The paper reviews some of the salient points arising in the design and development of the modern high-output air-cooled cylinder. It is based to a very large extent upon the work of Dr. A. H. Gibson at the Royal Aircraft Establishment, which in turn was principally a development of the pioneer efforts of Renault, supplemented by some post-war work of the author for British companies and tests made by the engineering division of the Air Service. While the paper may, therefore, lack somewhat in originality, many of the results presented, it is stated, have not been published previously. The problems of an aircraft cylinder of approximately 40 b.hp. are dealt with primarily, but some aspects of automobile-engine cylinder design are considered.
The first point treated is the heat to be dissipated, this being followed by a consideration of how to secure an even temperature-distribution in the various parts of the cylinder. Cooling by a direct air-blast and by conduction is discussed, the importance of removing the heat from the cylinder at the point where it is given to the head, the ports and the barrel being particularly emphasized. The effects of mixture-strength and cooling air-supply upon the cylinder temperature are commented on, the text being supplemented by a number of tables. Methods of finning different forms of cylinder; the cooling surface required; the effect of the compression-ratio on the output, fuel-consumption and wall-temperature; cylinder materials; types of cylinder, with a summary of the advantages and disadvantages of the different forms of construction, valve-seat inserts in aluminum cylinder-heads; exhaust-valve cooling; and valve-gears, all receive attention.
The conclusions reached are that (a) successful air-cooling is not limited to 50 b.hp. per cylinder, (b) fragility of the fins is a disadvantage of air-cooling and (c) the compromises necessary in the design of air-cooled cylinders have been made at the expense of the cooling efficiency.
The effect of the position of the spark-plugs on the power output and the fuel-consumption is discussed in an appendix and the use of two spark-plugs located on a common horizontal axis that passes through the vertical axis of the combustion-chamber in such a position that neither plug can project a flame-wave against the exhaust-valves is commended. The influence of gas velocity through the valves on the performance of an air-cooled engine is the subject of a second appendix. In this, as throughout the paper proper, numerous illustrations and tabulations of test results supplement the text.


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