« НазадПродовжити »
oil up to the distributor. As a constant pressure is maintained in the tank only a very slight additional pressure is required to force oil to the sight feeds and hence
- to the various points requiring lubrication. Aside from adjusting the needle valves on the distributor and keeping the tank full of oil, this device requires but little
Fig. 75.—Grease Cups
attention and is practically automatic; operating when the motor runs and ceasing as soon as the engine stops.
IGNITION.-PRINCIPLES OF ELECTRICAL IGNITION.—DYNAMOS AND
MAGNETOS.—HIGH AND Low-TENSION MAGNETOS.—THE
EVERY internal combustion engine must be provided with some method of igniting the charge of compressed gas in the cylinder, and while in the past this was accomplished in several ways, at present practically all gasolene engines use an electric spark for the purpose. Electric ignition in general use may be divided into two classes, Jump-spark and Make-and-break-spark ignition systems, but before discussing these two methods in detail a short explanation of the general principles of electricity as used for ignition purposes may be of interest. Electrical currents are said to "flow" through certain conductors, such as metallic wires, but the exact actions of these currents are not known. It is generally conceded, however, that the current does not pass through the body of the metal as much as it follows the surface. A clearer idea of electrical action may be obtained by comparing the current with the flow of liquid through a pipe. A liquid in a pipe is said to be under a certain pressure which causes it to move, the pressure being due to a difference in level of the source and outlet, or caused by some mechanical means as a pump. In the same way an electrical current has pressure or voltage caused
by a difference in what is known as a potential (see Glossary) between the source and the outlet; thus we have:
Volts=the unit of pressure dependent upon the difference in Potential, equivalent to lbs. per sq. inch = unit of pressure dependent upon difference in level.
The amount of water or liquid passing a given point during a certain time is known as the rate of flow in gallons per minute, etc. So a current of electricity has a similar rate of flow, which is measured by a unit known as an Ampere, which represents quantity just as in a water pipe the quantity is reckoned in gallons. The quantity of water is dependent upon the pressure used to force the water through the pipe and upon the resistance or friction of the pipe. In the same way the number of amperes in an electrical current depends upon the pressure or voltage and upon the resistance to the passage of the current through the wires, or other conductors. Thus we have:
Ampere = unit of rate of flow, dependent upon voltage and resistance; equivalent to gallons per minute=unit of rate of flow, dependent upon pressure per sq. inch and frictional
resistance. The resistance to the flow of an electrical current is measured by units called Ohms, and is dependent upon the diameter, material, length, and temperature of the wires, exactly as the flow of water is resisted by friction dependent upon the size, shape, and length of a pipe. Therefore we may compare:
Ohm = unit of resistance dependent upon diameter, material, and length of wire; with coefficient of friction=unit of frictional resistance dependent upon diameter, shape, and length of pipe.
The two general sources of electricity for ignition are wet or dry cells and magnetos or dynamos. A storage battery does not generate electricity but merely stores that generated by a dynamo or other apparatus. By passing the electrical current through a contrivance known as a “spark coil” a certain change takes place in the electrical current and the voltage is increased. If the coil consists of a soft iron core with numerous coils of wire around it it is known as a Primary Coil, and is the kind used in the make-and-break system of ignition. If, however, the coil is composed of two different kinds of wire with the outer coil finer than the under and not connected with it, it is known as a Secondary Coil, such as is used in the jump-spark system.
Dynamos and magnetos are mechanical devices used to produce electricity, and a simple form of one of these is represented diagrammatically in Fig. 77. The pieces A and B are electro-magnets, or, in other words, are pieces of iron magnetized by means of a small current known as a shunt S, made to pass around them as shown. This shunt current is really a small part of the current produced by the dynamo itself. The ends of these two pieces of iron are known as poles and between them is a space known as the magnetic field. In this area an axis or armature, C, is rotated, upon which wires are wound so that the windings are continually passing through the lines of force between the poles and thus currents of electricity are generated in the wires. As the rotation of the armature causes currents in the wires which flow in opposite directions, these currents are gathered so they will flow in one direction through the outside wires