PREFACE This volume attempts two things: to formulate existing knowledge of the functioning of the airplane engine and its auxiliaries; and to present and discuss the essential constructive details of those engines whose excellence has resulted in their survival. The material here collected is largely new; very little of it could have been written before the war and only a small fraction was available for publication before 1919. It is based mainly on the researches and engine developments originating during the war and resulting from the war's urgencies. The researches have been carried out almost exclusively under governmental auspices; in the United States at the Bureau of Standards and at the Air Service experimental plant at McCook Field; in Great Britain at the Royal Aircraft Factory and the National Physical Laboratory; in France and Germany at equivalent institutions. Many of the results of these investigations were published confidentially during the war in Reports of the Bureau of Standards; in Bulletins and Technical Orders of the Airplane Engineering Division of the U.S. Army; in Reports of the (British) Advisory Committee for Aeronautics; in Bulletins de la Section Technique de l'Aéronautique Militaire; and in Technische Berichte. This material has now become available and much of it has been published in the Reports of the (U. S.) National Advisory Committee for Aeronautics and in the technical press.