DURING the past decade the general trend of aircraft-engine design has continued toward increased piston displacement, higher crankshaft speed, higher brake mean effective pressure, and improved materials.
These changes have had a marked influence on increasing the overall performance of the airplane by improving take-off, bettering climb, permitting higher cruising speeds at greater altitude, increasing periods between overhaul, and improving the reliability of the powerplant.
Although of secondary importance until quite recently, today fuel economy has become a major objective in both military and commercial operation.
Fuel consumption is a function, generally speaking, of engine design, of the properties of the fuel itself, and of the procedure for introducing and regulating the fuel-air mixture in the operation of the powerplant. Referring to engine design, lower specific fuel consumption may be obtained by careful attention to effective cooling of the combustion chamber and piston with optimum compression ratio for the fuel characteristics, and to good distribution and turbulence with efficient supercharging.
Ethylized fuels of high-knock rating have been a very important factor in the recent improvement of aircraft-engine performance. Despite means for regulating the fuel-air ratio to compensate for altitude or to obtain fuel economy, results have not measured up to the standards of economy desired. A major difficulty in the attainment of optimum fuel economy is described as the lack of some instrument which would definitely indicate to the pilot the immediate effect of manipulating the mixture control, and also would regulate the fuel-air mixture ratio automatically while the pilot is otherwise engaged. Devices which fulfill these requirements are described in detail.
A resume of the tests with 100-octane (Army method) fuel by the Air Corps and the Wright Aeronautical Corp., clearly indicates the marked improvement not only in increased take-off power but also in the extremely low fuel consumption at cruising output made possible by the use of fuels of ultra-high octane rating.