Fig. 112.—Two Methods of Operating Valves by Positive Cam Mechanism Which Closes as Well as Opens Them.
A part sectional view through a cylinder of an engine designed by G. Michaux is shown at [Fig. 112], A. There are two valves per cylinder, inclined at about ten degrees from the vertical. The valve-stems are of large diameter, as owing to positive control, there is no necessity of lightening this part in an unusual degree. A single overhead cam-shaft has eight pairs of cams, which are shown in detail at B. For each valve there is a three-armed rocker, one arm of which is connected to the stem of the valve and the two others are in contact respectively with the opening and closing cams. The connection to the end of the valve-stem is made by a short connecting link, which is screwed on to the end of the valve-stem and locked in position. This allows some adjustment to be made between the valves and the actuating rocker. It will be evident that one cam and one rocker arm produce the opening of the valve and that the corresponding rocker arm and cam result in the closing of the valve. If the opening cam has the usual convex profile, the closing cam has a correspondingly concave profile. It will be noticed that a light valve-spring is shown in drawing. This is provided to give a final seating to its valve after it has been closed by the cam. This is not absolutely necessary, as an engine has been run successfully without these springs. The whole mechanism is contained within an overhead aluminum cover.
The positive-valve system used on the De Lage motor is shown at D. In this the valves are actuated as shown in sectional views D and E. The valve system is unique in that four valves are provided per cylinder, two for exhaust and two for intake. The valves are mounted side by side, as shown at E, so the double actuator member may be operated by a single set of cams. The valve-operating member consists of a yoke having guide bars at the top and bottom. The actuating cam works inside of this yoke. The usual form of cam acts on the lower portion of the yoke to open the valve, while the concave cam acts on the upper part to close the valves. In this design provision is made for expansion of the valve-stems due to heat, and these are not positively connected to the actuating member. As shown at E, the valves are held against the seat by short coil springs at the upper end of the stem. These are very stiff and are only intended to provide for expansion. A slight space is left between the top of the valve-stem and the portion of the operating member that bears against them when the regular profile cam exerts its pressure on the bottom of the valve-operating mechanism. Another novelty in this motor design is that the cam-shafts and the valve-operating members are carried in casing attached above the motor by housing supports in the form of small steel pillars. The overhead cam-shafts are operated by means of bevel gearing.
FOUR VALVES PER CYLINDER
Fig. 113.—Diagram Comparing Two Large Valves and Four Small Ones of Practically the Same Area. Note How Easily Small Valves are Installed to Open Directly Into the Cylinder.
Mention has been previously made of the sixteen-valve four-cylinder Duesenberg motor and its great power output for the piston displacement. This is made possible by the superior volumetric efficiency of a motor provided with four valves in each cylinder instead of but two. This principle was thoroughly tried out in racing automobile motors, and is especially valuable in permitting of greater speed and power output from simple four- and six-cylinder engines. On eight- and twelve-cylinder types, it is doubtful if the resulting complication due to using a very large number of valves would be worth while. When extremely large valves are used, as shown in diagram at [Fig. 113], it is difficult to have them open directly into the cylinder, and pockets are sometimes necessary. A large valve would weigh more than two smaller valves having an area slightly larger in the aggregate; it would require a stiffer valve spring on account of its greater weight. A certain amount of metal in the valve-head is necessary to prevent warping; therefore, the inertia forces will be greater in the large valve than in the two smaller valves. As a greater port area is obtained by the use of two valves, the gases will be drawn into the cylinder or expelled faster than with a lesser area. Even if the areas are practically the same as in the diagram at [Fig. 113], the smaller valves may have a greater lift without imposing greater stresses on the valve-operating mechanism and quicker gas intake and exhaust obtained. The smaller valves are not affected by heat as much as larger ones are. The quicker gas movements made possible, as well as reduction of inertia forces, permits of higher rotative speed, and, consequently, greater power output for a given piston displacement. The drawings at [Fig. 114] show a sixteen-valve motor of the four-cylinder type that has been designed for automobile racing purposes, and it is apparent that very slight modifications would make it suitable for aviation purposes. Part of the efficiency is due to the reduction of bearing friction by the use of ball bearings, but the multiple-valve feature is primarily responsible for the excellent performance.
