PREVIOUS papers on Combustion-Chamber Design by three leading authorities on the subject showed enough points of real or apparent disagreement to leave the designing engineer in doubt on many of the details of design which they discussed. The author of this paper was asked to make a study of the works of these three authorities to discover points of agreement and clarify the subject for the benefit of engineers in general.
Requests were made that each of the three authors in question furnish a list of his writings to be considered in this connection. Such lists were received from Mr. Ricardo and Mr. Janeway, but not from Mr. Whatmough in time for use in preparing the original paper. After the paper was delivered, a letter was received from Mr. Whatmough, and revisions in the paper have been made on the basis of that letter.
Credit is given to Mr. Ricardo for initiating the study of combustion-chambers and inspiring other workers. He advocated turbulence in his earlier work and later promoted control of the rate of burning of that portion of the charge which is ignited first.
Whatmough is essentially a physical chemist; and his work is difficult for engineers to follow, partly because he abandons theories of thermodynamics.
Discovery of the principle of preventing detonation by cooling that portion of the charge which is last to burn is credited to Mr. Janeway, although conditions giving the same effect were found in Mr. Ricardo's designs. Mr. Ricardo has since subscribed to this principle. Mr. Whatmough has introduced the narrow clearance space in some designs, but has not definitely subscribed to it as a principle.
The rough performance obtained from the turbulent combustion-chambers called for improvement. Mr. Ricardo attacks this by controlling turbulence, Mr. Whatmough by controlling the temperature of the unburned mixture, and Mr. Janeway by control of the area of the flame front. Acceleration of the rate of pressure rise, as proposed by Mr. Janeway, is accepted by the author as a yardstick of roughness.
The paper concludes with an enumeration of five principles that the author finds to be common in the work of the three investigators, including cooling the last gas to burn by means of a shallow clearance space; locating the spark-plug near the exhaust valve; non-compact combustion-chambers; volume control for smoothness; and controlling the acceleration of the rise in pressure, to prevent roughness.
Discussion* presented at the meeting by Mr. Berry urged a study of combustion-chamber design as it influences steady idling and high-speed missing. Requirements as to these, which are considered largely as problems of carburetion, cannot be met unless conditions are right in the combustion-chamber. Mr. Janeway and Mr. Taub consider this to be largely a question of having the good part of the mixture at the spark-plug, and Mr. Taub showed how the trouble had been remedied in one combustion-chamber of peculiar form by modifications. Other discussers told of instances in which these difficulties had been caused by overlapping of the valve openings.
Written discussion has been received from all three of the writers whose works were reviewed. Mr. Ricardo expresses agreement with Mr. Janeway in nearly all his conclusions, and states that the conclusions have been reached by similar reasoning. He explains his early insistence on turbulence by the fact that engines of the slower speeds that formerly were common did not produce so much turbulence as present engines of higher speed. The only difference between himself and Mr. Janeway which he mentions as significant is that he considers the use of a variable-compression engine or of graded fuel mixtures to be a much better method of testing for detonation value than that of border-line ignition advance.
Mr. Whatmough, in his contribution, says that the difficulty in understanding his work applies equally to that of all modern heat theorists and that thermodynamics is virtually superseded in the newer sciences by the quantum heat theory. He refers to a summary which appeared at the end of a series of his articles in which the conclusions were couched in ordinary engineering terms.
The control of heat throughout the cycle is aimed at by Mr. Whatmough, who criticizes the paper as being confined to only a small portion of the cycle and infers that the means discussed are much more limited than those which he proposes. The cooling compression-space, for instance, he regards as one of several means of controlling the temperature of the compressed charge.
Mr. Janeway maintains that his method of securing variation in the rate of pressure rise by calculations based upon the fundamentals of reaction rate is superior to any cut-and-try method.