Chapter 4-Aviation Fuels
EARLY AIRCRAFT ENGINES WERE OPERATED on ordinary straight run motor gasoline well into the 1920s . Research then isolated uncontrolled combustion as a major source of engine overheating and failures, prompting a search for ways to cure the problem. The big step came in 1921 with the invention of tetraethyl lead (TEL), an unequaled knock resistance enhancer. The same period saw the development of the heptane-isooctane scale still in use today for rating antiknock properties in terms iof ocatne numbers. For aviation gasoline, the concept of rating knock resistance in special single cylinder engines resulted in the Aviation Octane Test Method (D 614), which tested fuels under lean fuel mixture conditions simulating cruise operation. (In 1970 this method was replaced in the specification by the Motor Octane Method, D 2700.) An aviation gasoline specification, issued by the U.S. Air Corps in 1938, listed a 68 octane grade containing no lead and a 92 octane grade with a maximum of 6 ml TEL/gallon. As engine power output was increased by supercharging the fuel air/mixture, a second rating method, D 909, came into use to evaluate performance under rich take-off conditions, so that by World War II both rating methods were required. Engine designers soon discovered the performance benefits of high octane, which permitted a higher octane fuel so develop more power in a given engine or allowed a reduction in engine size with the same power output. Research on high octane fuel thus received a high priority and resulted in a 100 octane fuel by the beginning of World War II. while the heroic performance of the RAF is widely recognized, it would have been impossible without the 100 octane fuel.
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