fig1 and 2 show an embodiment of a dishwasher of the present invention . the dishwasher mainly comprises a casing 1 , and a dishwashing tub 2 including a rotatable cover 3 . a sprinkler 4 is disposed in the lower section of the dishwashing tub 2 . the sprinkler 4 is provided with a plurality of nozzles 5 for emitting water towards tableware supported by a supporting rack 12 . a circulation pump 6 is disposed in a clearance formed between the casing 1 and the dishwashing tub 2 . the inlet side of the circulation pump 6 is communicated with a drain chamber 8 formed in the bottom wall of the dishwashing tube 2 through a drain conduit 7 , and the outlet side of the circulation pump 6 is communicated with the sprinkler 4 through a supply conduit 9 . a filter 10 is disposed in the drain chamber 8 . a drain valve 11 is provided for electro - magnetically controlling drain operation of water contained in the dishwashing tub 2 . a feed water opening 13 is formed in an upper portion of the dishwashing tub 2 for supplying water to the dishwashing tub 2 in a shower fashion . an electro - magnetic feed water valve 14 is provided for controlling water supply to the dishwashing tub 2 . a heater 15 is disposed at the lower section of the dishwashing tub 2 for heating the water in a main washing step , and for generating steam in a steam washing step . these steps will be described later in detail . fig3 shows a steam generator 16 , which mainly comprises the heater 15 and a heater cover 17 . the heater cover 17 includes a standing portion , where steam issuance openings 18 are formed . the dishwasher further comprises a level detection switch 19 for detecting a liquid level in the dishwashing tub 2 . a program timer 20 is disposed in a clearance formed between the casing 1 and the dishwashing tub 2 for controlling , in combination with the level detection switch 19 , operations of the circulation pump 6 , the drain valve 11 , the feed water valve 14 and the heater 15 . more specifically , the program timer 20 controls the operation cycle of the automatic dishwasher , for example , a preliminary washing step , a steam washing step , a main washing step , a rinse step , and a drying step . fig4 shows a control circuit of the dishwasher of the present invention . like elements corresponding to those of fig1 through 3 are indicated by like numerals . the control circuit mainly comprises a program cam switching unit 21 controlled by the program timer 20 . more specifically , the program cam switching unit 21 comprises eight cam plates which are driven to rotate by the program timer 20 . each cam plate is associated with switches s 1 through s 8 for controlling various operations in the automatic dishwasher . the operation cycle of the automatic dishwasher of the present invention will be described with reference to fig4 and 5 . the operation cycle mainly comprises a sequence of the preliminary washing step , the steam washing step , the main washing step , the rinsing step and the drying step . the tableware is disposed in the supporting rack 12 , which is secured in the dishwashing tub 2 . when the rotatable cover 3 is closed , a door switch sw 1 is switched on . thereafter , when the program timer 20 is set , the switch contact s 1 is closed , whereby the timer motor begins to rotate . under these conditions , the above - mentioned sequential operations are automatically conducted . only the switch contact s 1 is maintained in its on condition in the start mode , which continues for one minute . the switch s 2 is turned on to open the feed water valve 14 . fresh water is applied to the dishwashing tub 2 through the feed water opening 13 . when the water level reaches a predetermined level , the level detection switch 19 is switched to terminate the energization of the solenoid associated with the feed water valve 14 , or to close the feed water valve 14 . thereafter , the switch s 3 is switched on to energize the circulation pump 6 . water is emitted from the nozzles 5 formed in the sprinkler 4 towards the tableware disposed in the dishwashing tub 2 . after a predetermined time period , the switch s 3 is switched off and the switch s 4 is switched on to open the drain valve 11 . the water contained in the dishwashing tub 2 is drained to complete the preliminary washing step which continues for about three minutes . the switches s 2 and s 5 are switched on , whereby fresh water is supplied to the dishwashing tub 2 and the heater 15 is energized . as in the case of the preliminary washing step , the water supply is terminated when the water level reaches a predetermined level . since the heater 15 heats up the water contained in the heater cover 17 , the steam is emitted from the steam issuance openings 18 , whereby starch tightly attached to the tableware is melted . the steam washing step continues for about eight minutes . the switches s 2 , s 3 and s 5 are switched on . the main washing requires more water than the steam washing . when the water level reaches a predetermined level , the water supply is terminated as in the case of the preliminary washing . the steam is not emitted because the circulation pump 6 is energized . warm water is emitted from the nozzles 5 . the switch 7 is temporarily connected to a terminal a at the beginning of the main washing step to energize a bimetal 22 , whereby a cleaning agent is supplied to the dishwashing tub 2 through a valve associated with the bimetal 22 . the cleaning agent may be any commonly used automatic dishwashing detergent . however , a preferred cleaning agent is a non - ionized ferment containing cleaning agent such as &# 34 ; finish &# 34 ; manufactured by sunstar dentifrice co ., ltd . the main washing step continues for about ten minutes . during the main washing step , the switch s 8 is connected to a terminal b to enable a thermostat th 1 , which functions to maintain the washing water at about 50 ° c . when the main washing step is completed , the switch s 4 is switched on to open the drain valve 11 . the switch contact s 8 is returned to the terminal a . the switch s 2 is switched on to open the feed water valve 14 . one minute later , the switch s 3 is switched on to enable the circulation pump 6 . further one minute later , the switches s 2 and s 3 are switched off and the switch s 4 is switched on to open the drain valve 11 . this cycle comprising the switching operation of switches s 2 , s 3 and s 4 is repeated three times . at the last cycle , the switch s 5 is switched on and the switch contact s 7 is connected to the terminal a in order to energize the bimetal 22 , whereby a rinse agent is supplied to the dishwashing tub 2 . any well - known rinsing agent for automatic dishwashers may be used . a preferred rinse agent is &# 34 ; pure rinse &# 34 ; manufactured by kabushiki kaisha adeka clean aid . the switches s 4 and s 6 are switched on , whereby the drain valve 11 is opened and a heater 23 for drying purposes is power supplied . the drying heater 23 is not necessarily required when the heater 15 is also used for drying purposes . the control circuit further includes a lamp 24 which is activated when the main switch s 1 is on for indicating the operation of the automatic dishwasher , a protection switch sw 2 which is operated when the level detection switch 19 is in an abnormal condition , and a relay rl which functions to close a relay contact rc only when the level detection switch 19 detects that the water is supplied to a predetermined level . a thermostat th 2 functions to maintain the washing water below 70 ° c ., and another thermostat th 3 functions to prevent an abnormal high temperature . as discussed above , the present automatic dishwasher includes the steam washing step interposed between the preliminary washing step and the main washing step . the present inventors have discovered that the cleaning efficiency is greatly enhanced when the steam washing is interposed between the preliminary washing and the main washing . the following are experimental data showing the above - mentioned cleaning efficiency . the experimentation was conducted by the present inventors . the cleaning efficiency is compared by three different washing sequences as follows : 2 . preliminary wash ( three ( 3 ) minutes )→ steam wash ( eight ( 8 ) minutes )→ main wash ( eighteen ( 18 ) minutes ) the water temperature is about 20 ° c . in the preliminary washing , and the water temperature reaches 65 ° c . at the end of the main washing . the cleaning agent &# 34 ; finish &# 34 ; of ten ( 10 ) grams is supplied to the dishwashing tub at the beginning of the main washing step . test samples are three large plates , four plates , four bowls , four large cups , four cups and four glasses . rice grains of five ( 5 ) grams were attached to each test sample and dried by application of an heated air flow ( 60 ° c .) for thirty minutes before the washing . the cleaning efficiency is calculated through the use of the following formula and score table . score table______________________________________cleaning efficiency = ## str1 ## score condition______________________________________5 completely cleaned4 one or two small stains ( about 1mmφ ) are observed ; or a starch film is slightly observed only a portion of the sample3 one , two or three rice grain size stains are observed ; or a starch film of a size below 4 cm . sup . 2 is apparently observed2 four through seven rice grain size stains are observed ; or a starch film of a size 5 to 24 cm . sup . 2 is apparently observed1 rice grain size stains are observed more than eight ; or a starch film excess of 25 cm . sup . 2 is apparently observed______________________________________ the invention being thus described , it will be obvious that the same may be varied in many ways . such variations are not to be regarded as a departure from the spirit and scope of the invention , and all such modifications are intended to be included within the scope of the following claims .
soil , including starch deposits , are removed from tableware in a dishwasher by a preliminary wash with cool water at approximately 20 ° c ., followed by a steaming operation and then a washing operation with water containing a cleaning agent . a portion of the water to be used in the washing operation is preheated during the steaming operation .