AIRCRAFT PROPULSION — TAYLOR 277 



For successful use in spark-ignition engines, gasoline must have the 

 proper volatility range, and the highest possible resistance to "knock," 

 or "detonation." Control of volatility seems never to have been a 

 serious problem, and development work in aircraft fuels has centered 

 around increasing their antiknock value. 



Earliest work on the relation of detonation to fuel composition 

 seems to have been by Eicardo in England and by Kettering in the 

 U.S.A. Intensive work was started by Midgeley and Boyd, under 

 Kettering's direction, in Dayton, Ohio, in 1917. In the course of this 

 work it was discovered that some substances, notably iodine, had a 

 strong antilmock effect even in very small concentrations. This dis- 

 covery led to an intensive search for powerful antiknock agents. 



Midgeley's work was done on a tiny smgle-cylinder engine in an 

 old Dayton kitchen, and when a promising substance was found there, 

 he would bring it to the McCook Field engine laboratory for test in 

 an aircraft engine. I was closely associated with his work during 

 my administration of that laboratory, 1919-1923. By 1920 toluene 

 and its related compounds appeared promising as an additive and 

 were used in flight tests, notably by Schroeder for the 1920 altitude 

 record with a turbo-supercharged Liberty engine. By 1921 the ex- 

 treme antiknock effects of metallo-organic compounds was evident, 

 and in 1922 Midgeley brought the first samples of tetraethyl lead, 

 Pb (02115)4, to McOook field for tests in a full-scale aircraft engine. 

 This important substance was accepted for use in aviation gasoline by 

 the U.S. Navy in 1926 and by the Army in 1933, and it is now uni- 

 versally accepted as an additive for gasoline. 



Another important contribution was Edgar's work, about 1926, in 

 determining the effect of fuel structure on antiknock quality and, 

 specifically, discovering the high antiknock properties of the 

 branched-chain parafins such as iso-octane. 



Specifications and laboratory tests for antiknock quality of aviation 

 fuels were sponsored by the Cooperative Fuel Research Committee in 

 1933, and led to good control of this quality in U.S. aviation fuels soon 

 after. S. D. Heron was also an important contributor to this result. 

 The "performance number" of a fuel, used from about 1935, is the 

 ratio of Imock-limited indicated mean effective pressure (klimep) 

 with that fuel, to the klimep in the same engine using iso-octane. Fig- 

 ure 15 shows the improvement of klimep vs. time, achieved both by 

 tetraethyl lead and the control of fuel composition. 



The powerful effect of water or water-alcohol injection is also illus- 

 trated in figure 15. This development was promoted chiefly by the 

 N.A.C.A. during the last war, and by 1946 water-alcohol injection was 

 generally used for takeoff by both military and transport airplanes. 

 The high consumption of the auxiliary fluid (about 50 percent of the 

 fuel flow) limits its use to the takeoff period and to engines with suffi- 



