If he is renewing a connecting rod or a wrist pin bearing, he must also remember that the piston has formerly been traveling over a certain area of cylinder surface that has not varied in length the ten-thousandth part of an inch between one stroke and the next. Consequently, the babbitts or bushings should be so replaced that the piston shall occupy the same position relative to the cylinder walls at the top and bottom of its stroke that it did formerly. In other words, by varying the thickness of the top of the babbitt he is replacing, he may change the "center" of the bearing so that the piston will start on its upward stroke from a different point than was previously the case. Thus, while the length of travel of the piston will be the same, it will traverse a slightly different portion of the cylinder walls under the new conditions, and this will have the effect of changing the compression and, possibly, of wearing the piston and rings unduly.
[CHAPTER IV]
The Ignition System
It was the application of the electric current to the ignition system of the gasoline engine that first enabled these new forms of power plants to be designed with sufficient compactness and to possess enough flexibility to render their use practical on self-propelled vehicles. Without the electric ignition system, the speed and power of the vehicle could not well be controlled, and the explosions would be uncertain and irregular, at best.
Those of us who are familiar with the electric gas lighters that were in popular use a few years ago are furnished with a convincing demonstration of the operation of the first electric ignition systems. By pulling a chain, a wire, or arm was rubbed across a metal point until the contact thus formed was suddenly broken. This arm and the stationary point formed the two terminals of an electric circuit, which caused a flash of blue flame when the contact was broken as the one was "wiped" across the other. The flame thus formed at the instant the contact was broken contained sufficient heat to ignite the gas escaping from the burner to which the device was attached.
Sparks will be formed in the same manner if we hold two wires, connected to the opposite poles of a set of batteries, in both hands and wipe the bare ends across each other. If an arrangement producing this effect is introduced into the gas engine cylinder at the portion in which the charge is compressed, the flash resulting when the terminals are separated will serve to ignite the explosive mixture. The movable terminal is connected to a rod which passes through the cylinder walls and is attached to a mechanism actuated by a cam revolved by the engine. This mechanism is termed the "make-and-break" ignition system for the reason that contact of these terminals is alternately made and broken to produce the flash of electricity that explodes the surrounding charge.
In order to produce a flash of sufficient size when the contact is broken, the nature of the current, obtained from the dry cells or storage battery is changed somewhat by conducting it through a coil of wire surrounding a bundle of bare copper wires. This is known as a spark coil, and while it is generally used with battery ignition of the make-and-break type, magnetos may be designed which produce the proper kind of current direct, without the aid of the coil.
An ordinary set of six dry cells, connected in series—or like with unlike poles—will produce a current of between twenty and twenty-five amperes at a pressure of about nine volts—assuming each battery, when new, to deliver twenty-five amperes at a pressure of one and one-half volts. The "series" wiring gives the entire set the combined voltage of all with the average amperage of one. For the benefit of those who have forgotten their elementary physics, let it be remembered that the ampere is the measure of current amount, or flow, while the voltage is concerned only with the pressure of the current. By the use of various arrangements of windings of wires, the voltage may be raised with a corresponding decrease in the amperage—and vice versa. Thus, if a coil is used that doubles the original number of amperes produced by the battery, the voltage will be halved.
The make-and-break type of ignition has been used successfully for many years, but with the perfection of the magneto, it has been largely supplanted, in automobile practice, at least, by the "jump spark," or "high-tension" system. Because of the fact that the latter system is less expensive to construct and is highly efficient, it will be found also on the majority of the older cars not equipped with a magneto.
It was found, after the general adoption of the make-and-break ignition system, that a flame was not necessary for the combustion of a properly-mixed charge in the engine cylinder. In fact, a tiny spark, scarcely one-sixteenth of an inch long and no larger around than a pin, was discovered to be sufficient to produce the ignition of the charge. Although, of small volume, such a spark generates intense heat, and it is upon this quality, rather than upon area, that the charge depends for its ignition—although it is claimed that a large flame will produce more complete, rapid, and consequently more efficient, combustion. But the jump spark possesses the advantage of requiring no moving parts projecting through the cylinder walls into the combustion chamber, and its greater simplicity over that of the make-and-break system has resulted in its almost universal adoption by automobile manufacturers.