Abstract:
A spark plug for internal combustion engines has a labyrinth groove formed in the vicinity of the open end of a space defined by the outer surface of an insulator leg, the inner surface of a metal plug body and a packing so as to restrict entry of fresh air-fuel mixture therein.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to spark plugs for internal combustion engines, and more particularly the invention relates to an improved spark plug which is highly resistant to carbon fouling. 
     It has been the tendency of the previously known spark plugs to cause incomplete combustion due to the flame propagation, thus causing a carbon deposition on the surface of the insulator leg portion and thereby causing insulation failure and misfiring. This has also been confirmed from the fact that carbon fouling increases as the spark timing is advanced even if the other conditions are maintained the same. 
     SUMMARY OF THE INVENTION 
     With a view to overcoming the foregoing deficiencies, it is a main object of the present invention to provide an improved spark plug wherein a metal plug body or shell is provided with a restrictor section disposed to reduce the opening area of the open end of a space in the plug, and the restrictor section is provided with a depressed labyrinth groove formed in the portion opposing the leg portion of an insulator, whereby during the compression stroke of the engine, the velocity of a fresh mixture flowing into the space is decreased by the restrictor section with the labyrinth groove, thus retarding the entry of the fresh mixture into the space to thereby create an atmosphere in the space which tends to prevent incomplete combustion during the initial ignition and also preventing the propagation of the initial flame into the space by virtue of the decreased flow velocity of the fresh mixture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1(a) is a sectional view showing the principal parts of an embodiment of a spark plug according to the invention. 
     FIG. 1(b) is a bottom view of FIG. 1. 
     FIG. 2(a) is an enlarged sectional view showing a part of FIG. 1. 
     FIG. 3(a) is a sectional view showing the principal parts of a second embodiment of the spark plug according to the invention. 
     FIG. 3(b) is a bottom view of FIG. 3(a). 
     FIGS. 4 to 6 are sectional views showing the principal parts of other embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described. 
     Referring to FIGS. 1(a) and 1(b) and FIG. 2 showing a first embodiment of the invention, an insulator 2 is fixedly mounted in place inside a metal plug body or shell 1 through the intermediary of a ring-shaped metal packing 5. A center electrode 3 is fixedly mounted in a leg portion 2a of the insulator 2 which is exposed into the combustion chamber (not shown) of the engine. Positioned opposite to the tip of the center electrode 3 is the end side of a ground electrode 4 fixed to the end of the plug body 1 and a spark gap 7 is provided between the electrodes. A space portion 6 defined by the leg portion 2a of the insulator 2, the plug body 1 and the packing 5 is opened to the combustion chamber in a direction of the spark gap 7. Formed integral with the end of the plug body 1 is an annular restrictor section 8 which reduces the opening area of an open end 6a of the space portion 6. A depressed labyrinth groove 8a is formed in the portion of the annular restrictor section 8 which is opposite to the leg portion 2a of the insulator 2, and first and second ridge portions 9a and 9b are formed on both sides of the labyrinth groove 8a. Numerals 10a and 10b designate first and second restriction gaps which are respectively defined between the first and second ridge portions 9a and 9b and the leg portion 2a of the insulator 2. Numerals 11a and 11b designate first and second creeping gaps extended along the surface of the leg portion 2a of the insulator 2 respectively to the first and second restriction gaps 10a and 10b and connected in series therewith. Assuming that the distance of the spark gap 7 defined between the center electrode 3 and the ground electrode 4 is G7 mm, and the distances of the first restriction gap 10a, the second restriction gap 11b, the first creeping gap 11a and the second creeping gap 10b are respectively G10a mm, G10b mm, G11a mm and G11b mm, the proper values are ones which satisfy the relations of the following formulas: 
     
         1/2 G11a+G10a≧G7≧G10a                        1. 
    
     
         1/2 G11b+G10b≧G7≧G10b                        2. 
    
     With the construction described above, the operation of the spark plug will now be described. The spark plug must cause sparks to jump across the spark gap 7 to ignite the mixture and thus it is undesirable to allow the spark to travel from the center electrode 3 along the first and second creeping gaps 11a and 11b and jump to the first and second ridge portions 9a and 9b of the restrictor section 8 through the first and second restriction gaps 10a and 10b. To prevent this, among (a) the spark gap 7, (b) the sum of the first creeping gap 11a and the first restriction gap 10a and (c) the sum of the second creeping gap 11b and the second restriction gap 10b, the dielectric breakdown voltage of the spark gap 7 in (a) must be minimized. Generally, in order that the same dielectric breakdown voltage may be ensured, the distance of the first and second creeping gaps 11a and 11b, respectively, must be about two times that of the spark gap 7, and consequently the relations 1/2 G11a+G10a≧G7 and 1/2 G11b+G10b≧G7 must be satisfied to ensure that sparks jump across the spark gap 7. On the other hand, to prevent the propagation of the initial flame into the space portion 6, it is desirable that the first and second restriction gaps 10a and 10b satisfy the relations G7≧G10a and G7≧G10b. By satisfying these relations, the combustion heat of the initial flame can be removed by virtue of the heat dissipation by the restrictor section 8 through the plug body 1 to thereby prevent the propagation of the flame into the space portion 6. 
     The following two conditions exist within the space portion 6, that is, (a) the condition which allows mixing with a fresh mixture of a relatively large amount of the burned gas produced by the previous cycle and (b) the combustion head tends to be dissipated by the plug body surrounding the space portion 6, and thus the propogation of a relatively low temperature initial flame into the space portion 6 will cause an incomplete combustion and hence a carbon fouling. If the deposition of carbon on the leg portion 2a extends to the packing 5, the current flow from the center electrode 3 will leak to the plug body 1 through the carbon deposits and the packing 5, and thus no spark will jump across the spark gap 7, thus causing misfiring. With this embodiment, however, by virture of the fact that the restrictor section 8 including the depressed labyrinth groove 8a is provided at the end of the plug body 1, the first restriction gap 10a, a gap 1a between the bottom portion of the labyrinth groove 8a and the leg portion 2a of the insulator 2 and the second restriction gap 10b exist at the open end 6a of the space portion 6, with the result that when a fresh mixture is introduced into the space portion 6 during the compression stroke of the engine, the mixture flows through the first restriction gap 10a, the gap 1a which is greater in size than the first restriction gap 10a and the second restriction gap 10b which is smaller in size than the first restriction gap 10a in such a manner that the mixture flows through the smallest second restriction gap 10b without causing any turbulence in the gap 1a, and the velocity of the fresh mixture flowing into the space portion 6 through the open end 6a is decreased. As a result, during the initial period of the ignition, there is practically no possibility of the initial flame propagating and develop in the direction of the space portion 6. Also, by virtue of the fact that the velocity of the fresh mixture flowing into the space portion 6 is decreased in this way, the rate of the fresh mixture flowing into the space portion 6 is decreased and an atmospher is produced within the space portion 6 which tends to prevent the occurrence of misfiring during the initial ignition. This positively prevents the propagation of the initial flame. Further, the prevention of the fresh mixture flow into the space portion 6 has the effect of reducing the pressure loss during the burning of the mixture in the combustion chamber and increasing the engine power output. 
     While, in the embodiment described above, the number of the labyrinth groove 8 in the restrictor section 8 is one, a plurality of such labyrinth grooves may be provided. The latter arrangement is more effective. 
     Further, the known multi-gap spark plugs have been used in engines of the type employing lean mixtures, engines with sub-chambers, rotary piston engines and the like which have poor ignition properties and tendency to cause carbon fouling, and the incorporation of the present invention in such multi-gap spark gaps will be extremely advantageous. 
     Moreover, while, in the embodiment described above, the restrictor section 8 is provided on the inner side of the end of the plug body 1, its location is not limited to the plug body end and the restrictor section 8 may be provided at a position located inwardly of the end of the plug body 1. In this case, the open end 6a of the space portion 6 means the open end portion near the restrictor section 8. 
     Still further, the restrictor section 8 need not always be annular in shape, and it may be formed with a plurality of kerfs. 
     The second embodiment of the invention shown in FIGS. 3(a) and 3(b) will now be described. In the Figures, an insulator 2 is fixedly positioned inside a metal plug body 1 through the intermediary of a metal packing 5. A center electrode 3 is fixedly mounted in a leg portion 2a of the insulator 2 which is exposed to the inside of an engine combustion chamber, and a spark gap 7 is provided between the center electrode 3 and the forward end of a ground electrode 4 fixed to the end of the plug body 1. A space portion 6 defined by the plug body 1, the leg portion 2a of the insulator 2 and the packing 5 is opened in the direction of the spark gap 7 through an open end 6a. The leg portion 2a of the insulator 2 is provided, in the vicinity of the open end 6a of the space portion 6, with a pair of annular projections 8 formed integral therewith in a tandem relation and arranged at a suitable distance from the forward end of the leg portion 2a (i.e., that end from which the tip of the center electrode 3 is projected). An unprocessed straight portion 9 is provided between the annular projections 8 and between the lower annular projection 8 and the forward end of the leg portion 2a of the insulator 2. By thus providing the annular projections 8, restrictions 10 are discontinuously provided in the open end 6a of the space portion 6. 
     With the construction described above, the operation of the second embodiment is as follows. When a fresh mixture is introduced into the space portion 6 through the open end 6a during the compression stroke of the engine, by virtue of the fact that the two annular projections 8 are formed on the leg portion 2a of the insulator 2 thus discontinuously providing the restrictions 10 in the open end 6a of the space portion 6, the velocity of the fresh mixture flowing into the space portion 6 is decreased considerably by the restrictions 10 and consequently the flame during the initial ignition period is retard from propagating into the space portion 6 along with the fresh mixture flowing thereinto. Further, by virtue of the fact that the fresh mixture tends to stagnate at the straight portions 9 due to the restrictions 10, if in this condition the initial ignition takes place, the temperature rises satisfactorily at the straight portions 9 so that even if carbon is deposited on the leg portion 2a of the insulator 2 due to the incomplete combustion within the space portion 6, the carbon deposits will be readily burned off by virtue of the temperature of the straight portions 9 and no leakage will be caused. Moreover, since the two restrictions 10 are provided by the annular projections 8, the creeping distance on the forward end side of the leg portion 2a of the insulator 2 is increased, with the result that the danger of erratic ignition from the center electrode 3 to the inner side of the plug body 1 is eliminated and this also tends to prevent the propagation of the initial flame into the space portion 6. 
     FIG. 4 illustrates a third embodiment of the invention in which the corners of the two annular projections 8 are tapered. 
     FIG. 5 illustrates a fourth embodiment of the invention in which the open end side of a hole 2ab formed in the leg portion 2a of the insulator 2 is expanded so as to provide an electrode gap 11 between the resulting expanded hole 2ac and the tip of the center electrode 3. In this embodiment, as compared with the previously mentioned embodiments, the creeping distance is increased by the provision of the electrode gap 11 thus preventing the occurrence of erratic ignition and moreover the heat receiving area of the leg portion 2a at its forward end is increased thus improving the carbon burn-off effect. 
     FIG. 6 illustrates a fifth embodiment of the invention in which the diameter of the center electrode 3 is decreased at its forward end as compared with the other component parts and thus an electrode gap 11 is provided between the hole 2ab of the leg portion 2a and the tip of the center electrode 3, and the functions and effects of this embodiment are the same as the embodiment of FIG. 5. 
     While, with the embodiments described above, when the combustion stroke proceeds from the initial combustion stage so that the burning of the mixture within the combustion chamber becomes more vigorous and the pressure increases thus allowing the flame to propagate into the space portion 6, in this case the flame temperature is so high that there is no danger of carbon fouling. 
     Further, the known multi-gap spark plugs are currently in use with the engines of the type employing lean mixtures, engines with sub-chambers, rotary piston engines and the like which have poor ignition properties and tendency to cause carbon fouling, in preference to the conventional spark plugs of the ordinary type, and consequently the present invention may be exceedingly advantageously incorporated in such multi-gap spark plugs. 
     Still further, the present invention is not intended to be limited to the spark plugs of the type in which the forward end of the leg portion 2a of the insulator 2 is projected beyond the end of the plug body 1 as in the case of the previously mentioned embodiments, and the invention may be incorporated in spark plugs of the type in which the forward end of the leg portion 2a is positioned inwardly of the end of the plug body 1. 
     Still further, the member of the annular projections 8 may be three or four, and at least two such annular projections are required to ensure the advantages of the present invention.