Abstract:
A method and a cooking appliance has a pyrolytic cleaning control in which multiple stages permit volatile residue to be incinerated before inducing air current flow that can complete the cleaning process throughout the oven housing. The oven combines cooking energy sources, including convection current heating and preferably, jet impingement heating, whose blowers are disabled during a first stage of pyrolytic cleaning to avoid uncontrolled ignitions within the appliance. The second stage of the pyrolytic cleaning process induces current flow to distribute pyrolytically heated air throughout the housing and complete the cleaning process in the apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for controlling pyrolytic cleaning in a combination microwave and convection heating oven in which blower actuation, such as jet impingement actuation, is deferred to a time or an event near the terminal portion of the cleaning cycle. 
     2. Background Art 
     Many cooking ovens have a pyrolytic cleaning function in which the heating elements are operated to obtain a pyrolytic cleaning temperature, for example 800° F. (427° C.) within the oven chamber. Typically, the oven is sealed so that accumulated debris on the walls of the oven can be incinerated and removed as ash residue after the pyrolytic cleaning cycle has terminated. The operation of such cleaning cycles is regulated and test results must comply with standards established, for example Underwriters Laboratory (UL) 858 standard for household electric ranges. 
     Although recent improvements of cooking devices have employed multiple cooking energy sources such as microwave generators, convection heating sources, and conventional radiant heating elements, the control of pyrolytic cleaning cycles is limited to actuation of radiant heating elements which can be powered to achieve the high temperatures in the oven that are required for pyrolytic cleaning. The operation of additional heating sources during a cleaning cycle is problematic. 
     One previously known oven construction included a passageway surrounding the oven chamber in which a suction fan operates to cool the outer housing of the cooking appliance. The passageway communicates with the interior oven chamber through a throttled passageway to provide exhaustion of gases emanating from food being processed in the oven chamber. The throttled opening is controlled by a thermally responsive gate so that the throttle passageway is closed during pyrolytic heating of the oven chamber to avoid the release of volatile gases from the oven chamber during cleaning. Accordingly, pyrolytic cleaning is limited to the interior oven chamber. 
     Another combination microwave and electric self-cleaning oven is disclosed in U.S. Pat. No. 4,547,642. This combination oven includes an air vent from the cooking cavity communicating with an air exhaust passageway including a catalytic oxidation unit that decomposes gases generated when the oven is operating in a self-cleaning mode. A restricted air inlet is configured to limit unaugmented airflow into the oven chamber during the self-cleaning mode. A humidity sensor controls the operation of a blower that augments the flow of air into the cavity when the humidity level caused by microwave only heating is used for cooking. 
     More recent developments in cooking appliances have combined jet impingement heating sources with microwave cooking sources and radiant heat elements to additionally speed up and control the cooking of foods within the oven chamber. However, while such combined cooking sources accelerate the time of cooking processes, such combinations have not been useful in pyrolytic cleaning operations of those appliances within the test standards of regulations previously discussed. 
     DESCRIPTION OF THE INVENTION 
     The present invention overcomes the above mentioned disadvantages by providing a combination cooking source oven having a control that generates a multiple stage pyrolytic cleaning operation. The present invention provides a method and apparatus for pyrolytically cleaning an oven chamber by sealing the chamber, raising the temperature to a pyrolytic cleaning level for a first operating period in which the blower operation is disabled, maintaining the temperature of the pyrolytic cleaning level and actuating the blower operation in a second time period following the first operating period. 
     In the preferred embodiment, a blower of a jet impingement cooking source is disabled while up to four heating elements are operated to obtain a pyrolytic cleaning temperature within a sealed oven chamber. An additional stage is initiated by the oven control to generate jet impingement currents throughout the oven chamber and the passageways in communication with the chamber that may have been soiled during cooking operations. Preferably, the pyrolytic cleaning temperature is maintained in the second stage while the risk of volatile gases and their inadvertent ignition is avoided by consumption of the gases in the initial cleaning stage. The duration of each stage may be fixed or variable as desired. The preferred embodiment of the method is preferably open loop, and relies only on a fixed time duration for each stage to simplify the control. However, the invention includes closed loop systems, wherein the initiation of the second stage is triggered by an event, such as a sensor detection that enables the control to determine when a level of volatile gases or oxygen level has been sufficiently reduced, so as to avoid excessive operation of the heaters in the cleaning stage. 
     As a result, the present invention provides a combination cooking source oven with a self-cleaning operation that complies with regulatory guidelines for pyrolytic cleaning operation. Unlike previous controls that operate in response to cooking conditions occurring within the oven, the present invention provides a pyrolytic cleaning operation that effectively cleans additional surfaces in communication with the oven chamber that may be subjected to the accumulation of debris during cooking. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood by reference to the following detailed description when read in conjunction with the accompanying drawing in which like reference characters refer to like parts throughout the views and in which 
     FIG. 1 is a perspective view of a cooking appliance constructed in accordance with the present invention; 
     FIG. 2 is a perspective view of a portion the device shown in FIG. 1 taken from a different direction; and 
     FIG. 3 is a rear perspective view of the portion shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a cooking appliance  10  comprises a housing  12  supporting an oven  14 . The oven  14  comprises a housing  16  carrying a plurality of cooking sources for generating heat in cooking conditions within the oven chamber  18 . The housing  12  carries a control  20 , preferably a microprocessor based control, for operating the multiple cooking sources, and preferably including a control panel  22 , for example a keypad  24  or other arrangement of knobs and switches, that can be manipulated by a user in order to control operation of the cooking sources. 
     In the preferred embodiment, the cooking sources include a microwave generator that is coupled to waveguide  26 , a jet impingement heater  28 , that generates and distributes strong convection currents of heated air, and a radiant heating element  30  (FIG. 2) mounted for heating the oven chamber  18 , for example by exposure or as in the preferred embodiment, radiant heating from the chamber walls covering the bottom element and mullion element. The control  20  provides for selective actuation of one or more sources by a user as desired, including combinations of sources in a single cooking process. 
     The jet impingement heater  28  includes ducting  32  that carries at least one blower  34  to induce currents of air within the cooking chamber  18 . In the preferred embodiment, an upper duct assembly  36  carries a blower  34  so that its output is delivered to an upper manifold  38  (FIG. 2) through a channel containing a moving vane. The manifold  38  includes a plurality of outlets or nozzles that communicate with the oven chamber  18 . In the preferred embodiment, the outlets are in a form of nozzles, preferably arranged in a pattern carried by a manifold plate  40  that is positioned below the moving vane. The vane displacement directs the movement of convection currents throughout the oven chamber. A lower ducting  42  carries a second blower  34  for communication with a lower manifold  46 . The lower manifold  46  is enclosed from the ducting  42  to the oven chamber  18 . The manifold includes bottom wall  48  of the housing that includes outlets or nozzles communicating currents from the ducting to the oven chamber  18 . The outlets are arranged in a pattern that cooperates with the moving nozzles in the vane in the upper manifold  38  to deliver jet impingement currents throughout the cooking area of the oven chamber  18 . 
     While the oven housing  16  defines five walls enclosing the oven chamber  18 , an open wall of the chamber is enclosed by a door  50  that is hinged in a well-known manner to enclose the open end of the oven chamber  18 . A seal membrane, preferably carried by at least one of the face frame  54  and the door  50 , is pliable and compressible between the door  50  and the face frame  54  when the door  50  is in its closed position. Preferably, a latch mechanism  56  mechanically locks the door  50  in a position against the face frame  54  so that the seal  52  is compressed between the surfaces and seals the oven chamber  18  from communication exteriorly of the housing  12 . The latch  56  may be operated during cooking modes but is more importantly used for locking the oven door against the face frame when a pyrolytic cleaning operation is to be performed by the appliance  10 . 
     When the pyrolytic cleaning mode has been selected by a user on the control panel  22  the oven cavity temperature is raised to approximately 850° F. (454° C.) for a first period. At this temperature, any soil or food residues on the walls on the inside of the oven chamber  18  are burned off, turning them into ash. The jet blowers are inactive during the first period, defining the first stage of the cleaning operation, since energy is provided to the radiant heater  30  only. Of course, the heating element used in the jet impingement heater may also be actuated, preferably for a period not greater than the first time period, to incinerate any debris that has reached the ducting  36  and  42 . The first stage or period of the pyrolytic cleaning operation may be simply fixed as a predetermined time duration that is considered long enough to burn off most of the incineratible residue. After a predetermined time period or event responsive condition has occurred, the jet blowers  34  are turned on by the control to complete the self-cleaning process. In the later stage, the jet blowers insure that all air channels are raised to the clean temperature by moving hot air across all the air passages. 
     The duration of the self-clean mode may be user selectable and preferably ranges from 2 to 4 hours. The user selects the proper duration depending on how much soil has been detected in the oven. In the preferred production embodiment, a second stage may be set at a 30 minute time duration of a self-clean cycle regardless of the selected duration of the self-clean mode. Nevertheless, the duration of the first stage may also be set to a predetermined amount. Moreover, if a variable cleaning time may be set, the second stage may also be variably selected by the user to occur at a selectable duration during the terminal portion of the cleaning operation. For example, if a pyrolytic cleaning time of 4 hours is selected, the jet blowers  34  could turn on during the last half-hour of the self-clean mode. 
     In addition to the above described open loop systems, a closed loop system may be employed. For example, a sensor, such as a volatile gas sensor or oxygen sensor, may be placed within the oven to detect the amount of incineratible fuel or vapor remaining in the oven chamber. When the amount of volatile gas has dropped to a predetermined value, the control may be employed to compare the reduced value with a predetermined value so that the comparison will trigger actuation of the jet blowers to complete the pyrolytic cleaning process. Accordingly, the stages may be dynamically changed depending upon the dissipation of volatile gases. 
     Having thus described the present invention, many modifications will become apparent to those skilled in the art to which it pertains without departing from the scope and spirit of the present invention as defined in the appended claims.