Abstract:
A glow plug is maintained at a desired operating temperature by using as its heat generating element a material which may be quickly heated, has a non-zero resistance-temperature coefficient and has a repeatable resistance value at the desired operating temperature. The heat generating element is used as one leg of a bridge circuit and when the bridge is balanced, the engine starter is energized and a short time later the current through the bridge is interrupted.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a device for promoting the starting operation of engines, more particularly to a device for promoting the starting of a diesel engine having glow plugs. 
     In most conventional diesel engines, especially in diesel engines having a preheating in a swirl chamber, the engines are started by using glow plugs. This method of utilizing glow plugs is advantageous in that the necessary device is simple and the engine can be readily started. However, the method is still disadvantageous in that it takes 20-30 seconds to preheat the glow plugs; that is, the engine starting characteristic is very poor. 
     Furthermore, in most conventional glow plug starting systems, separate preheating and starting circuits are provided and in some cases a voltage drop, which occurs during starter motor operation will hinder the heating of the glow plug due to inefficient design of the preheating control circuitry. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to eliminate the above-described drawback accompanying the conventional method in which glow plugs are utilized for starting the engine. More specifically, an object of the invention is to provide a novel engine start promoting device in which the preheating time of the glow plug is considerably reduced and the engine starting characteristic is excellent. 
     Briefly, this is accomplished by using a glow plug having a heat generating element which is quickly heated, has a non-zero resistance-temperature coefficient and has a repeatable resistance value at the desired operating temperature. The element is used as one leg of a bridge and when the bridge is balanced, the starter is energized through a self-holding amplifier. After a short delay the current through the bridge is interrupted and the element cools down for a time after which the current is resupplied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram, partly as a block diagram, showing one embodiment of the invention; 
     FIG. 2 is a graphical representation indicating the resistance-temperature characteristic of the heat generating element of a glow plug in the embodiment shown in FIG. 1; and 
     FIG. 3 is a graphical illustration of the operating cycle of the device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of this invention will be described with reference to the accompanying drawings, in which reference number 1 designates power supply; reference numeral 2, a start switch; reference numeral 3, a glow plug which is provided in correspondence to the number of cylinders; reference numeral 31, a heat generating element of the glow plug 3 having a resistor R; and reference numerals 4, 5 and 6, resistors having resistances r 1 , r 2  and r 3 . The resistors R, r 1 , r 2  and r 3  form a Wheatstone bridge circuit 7. A comparator 8 is connected to middle points m 1  and m 2 . When the bridge is balanced, the following equation is established, and a signal is generated from the comparator. 
     
         r.sub.2 /r.sub.1 =r.sub.3 /R                               (1) 
    
     Reference numeral 9 designates a signal delaying means, and reference numeral 10 designates a relay contact operating means having a high speed intermittent function at a predetermined period and intermittently controlling change-over of an electric current controlling element 11 provided between the starter switch 2 and the bridge circuit 7, for example, a relay contact 11. Reference numerals 11a and 11b are contact points for the change-over. 
     Reference numeral 12 designates a transmission signal converting amplifier having a self-holding function; reference numeral 13, a starter relay; and reference numeral 14, a starter. All of the above-described components are connected as shown in FIG. 1 to form a control circuit 15. 
     A heat generating element 31 of the glow plug 3 is made of a metallic resistance material, e.g., nickel, such that its resistance R is lower than 2/3 of the rated resistance Rt of a conventional glow plug at all temperatures up to the set operating temperature Ts; the resistance Rs at the set temperature Ts is repeatable. 
     The operation of the engine start promoting device constructed as mentioned above according to the present invention will now be described 
     When the starter switch 2 is closed to start the engine, the electrical current flows from the power supply 1 through the closed contact point 11a of the relay contact 11, resistors 4, 5 and 6 and the heat generating element 31 of the glow plug 3 and at the same time the current flows through the comparator 8 until the resistance R of the heat generating element 31 satisfies the equation (1). As a result, the heat generating element 31 is abruptly heated until the resistance R becomes the set resistance Rs corresponding to the set temperature Ts. When the equation (1) is established, that is, the resistance R becomes the set resistance Rs, the comparator 8 is actuated so that an output signal is generated from the output terminal 8a thereof. 
     The output signal of the comparator 8 is converted from a theoretical signal into an electrical signal by the amplifier 12. The electrical signal is transmitted to the starter relay 13 to thereby actuate the starter 14 and, at the same time is transmitted to relay contact operating means 10 through the signal delay means 9. The relay contact 11 is changed-over from the contact point 11a to 11b by the operation of the operating means 10 so that the contact point 11a is released during a short period of time. As a result, the current does not flow to the bridge circuit 7 including the glow plug 3, thereby interrupting intermittently the heating of the heat generating element. 
     This will be more specifically illustrated in FIG. 3 in which the relations between the temperature T of the heat generating element and time, between the output signal of the comparator 8 and time and between the operation of the relay contact 11 and time are shown on the same scale of the time period, respectively. 
     When the temperature of the heat generating element reaches the set temperature Ts as shown by a, the first output signal S 1  is generated by the comparator 8, and delayed by the signal delay means 9 by a short period dt 1  of time and introduced into the relay contact operating means 10 to thereby release the relay contact point 11a of the relay contact 11. The temperature of the heat generating element 31 further increases to point b during the short period dt 1  of time, and, when the electical current is stopped by the relay contact 11, the temperature of the heat generating element falls from b toward c. After the period dt 2  of time passes, the contact point 11a of the relay contact 11 is again closed by the reoperation of the relay contact operating means 10. Then, the temperature again increases from c to the set temperature. When the temperature reaches the set temperature Ts (d), a new output signal S 2  is generated in the comparator 8, and thereafter, the same operation is repeated as shown by the curve a-b-c-d-b&#39;-c&#39;. With such repetition of the relay contact operating means 10, even if a voltage drop occurs during the actuation of the starter 14, the temperature of the heat generating element 31 can be maintained at a value adjacent the set temperature Ts. 
     Further, because the amplifier 12 has a self-holding function, the actuation of the starter 14 is not interrupted even during the intermittent operation of the comparator 8. 
     The heat generating element 31 of the glow plug 3 is made of a metallic resistance material such that its resistance R is lower than 2/3 of the rated resistance Rt of a conventional glow plug at all temperatures up to the set temperature Ts during the application of the output supply voltage E 0 , the resistance Rs at the set temperature Ts is repeatable, and the resistance R is repeatable at any temperature up to the set temperature. Therefore, it is possible to apply a current 1.5 times the conventional rated current to the heat generating element 31. Accordingly, the heat generating element can be heated up in a very short time, that is, the preheating time can be considerably reduced. 
     The resistance is always less than two-thirds of conventional heating element resistances at corresponding temperatures. Thus, from the following equation (2), the maximum time t 1  required to heat the glow plug would be given by 
     
         E.sup.2 t/R=E.sup.2 t.sub.1 /0.66R                         (2) 
    
     or 
     
         t.sub.1 =0.66t                                             (2)&#39; 
    
     Since t, the conventional time requirement, is usually 20-30 seconds, the time t 1  according to the invention would be at longest 13 to 20 seconds. However, since the heat generating element resistance has a positive temperature coefficient, the time requirement will be much lower, for example, less than 10 seconds when a conventional 12 or 24 volt diesel engine power supply is used. 
     Further, in the engine start promoting device, the temperature of the heat generating element 31 is detected on the basis of its set resistance Rs and, therefore, the heating temperature T of the heat generating element 31 is detected with high accuracy. Therefore, even though the resistance R of the heat generating element 31 is lower than 2/3 of the rated resistance Rt, the damage due to over-heating, such as the fusing of the heat generating element 31 due to over-current, can be prevented. 
     In addition, the engine start promoting device according to the invention is so designed that the four resistors including the heat generating element 31 form the bridge circuit 7 in the control circuit 15, and the comparator 8 is connected to the middle points thereof, and the starter 14 is actuated through the amplifier 12 having the self-holding function and the starter relay 13 is actuated by the output signal from the comparator 8. Accordingly, in the engine start promoting device according to the present invention, unlike the conventional one, the provision of independent preheating and starting devices is unnecessary. Therefore, the engine start promoting device according to the invention is very simple in construction. This is one of the significant advantages of the invention. 
     In the above-described embodiment, the transmission signal converting amplifier 12 has a self-holding function; however, the engine start promoting device may be so designed that the starter relay 13 has a self-holding function, or both of the amplifier 12 and the starter relay 13 have the self-holding function. 
     As is apparent from th above description, according to the invention, an engine start promoting device is provided in which the preheating time of the glow plug can be greatly reduced, the construction is simple and the engine start characteristic is excellent, and an engine start promoting device according to the invention is applicable to a ship engine, an agricultural engine, and other industrial engines.