Engine Fuel
A fuel is a substance composed principally of hydrogen and carbon in such proportions that it will burn in the presence of oxygen and liberate heat energy. By burning fuel in an internal com bustion engine this heat energy can be trans formed into mechanical energy. Liquid fuels are ideal for internal combustion engines becau se they can be economically produced, have a high heat value per pound, an ideal rate of burning and can be easily handled and stored.
The most common internal combustion engine fuels are gasoline, kerosene and Diesel fuel oil. Gasoline, because of its many advantages, is used to a much greater extent than any other fuel in internal combustion engines having spark ignition. It has a better rate of burning than other fuels, and due to the ease with which it vaporizes, it gives quick starting in the coldest weather, smooth acceleration and maximum power.
Diesel fuel oil ranks next to gasoline in quantity used. It can be produced as economically as gaso line but its use is limited to Diesel type engines.
The use of kerosene as a fuel for internal com bustion engines is usually limited to farm tractors, marine and stationary engines which operate at a fairly constant speed. Its characteristics are such that it cannot be properly mixed with air and controlled in variable speed engines.
COMBUSTION
The burning of a fuel in the presence of oxygen is called combustion.
A gallon of gasoline weighs about 6 pounds of which 5 pounds is carbon and the remainder hydrogen. The air necessary to burn a gallon of gasoline is composed of about 19 pounds of oxy gen and 72 pounds of nitrogen and other gases. In burning a g;:: Jlon of gasoline, about 11 of the 19 pounds of oxygen from the air combine with the 5 pounds of carbon in the fuel to form carbon monoxide and carbon dioxide gases. Complete combustion converts all the carbon to carbon diox ide gas. However, complete combustion is never attained in the engine and, as a result, carbon
monoxide is also formed.
While the carbon and oxygen are uniting to liberate heat, which is converted into energy to run the engine, the 1 pound of hydrogen in the fuel unites with the remaining 8 pounds of oxygen in the air to form about 9 pounds of water; that is, over 1 gallon of water is formed for every gallon of gasoline burned. The water passes off with the burned exhaust gases in vapor form. In hot weather it is not of great importance, but in cold weather the water vapor partially condenses, which usually causes rust and corrosion in the muffier.
The nitrogen in the air is not affected. It re mains stable and acts as a cooling agent, reducing the maximum burning temperature that could be obtained if pure oxygen were supplied instead of air.
A Diesel engine differs from a gasoline engine. Operating on an excess amount of air, a Diesel engine burns the fuel more completely and the exhaust is ordinarily free f!om carbon monoxide gas. It is, however, given off when the combustion in a Diesel engine is incomplete, usually due to faulty fuel injection.
When fuel burns inside the cylinders of an internal combustion engine, the temperature of the mixture is raised by the heat given off. Actual burning temperatures of upwards of 4,000 degrees have been recorded in operating engines.
COMPRESSION PRESSURE
Within practical limits, the more fuel that is compressed in the combustion chamber (com pression pressure) the more efficient it is and the more power it produces. As a fuel burns the pressure it creates (combustion pressure) is about four times greater than the compression pressure. A fuel subjected to a compression pressure of 100 pounds per square inch will develop about 400 pounds per square inch combustion pressure as it burns. Should the compression pressure be raised to 150 pounds per square inch, the power produced by the engine will be greatly increased because the combustion pressure will have been raised to about 600 pounds per square inch.
DETONATION
When the compression pressure is very high, the fuel mixture tends to explode instead of burn ing uniformly and slowly, causing detonation, knock or ping. Fuel knock, besides being an an noying sound, results in loss of power, overheat ing, increased fuel consumption and severe shock to spark plugs, pistons, connecting rods, bearings and crankshaft. It has been known in extreme cases to chip porcelain from spark plugs and crack cylinder and valve heads.
Improvements in combustion chamber design have helped to reduce the detonation. When the temperature of an unburned fuel can be kept below the detonation temperatures by engine cooling, the – increased compression pressures re sult in increased combustion pressures. Hot areas, like the exhaust valve, radiating heat to the un burned charge, may cause detonation. By placing the spark plugs close to these hot areas so that the fuel starts to burn near them and then moves away from them, the tendency toward engine knocking can be minimized.
Carbon formation in a combustion chamber in creases the tendency of a fuel to knock because, being an excellent heat insulator, it reduces the effective cooling surface in the region of the last-to burn portion of the fuel charge.
OCTANE RATING
The ability of a gasoline to resist detonation is called its octane or anti-knock rating. A gasoline from asphaltic base crude oil produces less knock than one from paraffinic base crude. Cracked gasoline has less tendency to knock than straight run gasoline. All marketed gasolines are a blend of straight run and cracked gasolines, so unless their blending is controlled, their anti-knock qualities will vary.
Engineers and refiners have devised a method of determining and comparing the anti-knock qualities of gasolines by using a special one cylinder engine, known as the C.F.R. (Co-opera tive Fuel Research Committee) fuel testing en gine, in which the compression pressure can be raised or lowered. A device records and measures the knocking effect of the fuel being tested.
A mixture of iso-octane, which has a very high anti-knock rating, and heptane, which produces a pronounced knock, is used as a reference fuel to establish an anti-knock standard. The anti-knock value or octane number of a gasoline being tested is represented by the percentage of volume of iso octane that must be mixed with normal heptane in order to duplicate the knocking of the gasoline being tested. Octane numbers range from 50 in third grade gasoline to llO in aviation gasolines. Since an octane number of 100 indicates a fuel having an anti-knock value equal to that of iso octane, a number higher than 100 indicates that the anti-knock value is that much greater than that of iso-octane.
If the octane rating of a gasoline is naturally low, the fuel will detonate as it burns and power will be applied to the pistons in hammer-like blows. The ideal power is that which pushes steadily rather than hammers against the piston.
The octane rating of a gasoline can be raised by treating it with a chemical.
A treated fuel is one which contains a chemical that is not a fuel. The most satisfactory chemical known is tetra-ethyl lead compound, which is added to the gasoline in the proportion of about 1 to 1200 by volume, depending on the fuel and the anti-knock value desired.
Tetra-ethyl lead is a liquid which mixes thor oughly with gasoline and vaporizes completely. Ethylene dibromide prevents the tetra-ethyl lead from forming lead oxide deposits on spark plugs and on valve seats and stems. Red dye is added to identify an ethyl treated gasoline and to warn against its being used as anything but an engine fuel.
An engine that does not knock on a low octane fuel does not increase in efficiency when operated on fuel with a higher octane rating. If the knock does not stop, some mechanical adjustments are probably necessary. By adjusting the spark timing of an engine using a low anti-knock gasoline so that it will fire later (retarding the spark), the knocking will be eliminated, but fuel consumption will be increased and the engine will overheat. It may be less expensive to use a higher priced, high octane gasoline with an advanced spark than to use a cheap, low octane gasoline with a retarded spark.