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spark plugs do not differ essentially in design. Four different forms of plugs using porcelain insulation are shown in part section at Fig. 51. Porcelain is the material most widely used because it can be glazed so that it will not absorb oil, and it is subjected to such high temperature in baking that it is not liable to crack when heated.

The spark plugs may be screwed into any convenient part of the combustion chamber, the general practice being to install them in the caps over the inlet valves, or in the side. of the combustion chamber, so the points will be directly in the path of the entering fresh gases from the carburetor. The methods of spark plug installation commonly used are shown at Fig. 52. At A the plug is screwed into a threaded hole which passes through the valve cap in such a manner that the points are in a pocket. This is not considered to be as good as the method depicted at B, where the interior of the valve cap is recessed out so there is considerable clear space around the spark points. When the electrodes are carried in a pocket they are more liable to become short circuited by oil or carbon accumulations, because it is difficult for the fresh gases to reach them and the pocket tends to retain heat. Ignition is not so certain because some of the burned gases may be retained in the pocket and prevent the fresh gas from getting in around the spark gap. With a recess, as shown at B, conditions are more favorable because the fresh gases can sweep the points of the spark plug and keep them clear, and also because of the larger space any burned products retained in the cylinder are not so apt to collect around the plug point. The method of installation shown at C causes the plug to heat and is not as efficient as that outlined at D, which permits ready transference of heat to the cooling water in the jacket spaces.

On some types of engines which are not provided with compression relief, or priming cocks, plugs are sometimes installed, as shown at Fig. 51, C. A special fitting, which carries a priming cup at one side, is screwed into the cylinder and the spark plug is fitted to its upper portion. When it is desired to relieve the compression, the valve portion is turned in such a way that a passage is provided from the interior of the fitting to the outer air.

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At the same time when the valve is in the position shown in illustration, gasoline may be introduced into the cylinder for priming purposes. It is advanced that this method of construction also provides a simple means of freeing the plug points from oil or particles of carbon if the cock is opened while the engine is running. The high pressure gas which brushes by the points on its way out of the cylinder tends to dislodge any particle of foreign matter which may be present near the spark gap. The same objections apply to this method of mounting as to that illustrated at A.

Some spark plugs have been designed with a view of permitting one to see if the charge is being exploded regularly in the cylinder by some form of transparent material for insulation, so that the light produced by the explosion could be seen from the outside of the cylinder. The simplest method of determining if a spark is occurring regularly between the points is to use some form of spark gap which is interposed between the source of current and the plug terminal. A device of this nature is shown at Fig. 51, G. It consists of a body of insulating material which carries in a glass tube two points, which are separated by a slight air space. The eye or hook end is attached to the plug terminal, while the other end is attached to the secondary wire. If the current is passing between the points of the plug, a spark will take place between the points of the auxiliary spark gap every time one occurs between the points of the plug in the cylinder.

It is claimed that there are certain advantages obtained when a spark gap is used in the circuit, in that the spark in the cylinder is more effective and less liable to be short circuited by particles of foreign matter. At the other hand, others contend that the current must be stronger to jump two gaps than would be required if only the resistance of one was to be overcome. While very popular at one time, the spark gap is of rather doubtful utility and is seldom used at the present time, except as a means of indicating if spark has taken place between the points of the spark plug. It is apt to be somewhat misleading, however, because even if the points of the plug are short circuited and no spark is taking place between the plug points, and yet current is passing to the ground, a spark will continue to take place at the auxiliary spark gap. The device is useful in showing when there is a break or derangement of the wiring or coils.

A form of spark plug having glass bull's-eyes set into the plug shell or body is shown at Fig. 51, H. These simple lenses are made of specially compounded glass, which has a high resistance to heat and every time an explosion takes place in the cylinder the light resulting causes a flash which is readily seen through the lens. If the flashing is regular it is safe to assume that the cylinder is functioning properly, but should the flashes be intermittent or separated by unequal intervals of time the cylinder is missing explosions.

It is often desirable to have a water-tight joint between the high-tension cable and the terminal screw on top of the insulating bushing of the spark plug, especially in marine applications. The plug shown at Fig. 51, F, is provided with an insulating member or hood of porcelain, which is secured by a clip in such a manner that it makes a water-tight connection. Should the porcelain of a conventional form of plug become covered with water or dirty oil, the high-tension current is apt to run down this conducting material on the porcelain and reach the ground without having to complete its circuit by jumping the air gap and producing a spark. It will be evident that wherever a plug is exposed to the elements, which is often the case in motor-cycle or motor-boat service, that it should be protected by an insulating hood which will keep the insulator dry and prevent short circuiting of the spark.

Spark plugs are made in infinite variety, more simple forms being shown at Fig. 50. Those in section at A, B and C utilize a porcelain insulator through which a central rod or electrode passes. This terminates at the top in a threaded member, to which the thumb nut is screwed. In most plugs using porcelain insulators a cap is cemented to the top of the porcelain in order to form a seating for the thumb nuts. The form outlined at A is the type of plug most generally used, as it is a simple and effective design. It is easier to clean the points or the interior of the body than in the form shown at B, which has a closed end and which must be dismembered in order to remove the sooty deposit from the insulator surface. The type of plug at C has a very fine wire imbedded in the lower portion of the porcelain, which is in connection with a conductor of heavier material used to transmit the current from the terminal nuts to the fine wire. The theory of action of a plug of this nature is that the fine wire is not so apt to be short circuited by soot as the projecting electrode forms are, and that the spark tends to clear away material that might short circuit the current by burning it.

The plugs shown at D and E have mica insulators instead of porcelain. When mica is used a sheet of that material is wrapped around the central electrode several times, after which a series

A

-B

Fig. 53.-Conventional Type of Spark Plug at A, Showing Air Gap Be

tween the Points. B—Priming Plugs. C—Two-Point Spark Plug.

of mica washers is clamped tightly together and turned down to form a smooth insulator. The plug at F is the only one marketed using glass insulation. Other plug forms made on the same general principles as that at A use lava or steatite as an insulator instead of the porcelain or mica. For all-around service the porcelain insulator gives the best results, as the mica and lava insulators are apt to become oil soaked and permit the current to short circuit through the insulator and the plug body instead

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