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Q. How is spark produced in cylinder?
A. There are two methods of causing an electric spark to occur in the combustion chamber. That outlined at B in Fig. 76-A involves a separation of two contact members, through which a current of electricity is flowing. The spark is produced when the points separate. That outlined at C is known as the high tension system and a spark overcomes the air space between the points of the spark plug because it has sufficient power or pressure to overcome the resistance at that point.
Fig. 76.-Section Through Vaporizer and Valve Chamber of Station.
ary Engine Utilizing Hot Tube Ignition.
Q. What are the important advantages of the electrical ignition systems?
A. Electrical ignition systems, as a rule, are easily controlled, ignite the charge unfailingly and exactly when desired, and are the most reliable of the ignition methods that permit flexible engine operation.
Q. Why is variable ignition desirable ?
automobile motors, it is necessary to fire the charge earlier when the engine is running fast than when it is running slow.
Q. Why should spark be retarded when starting an engine?
A. When an engine is started by hand it is desirable to have the ignition late or retarded in order that the gas will not explode until the piston has started down on its power stroke. If the explosion occurred before the piston started down it might be driven around in the opposite direction to that it should go and the operator would be injured by the reversal of motion of the crank or flywheel.
Fig. 76A.-Diagrams Outlining Possible Methods of Electrical Igni
tion. A-By Incandescent Platinum Wire. B-By Make and Break or Low Tension Spark. C—By High Tension or Jump Spark.
Q. Why is it necessary to advance spark?
A. When the engine speed is increased faster than a normal number of revolutions, a greater number of explosions occur in a given space of time than when the engine is running slowly. The gas requires a certain amount of time to become ignited, and when an engine is running faster and more gas charges must be exploded, it is necessary to start the ignition earlier, which means that the spark time in relation to the crankshaft or piston travel must be advanced.
Q. Why is throttle control alone inadequate?
A. Controlling the engine speed by varying the amount of gas does not permit exploding the gas charges fast enough in order to speed up the engine.
Q. What would be the effect when using throttle control without variable ignition at various engine speed?
A. When a set spark is used it is customary to set the time of ignition so that a condition best suited for normal running conditions is obtained. This means that the spark is set too early to insure absolute safety when starting the engine and yet it is too late to permit the engine attaining its highest possible number of revolutions. If the spark is set a little late and speed is desired it will be noticed that the engine heats up with full throttle openings and yet it is liable to fire back due to an advanced position when attempt is made to start the motor.
Q. Is set spark used to any extent?
A. Very few automobiles are controlled by varying the supply of gas solely, though in a few cases where magneto ignition is employed the spark time is fixed and engine speed is regulated solely by controlling the admission of explosive vapor to the cylinders. This is possible because a magneto will generate more current and produce a spark of greater intensity as its speed of rotation is increased. This means that a hotter spark is present in the cylinder at high speed and in a measure this provides for positive ignition and corresponds to some extent to advancing the time of ignition. Practically all automobiles in use at the present time are provided with variable ignition systems, and do not depend on throttle control alone.
PARTS OF THE IGNITION SYSTEM AND THEIR
Q. Name the principal parts of an electrical ignition system.
A. The principal parts of typical ignition systems of the high tension type are outlined at Figs. 77 and 78-A. The prime requisite is a source of electrical current, a device by which it will produce a spark in the combustion chamber, auxiliary devices for intensifying it or increasing its value, a suitable timing appliance to insure that the spark will take place in the cylinder only at the proper time and wires or conductors to permit the electrical energy to flow from one unit of the ignition system to the others. The important parts of a high tension system are: The battery or other current producer; the timer, which regulates the time of ignition; the induction coil, which is a transformer to step up or increase the current pressure of the batteries; and the spark plug which is screwed into the combustion chamber, and between the points of which the igniting spark takes place.
Q. What are the requirements of the current producer?
A. The current producer must supply the electrical energy required to produce the spark, and must be reliable in action, as well as relatively simple in construction.
Q. How is the current of electricity transmitted?
A. The electrical energy is transmitted over a number of metal conductors or wires insulated from each other and the metal parts of the engine to form circuits.
Q. What is a circuit?
A. A circuit is a path for the electric current to flow over and may be composed entirely of wire or of wire which joins various
elements or devices interposed so that the current must pass through these before it will complete its circuit. Q. What is the difference between a closed and open circuit?
A. A closed circuit is one that provides an unbroken or continuous path for the transmission of electrical energy while an open circuit is one in which there is a gap or resistance sufficiently great to prevent the electrical energy from passing between the various members comprising the circuit. Electricity can flow only over a closed circuit, except in cases where the pressure of the current is great enough so that it may overcome or bridge the gap or resistance in an open circuit.
Q. How many circuits are there in
usual ignition system?
A. There are two circuits in the conventional
high tension ignition sysFig. 77.—Vertical Engine Employed in
tems, these being the priStationary Service Showing Method of
mary and secondary circuits Using Low Tension Ignition System.
respectively. Q. What parts are included in the primary circuit?
A. The primary circuit includes the source of current or battery, the switch, the ignition timer and the primary winding of the transformer or induction coil. (See Figs. 77A and 78.)