting to the right. A through hole  29  is formed along the axial direction at the large diameter portion  27   a  and the small diameter portion  27   b,  and a check valve (outlet valve)  30  is disposed at the through hole  29  in the small diameter portion  27   b  and is made to sit at a valve seat  32  with a spring  31 . In addition, the small diameter portion  27   b  is slidably inserted at a cylinder  34  mounted at the magnetic rod  13  via an O-ring  34   a.    
      Pressure is applied to the electromagnetic plunger  27  from a return spring  35  which imparts a strong force and, as a result, although there is also a spring  37  imparting a force along the opposite direction, the left end of the electromagnetic plunger  27  is placed in contact with the stopper  14 . Namely, if no power is supplied to the electromagnetic coil  6 , the electromagnetic plunger  27  is set at the return position at which its left end is in contact with the stopper  14 , but whenever a pulse is applied to the electromagnetic coil  6 , the electromagnetic plunger  27  is allowed to start moving away from the stopper  14 . It is to be noted that the spring  37 , which imparts only a weak force, may be omitted depending upon the particulars of the design requirements.  
      The left end of the electromagnetic plunger operation chamber  28  is made to communicate with an outlet flow passage  39  formed at the apparatus main unit  5  via a hole  38  at the stopper  14 , and the outlet flow passage  39  extends to a mixing chamber  64  detailed below.  
      As a pulse current that can be varied over wide range is supplied to the electromagnetic coil  6  in the metering pump  2  structured as described above, the electromagnetic plunger  27  makes reciprocal movement. Namely, as the pulse is supplied, the magnetic rod  13  becomes magnetized and the attraction of the magnetized magnetic rod  13  causes the electromagnetic plunger  27  to move against the force imparted by the return spring  35 .  
      Then, as the pulse ceases, the energy stored in the return spring  35  resets the left end of the electromagnetic plunger  27  to the position at which it comes in contact with the stopper  14 . When the pulse is applied to the electromagnetic coil  6  again, the electromagnetic plunger  27  is caused to move as described above and thus, a pump function is achieved with the check valves  21  and  30  through the repeated motion of the electromagnetic plunger  27 . Namely, the liquid, i.e., the urea water, is force-fed into the mixing chamber  64  with its quantity increased substantially in proportion to the application frequency.  
      While the metering pump  2  is operated over a wide range with regard to the pulse applied to the electromagnetic coil  6 , the characteristics of the electromagnetic pump poses a hindrance to increasing the output volume to a desired level simply by increasing the frequency. Accordingly, the metering pump is constituted as a pulse-width dependent constant-volume electromagnetic pump that varies the pulse width in proportion to the frequency so as to increase the proportion of the output volume relative to the proportion of the frequency. The specific ranges of frequency between 2 Hz to 40 Hz and pulse width between 5 ms and 12.5 ms are selected for illustration in  FIG. 6 . It is to be noted that the pulse width and the output volume in the low output volume range (Min shown in  FIG. 6 ) are respectively 5 (ms) and 1.5(g/min), the pulse width and the output volume in the middle output volume range (Mid shown in  FIG. 6 ) are respectively 7.5 (ms) and 30.0 (g/min) and the pulse width and the output volume in the high output volume range (Max shown in  FIG. 6 ) are respectively 12.5 (ms) and 123.4(g/min). Since “1 g” and “1 cc” of pure water are equal in quantity, the unit “g” could be replaced with “cc” if the liquid was pure water.  
      Now, a mixing device  43  is explained in reference to  FIGS. 1 and 3 . The mixing device  43  located on the left side of the apparatus main unit  5  includes an electromagnetic valve  44  provided at the left end of the outlet flow passage  39  to control the open/closed state of the outlet flow passage  39 . The electromagnetic valve  44  includes a case  45  which is located on the outside and having an open end thereof attached to the apparatus main unit  5 , and also an electromagnetic coil  46  located inside the case  45 .  
      At the electromagnetic coil  46 , which is formed by winding an electric wire around a resin bobbin  47 , a non-magnetic guide pipe  48  is fitted in a through hole passing through the center of the bobbin  47 . A right plate  50  and a left plate  51  are provided at the right end and the left end respectively of the bobbin  47 , to constitute a magnetic circuit together with the case  45 .  
      A magnetic rod  52  to constitute a magnetic pole is provided to the right of the guide pipe  48 , whereas a valve seat  53  is provided to the left of the guide pipe  48 . At the magnetic rod  52 , constituted of a magnetic material such as iron, a communicating hole  54  with an orifice  57  is formed so as to extend along the axis of the magnetic rod  52 . In addition, an electromagnetic plunger operation chamber  56 , in which an electromagnetic plunger  55  constituted of a magnetic material is housed, is formed inside the guide pipe  48 . The electromagnetic plunger  55  includes a communicating hole  58  formed so as to extend along the central axis, and the electromagnetic plunger  55  is made to sit at the valve seat  53  by the force applied by a spring  59  to close the outlet flow passage  39 . Then, as power is supplied to the electromagnetic coil  46 , the electromagnetic plunger  55  becomes displaced against the force applied by the spring  59 , thereby opening the outlet flow passage  39 . An O-ring  60  is mounted at the front end of the electromagnetic plunger  55  located on the side opposite from the side where the magnetic rod is present with a needle  61  projecting out at the same end. The needle  61  is inserted at an orifice  62  at the valve seat  53 .  
      The orifice  62  through which the flow rate of the liquid supplied (injected) into the mixing chamber  64  is raised is formed at the center of the valve seat  53  located at the left end of the guide pipe  48 . As described above, the needle  61  is inserted at the orifice  62  so that as the electromagnetic valve  44  is turned on/off, the needle  61  becomes displaced to clean the inside of the orifice  62 .  
      The mixing chamber  64  is formed inside a connection member  66  having an outlet port  65 , with the orifice  62  described above and an air supply hole  68  formed at the right end thereof. Thus, air is supplied into the mixing chamber  64  in the required quantity from an air tank or the like (not shown) via an air control valve  72  to be detailed below, and the urea water having been injected into the mixing chamber  64  becomes aerated with the air and atomized. Since the air supply hole  68  is connected to the inner circumferential surface of the mixing chamber  64  along the tangential direction, the air is supplied into the mixing chamber  64  in a rotary motion to further promote the aerated atomization of the urea water. The urea water having been aerated and atomized is sent out from the outlet port  65  via a nozzle  69  into a discharge pipe which is an external device.  
      The air control valve  72  is now explained in reference to  FIGS. 1 and 4 . The air control valve  72  located above the apparatus main unit  5  includes a case  73  constituted of a magnetic material, provided on the outside and having an open end thereof mounted at the apparatus main unit  5 , and also includes an electromagnetic coil  74  provided inside the case  73 . At the electromagnetic coil  74 , which is formed by winding an electric wire around a resin bobbin  75 , a non-magnetic guide pipe  76  is fitted in a through hole passing through the center of the bobbin  75 . An upper plate  77  and a lower plate  78  are provided at the upper end and the lower end of the bobbin  75  respectively, to constitute a magnetic circuit together with the case  73 .  
      At the top of the guide pipe  76 , a magnetic rod  80  to constitute a magnetic pole is provided, whereas toward the bottom of the guide pipe  76 , a valve seat  81  is provided. The magnetic rod  80  constituted of a magnetic material such as iron includes a communicating hole  82  extending along its axis. Above the magnetic rod  80 , an intake coupling  85  connecting with an air flow passage  83  through which the air is supplied from the air tank provided. The valve seat  81  includes a communicating hole  84  which communicates with the mixing chamber  64  on its downstream side via the airflow passage  83 . Inside the guide pipe  76  partitioned into spaces housing the magnetic rod  80  and the valve seat  81  as described above, an electromagnetic plunger operation chamber  87  in which an electromagnetic plunger  86  is disposed, is formed.  
      The electromagnetic plunger  86  includes a communicating hole  89  extending along the central axis, and also has a spherical valve element  90  mounted at one end thereof. The valve element  90  at the electromagnetic plunger  86  supported by a pair of springs  91  and  92  and provided in the electromagnetic plunger operation chamber  87  is made to sit at the valve seat  81  and thus, the communicating hole  84  is closed when no power is supplied. Then, as power is supplied, the valve element  90  departs from the valve seat  81  to open the communicating hole  84 .  
      The air control valve  72  structured as described above is controlled by applying a pulse current to the electromagnetic coil  74 . The air control valve  72  is driven synchronously with the drive pulses of the metering pump  2  when a pulse width applied to the metering pump  2  is narrow (namely, a low output volume range Min), as shown in  FIG. 6 , in relation to the metering pump  2 .  
      Namely, a drive pulse with a rising side synchronous with a falling side of the drive pulse of the metering pump is made at the low output volume range (Min) of the metering pump  2 . It is preferred that a delay processing which delays the up of the drive pulse is operated. A width of the drive pulse of the air control valve  72  is limited by a rising side of a next drive pulse of the metering pump  2 .  
      Since the air to be mixed with the urea water achieves a constant pressure of  15  psi and thus there is a risk of the air flowing backward unless the air is supplied synchronously when the injection quantity of the urea water injected from the metering pump  2  is small, i.e., in a so-called low pulse rate condition (Min shown in  FIG. 6 ), and the pulsating pressure inherent to the electromagnetic pump dips lower than the air pressure. The drive pulse of the air control valve can resolve the risk.  
      A pressure sensor  93  is described in reference to  FIGS. 1 and 5 . A pressure sensor main unit  94  fitted in the apparatus main unit  5  assumes a tubular shape and includes a piston  96  disposed inside a central chamber  95  and having a magnet  98 , with a spring  97  applying a force to the piston  96 . At the center of the piston  96 , a magnetic sensor  99 , which may be a Hall IC or a magnetic resistor element that reacts to magnetism, is provided. The magnetic sensor  99  is located at a rod  100  screwed onto the pressure sensor main unit  94  and the sensor sensitivity is adjusted by varying the position of the rod  100 .  
      The pressure sensor main unit  94  assuming the structure described above is fitted in the apparatus main unit  5  via a diaphragm  101  which is connected to the outlet flow passage  39  formed at the apparatus main unit  5  via a branch flow passage  39   a.  Thus, as the pressure in the outlet flow passage  39  increases, the diaphragm  101  becomes displaced and, at the same time, the piston  96 , too, becomes displaced against the force applied by the spring  97 . The displacement of the piston  96  is detected with the magnetic sensor  99 , and it becomes possible to infer the proper function of the metering pump according to displaying the sensor output (an output characteristic of the pressure sensor shown in  FIG. 6 ).  
      Based upon the output from the pressure sensor  93 , any abnormal increase in the pressure in the outlet flow passage  39  can be detected, and if the pressure rises to an abnormally high level, power is supplied to the electromagnetic coil  46  at the electromagnetic valve  44  described earlier to open the electromagnetic valve  44 , thereby releasing the pressure to the outside and, as a result, any rupture is prevented.  
      Besides, it is not necessary to define the pressure sensor  93  to only a structure for detecting displacement as above-mentioned. It may have a structure which is provided with a means for detecting distortion by the pressure, a means for detecting thermoelectromotive force by the pressure dependence of the thermal conductivity, a means for detecting a voltage by the pressure dependence of the break-down voltage, a means for detecting an ionic current due to gaseous ionization phenomenon, a means which detects a phase due to the interference phenomenon of the light, or a means for detecting the strength of the light due to micro vent loss.  
      Now, in reference to  FIGS. 1 and 7 , a temperature sensor  103  is described. The temperature sensor  103  constituted of a thermistor provided near the outlet flow passage  39  in the apparatus main unit  5  detects the temperature of the apparatus. It becomes engaged in operation as the external air temperature becomes low in a non-operating state to prevent the urea water from freezing. Besides, it is not necessary to define the temperature sensor  103  to the thermistor, but a thermo couple, a metal resistance temperature sensor (a resistance bulb), heat sensitive magnetic material such as a heat sensitive ferrite, a bimetal thermostat, an IC temperature sensor, an infrared ray detecting element, a crystal temperature sensor, or a fluorescence type fiber temperature sensor can be used.  
      Namely, as shown in  FIG. 7  presenting its control flow, a temperature signal provided by the temperature sensor  103  is taken in during a temperature detection step  201 . Then, the operation proceeds to step  202  to judge the temperature. In this step, a decision is made as to whether or not the temperature has become equal to or lower than −7° C., and if it is decided that the temperature is equal to or lower than −7° C. and thus, there is a risk of the urea water freezing, the operation proceeds to step  203  to apply a DC current (DC 24 V) to the electromagnetic coil  6  at the metering pump  2 . Thus, the electromagnetic valve generates heat. Then, proceeding to steps  204  and  205 , the electromagnetic valve  44  and the air control valve  72  are opened.  
      After that, the temperature sensor  103  monitors the temperature of the apparatus main unit  5 , and once the heat rises above 0° C., the operation proceeds to steps  206 ,  207  and  208  to stop applying DC current to the metering pump  2 , for the electromagnetic valve to be closed and for the air control valve to be closed. The urea water is prevented from freezing through this control. It is to be noted that since the internal pressure rises if the urea water starts to freeze, the rise in the pressure is detected with the pressure sensor  93  and once the pressure rises to a level exceeding a predetermined level, the electromagnetic valve  44  is opened to preempt any possible problem in conjunction with the temperature sensor  103 .  
      In the structure described above, a pulse current (2 to 40 Hz) is applied to the electromagnetic coil  6  at the metering pump  2  and the electromagnetic plunger  27  is thus caused to vibrate 2 to 40 times per second to achieve a pump function. This metering pump  2  achieves a linear output which is in proportion to the pulse rate. The liquid supplied from the metering pump (i.e., the urea water) travels through the outlet flow passage  39  and is injected into the mixing chamber  64  via the orifice  62 , and in the mixing chamber  64 , it becomes mixed with the air supplied thereto.  
      The orifice  62 , which is cleaned with the needle  61 , never becomes clogged since urea having been deposited and crystallized which then adheres to the orifice  62  is removed through the movement of the needle  61  caused by the electromagnetic valve  44  at an operation start. In addition, in the low output volume range (Min), since control is implemented with the air control valve  72  to supply the air in synchronization with the supply of the liquid from the metering pump  2 , the air is not allowed to flow back toward the metering pump  2 , thereby achieving stable injection through the nozzle.  
      The first embodiment described above is to use an engine of a large vehicle such as a truck, and it is difficult to use in a small size vehicle with a small displacement because it is too large. Therefore, a second embodiment of this invention is to use the electromagnetic valve  44  installed in the device as the means for preventing backward flow. Thus, the air control valve  72  can be omitted.  
      FIGS.  8  though  13  show a liquid aeration delivery apparatus  301  according to a second embodiment of the present invention. A metering pump  302  includes a case  304  constituted of a magnetic material such as iron and mounted at an apparatus main unit  305  at an open end thereof as shown in  FIG. 9  too, and also an electromagnetic coil  306  disposed inside the case  304 , to which a pulse current is applied from a control unit (not shown).  
      At the electromagnetic coil  306 , which is formed by winding an electric wire around a resin bobbin  303 , a non-magnetic guide pipe  309  is fitted at a through hole  308  passing through the center of the bobbin  303 . A right plate  310  and a left plate  311  are provided at the right end and the left end of the bobbin  303  respectively, to constitute a magnetic circuit together is with the case  304 .  
      To the right of the guide pipe  309 , a magnetic rod  313  to constitute a magnetic pole is disposed, whereas a stopper  314  is fitted at the left end of the guide pipe  309 . The magnetic rod  313  is constituted of a magnetic material such as iron, with substantially half of the magnetic rod  313  on left side inserted at the guide pipe  309  via an O-ring  315  and the remaining half, i.e., the right half, inserted at a barrel portion  319  of an intake coupling  317  to be detailed later via an O-ring  316 . In addition, a communicating hole  318  passing through along the lateral direction is formed inside the magnetic rod  313 , and the communicating hole  318  is connected to a urea water tank (not shown). Reference numeral  324  indicates a filter provided at the communicating hole  318 .  
      In a communicating hole  320  formed at the magnetic rod  313 , a check valve (intake valve)  321  constituted of rubber, resin or the like is disposed, and the check valve  321  made to sit at a valve seat  323  provided at the communicating hole  320  with a pressing force imparted by a spring  322 .  
      An electromagnetic plunger operation chamber in which an electromagnetic plunger  327  constituted of a magnetic material such as iron is disposed is formed inside the guide pipe  309 . The electromagnetic plunger  327  includes a large diameter portion  327   a  and a small diameter portion  327   b  continuous to the large diameter portion  327   a  and projecting to the right. A through hole  329  is formed along the axial direction at the large diameter portion  327   a  and the small diameter portion  327   b,  and a check valve (outlet valve)  330  is disposed at the through hole  329  in the small diameter portion  327   b  and is made to sit at a valve seat  332  with a spring  331 . In addition, the small diameter portion  327   b  is slidably inserted at a cylinder  334  mounted at the magnetic rod  313  via an O-ring  334   a.    
      Pressure is applied to the electromagnetic plunger  327  from a return spring  335  which imparts a strong force and, as a result, although there is also a spring  337  imparting a force along the opposite direction, the left end of the electromagnetic plunger  327  is placed in contact with the stopper  314 . Namely, if no power is supplied to the electromagnetic coil  306 , the electromagnetic plunger  327  is set at the return position at which its left end is in contact with the stopper  314 , but whenever a pulse is applied to the electromagnetic coil  306 , the electromagnetic plunger  327  is allowed to start moving away from the stopper  314 . It is to be noted that the spring  337 , which imparts only a weak force, may be omitted depending upon the particulars of the design requirements.  
      The left end of the electromagnetic plunger operation chamber  328  is made to communicate with an outlet flow passage  339  formed at the apparatus main unit  305  via a hole  338  at the stopper  314 , and the outlet flow passage  339  extends to a mixing chamber  364  detailed below.  
      As a pulse current that can be varied over a wide range is supplied to the electromagnetic coil  306  in the metering pump  302  structured as described above, the electromagnetic plunger  327  makes reciprocal movement. Namely, as the pulse is supplied, the magnetic rod  313  becomes magnetized and the attraction of the magnetized magnetic rod  313  causes the electromagnetic plunger  327  to move against the force imparted by the return spring  335 .  
      Then, as the pulse ceases, the energy stored in the return spring  335  resets the left end of the electromagnetic plunger  327  to the position at which it comes in contact with the stopper  314 . When the pulse is applied to the electromagnetic coil  306  again, the electromagnetic plunger  327  is caused to move as described above and thus, a pump function is achieved with the check valves  321  and  330  through the repeated motion of the electromagnetic plunger  327 . Namely, the liquid, i.e., the urea water, is force-fed into the mixing chamber  364  with its quantity increased substantially in proportion to the application frequency.  
      While the metering pump  302  is operated over a wide range with regard to the pulse applied to the electromagnetic coil  306 , the characteristics of the electromagnetic pump poses a hindrance to increasing the output volume to a desired level simply by increasing the frequency. Accordingly, the metering pump is constituted as a pulse-width dependent constant-volume electromagnetic pump that varies the pulse width in proportion to the frequency so as to increase the proportion of the output volume relative to the proportion of the frequency. The specific ranges of frequency between 2 Hz to 40 Hz and pulse width between 5 ms and 12.5 ms are selected for illustration in  FIG. 13 . It is to be noted that the pulse width and the output volume in the low output volume range (Min shown in  FIG. 13 ) are respectively 5 (ms) and 1.5(g/min), the pulse width and the output volume in the middle output volume range (Mid shown in  FIG. 13 ) are respectively 7.5 (ms) and 30.0 (g/min) and the pulse width and the output volume in the high output volume range (Max shown in  FIG. 13 ) are respectively 12.5 (ms) and 123.4(g/min). Since “1 g” and “1 cc” of pure water are equal in quantity, the unit “g” could be replaced with “cc” if the liquid was pure water.  
      Now, a mixing device  343  is explained in reference to  FIGS. 8 and 10 . The mixing device  343  located on the left side of the apparatus main unit  305  includes an electromagnetic valve  344  provided at the left end of the outlet flow passage  339  to control the open/closed state of the outlet flow passage  339 . The electromagnetic valve  344  includes a case  345  which is located on the outside and having an open end thereof attached to the apparatus main unit  5 , and also an electromagnetic coil  346  located inside the case  345 .  
      At the electromagnetic coil  346 , which is formed by winding an electric wire around a resin bobbin  347 , a non-magnetic guide pipe  348  is fitted in a through hole passing through the center of the bobbin  347 . A right plate  350  and a left plate  351  are provided at the right end and the left end respectively of the bobbin  347 , to constitute a magnetic circuit together with the case  345 .  
      A magnetic rod  352  to constitute a magnetic pole is provided to the right of the guide pipe  348 , whereas a valve seat  353  is provided to the left of the guide pipe  348 . At the magnetic rod  352 , constituted of a magnetic material such as iron, a communicating hole  354  with an orifice  357  is formed so as to extend along the axis of the magnetic rod  352 . In addition, an electromagnetic plunger operation chamber  356 , in which an electromagnetic plunger  355  constituted of a magnetic material is housed, is formed inside the guide pipe  348 . The electromagnetic plunger  355  includes a communicating hole  358  formed so as to extend along the central axis, and the electromagnetic plunger  355  is made to sit at the valve seat  353  by the force applied by a spring  359  to close the outlet flow passage  339 . Then, as power is supplied to the electromagnetic coil  346 , the electromagnetic plunger  355  becomes displaced against the force applied by the spring  359 , thereby opening the outlet flow passage  339 . An O-ring  360  is mounted at the front end of the electromagnetic plunger  355  located on the side opposite from the side where the magnetic rod is present with a needle  361  projecting out at the same end. The needle  361  is inserted at an orifice  362  at the valve seat  353 .  
      The orifice  362  through which the flow rate of the liquid supplied (injected) into the mixing chamber  364  is raised is formed at the center of the valve seat  353  located at the left end of the guide pipe  348 . As described above, the needle  361  is inserted at the orifice  362  so that as the electromagnetic valve  344  is turned on/off, the needle  61  becomes displaced to clean the inside of the orifice  362 .  
      The mixing chamber  364  is formed inside a connection member  366  having an outlet port  365 , with the orifice  362  described above and an air supply hole  368  formed at the right end thereof. Thus, air is supplied into the mixing chamber  364  in the required quantity from an air tank or the like (not shown) via an air control valve  372  to be detailed below, and the urea water having been injected into the mixing chamber  364  becomes aerated with the air and atomized. Since the air supply hole  368  is connected to the inner circumferential surface of the mixing chamber  364  along the tangential direction, the air is supplied into the mixing chamber  364  in a rotary motion to further promote the aerated atomization of the urea water. The urea water having been aerated and atomized is sent out from the outlet port  365  via a nozzle  369  into a discharge pipe which is an external device.  
      A pressure sensor  393  is described in reference to  FIGS. 8 and 11 . A pressure sensor main unit  394  fitted in the apparatus main unit  305  assumes a tubular shape and includes a piston  396  disposed inside a central chamber  395  and having a magnet  398 , with a spring  397  applying a force to the piston  396 . At the center of the piston  396 , a magnetic sensor  399 , which may be a Hall IC or a magnetic resistor element that reacts to magnetism, is provided. The magnetic sensor  399  is located at a rod  400  screwed onto the pressure sensor main unit  394  and the sensor sensitivity is adjusted by varying the position of the rod  400 .  
      The pressure sensor main unit  394  assuming the structure described above is fitted in the apparatus main unit  305  via a diaphragm  401  which is connected to the outlet flow passage  339  formed at the apparatus main unit  305  via a branch flow passage  339   a.  Thus, as the pressure in the outlet flow passage  339  increases, the diaphragm  401  becomes displaced and, at the same time, the piston  396 , too, becomes displaced against the force applied by the spring  397 . The displacement of the piston  396  is detected with the magnetic sensor  399 , and it becomes possible to infer the proper function of the metering pump according to displaying the sensor output (an output characteristic of the pressure sensor shown in  FIG. 13 ).  
      Based upon the output from the pressure sensor  393 , any abnormal increase in the pressure in the outlet flow passage  339  can be detected, and if the pressure rises to an abnormally high level, power is supplied to the electromagnetic coil  346  at the electromagnetic valve  344  described earlier to open the electromagnetic valve  344 , thereby releasing the pressure to the outside and, as a result, any rupture is prevented.  
      Besides, it is not necessary to define the pressure sensor  393  to only a structure for detecting displacement as above-mentioned. It may have a structure which is provided with a means for detecting distortion by the pressure, a means for detecting thermoelectromotive force by the pressure dependence of the thermal conductivity, a means for detecting a voltage by the pressure dependence of the break-down voltage, a means for detecting an ionic current due to gaseous ionization phenomenon, a means which detects a phase due to the interference phenomenon of the light, or a means for detecting the strength of the light due to micro vent loss.  
      Now, in reference to  FIGS. 8 and 12 , a temperature sensor  403  is described. The temperature sensor  403  constituted of a thermistor provided near the outlet flow passage  339  in the apparatus main unit  305  detects the temperature of the apparatus. It becomes engaged in operation as the external air temperature becomes low in a non-operating state to prevent the urea water from freezing. Besides, it is not necessary to define the temperature sensor  403  to the thermistor, but a thermo couple, a metal resistance temperature sensor (a resistance bulb), heat sensitive magnetic material such as a heat sensitive ferrite, a bimetal thermostat, an IC temperature sensor, an infrared ray detecting element, a crystal temperature sensor, or a fluorescence type fiber temperature sensor can be used.  
      Namely, as shown in  FIG. 12  presenting its control flow, a temperature signal provided by the temperature sensor  403  is taken in during a temperature detection step  501 . Then, the operation proceeds to step  502  to judge the temperature. In this step, a decision is made as to whether or not the temperature has become equal to or lower than −7° C., and if it is decided that the temperature is equal to or lower than −7° C. and thus, there is a risk of the urea water freezing, the operation proceeds to step  503  to apply a DC current (DC 24 V) to the electromagnetic coil  306  at the metering pump  302 . Thus, the electromagnetic valve generates heat. Then, proceeding to step  504 , the electromagnetic valve  344  is opened.  
      After that, the temperature sensor  403  monitors the temperature of the apparatus main unit  305 , and once the heat rises above 0° C., the operation proceeds to steps  506  and  507  to stop applying DC current to the metering pump  2  and for the electromagnetic valve  344  to be closed. The urea water is prevented from freezing through this control. It is to be noted that since the internal pressure rises if the urea water starts to freeze, the rise in the pressure is detected with the pressure sensor  393  and once the pressure rises to a level exceeding a predetermined level, the electromagnetic valve  344  is opened to preempt any possible problem in conjunction with the temperature sensor  403 .  
      In the structure described above, a pulse current (2 to 40 Hz) is applied to the electromagnetic coil  306  at the metering pump  302  and the electromagnetic plunger  327  is thus caused to vibrate 2 to 40 times per second to achieve a pump function. This metering pump  302  achieves a linear output which is in proportion to the pulse rate. The liquid supplied from the metering pump (i.e., the urea water) travels through the outlet flow passage  339  and is injected into the mixing chamber  364  via the orifice  362 , and in the mixing chamber  364 , it becomes mixed with the air supplied thereto.  
      The orifice  362 , which is cleaned with the needle  361 , never becomes clogged since urea having been deposited and crystallized which then adheres to the orifice  362  is removed through the movement of the needle  361  caused by the electromagnetic valve  344  at an operation start. In addition, the electromagnetic valve  344  is operated synchronously with the drive pulse of the metering pump  302  in order to prevent the air backward flow to the metering pump  302  in a range from the middle output volume range (Mid) to the low output volume range (Min), as shown in  FIG. 13 .  
      Namely, in the middle and the law outlet volume ranges, the electromagnetic valve  344  is opened by a rising side of the drive pulse synchronously with a falling side of the drive pulse of the metering pump  302 , and is closed by falling down the drive pulse before the next drive pulse of the metering pump  302 . As a result, since the liquid flows into the mixing chamber when the outlet pressure from the metering pump  302  is high and the outlet passage  339  is closed to prevent the air backward flow when the outlet pressure lowers, the injection quantity of the liquid is stabilized. Note that a rising of the drive pulse of the electromagnetic valve  344  is given about 2 ms delay.