Fig. 25.-Sectional View Through Cylinder and Crank Case of Benz Racing Motor Showing Valve-in-the-Head Construction. Q. What is the advantage of "valve-in-the-head" construction? A. When the valves are placed so that they open directly into the combustion chamber it is possible for the gases to flow with less friction on account of the direct passages and a combustion chamber is provided which has greater efficiency as regards heat losses than either the "L" or "T" form. (Figs. 25 and 26.) n Fig. 26.-Sectional View of Individual Cylinder Casting with Valves-in-the-Head. Q. What determines sizes of valves? A. Valve sizes are determined by the cylinder volume and by the speed at which the engine operates. High speed engines require larger diameter valves in propor tion to the cylinder bore than do low speed engines. The form of the cam and shape of the valve head also have a bearing on the size of the valve. Q. Name forms of mushroom valves. A. Mushroom valves may have either a flat seat (Fig. 28) or a beveled seat (Fig. 30). They are usually a one-piece con struction and are machined from a forging of nickel steel. Sometimes the head is of one material, such as a 35% nickel steel, electrically welded to a carbon steel stem. Valves are sometimes made with a head of cast iron which is attached to the steel stem by riveting the end of the stem over after the cast piece is forced on the upper end. Fig. 26A.-Cylinder Pair of Knox Engine With Valves-in-the-Head. Q. Are other forms of valves practical? A. A number of other forms of valves have been developed recently that differ radically from the mushroom form. These have given satisfactory service in some instances. Fig. 27.-Sectional View of Four-Cylinder Motor Having the Cylinders Cast in Block and Using a TwoBearing Four-Throw Crankshaft. Q. Name the other systems of valve construction. A. The other forms of valves that have been evolved to replace the poppet type usually consist of some rotating or sliding member that uncovers ports, through which the gases pass. Q. What is the Knight engine? The Knight engine, wh ch is illustrated at Figs. 31 and 32, |