tioned that the cam shaft revolves at half the engine speed, while the magneto is speeded up so it will have the same speed as the crank shaft.

Fig. 80.-Front View of Warren-Detroit " 30" Motor with Timing Gear Case Cover Removed to Show Arrangement of Cam Shafts and Water Pump Driving Gears.

At Fig. 81, B, the silent chain drive on the White & Poppe engines is shown. This installation is similar in the main to that previously described, and further description is not needed. The advantages cited for the application of chains are, first, silent operation which obtains even after the chains have worn considerably; second, in designing it is not necessary to figure on maintaining certain absolute center distances between the crank shaft and cam shaft sprockets,

Fig. 81. Showing Use of Silent Chain Connection Between Crank Shaft and Cam Shaft, and also for Driving Water Pump and Magneto Shafts. A-Chain Drive on Wolseley (English) 1912 Motor. B-Method of Using Silent Chains on White & Poppe (English) Power Plant.

as would be the case if conventional forms of gearing were used. On some forms of motor employing gears, three and even four members are needed to turn the cam shaft. With a chain drive but two sprockets are necessary, the chain forming a flexible connection which permits the driving and driven members to be placed at any distance apart that the exigencies of the design demand. When chains are used it is advised that some means for compensating chain slack be provided or the valve timing will lag when chains are worn. Many combination drives may be worked out with chains that would not be possible with other forms of gearing. It is expected that there will be a gradual tendency on the part of American designers to incorporate the silent chain drive in their product.

Valve Springs. Another consideration of importance is the use of proper valve springs, and particular care should be taken with those of automatic valves. The spring must be weak enough to allow the valve to open when the suction is light and must be of sufficient strength to close it in time at high speeds. It should be made as large as possible in diameter and with a large number of convolutions, in order that fatigue of the metal be obviated, and it is imperative that all springs be of the same strength when used on a multiple-cylinder engine. On the exhaust valve the spring must be strong enough so that the valve will not be sucked in on the inlet stroke. It should be borne in mind that if the spring is too strong a strain will be imposed on the valve-operating mechanism and a hammering action produced which may cause deformation of the valve seat. Only pressure enough to insure that the operating mechanism will follow the cam is required. It is common practice to make the inlet and exhaust valve springs of the same tension when the valves are of the same size and both mechanically operated. This is done merely to simplify manufacture and not because it is necessary for the inlet valve spring to be as strong as the other.

Piston and Rotary Valve Motors.-Mention has been previously made of the interest obtaining in various forms of valves which permit more silent operation than the conventional poppet type. The main features of the Knight engine and its advantages have been considered, but a more complete description of the valve action may be timely. The sectional view through the cylinder at Fig. 82 shows the Knight sliding sleeves and their actuating means very clearly. The diagrams at Fig. 83 show graphically the sleeve movements and their relation to the crank shaft and piston travel. At A the piston has reached the top of the exhaust stroke and the exhaust port is barely open. The inlet port is just beginning to open. At B the piston is about twothirds down on the inlet stroke and the inner sleeve has moved down, this bringing the two ports in alignment. This movement of the sleeve has closed off the exhaust port. At C the position of the sleeves at the end of the intake stroke is shown. The inner sleeve continues to go up, the outer sleeve is still moving down. Here we see the inlet port is almost closed; the exhaust port entirely so. D represents the position assumed by the sleeves at the end of the compression stroke,

both ports are closed and the compressed charge is ready for ignition. At E the piston has covered about three-quarters of the power stroke

Fig. 82.-Section Through Cylinder of Knight Motor, Showing Important Parts of Valve Motion.

and the exhaust port begins to open. Both sleeves are now traveling down. At F the piston has reached the bottom of the power stroke

and the exhaust port is almost fully opened. At G the piston is moving upward and the burned gas is being discharged through the fully opened exhaust port. At H the piston has started down on the intake stroke. The exhaust port is fully closed and the inlet port is just be

Fig. 83.-Diagram Showing Relative Movement of Sleeves and Cam Shaft of Knight Type Motor. Note Port Opening at Various Piston Positions. Shaded Portions of Sleeves Represent Ports.

ginning to open. The action may be summed up as follows: The inlet port begins to open when the lower edge of the opening of the outside sleeve which is moving down passes the top of the slot in the inner member also moving downwardly. The inlet port is closed when the