Fuel Requirements of the Gasoline Aircraft-Engine 300024
FUELS for use in aircraft engines are discussed with reference to their antiknock value, volatility, vapor-locking and engine-starting properties, gum content and availability, and to antiknock agents.
The usefulness of a fuel for spark-ignition engines is stated to be limited by its tendency to heat the cylinder and the piston unit. Definite evidence is available that the tendency of fuels to heat the cylinder unit is not always in accord with their tendency to cause audible knocking.
The fuel required depends upon the compression ratio of the engine, its volumetric efficiency, the design, size and temperature of the cylinder unit, and the rate of revolution. Mid-Continent Domestic Aviation gasoline having an approximate antiknock value of 50 octane-50 heptane gives excellent results if the engine output is kept within the limitations of this fuel but is not suitable for many modern aircraft engines if flown wide open at sea level. Fuel having an antiknock value of 70 to 75 octane, 30 to 25 heptane is amply good for most modern commercial and military airplanes, except certain fuels that lose antiknock value excessively at high cylinder-temperature. For engines of the ground-boosted, high-compression and high-cylinder-temperature types, the Air Corps intends to standardize on an antiknock value of 87 octane 13 heptane.
Engine performance that is now being secured by the Materiel Division of the Air Corps does not seem to be commercially possible without the addition of tetraethyl lead to the fuel.
In present type engines Domestic Aviation gasoline does not seem to be volatile enough, as somewhat excessive heating of the induction system is necessary for the minimum specific fuel consumption, and such heating results in a reduction of power output. Therefore the Air Corps has adopted for use in super-performance engines a fuel meeting the volatility requirements of the Federal Specifications Board for Fighting Grade gasoline, with certain modifications called for by United States Army Specification Y-3557-A.
Vapor-locking trouble in present military equipment is violently accentuated by gasoline having a Reid vapor pressure in excess of 7 lb. per sq. in. at 100 deg. fahr. Therefore, pending development of improved fuel systems, Specification Y-3557-A fixes an upper limit of 167 deg. fahr. on the 10-per cent-evaporated point and a maximum Reid vapor pressure of 6½ lb. at 100 deg. fahr. The Air Corps, however, is working to improve fuel systems, which it believes should embody pressure feed of the fuel by a pump the suction of which is below the lowest point of the fuel tanks, which is not subjected to heat and which has a capacity greatly in excess of the maximum engine-demand. It plans a system that will avoid pump vapor-locking with fuel having a vapor pressure of 16 to 17 lb. per sq. in. at 100 deg. fahr.
Little reason seems to exist at present for excluding cracked gasoline for aviation use. Endurance tests in flight have shown no undesirable effects from gum in such gasolines containing less than 10 mg. per 100 cc.
Ample supplies of Fighting Grade gasoline having an antiknock value of 70-75 octane 25-30 heptane, of both gum-stable cracked and straight-run types, seem to be available. Apparently the best method of increasing the antiknock value is the addition of tetraethyl lead and aromatics. Benzol seems to be relatively ineffective at high cylinder-temperatures and causes ice formation in the fuel system if more than 20 per cent is added to the gasoline. Aromatics of low freezing-point, synthesized from benzol, apparently are the only other generally available antiknocks suitable for sufficiently raising the antiknock value. The Air Corps feels that troubles arising from the use of tetraethyl lead in aircraft engines can be overcome by changes in engine design, material and lubrication.
Very large supplies of straight-run California gasoline that will meet the antiknock value required by Army Specification Y-3557-A with less than 6 cc. of lead per gallon are available; and straight-run gasolines from other crudes that will meet the requirements seem to be obtainable in considerable quantity. Some cracked gasolines without lead additions are found to be superior to straight-run California gasolines without lead additions, but their response to added lead seems to be slight.