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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.

LESSON EIGHT

PARTS OF THE IGNITION SYSTEM AND THEIR

PURPOSE

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

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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

there in the 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 Using Low Tension Ignition System.

mary and secondary circuits

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.)

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Fig. 77A.—Simple High Tension Ignition System for One Cylinder

Motor Showing Arrangement and Wiring of Principal Parts.

Q. What parts are in the secondary circuit?

A. The secondary circuit includes the high tension winding of the induction coil and the spark plug.

Q. Describe parts of simple battery ignition system.

A. A simple battery ignition system for a single cylinder engine is shown at Fig. 77A and a corresponding system for a two cylinder engine is outlined at Fig. 78. The current supply is obtained from two sets of cells, one of these being a dry battery, the other a storage

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Fig. 78.-High Tension Ignition System for Two-Cylinder Opposed

Motor.

battery. One wire from the negative or minus terminal (-) of each battery leads to a button on the switch. The plus terminal (+) or positive, as it is commonly termed, of each battery is grounded to the engine frame. A single wire leads from the top of the switch to one of the primary terminals of the induction coil. The other primary terminal of the induction coil is connected to the insulated terminal on the contact maker or timer. A heavily insulated wire joins the secondary terminal of the induction coil with the insulated terminal of the spark plug.

The action of this method of ignition may be easily understood

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