The gradual trend toward overhead valves in automobile engines, as indicated by an increase in their use on American cars from 6 per cent in 1914 to 31 per cent in 1922, has been accelerated, in the opinion of the author, by their successful application to aircraft engines and by the publicity given them by their almost universal adoption on racing machines. Tractor engines recently brought out show the advantage of this construction. Methods of operating valves in the cylinder-head; the advantages of the valve-in-head construction as regards the form of combustion space, engine cooling and high-speed operation; the reason for using an overhead camshaft to operate the valves on racing engines, the question of noisy operation and the possibility of having an overhead camshaft engine operate as quietly as one in which the camshaft is enclosed in the crankcase; the location of the drive in the various foreign engines of the overhead camshaft type; the silent operation that is possible with a rear drive; the use of chains and spur, helical, worm and spiral bevel gears for the camshaft drive, with the advantages and disadvantages of each method and descriptions of specific applications; and some radical designs of overhead camshaft drive and valve-actuating mechanism that have been developed abroad are among the topics discussed. The three methods of operating the valves: (a) directly through the action of cams on followers secured to the end of the valve-stem, (b) through the interposition of single-armed levers or adjusting blades between the cams and the valve-stems, and (c) by the use of tappet levers, are also outlined with particular reference to the specific applications of each. Numerous illustrations supplement the text.
