Aviation gasoline consists almost entirely of hydrocarbons, namely, compounds consisting of hydrogen and carbon. Some impurities in the form of sulfur and dissolved water will be present. The water cannot be avoided, since the gasoline is exposed to moisture in the atmosphere. A small amount of sulfur, always present in crude petroleum, is left in the process of manufacture.
Tetraethyl lead (TEL) is added to the gasoline to improve its performance in the engine. Organic bromides and chlorides are mixed with TEL so that during combustion volatile lead halides will be formed. These then are exhausted with the combustion products. TEL, if added alone, would burn to a solid lead oxide and remain in the engine cylinder. Inhibitors are added to gasoline to suppress the formation of substances that would be left as solids when the gasoline evaporates.
Certain properties of the fuel affect engine performance. These properties are volatility, the manner in which the fuel burns during the combustion process, and the heating value of the fuel. Also important is the corrosiveness of the gasoline as well as its tendency to form deposits in the engine during use. These latter two factors are important because of their effect on general cleanliness, which has a bearing on the time between engine overhauls.
Volatility is a measure of the tendency of a liquid substance to vaporize under given conditions. Gasoline is a complex blend of volatile hydrocarbon compounds that have a wide range of boiling points and vapor pressures. It is blended in such a way that a straight chain of boiling points is obtained. This is necessary to obtain the required starting, acceleration, power, and fuel mixture characteristics for the engine.
If the gasoline vaporizes too readily, fuel lines may become filled with vapor and cause decreased fuel flow. If the fuel does not vaporize readily enough, it can result in hard starting, slow warmup, poor acceleration, uneven fuel distribution to cylinders, and excessive crankcase dilution.
The lower grades of automobile fuel are not held within the tolerances
required for aviation gasoline and usually contain a considerable amount
of cracked gasoline, which may form excessive gum deposits. For these reasons,
automobile fuels should not be used in aircraft engines, especially air
cooled engines operating at high cylinder temperatures.
Vaporization of gasoline in fuel lines results in a reduced supply of gasoline to the engine. In severe cases, it may result in engine stoppage.
This phenomenon is referred to as vapor locking. A measure of a gasoline's tendency to vapor lock is obtained from the Reid vapor pressure test. In this test a sample of the fuel is sealed in a "bomb" equipped with a pressure gauge. The apparatus (see figure 4-1) is then immersed in a constant temperature bath and the indicated pressure is noted. The higher the corrected vapor pressure of the sample under test, the more susceptible it is to vapor locking. Aviation gasolines are limited to a maximum of 7 psi because of their increased tendency to vapor lock at high altitudes.
Carburetor icing is also related to volatility. When the fuel changes from a liquid to a vapor state, it extracts heat from its surroundings to make this change. The more volatile the fuel, the more rapid the heat extraction will be. As the gasoline leaving the carburetor discharge nozzle vaporizes, it can freeze water vapor contained in the incoming air. The moisture freezes on the walls of the induction system, the venturi throat, and the throttle valves. This type of ice formation restricts the fuel and air passages of the carburetor. It causes loss of power and, if not eliminated, eventual engine stoppage. Extreme icing conditions can make operation of the throttle controls impossible. This icing condition is most severe in the temperature range of 30° to 40° F outside air temperature.
Some fuels may contain considerable quantities of aromatic hydrocarbons, which are added to increase the rich mixture performance rating of the fuel. Such fuels, known as aromatic fuels, have a strong solvent and swelling action on some types of hose and other rubber parts of the fuel system. For this reason, aromatic resistant hose and rubber parts have been developed for use with aromatic fuels.
In an engine that is operating in a normal manner, the flame front traverses the charge at a steady velocity of about 100 feet per second until the charge is consumed. When detonation occurs, the first portion of the charge burns in a normal manner but the last portion burns almost instantaneously, creating an excessive momentary pressure unbalance in the combustion chamber.
This abnormal type of combustion is called detonation. This tremendous increase in the speed of burning causes the cylinder head temperature to rise. In severe cases, the increase in burning speed will decrease engine efficiency and may cause structural damage to the cylinder head or piston. During normal combustion, the expansion of the burning gases presses the head of the piston down firmly and smoothly without excessive shock. The increased pressure of detonation exerted in a short period of time produces a heavy shock load to the walls of the combustion chamber and the piston head. It is this shock to the combustion chamber that is heard as an audible knock in an automobile engine. If other sounds could be filtered out, the knock would be equally audible in an aircraft engine. Generally, it is necessary to depend upon instruments to detect detonation in an aircraft engine.
Ignition of the fuel/air mixture by hot spots or surfaces in the combustion chamber is called surface ignition. If this occurs before the normal ignition event, the phenomenon is referred to as preignition. When it is prevalent, the result is power loss and engine roughness. Preignition is generally attributed to overheating of such parts as spark plug electrodes, exhaust valves, carbon deposits, etc. Where preignition is present, an engine may continue to operate even though the ignition has been turned off. Present information indicates that gasoline high in aromatic hydrocarbon content is much more likely to cause surface ignition than fuels with a low content.
Octane and Performance Number Rating
Octane and performance numbers designate the antiknock value of the fuel mixture in an engine cylinder. Aircraft engines of high power output have been made possible principally as a result of blending to produce fuels of high octane ratings. The use of such fuels has permitted increases in compression ratio and manifold pressure, resulting in improved engine power and efficiency. However, even the high octane fuels will detonate under severe operating conditions and when certain engine controls are improperly operated.
Antiknock qualities of aviation fuel are designated by grades. The higher the grade, the more compression the fuel can stand without detonating. For fuels that have two numbers, the first number indicates the lean mixture rating and the second the rich mixture rating. Thus, grade 100/130 fuel has a lean mixture rating of 100 and a rich mixture rating of 130. Two different scales are used to designate fuel grade. For fuels below grade 100, octane numbers are used to designate grade. The octane number system is based on a comparison of any fuel with mixtures of iso octane and normal heptane. The octane number of a fuel is the percentage of iso octane in the mixture that duplicates the knock characteristics of the particular fuel being rated. Thus, grade 91 fuel has the same knock characteristics as a blend of 91 percent iso octane and 9 percent normal heptane.
With the advent of fuels having antiknock characteristics superior to iso octane, another scale was adopted to designate the grade of fuels above the 100 octane number. This scale represents the performance rating of the fuel - its knock free power available as compared with that available with pure iso octane. It is arbitrarily assumed that 100 percent power is obtained from iso octane alone. An engine that has a knock limited horsepower of 1,000 with 100 octane fuel will have a knock limited horsepower of 1.3 times as much (1,300 horsepower) with 130 performance number fuel.
The grade of an aviation gasoline is no indication of its fire hazard. Grade 91/96 gasoline is as easy to ignite as grade 115/145 and explodes with as much force. The grade indicates only the gasoline's performance in the aircraft's engine.
A convenient means of improving the antiknock characteristics of a fuel is to add a knock inhibitor. Such a fluid must have a minimum of corrosive or other undesirable qualities, and probably the best available inhibitor in general use at present is TEL (tetraethyl lead). The few difficulties encountered because of the corrosion tendencies of ethylized gasoline are insignificant when compared with the results obtained from the high antiknock value of the fuel. For most aviation fuels the addition of more than 6 ml. per gallon is not permitted. Amounts in excess of this have little effect on the antiknock value, but increase corrosion and spark plug trouble.
There are two distinct types of corrosion caused by the use of ethyl gasoline. The first is caused by the reaction of the lead bromide with hot metallic surfaces, and occurs when the engine is in operation; the second is caused by the condensed products of combustion, chiefly hydrobromic acid, when the engine is not running.
Aviation fuels must be free of impurities that would interfere with the operation of the engine or the units in the fuel and induction system.
Even though all precautions are observed in storing and handling gasoline, it is not uncommon to find a small amount of water and sediment in an aircraft fuel system. A small amount of such contamination is usually retained in the strainers in the fuel system. Generally, this is not considered a source of great danger, provided the strainers are drained and cleaned at frequent intervals. However, the water can present a serious problem because it settles to the bottom of the fuel tank and can then be circulated through the fuel system. A small quantity of water will flow with the gasoline through the carburetor metering jets and will not be especially harmful. An excessive amount of water will displace the fuel passing through the jets and restrict the flow of fuel; it will cause loss of power and can result in engine stoppage.
Under certain conditions of temperature and humidity, condensation of moisture (from the air) occurs on the inner surfaces of the fuel tanks. Since this condensation occurs on the portion of the tank above the fuel level, it is obvious that the practice of servicing an airplane immediately after flight will do much to minimize this hazard.
|Gasolines containing TEL must be colored to conform with
the law. In addition, gasoline may be colored for purposes of identification.
For example, grade 100 low lead aviation gasoline is blue, grade 100 is
green and grade 80 is red. See figure 4-2.
100/130 gasoline is manufactured (1975) in two grades - high lead, up to 4.6 milliliters of lead per gallon and low lead, not over 2.0 milliliters per gallon. The purpose being to eliminate two grades of lower octane fuel (80/87) and 91/96). The high lead will continue to be colored green whereas the low lead will be blue.
The low lead will replace the 80/87 and 91/96 octane fuels as they are phased out. Engine manufacturers have prepared instructions to be followed in making adjustments necessary for changeover to the 100 octane fuel.
A change in color of an aviation gasoline usually indicates contamination with another product or a loss of fuel quality. A color change can also be caused by a chemical reaction that has weakened the lighter dye component. This color change in itself may not affect the quality of the fuel.
A color change can also be caused by the preservative in a new hose. Grade 115/145 gasoline that has been trapped for a short period of time in new hose may appear green. Flushing a small amount of gasoline through the hose usually removes all traces of color change.
Fuel Identification Markings
The most positive method of identifying the type and grade of fuel includes the following:
1. Marking of Hose. A color band not less than one foot wide painted adjacent to the fitting on each end of hose used to dispense fuel. The bands completely encircle the hose, and the name and grade of the product is stenciled longitudinally in one inch letters of a contrasting color over the color band.
2. Marking of Fuel Carriers, Pits and Fill Stands. Tags identifying the name and grade of the product permanently affixed to each discharge meter and fill pipe. Porcelain tags (4" x 6") carrying the same information permanently bolted to the outside of the rear compartment of fuel servicing equipment. The delivery pipes of truck fill stands are banded with colors corresponding to that used on the dispensing hose.