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Berriman, 1913, Aviation, pp. 249–250:

There is one technical word in particular to which it seems well to call special attention—efficiency. This term has but one meaning to the engineer, to whom it represents the ratio of the useful work done to the energy expended in doing it. Thus, technically, the only justifiable use of the word is in the form of a percentage, e.g. 80% efficiency, etc. The sole criterion as to the technical justification of the use of the word in any particular case is, therefore, determined by whether or no it is possible to define 100% efficiency. When there is obviously no such theoretical limit to the relationship to which the term efficiency has been applied, its use, technically speaking, is improper.
But the ramparts of this exclusiveness have been much battered. The lay public uses the term in an indefinite sense, implying a vague merit.

Textbook of Aero Engines, 1920, p. 83:

During the past fifteen years the commercial development of the internal combustion engine has come forward from a place of questionable utility to a necessity of the first rank in the public mind, and has undergone a process of remarkable improvement from a mechanical viewpoint. Perhaps no other invention has brought the meaning of the word "efficiency" into so much prominence, in this industrial era, than has the internal combustion engine.

See also Maxim, 1892, Progress in Aerial Navigation.

w:wikt:efficiency, w:Efficiency, w:Lift-to-drag ratio, w:Thrust-to-weight ratio,

Enclosing categories Propulsion
Keywords Engine
Start year
End year