Q. What is the cylinder head? A. The cylinder head is the name generally given to the upper or closed end of the cylinder. Q. Name two methods of constructing cylinder heads. A. Cylinder heads may be a separate casting attached to the cylinder member by bolts or they may be formed integrally with the cylinder during the casting process. The latter is the conventional method. Q. What are the advantages of individual cylinders? A. It is claimed that individual cylinders are simpler to form at the foundry, easier to handle in the machine shop and that they are more cheaply replaced in event of damage than where cylinders are cast in a block. With some form of engines it is possible to use only individual cylinder castings. This is true of the two cylinder opposed form, engines having mechanically applied sheet metal water jackets and air cooled engines. (Fig. 26) Q. What are the advantages of block castings? A. Block castings, in which three or more cylinders are cast together, not only make a shorter and more compact cylinder assembly possible, but produce a stronger engine because the cylinder block actually reinforces the engine base. With unit castings of the individual type the engine is longer and the crankcase must be made heavier in order to secure the proper degree of strength. (Fig. 27.) Q. What are the valves? A. The valves are members that control the ports through which the gases enter and leave the combustion chamber. Q. Name two types of valves. A. Valves are either automatically or mechanically operated and may be of the conventional mushroom or poppet valve type or of the sleeve, slide, piston, ring or rotary disc forms. Q. Can automatic valves be used for exhaust? A. Automatic valves cannot be used for exhaust because they can only open when there is a partial vacuum in the cylinder or when the pressure in the cylinder is less than that of the outer air. Auto matic valves cannot be used except for letting in a charge of fresh gas as they will only open during the suction stroke of the piston. Q. Why must exhaust valves be mechanically operated? A. Exhaust valves must be mechanically operated because they are forced to open against the pressure of the burnt gas in the cylinder which may be three or four times that of the atmosphere. Q. Where are valves placed as a rule? A. Conventional practice is to place the valves in pockets or valve chambers at one or both sides of the cylinder castings. They are commonly placed so that when opened the entering fresh gas stream will flow directly into the combustion chamber or so the exhaust gas will find a free passage to the outer air. Q. Describe the three common methods of valve placing. A. The three common methods of valve placing are at the top of the cylinder opening inward, all valves at one side, or half on one side and the remainder on the other. In some engines one valve may be placed at the top of the cylinder while the other is carried in a side pocket. Q. How are valves operated? A. Valves are usually operated by means of push rods which are actuated by revolving cams. The method of valve operation depends upon its placing in the cylinder. If placed in a pocket at the side it may be operated directly by means of a movable p unger interposed between the cam and the valve stems. If placed in the cylinder head it is usually actuated by a push rod and centrally fulcrumed lever which changes the upward movement of the valve plunger to a downward movement of the valve stem. (See Fig. 29.) Q. What is the advantage of L head construction? A. It is claimed that greater compactness is obtained by placing all valves on one side of the engine and that the piping is more easily installed. (Figs. 23 and 27.) Q. What is the advantage of T head construction? A. Carrying the valves on both sides of the cylinder makes it possible to use larger valve members and ports of greater area for the passage of the gases. (Fig 24.) 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. What determines sizes of valves? A. Valvesizes are determined by the cylinder volume and by the speed at which the engine operates. High speed engines require larger diameter valves in propor t'on 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. |