Combustion-Chamber Design in Theory and Practice 290015
POINTING out the difference between scientific and industrial progress as manifested by heat theory and engine design and the Carnot and Otto cycles, the author discusses the working principles of combustion. A simple synopsis of internal combustion is presented, followed by a discussion of influence of spark-plug location on detonation and pressure rise and some observations on overcooling and flame quenching.
The reasons underlying the decision to use a particular type of engine in an automobile are commented on, and this is followed by a discussion of the limitations imposed upon induction and combustion by such a choice. The advance in combustion-chamber design is traced from the early T-head through the L-head, in various forms, the overhead-valve, the hemispherical and four-valve types. Drawings of the different heads supplement the text and some comparative power curves are included.
New developments receive some consideration and a statement of the Whatmough principles of combustion control is given. The coordinated application of the heating, flow and cooling effects to engine design is discussed at some length and a combustion-chamber head requiring mixture control and compensatory water circulation is illustrated. This design uses a curved venturiform annulus for the inlet port to deflect the gas-flow into the cylinder and lessen the effect of varying suction during sudden opening and closing of the valve and differential water circulation with two separate outlets controlled by thermostats to assure smooth burning and fuel efficiency by preventing overheating and overcooling.
Compactness, turbulence and central spark-plug location are postulates from heat theory which have become traditions in combustion-chamber design. The conflict between theory and practice in these respects is discussed, with reasons given for the failure of the former and the success of the latter. The relative importance of the factors of service, economy, power, smoothness and flexibility in commercial vehicles and high-grade passenger-cars is stated and their dependence upon combustion control is emphasized.
The discussers differ with the author on many points of theory while acknowledging the improvement in performance made by changes of combustion-head design on English engines. These improvements are asserted to be the result mainly of change in location of the spark-plug with resulting increase in length of flame travel, less compact design of the combustion space and provision of clearance space, increase in volumetric efficiency by reduction of restrictions, reduction in offsetting of the chamber over the piston, and better distribution of the volume of the charge in the chamber with respect to the spark-plug.
Derogatory comparison is made of designs of English engines shown by the author with American practice, which is said to follow closely the Ricardo design. In reply to these criticisms, the author points out that he purposely selected bad examples of English design to emphasize the changes made and the improvement in performance, and states that the only point at issue concerns the relationship of cooling in clearance space to engine detonation. He presents curves from bench-test data showing a 50-per cent increase in power of the high-compression anti-turbulent head as compared with the sloping-roof and turbulent L-heads.