GB2051225A

GB2051225A - Auxiliary Combustion Chamber Preheating Device For Internal Combustion Engines

Info

Publication number: GB2051225A
Application number: GB7921057A
Authority: GB
Original assignee: NGK Spark Plug Co Ltd
Current assignee: Niterra Co Ltd
Priority date: 1979-06-18
Filing date: 1979-06-18
Publication date: 1981-01-14
Also published as: GB2051225B

Abstract

The device includes a metal shell 1 threadedly mountable on the engine and a tubular heat generating element 2 having an open end inserted within the connected to the metal shell 1. The element 2 is formed of a nonmetallic resistance material selected from silicon carbide and molybdenum disilicide. A rod-like centre electrode 3 of nickel, tungsten or molybdenum extends through the shell 1 into the element 2 and at its lower end surface 31 is electrically connected to an inner wall portion of a closed end 22 of the element 2, either directly or, as shown, via carbon powder 5 which may be enclosed in a sleeve 8 of silicon nitride. An electrically conducting layer is provided on an outer wall portion of the element 2 connected to the metal shell 1 . The energization of the device is controlled by a circuit which uses the element 2 and the electrode 3 as a temperature-sensing thermocouple. <IMAGE>

Description

SPECIFICATION Auxiliary Combustion Chamber Preheating Device The present invention relates to a device for preheating an auxiliary combustion chamber of an internal combustion engine.

It is known to preheat an auxiliary combustion chamber of an internal combustion engine by means of a plug having a heat generating element in the form of a heat generating resistance such as silicon carbide (SiC) or the like. Typically, the heat genertaing element made of silicon carbide is in the form of a rod and is inserted into a metallic sheath tube or is of generally U-cross section and is mounted on a metal shell. In the former arrangement, electrically insulative powders such as magnesia fill the sheath tube and reduce the rate of conduction of heat from the heat generating element so that there is an increase in the time required for the outer surface of the sheath tube to reach its operating temperature. Moreover the durability of the glow plug is reduced as a result of the complexity of the connection to the associated electrode.In the latter arrangement, the shape of the heat generating element is complicated and the connection between the heat generating element and the metal shell is liable to result in difficulty.

In a diesel engine, the higher the surface temperature of a glow plug, the more easily or positively the engine can be started even at low ambient temperatures in winter, particularly when the glow plug is heated to the extent of 1,3000C.

Namely, when a surface temperature of the heat generating portion of the glow plug is low, it takes a long period of time from the initial explosion to the complete explosion, so that an electric current is expended in vain. At high temperatures, a quick start can be smoothly carried out.

It is impossible to obtain satisfactory starting characteristics using a metallic glow plug in which a metallic heat generating element made of a material such as nickel-chrome and iron-chrome is wound in a spiral manner in the metallic tube.

The metallic glow plug has a low allowance temperature of at most 1,1000C.

An object of the present invention is to alleviate or overcome the above-noted defects experienced in the prior art.

Accordingly, the invention resides in an auxiliary combustion chamber preheating device for a diesel engine, the device having a glow plug comprising a metal shell threadedly mountable on the engine, a centre electrode having a rod-like configuration, a tubular heat generating element having an open end and a closed end, said element being made of a non-metallic resistance material selected from silicon carbide (SiC) and molybdenum disilicide (MoSi2) and havingits open end inserted within and connected to the metal shell; electrical connecting means for electrically connecting a lower end surface of the centre electrode and an inner wall portion of the closed end portion of the heat generating element; and an electrically conducting layer on an outer wall portion of the heat generating element.

In the accompanying drawings: Figures 1 to 6 show glow plugs according to first to sixth embodiments respectively of the present invention, and Figure 7 shows an electrical circuit for controlling the preheating of the auxiliary combustion chamber of an internal combustion engine using a glow plug according to the present invention.

Referring to Figure 1 , the glow plug of the first embodiment includes a metal shell 1 which is externaliy screw threaded so as to be mountable in a wall of a diesel engine. Supported by the shell 1 is a heat generating tube 2 made of nonmetallic resistance material, such as silicon carbide (SiC) or molybdenum disilicide (MoSi2), and having an open end 21, a closed end 22 and a hollow space therein. The open end 21 of the tube 2 is inserted into the metal shell as shown in Figure 1 and an electrode layer of silver, copper or the like, which is secured to an outer wall of the open end 21 by fusing-injection is connected to an inner wall 11 of the metal shell 1 by means of a brazing material 10 such as silver, copper or the like.A rod-like centre electrode 3 made of a metal such as tungsten, molybdenum or the like is coaxially mounted in the metal shell 1 and the heat generating tube 2 so that an end portion 31 of the centre electrode 3 abuts directly against an inner end surface of the closed end 22 of the heat generating tube 2. An electrically insulative ceramic powder 4 such as magnesia or alumina fills the annular space defined between the metal shell 1, the heat generating tube 2 and the centre electrode. The ceramic powder 4 is stably maintained within said annular space by caulking a metal packing 14 and an electrically insulative annular plug 13 made, for example, of silicone rubber at a top end portion 12 of the metal shell 1.

The thus constructed glow plug is mounted in an auxiliary combustion chamber of an internal combustion engine so that an electric current can flow thereto through the metal shell forming an outer electrode. Since the heat generating tube 2 is directly exposed, the outer wall of the heat generating tube 2 can be rapidly heated by the heat generation therein, thereby enhancing the efficiency of heating of the auxiliary combustion chamber. Further, since the heat generating tube 2 is formed into a tubular shape having a closed end, the construction is relatively simple and the connection to the metal shell is facilitated by the brazing or the like.

Figure 2 shows a second embodiment of a glow plug according to the present invention. In the foliowing embodiments, like members or parts will be identified by the same reference numerals as used in Figure 1. Figure 2 shows a second preferred embodiment of a glow plug in which centre electrode 3 is inserted into the heat generating tube 2 while the end surface 31 of the centre electrode 3 is separated from the inner surface of the closed end 22 of the heat generating tube 2, by a layer of carbon powder 5 filling the closed end 22. Non-oxidizing electrically insulative powder 6, such as silicon nitride (Si3N4), boron nitride (BN), aluminum nitride (AIN) or the like is disposed in the tube 2 adjacent the carbon powder 5. The open end 21 of the heat generating tube is sealed by a sealing member 7 made of glass or the like.

When the thus constructed glow plug is in use an electric current flows between the centre electrode 3 and the metal shell 1. The carbon powder 5 serves to provide the required electrical connection between the heat generating tube 2 and the centre electrode 3 and, by virtue of its fluidity, is able to absorb differential thermal expansion between the electrode 3 and the tube 2 and thereby prevent damage of the heat generating tube 2. The electrically insulative powder such as Si3N4, BN or AIN reacts with oxygen contained in any air enclosed in the heat generating tube to form silicon dioxide. Therefore, the insulative powder 6 serves to consume the free oxygen in the heat generating tube and has a reductive effect to prevent the oxidation of the centre electrode and the carbon powder and thereby also prevent an increase in the resistance of the glow plug.

Figure 3 shows a third embodiment of the present invention. In the same manner as described above, the centre electrode is inserted into the closed end portion of the heat generating tube but is separated from the closed end 22 of the heat generating tube. In Figure 3, a sleeve 8 made of silicon nitride is provided so as to partially encircle the end portion 31 of the centre electrode 3. The outer end of the sleeve 8 is in abutment with the closed inner end portion 22 of the heat generating tube 2. Also, the inner hollow space of the sleeve 8 is filled with carbon powder 5. In this embodiment, electric current flows through the end portion 23 of the heat generating tube 2 in the directions shown by arrows K.

Accordingly, the end portion 23 can contribute to heating more effectively. In contrast, in Figure 2, electric current flows as shown by arrows R. As a result, the heat generation at the end portion 23 in Figure 2 occurs somewhat slower than that in Figure 3.

An electric current heating experiment was carried out repeatedly ten-thousand times using the glow plug in Figure 2 according to the pgesent invention, in which the surface temperature of the heat generating tube was heated to 1 0000C from the room temperature. As a result, no increment of the resistance value occurred and no damage of the heat generating tube occurred.

In Figure 4, element 2 is a heat generating tube. A side portion of the open end of the heat generating tube 2 is designated by 2a. An electrically conductive layer 35 made of alloy or pure metal, such as silver, copper or the like, is secured to an end portion of a large diameter portion 2b of the heat generating tube 2 by fusing or baking. The electrically conductive layer 25 is connected to the inner wall of the metal shell 1 by brazing material such as silver, copper or the like thereby allowing an electric current to flow through the heat generating tube 2 and the metal shell 1.A heat resistant adhesive agent 71 which fills an annular space defined by a narrow portion 2a of the open end portion of the heat generating tube 2 and the metal shell 1 and which is made of material having a high heat insulative performance, such as ceramic bond or the like serves to secure the heat generating tube 2 and the metal shell 1 and at the same time to seal them. In this embodiment, the overall surface of the electrical conductive layer 25 is reduced by the specific construction of the plug.

When an electric current flows between the centre electrode 3 and the metal shell 1 in the above described construction, conduction of heat from the part of the heat generating tube 2 surrounded with the adhesive agent 71 to the metal shell 1 is substantially prevented. The rate of the escape of the heat to the engine body is therefore reduced.

Figure 5 shows a modification to the previous example, wherein a heat insulative layer 71' is made by ceramic coating the annular space defined by the upper portion of the heat generating tube 2 and the metal shell 1.

Figure 6 shows another modification to the Figure 4 example, wherein an expanded portion 2c is formed on the metal shell covered portion of the heat generating tube 2, an electrically conductive layer 25 is secured to the lower portion thereof, and a packing member such as copper, silver or the like is disposed between the lower end portion 15 of the metal shell 1 and the heat generating tube 2.

In the preceding embodiments, a thermocouple is formed by silicon carbide (SiC) of heat generating tube 2 of the glow plug and nickel, tungsten or molybdenum of the centre electrode 3 due to their thermal electromotive force. The output voltage is at 90 to 110 mV when the temperature at the closed end portion 23 of the heat generating tube 2 is at 1 ,0000C. Such output voltage is approximately twice that of a chromel-alumel thermocouple, which is 41.31 mV/1 ,0000C. For this reason, an operation in which another thermocouple is inserted into the glow plug can be dispensed with. At the same time, disconnection of the thermocouple hardly occurs because the centre electrode and the heat generating tube are sufficiently thick.

Figure 7 shows a preheating circuit used in the preheating device according to the present invention*, wherein V designates a battery or electric source, M a motor, S a switch, G a signal lamp and 50 an electric flow control circuit for controlling operation of the glow plug A. The control circuit is composed of a relay circuit and relay elements having a switching operation such as a lead switch which is opened or closed by the output signal of the thermocouple defined by the centre electrode 3 and the heat generating tube 2. The centre electrode 3 is connected to the electric flow control circuit 50 by line L. The heat generating tube 2 is connected to the flow control circuit 50 by line L2. Switch S is connected through the signal lamp G to the glow plug A by line L3.Also, the switch S is connected through the control circuit 50 to the glow plug A by line L4.

Further, the switch S is connected directly to the glow plug A by line L5 without the signal lamp G when the starter motor M is driven.

In starting a diesel engine, the starter line L3 is coupled to the battery V for a time period of 10 to 30 seconds to preheat the auxiliary combustion chamber. Next, the starter motor M and the circuit L5 are connected to the battery V to pass an electric current through the glow plug and at the same time, the engine is started by the motor M.

After the engine is started, the above described switch connections are released, and the circuit L4 is coupled to the battery V, and while the engine is running, such a state is maintained. In this condition, in the event that mistimed combustion is liable to occur and the temperature of the combustion chamber is reduced, such as for example when the engine is idling just after starting or when in a cold country a low load engine running condition is maintained for a long period of time, the output voltage of the thermocouple defined by the centre electrode and the heat generating tube 2 is small, and the control circuit 50 energises the line L4 to allow electric current to flow through the glow plug A and thereby heat the preheating chamber. On the other hand, when the combustion chamber temperature is high during normal or high speed engine running conditions, that is, at more than 5000C, the output voltage of the thermocouple is high and control circuit 50 opens the line L4 to thereby prevent the unnecessary consumption of the battery V.

Claims

Claims

1. An auxiliary combustion chamber preheating device for a diesel engine, the device having a glow plug comprising: a metal shell threadedly mountable on the engine; a centre electrode having a rod-like configuration; a tubular heat generating element having an open end and a closed end, said element being made of a nonmetallic resistance material selected from silicon carbide (SiC) and molybdenum disilicide (MoSi2) and having its open end inserted within and connected to the metal shell; electrical connecting means for electrically connecting a lower end surface of the centre electrode and an inner wall portion of the closed end portion of the heat generating element; and an electrically conducting layer on an outer wall portion of the heat generating element.

2. A device as claimed in Claim 1, wherein said electrical connecting means includes carbon powder and a space defined between the centre electrode and the tubular heat generating element contains a non-oxidizing electrically insulative material selected from silicon nitride (Si3N4), boron nitride (BN) and aluminium nitride (AIN).

3. A device as claimed in Claim 2, wherein a ceramic sleeve member is disposed around the lower end of the centre electrode and between the bottom wall of the closed end of the heat generating element and the non-oxidizing electrically insulative material, the carbon powder between contained within a portion of the sleeve member projecting from the centre electrode.

4. A device as claimed in Claim 1, wherein said tubular heat generating element is formed intermediate its ends with a large-diameter portion which is partially inserted into the metal shell, and an annular space is defined betwee'n the open end portion of said element and the metal shell, the annular space being filled with a heat insulative adhesive agent to provide a seal between the shell and said element.

5. A device as claimed in Claim 1, wherein said tubular heat generating element is formed intermediate its ends with a large-diameter portion which is partially inserted into the metal shell, and an annular space is defined between the open end portion of said element and the metal shell, the annular space being filled with a ceramic coating layer to provide a seal between the shell and said element.

6. A device as claimed in Claim 1, wherein said tubular heat generating element is formed intermediate its ends with a large-diameter portion which is fully inserted into the metal shell, an upper annular space defined between the open end portion of said element and the metal shell is filled with a heat insulative adhesive agent, and packing member made of an electrical conductive material is fixedly disposed in a lower annular space defined between the shell and said element to provide a seal therebetween.

7. A device as claimed in any preceding claim, and further comprising electrical circuit means which includes a thermocouple defined by the centre electrode and the heat generating element and which is arranged so that, in use, the supply of electrical current to the glow plug is controlled by the output of said thermocouple.

8. An auxiliary combustion chamber preheating device containing the combination and arrangement of parts substantially as hereinbefore described with reference to, and as shown in, any one of the accompanying drawings.

New Claims or Amendments to Claims filed on 14 August 1980.

Superseded Claim 1.

New or Amended Claims

1. An auxiliary combustion chamber preheating device for a diesel engine, the device having a glow plug comprising: a metal shell threadedly mountable on the engine; a centre electrode having a rod-like configuration; a tubular heat generating element having an open end and a closed end, said element being made of a nonmetallic resistance material selected from silicon carbide (SiC) and molybdenum disilicide (MoSi2) and having its open end inserted within and connected to the metal shell; electrical connecting means for electrically connecting a lower end surface of the centre electrode and an inner wall portion of the closed end portion of the heat generating element; and an electrically conducting layer on an outer wall portion of the heat generating element and in contact with said metal shell.