It is a fact well known to experienced repairmen and motorists that atmospheric conditions have much to do with carburetor action. It is often observed that a motor seems to develop more power at night than during the day, a circumstance which is attributed to the presence of more moisture in the cooler night air. Likewise, taking a motor from sea level to an altitude of 10,000 feet involves using rarefied air in the engine cylinders and atmospheric pressures ranging from 14.7 pounds at sea level to 10.1 pounds per square inch at the high altitude. All carburetors will require some adjustment in the course of any material change from one level to another. Great changes of altitude also have a marked effect on the cooling system of an airplane. Water boils at 212 degrees F. only at sea level. At an altitude of 10,000 feet it will boil at a temperature nineteen degrees lower, or 193 degrees F.
In high altitudes the reduced atmospheric pressure, for 5,000 feet or higher than sea level, results in not enough air reaching the mixture, so that either the auxiliary air opening has to be increased, or the gasoline in the mixture cut down. If the user is to be continually at high altitudes he should immediately purchase either a larger dome or a smaller strangling tube, mentioning the size carburetor that is at present in use and the type of motor that it is on, including details as to the bore and stroke. The smaller strangling tube makes an increased suction at the spray nozzle; the air will have to be readjusted to meet it and you can use more auxiliary air, which is necessary. The effect on the motor without a smaller strangling tube is a perceptible sluggishness and failure to speed up to its normal crank-shaft revolutions, as well as failure to give power. It means that about one-third of the regular speed is cut out. The reduced atmospheric pressure reduces the power of the explosion, in that there is not the same quantity of oxygen in the combustion chamber as at sea level; to increase the amount taken in, you must also increase the gasoline speed, which is done by an increased suction through the smaller strangling aperture. Some forms of carburetors are affected more than others by changes of altitude, which explains why the Zenith is so widely employed for airplane engine use. The compensating nozzle construction is not influenced as much by changes of altitude as the simpler nozzle types are.
CHAPTER VI
[Early Ignition Systems]—[Electrical Ignition Best]—[Fundamentals of Magnetism Outlined]—[Forms of Magneto]—[Zones of Magnetic Influence]—[How Magnets are Made]—[Electricity and Magnetism Related]—[Basic Principles of Magneto Action]—[Essential Parts of Magneto and Functions]—[Transformer Coil Systems]—[True High Tension Type]—[The Berling Magneto]—[Timing] and [Care]—[The Dixie Magneto]—[Spark Plug Design and Application]—[Two-Spark Ignition]—[Special Airplane Plug].
EARLY IGNITION SYSTEMS
One of the most important auxiliary groups of the gasoline engine comprising the airplane power plant and one absolutely necessary to insure engine action is the ignition system or the method employed of kindling the compressed gas in the cylinder to produce an explosion and useful power. The ignition system has been fully as well developed as other parts of the engine, and at the present time practically all ignition systems follow principles which have become standard through wide acceptance.
During the early stages of development of the gasoline engine various methods of exploding the charge of combustible gas in the cylinder were employed. On some of the earliest engines a flame burned close to the cylinder head, and at the proper time for ignition a slide or valve moved to provide an opening which permitted the flame to ignite the gas back of the piston. This system was practical only on the primitive form of gas engines in which the charge was not compressed before ignition. Later, when it was found desirable to compress the gas a certain degree before exploding it, an incandescent platinum tube in the combustion chamber, which was kept in a heated condition by a flame burning in it, exploded the gas. The naked flame was not suitable in this application because when the slide was opened to provide communication between the flame and the gas the compressed charge escaped from the cylinder with enough pressure to blow out the flame at times and thus cause irregular ignition. When the flame was housed in a platinum tube it was protected from the direct action of the gas, and as long as the tube was maintained at the proper point of incandescence regular ignition was obtained.
Some engineers utilized the property of gases firing themselves if compressed to a sufficient degree, while others depended upon the heat stored in the cylinder-head to fire the highly compressed gas. None of these methods were practical in their application to motor car engines because they did not permit flexible engine action which is so desirable. At the present time, electrical ignition systems in which the compressed gas is exploded by the heating value of the minute electric arc or spark in the cylinder are standard, and the general practice seems to be toward the use of mechanical producers of electricity rather than chemical batteries.