Glow Plug for an Internal Combustion Engine

The present invention relates to a glow plug for an internal combustion engine and more particularly to a glow plug suitable for a diesel engine.

In diesel engines, a glow plug is now used as a part for starting at low temperatures, and in order to improve the starting characteristic of diesel engines, a small-sized, fast-heating glow plug is now desired.

Most conventional glow plugs are sheathed type glow plugs, in which a heating wire made of Ni-Cr alloy or like material is wound in the form of a coil and placed in a sheath which is formed of a heat- and corrosion-resistant alloy such as stainless steel or inconel and whose one end is closed, and the interior portion of the sheath around the heating wire is filled with an insulating material such as magnesium oxide. In glow plugs of this type, therefore, since the heat conduction between the heating wire and the sheath is effected through the insulating material, it takes time for the surface of the sheath to be heated to redness for igniting a fuel-air mixture, that is, the preheating time is prolonged.

In an effort to solve such problems, that is for attaining rapid heating in sheathed type glow plugs, there is adopted a method in which the rated voltage is set at a low level, and during preheating, a battery voltage higher than the rated voltage is temporarily applied directly for rapid glowing, while during after-glow after start-up of the engine, the battery voltage is dropped by means of a resistor and the rated voltage is applied. In such method, however, it is necessary that a resistor having a fairly large capacity and hence involving generation of heat should be provided separately or as a built-in component, thus causing problems such as an increase in installation space required and a more complicated structure of glow plug, as well as wasteful power consumption in the resistor.

It is an object of the present invention to provide a glow plug for an engine superior in the rapid heating characteristic.

It is another object of the present invention to provide a glow plug for an engine free from wasteful power consumption.

It is a further object of the present invention to provide a glow plug for an engine capable of appropriately controlling the quantity of generated heat according to the state of preheating and that of after-glow.

In the glow plug of the present invention, a heater member formed of an electrically conductive ceramic material superior in heat- and oxidation-resistance is affixed to the surface of an electrically insulative heater support member projecting into a combustion chamber of an engine. In the heater support member are embedded at least three lead wires for power supply, one ends of which are each independently connected to the heater member and the other ends of which are connected to a power source through a power switching means. The power switching means has a contact for connecting the power source selectively between the lead wires according to the state of preheating in an engine combustion chamber. Through this switching operation of the power switching means there are formed plural heater elements having different resistance values in the heater member.

FIGS. 1 through 4 illustrate a first embodiment of the present invention;

FIG. 1 is a longitudinal sectional view of a projecting end portion of a glow plug projecting into a combustion chamber of an internal combustion engine;

FIG. 2 is a perspective view showing a lamination structure of ceramic green sheets which are integrally laminated and sintered into a heater support member and a heater member;

FIG. 3 is a circuit diagram of a power switching circuit;

FIG. 4 is a view showing changes with time of the temperature of the glow plug.

FIGS. 5 and 6 illustrate a second embodiment of the present invention;

FIG. 5 is a sectional view showing a structure and an electrical connection of a heater support member and a heater member; and

FIG. 6 is an electric equivalent circuit showing states of power supply to the heater member.

The present invention will be described hereinunder on the basis of its illustrated embodiments.

Referring first to FIG. 1, there is shown a tip end portion of a glow plug G projecting into a combustion chamber C through a combustion chamber wall C1 of an internal combustion engine. In a tip-end opening of a cylindrical metallic case 1 of the glow plug G is disposed a rod-like heater support member 2 formed of an insulating ceramic material, the heater support member 2 projecting from the tip-end opening of the case 1. To the surface of the tip end portion of the heater support member 2 if affixed a heater member 3 formed of an electrically conductive ceramic material. The heater member covers in U-shaped form the tip end face of the heater support member as well as the upper and lower faces of the heater support member contiguous to the end face. Fitted over the outer periphery of the support member 2 is a metallic sleeve 4, which is fixed to the opening portion of the case 1 by soldering. Metallic caps 5a and 5b are attached to the base end of the support member 2. Within the support member 2 are embedded first to third lead wires 6a, 6b and 6c for the supply of electricity. The first lead wire 6a is connected at one end thereof to the heater member 3 and at the other end thereof to the sleeve 4. The second and third lead wires 6b and 6c are connected at one end to the heater member 3 and at the other ends thereof to the caps 5b and 5a, respectively. One end of the third lead wire 6c is connected to the heater member substantially intermediately between the connection points of the other lead wires 6a and 6b. Therefore, the resistance value of a heater element 31 formed between the first and third lead wires 6a and 6c and that of a heater element 32 formed between the second and third lead wires 6b and 6c are almost equal.

To the caps 5a and 5b are connected one end of lead wires 6c' and 6b', respectively, the other ends of the lead wires 6c' and 6b' extending up to outlet terminals (not shown) of the glow plug G. On the outer periphery of the case 1 is formed a screw portion 1a, whereby the body of the glow plug G is fixed to the combustion chamber wall C1.

As the insulating ceramic material which forms the support member 2, there is used a sintered product of a mixture of alumina (Al.sub.2 O.sub.3) and silicon nitride (Si.sub.3 N.sub.4), and as the electrically conductive ceramic material which forms the heater member 3, there is used a sintered product of a mixture of molybdenum disilicate (MoSi.sub.2) and Si.sub.3 N.sub.4. MoSi.sub.2 is extremely superior in the resistance to oxidation and to heat, and Si.sub.3 N.sub.4 is used for imparting resistance to thermal shock and also for adjusting the resistance value. As the first to third lead wires 6a-6c, nickell wires are used.

An example of a procedure for forming an integral structure of the support member 2 and the heater member 3 will now be described with reference to FIG. 2. Predetermined amounts of Al.sub.2 O.sub.3 powder and Si.sub.3 N.sub.4 powder are mixed, to which is added a suitable organic binder, and the mixture is formed into ceramic green sheets according to the doctor blade method. The ceramic green sheets thus obtained are laminated into two sets of ceramic sheets 2'a and another two sets of ceramic sheets 2'b, each set comprising several ceramic green sheets. Next, predetermined amounts of MoSi.sub.2 powder and Si.sub.3 N.sub.4 powder are mixed, and then in the same way as above there are obtained two sets of ceramic sheets 3'a and another two sets of ceramic sheets 3'b. Then, the ceramic sheets 2'a and 3'a are affixed together, and the ceramic sheets 2'b and 3'b also joined. The ceramic sheets thus combined are then laminated so that the lead wires 6a, 6b and 6c are disposed in a sandwiched manner. This laminating operation is performed while applying pressure in the arrowed directions in FIG. 2. The laminate thus obtained is then hot-pressed and sintered at a temperature of about 1,600.degree. C. in an inert atmosphere to obtain an integral ceramic body having the support member 2 and the heater member 3. The surface of the ceramic body is then polished and metallized, and thereafter the sleeve 4 and the caps 5a and 5b are attached thereto.

In FIG. 3, there is shown a circuit for a power supply to the glow plug, in which the numeral 7 denotes a battery, numeral 8 denotes a resistor of a small resistance value (about 10 m.OMEGA.) and numeral 9 denotes a power switching circuit.

A timer circuit 91 which is a constituent of the power switching circuit 9 is connected to a starter switch 10. Its output continues to be "1" state for 10 minutes after operation of the starter switch 10.

A vehicle speed detecting circuit 92, which is connected to a vehicle speed sensor 11, produces a "1" state output at a vehicle speed not higher than 20 km/hr.

An engine rotation detecting circuit 93, which is connected to a neutral point of a stator coil (not shown) incorporated in an alternator 12, produces a "1" state output during rotation of the engine.

A water temperature detecting circut 94, which is connected to a water temperature sensor 13, produces a "1" state output at a water temperature not higher than 60.degree. C.

A glow temperature detecting circuit 95 is supplied with the voltage across the resistor 8 which is disposed between the battery 7 and the glow plug G. As the resistance value of the heater elements 31 and 32 of the glow plug G varies with increase in temperature, the said input voltage to the glow temperature detecting circuit 95 also varies, and the circuit 95 detects this change in voltage and produces an output having such a hysteresis as exhibits a "1" state at a glow plug G temperature not lower than 1,000.degree. C. and a "1" level at not higher than 750.degree. C.

The power supplying lead wire 6b of the glow plug G is connected to the resistor 8 through a contact 97a of an output relay 97 of the power switching circuit 9, and the power supplying lead wire 6c is connected to the resistor 8 through a normally open contact 96a of an output relay 96. The power supplying lead wire 6a is grounded through the case 1 (see FIG. 1) of the glow plug G. The contact 97a conducts between C1 and C3 when the relay 97 is energized.

The power switching circuit 9 having the above-described configuration operates in the following manner.

When the power source is turned on to start preheating, the output of the engine rotation detecting circuit 93 and that of the glow temperature detecting circuit 95 are both at "0" state, so that the output of a NOR gate 98 becomes "1" state and the relay 96 is energized to close its contact 96a. At this time, the relay 97 is in a non-energized state, with its contact 97a conducting between C1 and C2, and the second lead wire 6b of the glow plug G is grounded. Consequently, the battery 7 is connected between the third lead wire 6c and the other two lead wires 6a, 6b, that is, the heater elements 31 and 32 are connected in parallel with the battery 7. In this state, if the resistance of each of the heater elements 31 and 32 is R and the voltage of the battery 7 is E, the electric power applied to the heater elements 31 and 32 is approximately 2E.sup.2 /R, so that the temperature of the glow plug G rises rapidly from point S to point T in the graph of FIG. 4.

When the temperature of the glow plug G reaches 1,000.degree. C. (point T in FIG. 4), the temperature detecting circuit 95 produces "1" state output signal, then AND gate 99a and NOR gate 98 produce "1" state output signals, whereby the relay 97 is energized through OR gate 100 in place of the relay 96 and its contact 97a conducts between C1 and C3, while the contact 96a opens. In this state, the battery 7 is connected to the lead wires 6a and 6b, that is, the heater elements 31 and 32 are connected in series with the battery 7. Consequently, the electric power applied to the heater elements 31 and 32 is reduced to E.sup.2 /2R which corresponds to one fourth of the electric power applied in the foregoing parallel connection, so that the temperature of the glow plug G drops rapidly (from point T to point U in FIG. 4).

When the temperature reaches 750.degree. C. (point U in the figure), the output of the temperature detecting circuit 95 again becomes "0" state and the relays 97 and 96 assume non-energized and energized states, respectively, whereby the heater elements 31 and 32 of the glow plug G are again connected in parallel with the battery 7 and the temperature of the glow plug G begins to rise. In this way, the temperature of the glow plug G is held at a level between 750.degree. C. and 1,000.degree. C. during preheating.

When the starter switch 10 is turned on and the engine starts rotating, the timer circuit 91 and the engine rotation detecting circuit 93 produce "1" state output signals, whereupon the output of the NOR gate 98 and that of the AND gate 99b become "0" and "1" states, respectively, so that the relays 96 and 97 assume non-energized and energized states, respectively, and during the subsequent after-glow, the heater elements 31 and 32 of the glow plug G are connected in series with the battery 7 regardless of the temperature of the glow plug G. As a result, the temperature of the glow plug G begins to drop (point W in FIG. 4) and becomes an after-glow level (about 650.degree. C. in the illustrated embodiment) which is lower than the foregoing preheating temperature.

Then, in 10 minutes after operation of the starter switch 10, or when the vehicle begins to run, or when the cooling water temperature rises above 60.degree. C., the relay 97 becomes non-energized since the OR gate 100 inputs are "0" states, so that the power supply to the glow plug G is stopped (point X in FIG. 4).

Thus, in the glow plug of the present invention, during preheating, the heater elements formed between lead wires are connected in parallel with the power source to lower the resistance value of the entire heater member to thereby increase its quantity of generated heat for effecting a rapid heating, while during after-glow, those heater elements are connected in series with the power source to increase the resistance value of the entire heater member to thereby decrease its quantity of generated heat for maintaining the temperature at a lower level than that during preheating. Therefore, unlike conventional glow plugs, it is not necessary to provide a resistor of large capacity between the power source and the glow plug during after-glow, and hence there is no wasteful power consumption due to such a resistor.

Moreover, since the heater member is formed on the surface of the heater support member, an extremely rapid heating characteristic is attained. Further, since the heater member is formed of a ceramic material superior in oxidation- and heat-resistance, a prolonged service life is assured.

Referring now to FIG. 5, there is illustrated a second embodiment of the present invention, in which four, first to fourth, lead wires 6a, 6b, 6c and 6d for power supply are embedded within a support member 2, one end of the lead wires 6a-6d being connected to a heater member 3 and the other ends thereof connected to sleeves 4a, 4b, 4c and cap 5, respectively. A switching relay contact 96a has one end connected to ground and two other ends, to which are connected the sleeves 4a and 4b respectively through lead wires 6a' and 6b', while a switching relay contact 97a has one end connected to a battery 7 and two other ends, to which are connected the sleeve 4c and the cap 5 respectively through lead wires 6c' and 6b'. The relay contacts 96a and 97a conduct between C1 and C2 or between C1 and C3 according to whether they are energized or non-energized. One end of the first lead wire 6a and second lead wire 6b are connected embeddedly to one end portion of the U-shaped heater member 3. One end of the third lead wire 6c and fourth lead wire 6d are connected embeddedly to the other end portion of the heater member 3. The second lead wire 6b is embedded more deeply than the first lead wire 6a, and the third lead wire 6c is embedded more deeply than the fourth lead wire 6d.

By operating the relay contacts 96a and 97a in an appropriate manner, the connection between the power supplying lead wires 6a-6d and the battery 7 can be changed as shown in FIGS. 6(1), (2) and (3); that is, by the relay contacts 96a and 97a the battery 7 is selectively connected between the first lead wire 6a and the fourth lead wire 6d (FIG. 6(1)), between the first lead wire 6a and the third lead wire 6c (FIG. 6(2)), and between the second lead wire 6b and the third lead wire 6c (FIG. 6(3)). The resistance values of heater elements 31, 32 and 33 formed between the lead wires 6a-6d are set at 3R, 2R and R respectively in the order of (1), (2) and (3) in FIG. 6, whereby successively larger electric powers like ##EQU1## are generated in the heater elements 31-33. In this embodiment, even if the battery 7 is connected between the second lead wire 6b and the fourth lead wire 6d, the same resistance value as that of the heater element 32 is obtained. Since the quantity of generated heat in the heater member 3 can be changed in three stages, not only the same effect as in the first embodiment can be attained, but also the temperature of the combustion chamber can be adjusted more minutely.

In the glow plug of the present invention, as set forth hereinabove, since a heater member formed of an electrically conductive ceramic material is formed on the surface of a support member formed of an insulative ceramic material, the rapid heating characteristic of the glow plug can be remarkably improved. In the glow plug of the present invention, moreover, by supplying electricity to specific lead wires among at least three lead wires for power supply according to the state of preheating and that of after-glow, the resistance value of the heater elements formed therebetween is made variable to control the quantity of generated heat, and therefore it is not necessary to use a resistor of large capacity for lowering the voltage applied to the glow plug during after-glow, that is, wasteful power consumption can be prevented, and the temperature of the engine combustion chamber can be adjusted minutely in stages.