CHAPTER VII
IGNITION—(Continued)
While there are various methods by which an electric spark may be produced, but two are in use for automobile work, one being the high-tension or jump-spark system, and the other the low-tension or make-and-break system. The difference between these is in the pressure of the current, which in the first is great enough to enable it to jump from one terminal to another a short distance away, forming a spark as it passes. The pressure of the current of the low-tension system is not sufficient to permit it to do this, but when two terminals of the system are brought into contact so that the current may flow, a spark will form as they are separated.
MAKE-AND-BREAK SYSTEM
In the make-and-break system, the igniter, which is the device in the combustion space at which the spark occurs, is made with two metal points, one of which is stationary and the other movable, the latter being acted on by a cam, through a tappet. As the cam revolves, the tappet is lifted and the movable point brought into contact with the stationary; when the nose of the cam passes from under the tappet, the movable point is snapped away from the stationary through the action of a spring (Fig. 25). These points are so connected into the circuit that when they are in contact the current flows, the circuit being broken when they separate. Shortly before the spark is desired the movable point is brought into contact with the other so that the circuit is completed and the current flows, and they are separated at the instant when the spark is required to ignite the mixture, this occurring as they separate and break the circuit.
Fig. 25.—Make-and-Break Ignition.
The current that is supplied by the battery is not capable of producing this spark, for it flows with large volume and small pressure—i.e., high amperage and low voltage—and it is necessary to transform it to a current of higher voltage and lower amperage, which is done by means of a primary induction coil. A coil of this description consists of a core made of a bundle of soft iron wires, around which is the primary winding, formed by winding several layers of insulated copper wire on it. The core becomes a magnet when a current of electricity flows through the winding, and ceases to be a magnet when the flow ceases, the magnetization occurring slowly on the completion of the circuit, and demagnetization instantly on the breaking of the circuit.
The influence of a magnet, as, for instance, its attraction for a piece of iron, is felt throughout a field the extent of which depends on the strength of the magnetization. When a loop of wire forming a closed circuit is placed in the magnetic field, a current of electricity will be set up in it whenever the strength of the field changes. If the strength of the field does not change, no current will be set up; but the greater the change in strength, whether from strong to weak or from weak to strong, the greater will be the strength of the current. This principle is applied in the case of a coil, for the wire that forms the winding is in the magnetic field of the core, and a current is set up in it whenever the strength of the field changes. The strongest current will be set up, or induced, when the strength of the field changes from no magnetism to a degree when the core is magnetized to its fullest capacity or the reverse, and when the change occurs in the least possible time. The current that is set up will last only during the time when the change in strength occurs, ceasing to exist when the strength of the field is uniformly strong or weak. The more rapid the change, the greater will be the pressure of the current, but the shorter the period of its existence.
It is this induced current that forms the spark in the combustion space, the battery current serving only to establish the condition that will produce it. When the two points of the igniter come into contact and complete the circuit, the flow of the battery current through the winding of the coil will gradually magnetize the core, and on the igniter points being separated, this flow through the winding will cease, the strength of the magnetic field dropping instantly from full intensity to practically nothing. This change induces a powerful current in the winding, which current in passing over the circuit causes a spark between the points of the igniter as they separate. The induced current is set up so instantaneously on the breaking of the battery circuit that the spark is usually considered as being that of the battery current, but, as stated, the battery current serves only to magnetize the core, the spark resulting from the current induced by its demagnetization on the breaking of the circuit.
A magneto is a machine that when driven generates an electric current, and is in almost universal use for this system of ignition. In many cars the system is so arranged that the driver has his choice between ignition by battery and coil, as described, and ignition by magneto, but the mechanical production of a current is so reliable that many automobile builders rely on the magneto alone. Magnetos of the type used for ignition are described in the appendix.
JUMP-SPARK SYSTEM
The jump-spark or high-tension system of ignition depends on the production of a current at such a pressure that it can jump from one point to another through high resistance, such as is presented by air at ordinary or low temperature, and especially when compressed, the spark occurring as it passes.
This high pressure is obtained through the use of a secondary induction coil, which consists of a core and primary winding similar to those of the primary induction coil already described, and in addition has a secondary winding, formed by a great length of insulated copper wire, which to save bulk and weight is very fine, wound over the primary winding. The change in the strength of the magnetic field produced by the core affects this secondary winding, just as it affects the primary winding; that is, a current is set up in it at every change in the strength of the magnetic field, the pressure of the current depending on the greater or less extent of the change, and the rapidity with which it occurs. The magnetization of the core takes place but slowly, for the particles of iron must absorb it one from the other, but the demagnetization occurs instantly; the current that is induced during the demagnetization is therefore the greater, and is the current that has sufficient pressure to jump across the gap.
The ignition system employing this coil consists of two distinct parts: the primary circuit, which magnetizes the core, and the secondary circuit, which leads to the combustion space the current that is induced in the secondary winding.
The primary circuit includes the battery or generator that supplies the current, the primary winding of the coil, a timer, and a vibrator.
THE TIMER
The spark must occur in the combustion space only at the instant when the mixture is in a condition to be ignited, and it is obvious that only then is the secondary current required. As the secondary current results from the flow of the primary current through the primary winding, a device must be included in the primary circuit to permit the current to flow and magnetize the core at this instant. This device is called a timer, or commutator, and is nothing more than a revolving switch operated by the engine, that completes the primary circuit at such times as the secondary current is required, and breaks the circuit when the secondary current has done its work in the production of the spark. Because ignition occurs but once in each cylinder during two revolutions of the crank shaft, the timer completes the circuit but once in that interval, and is placed on the half-time shaft that operates the exhaust valves. In its simplest form, a timer consists of a disk of hard rubber, wood fiber, or other insulating material in which is set a metal plate, mounted on the half-time shaft and revolving with it. A flat spring carried on a plate of hard insulating material bears against the edge of the disk, and during one revolution of the latter the metal plate set in it will be brought into contact with the spring (Fig. 26, first diagram). If one wire of the primary circuit is connected to the metal plate of the disk and the other wire to the spring, it will be seen that the circuit will be complete when the revolution of the disk brings the two together, and broken when the further revolution of the disk separates them.
Fig. 26.—Types of Timers.
There is a great variety in the methods by which this result is secured, the chief essential being that the contact must be made as positively as possible in order that the resistance to the current in passing from one part of the timer to the other may be as low as possible, and broken equally positively. The number of contacts that the timer makes during one revolution depends on the number of cylinders of the engine, the revolving part touching two, three, four, or more springs or contact points on the stationary part as it turns. A timer for a two-cylinder engine is shown in the second diagram on Fig. 26, the third diagram being of a form of timer in which the revolving part is not connected into the circuit, but serves to bring together a spring and a contact point, the circuit being completed when they touch. A modern form of timer consists of a ring of hard rubber on the inside face of which are set the metal plates, while the moving part revolves within it, and may be a flat spring, or a plunger, or a roller, contact being maintained by a coil spring. The difficulty of attaching a wire to the moving part of the timer is overcome by having its contact in connection with the half-time shaft, the current flowing from the contact on the stationary part to that on the moving part, and from there flowing by the half-time shaft to the ground connection of the battery, for, as all parts are of metal, the current can pass from the shaft to the ground connection through the bearings and the engine.
If the spark were always required to pass in the combustion space at the same point in the stroke of the piston, this could be secured by setting the stationary part of the timer in such relation to the moving part that contact would be made when the half-time shaft reached any desired point in its revolution. As it is necessary to change the point at which ignition occurs, however, the timer must be so arranged that it closes the circuit earlier or later, according to conditions. To effect this, the stationary part of the timer is so made that it may rotate around the shaft for about a half turn, and it is controlled by a lever on the steering column, which holds it in any desired position. If the stationary part is rotated in the direction opposite to that in which the half-time shaft revolves, contact will be made and the circuit closed at an earlier point in its revolution than if it is rotated in the same direction. It must be remembered that the revolution of the half-time shaft, in operating the exhaust valve, revolves exactly in accordance with the crank shaft and the movement of the piston, and that as the moving part of the timer is attached to it, contact can be made at any desired point in the stroke of the piston by the position in which the stationary part is placed.
THE VIBRATOR
The secondary current occurs whenever the core is magnetized and demagnetized, and it is clear that the oftener the magnetization occurs the more constant will be its presence. The timer closes and opens the primary circuit, which results in the production of but two impulses of the secondary current, and as these are not sufficient to produce the necessary spark, additional means are employed to make and break the circuit while the timer makes contact. This is accomplished by means of a vibrator, or trembler, which consists of a flat steel spring secured at one end, so that it may vibrate after the manner of a tuning fork. About midway between its ends the point of an adjusting screw touches it, and when the flat spring, or blade, vibrates, it springs away from this screw and returns to it, both screw and blade being tipped with platinum to offset corrosion. If one wire of the circuit is attached to the blade and the other to the screw, the circuit will be complete when the blade touches the screw, and broken when it springs away from it. In the early type of vibrator the blade was set in vibration by mechanical means. At the end of the blade was a weight, which rested on the edge of a cam, the blade then being out of contact with the adjusting screw (Fig. 27). During the revolution of the cam the weight dropped into a deep notch, thus setting the blade into vibration and the circuit being made and broken. This type has been almost entirely superseded by the magnetic vibrator, which is more rapid and accurate. In this the blade is so placed that its free end is close to the end of the core of the coil (Fig. 28). When the timer makes contact and the current passes through the primary winding, the core becomes magnetized and attracts to it the free end of the steel blade, this movement drawing the blade away from the adjusting screw with which it was in contact. The vibrator is so connected into the primary circuit that the current flows from the battery to the adjusting screw, thence to the blade and through the primary winding; when the attraction of the blade by the core draws it away from the adjusting screw, the circuit is broken, and the magnetism then ceasing to exist, the elasticity of the blade causes it to spring back to the screw, to make contact, and to be again attracted by the magnetism.
Fig. 27.—Mechanical Vibrator. When the weight drops into the notch on the cam, the flat spring touches the adjusting screw and the current passes.
Fig. 28.—Magnetic Vibrator. When the magnetism of the core attracts the vibrator the contact is broken at the adjusting screw.
This action continues as long as the timer holds the circuit closed, and a wave of secondary current is set up in the secondary winding for every make and break that results. By means of the adjusting screw, the vibrations may be made long and slow, or short and quick, with a corresponding difference in the secondary current. As the object to be attained is the production of a spark that will ignite the mixture rapidly, the vibrator must be adjusted with this end in view; but it is also advisable to obtain the spark with the least possible expenditure of current, and with the least wear of the parts. The passing of the battery current through the primary winding establishes the condition that exists in the make-and-break ignition system, and the demagnetization of the core will set up an induced current that will result in a spark at the points where the circuit is broken; that is, at the vibrator and timer contacts. This spark can be greatly overcome in the timer by packing it with vaseline, which, being an insulator, breaks down the spark in flowing between the contacts of the stationary and moving parts as they separate. If the vibrator contacts presented no resistance—an impossible condition—there would be no spark, but as they will always be slightly burned and corroded, the spark must be kept as small as the requirements of the secondary current will justify by the adjustment of the screw. In other words, adjust the vibrator so that it gives the best results in the production of the secondary spark with as little sparking as possible at the vibrator contacts.
THE SPARK PLUG
The secondary circuit includes the secondary winding of the coil and the spark plug, which is the device in the combustion space at which the spark occurs. The spark plug provides two points of metal at a fixed distance apart, which are so connected into the circuit that the current must jump from one to the other in order to complete its circuit. The secondary current, as well as the primary, may make use of the metal of the engine as a ground return, and one of the points of the spark plug is therefore in contact with the metal, being supported on a metal sleeve that is screwed into the cylinder. The other point must of course be insulated from it, for if it were not the current could pass from one to the other without jumping the space between them. The insulation is secured by a tube of porcelain or mica set in the sleeve, the lower end of a metal rod that passes through it being the required distance from the wire or projection on the sleeve (Fig. 29). This device must be strong enough to resist the pressure in the combustion space during the compression and power strokes, and must be unaffected by the intense heat.
Fig. 29.—A, Spark plug; B, spark plug in section.
THE CIRCUIT
Fig. 30 shows the primary and secondary circuits of the jump-spark system, as applied to a one-cylinder engine, the light lines representing the primary circuit and the heavy lines the secondary. Each circuit forms a complete path by which the current passing over it may return to the point at which it started. In practice the wiring is simplified by the use of but one ground return for both circuits, the primary and secondary windings of the coil being connected at one point, and the secondary current returning either through the timer as it makes contact or through the battery.
Fig. 30.—Ignition Circuit. Wiring diagram for one-cylinder engine, with coil, timer, and two batteries; coil with magnetic vibrator.
The flow of the primary circuit is from the battery to the switch, to the vibrator, which is built into the coil, through the primary winding and the timer contacts, whence it passes by the metal of the engine and ground wire back to the battery. The secondary circuit consists of a wire from the secondary winding of the coil to the spark plug, and back to its winding by the metal of the engine and a ground wire, which, as has been explained, may be the separate ground wire shown in the diagram, or the return that is common to both circuits.
The timer makes contact as the half-time shaft revolves, and the vibrator operates as long as the circuit is closed; while this occurs, the secondary current is set up, and the spark passes.
Included in the primary circuit and built into the base of the coil is a condenser which, while of such delicate construction that it is most inadvisable for the automobilist to attempt to examine or repair it, is of such advantage that it should be understood. A current flowing in a circuit possesses momentum, and if the circuit is broken, the momentum will cause the current to flow across the break for a greater or less time, according to the pressure. This will produce a spark, and a spark at the vibrator as it breaks the circuit will not only burn away the contacts, but will prevent the instant demagnetization of the core. The passing of a spark between the vibrator contacts indicates that the current is still flowing through the primary winding, and that the core is still magnetized. The function of the condenser is to prevent sparking at the vibrator by absorbing the momentum of the current, and, what is more important, to cause the instant demagnetization of the core by the instant breaking of the circuit.
While it is usual to provide a multicylinder engine with one coil for each cylinder, it is possible to equip it with but one, in such a case a secondary distributer being used. This consists of two parts: a primary timer, and a somewhat similar device by which the secondary current induced in the coil is switched from cylinder to cylinder as it is required. For a four-cylinder engine the timer has four contacts, as in the usual type; but the primary winding is connected to all of them, so that as the moving part of the timer revolves, the core of the single coil is magnetized four times to two revolutions of the crank shaft. A single wire leads the secondary current that is thus induced to the distributer, by which it is switched to the several cylinders. The moving part of the distributer rotates in unison with the moving part of the timer, making contact as it does so with contact pieces, each of which is connected to one of the spark plugs.
When the timer makes its contact the distributer is in position to pass the induced current to the proper cylinder, the connections being made in accordance with the firing order of the engine. While this system has advantages in that there is but one vibrator to adjust, the fact that the coil is operating for the greater part of the time makes it more liable to injury through heating and the consequent breaking down of its insulation.