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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
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.)
Q. 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 proportion 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 Fig. 26.—Sectional View of Individual Cylinder Casting with Valves-in-the-Head.
seat (Fig. 30). They are
usually a one-piece construction 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 upper