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which turns against the roller Q pivoted in the pushrod P. As the engine shaft revolves in the direction indicated by the arrows, the wheel S revolves at half the speed and in the opposite direction. When the cam R runs against the push-rod P, it pushes it outward against the bell crank 0. This in turn presses the valve stem L upward and lifts the outlet valve H from its seat I, thus allowing the burnt gas to escape. The spring M, acting against the valve foot N, serves to bring the valve firmly on its seat and to cause the pushrod to follow back against the cam. In this motor it will be seen that the inlet valve E is not connected with any valve gear or other mechanism, but is provided with a spring C, which serves to keep it firmly seated. This is known as an automatically operated valve, while the outlet valve is a mechanically operated valve. Many motors are built in this way, for the suction of the piston on the intake stroke is sufficient to act on the inlet valve and cause it to open long enough to admit the proper charge

of gas.

Although several excellent motors utilize this system, yet they have many disadvantages. The springs soon lose their strength and liveliness, causing the valve to open slowly or unevenly, or else to open too readily and seat too lightly. In one case the charge admitted is insufficient, while in the other case the compressed gases are liable to escape backward into the inlet and cause loss of power and back-firing. Moreover, in case of a leakage around the piston, or in the firing chamber, the suction of the piston may prove unequal to the task of opening the inlet valve far enough or long

enough to admit a full charge. It is far better to operate both inlet and outlet valves by mechanical means as illustrated in Fig. 25, in which the exhaust valve is

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actuated by the push-rod and cam A, attached to the cam-shaft B, and the inlet valve VI is also operated by

another push-rod D through the cám E, on the same shaft with the exhaust-cam, but set at an angle with it. By this method there is no chance of the inlet valve sticking on its seat or failing to seat and if the cams and gears are set properly a uniform and correct charge will always be admitted to the cylinder at exactly the

Fig. 26.—“L”-head Cylinder

right time. This is a very common and widely used type of four-cycle motor. The exterior form is shown in Fig. 26. It is known as the T-head or L-head type from the shape of the cylinder and valve chamber. More recent still is the valve-in-head type in which the two valves are located in the head of the cylinder instead of in an offset, or separate, chamber. This type (Fig. 27) has many advantages over the T-head type, and is now generally acknowledged to be far more effective and

economical. The two valves being set in the cylinder head allows the interior to be machined smooth and free from corners, or pockets, where burnt gas might accumulate. The incoming charge also serves to thoroughly fill the chamber and the outgoing exhausted gas is more thoroughly discharged. The cylinder head and

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Fig. 27.—Valves in Head

valve seats, as well as the piston head, are also greatly cooled by the fresh charges of gas, while the arrangement of the valves allows the greatest cubical capacity of the cylinder with the least area of surface. Most of the various makes of poppet-valve motors vary principally in the valve mechanism, style of gear, and other minor details; but motors with rotary and sliding valves have now come into use and in their perfected state give results equal to, or even better than, the best poppetvalve engines.

The Knight Sleeve Valve motor is a comparatively recent invention, but has won its way rapidly to popularity and success by its remarkable performances in several well-known makes of automobiles. In this engine, which is shown in Fig. 28, the piston is surrounded by two cylindrical sleeves, or tubes, one within the other and both between the piston itself and the walls of the cylinder proper. The diagram (Fig. 28) shows a section cut away to show the piston P, the inner sleeve S, and the outer sleeve 0, while C represents the cylinder walls. The two sleeves are connected with connecting rods and eccentrics to the gear shaft, as illustrated in Fig. 29, in which P represents the piston, S the inner sleeve, O the outer sleeve, C the cylinder, A and B the connecting rods to the two sleeves, and D the eccentric attached to the gear shaft E. The gear is in the form of a sprocket-wheel E connected with the main shaft by a roller chain F. In the sleeves are openings, or ports, G, H, which are alternately brought opposite the exhaust and inlet ports in the cylinder, I, J. In operation the sleeve 0, sliding past the sleeve S, brings the ports G, H opposite one another as well as in line with the inlet port I. This occurs on the downward stroke of the piston and the charge of gas is thus drawn into the cylinder as in Fig. 30. On the upward or compression stroke, the sleeves move to the position shown in Fig. 31, in which the ports G, H, I, J are all out of line and thus closed against the escape of gas from the cylinder. The

The charge is compressed and fired on the limit of this upward stroke as usual and the piston driven downward. Near its lower limit two of the ports are again brought opposite the

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