The poppet type of valve has always been used for discharge valves and one of the first steps in replacing the Corliss valve for air cylinders was the introduction of a modified form of poppet valve which opens automatically, but is positively closed, an idea first brought out by the German engineer, Riedler.

This valve consists of a hollow steel pot drawn from a single piece of steel. This pot is from sixteen to twenty inches in diameter and of approximately the same depth, with a spherical bottom surface; the metal in it is about a quarter of an inch thick in the bottom, and three-sixteenths of an inch in the sides. This closes against a suitable seat in the cylinder head close to the piston. Inside of it works a plunger; sometimes this plunger is the same diameter as the inside of the pot in which it acts as a piston, sometimes the plunger is of smaller diameter.

This plunger is operated by a rock shaft driven from an eccentric so timed that the plunger forces the valve to its seat an instant before the crank passes the center and before any return current of air can be set up from the receiver back into the cylinder. The plunger is then withdrawn and the valve is held to its seat by the excess pressure within it, since it is provided with small holes around its circumference, just above the seat, which put it into constant communication with the air receiver, and which permit the escape of the air between the plunger and the cylinder, thus helping to bring it to its seat gently and without shock or jar.

Fig. 77. Reynolds' poppet valve.

One of these valves is shown in the upper portion of the right-hand end of Fig. 77. This type of valve has remained the predominant one throughout the period of supremacy of positively operated valves. The original form of this design was the invention of the late Edwin Reynolds.

Various types of air inlet valves were tried, all designed to keep the clearance down to a minimum and yet give ample area for the admission of air, not only to keep down the suction but to secure the complete filling of the cylinder before the beginning of the compression stroke.

Two of these designs were radical and interesting by their boldness. One was known from its inventor, Mr. Julian Kennedy, as the Kennedy inlet valve. It consists as is shown in Fig. 78, of a long slender cylinder passing entirely through both cylinder heads and the piston of the air cylinder, with suitable packing rings or glands in all three. This cylinder has near its upper and lower ends a system of ports cut through its walls on long spirals, one set for each end of the air cylinder. This valve is driven by an eccentric so that the ports in it are brought within the cylinder head during the admission stroke. Both ends of the valve are perfectly open to the air which passes in through its end and out through these ports into the cylinder. The valve is properly timed to open and close correctly at each end of the cylinder.

Fig. 78 shows a design in which this valve is set on one side of the cylinder and presumably in this case there are two of these valves. In the ordinary design there was one valve in the center of the cylinder, and the air piston was driven through two piston rods symmetrically spaced on each side of it, a design which has some merit.

The other design was the invention of Mr. E. E. Slick, formerly chief engineer of the Edgar Thompson Works, now Vice-President of the Midvale Steel Company. This was still more radical. The whole cylinder was perforated at each end with a set of ports like those in the Kennedy inlet valve, and this cylinder was moved back and forth by a heavy rock shaft so that during each suction stroke these ports came within the inner surface of the corresponding cylinder head, these being rigidly held independently of the cylinder and making a sliding joint with it like the piston. This obviously gave the whole circumference of the cylinder for a port and yet the clearance due to inlet ports was absolutely nil. This arrangement is shown in Fig. 79; it has been successfully used for a number of years on large and particularly on high-speed blowing engines, but many engineers and furnace operators objected to the awkwardness of the principle involved in sliding a cylinder weighing several thousand pounds back and forth through a travel approximating a foot at a rate of sixty, seventy-five, or more revolutions per minute.

Fig. 78. Kennedy piston inlet.

The Kennedy valve also was disliked by many engineers and opera-ators because of the great mass of metal to be pulled back and forth through a travel of nearly two feet, and at speeds up to fifty or sixty revolutions, also to the three moving joints necessary in that device, one in the piston, and one in each cylinder head. On several occasions these valves became detached from their driving mechanism and owing to their inertia, due to their weight and high speed, they generally made a wreck of serious proportions on such occasions.

Fig. 79. Slick blowing tuba at the central furnaces of the American Steel & Wire Co., Cleveland Ohio.

A modification of this valve has been very successful. This is shown in the lower half of the right-hand air cylinder head in Fig. 77 and by Fig. 80. It consists practically of one end of the Kennedy inlet working in a short cylinder set in the main cylinder-head.

The air passes in this case longitudinally through the valve and then outward through the ports in the sleeve which carries it, and so into the main cylinder. The only disadvantage under which this valve suffers as compared with the Kennedy is the slight increase in clearance necessary to carry the air from the ports in the sleeve into the cylinder proper.

The weight of the valve is only a small fraction of that of the Kennedy, and its travel is also much less, so that the inertia of this type of valve is almost insignificant in comparison with the inertia of the Kennedy valve and Slick valve types.

The construction is shown in the illustration. The Southwark air valve. The Southwark Foundry & Machine.

Company had enjoyed for many years the reputation of pioneer builders of high-speed engines, having developed a system of balanced slide valves for the steam cylinder which could be put under the control of a governor, and permitted these engines to be operated at a higher speed than that attainable by any other type of large engine at the time when the modern development of blowing engines began. They also appreciated the advantages of high speed from a purely mechanical standpoint, notably in regard to inertia stresses and the beneficial effects which they could be made to exercise upon the running of engines. This is a subject which we shall presently discuss.

These two conditions made it natural that the Southwark Company should desire to extend the benefits of their system of high speed engines to blowing engines, and they accordingly developed a system of positively operated valves arranged to open and close without shock and to provide ample port area for any speed at which blowing engines could be successfully operated. This system uses a multi-ported or a grid-iron valve both for inlet and outlet. The inlet valve is operated entirely by a cam driven by an eccentric. The inlet valve is on the inside of the head and is unbalanced because its movements take place only at times when the pressure on both sides of it is the same. The discharge valve is on the outside of the head and is balanced by a cover plate on its back. This valve is closed at the end of the stroke by a cam attached to the same mechanism as that which operates the inlet valve, but is opened by a small auxiliary cylinder in communication with the main cylinder, when the rising pressure in the latter during compression, equals that in the receiver.

A longitudinal section is shown in Fig. 81, and views of a cylinder head with its valves are shown from within and from without by Figs. 82 and 83.

Fig. 80. Kennedy inlet valve.

It is probably no more than fair to say that this system of valves was the earliest of any of the mechanically operated valves to be developed into its present condition and at the same time remain in general use. It is also probably fair to say that this valve was the first to be used in this country at the high speeds which are absolutely necessary with gas-driven blowing engines, and which are very desirable with many steam engines.

Fig. 81. Southwark air cylinder showing inlet and outlet valves, with gear.