A more modern way, which is fast becoming universal, is to use straight sides for the cams and take advantage of rapid closing in another way, the benefits of which more than offset the benefits of the old way, while having no corresponding disadvantages. In the ordinary automobile engine running at 1000 revolutions per minute, the gases are traveling into the cylinder at the rate of 5000 to 6000 feet per minute, and traveling out at from 7000 to 10,000 feet per minute. At this tremendous speed, the gas inertia is very high, and experiments go to show that the gases by means of this inertia will continue to force their way into the cylinder even against the return motion of the piston. So it is now common practice to hold the inlet valve open about 30 degrees on the upstroke of the piston, which results in a much larger piston charge. The same practice is carried out with the exhaust, but as the pressure is higher, so large an angle is not necessary. These actions take place on the back - flat side - of the cam surface, and have given to the highspeed automobile engine a larger charge and a more complete scavenging effect, resulting in more power and speed from the same sized cylinder.
As proof of this statement, the power curve of an engine of but 3½-inch diameter of cylinder is shown in Fig. 19. This size of six-cylinder engine would be rated by any formula at about 29 horsepower at the maximum speed, and a commercially obtainable type in this size would doubtless be guaranteed to deliver between 20 and 25 horsepower. This engine, not built for racing purposes, displays a power curve which continuously rises, a speed at which it would turn downward not having been obtainable in the tests. This curve shows also that the maximum power obtained was over 80, which is nearly three times the power of the ordinary engine of this same size. This result is ascribable to superior valves and superior attention to the valve angles as governed by the cams.
Fig. 19. Font Curve of an American Engine with Superior Cams and Balancing.
When it was stated that but two valves per cylinder were ordinarily used, with one cam for each, the majority case was spoken of. But, as it is a fact that there are other cases which differ from this, it would not be fair to close the subject without mentioning them. Thus, the most prominent advocate of air cooling in this country and the world, the H. H. Franklin Manufacturing Company, used three valves, and consequently three cams, per cylinder. These three were: the ordinary inlet; the usual exhaust; and the additional auxiliary exhaust. By re-designing later, this complication was avoided and the third valve eliminated.
A case in which the cam does differ is that of the use of two overhead valves operated by a single camshaft, Fig. 20. This practice originated with the F. I. A. T. Company, which brought it out for racing use only, where it was particularly useful in that it halved the weight of the camshaft, as well as saved much weight in push rods, etc. Later, this was taken up by other firms for regular use, although the company which first brought it out has never done so. In our own country, the device was adopted by the Pope Toledo Company, the Stoddard-Dayton, De Luxe, and others. The work of opening the extra valve is done by a spring, i.e., a depression in the back of the cam allows a strong spring to pull the push rod down, by which process the valve stem is depressed and the valve opened. The V-type of motor has made considerable difference in valve motions, for one thing bringing into use valves set at an angle with the vertical, a practice previously considered very bad because the weight of the valve adds to the tendency to wear the valve and seat on the low side. In the form shown in Fig. 21, the eight-cylinder motor used in the Briscoe 38, made by the Briscoe Motor Company, Jackson, Michigan, there are no unusual features. The single camshaft with 16 cams is centrally placed in the middle of the V and operates the push rods, inclined outward, parallel to their respective groups of cylinders. A rocker arm, or follower, is used at the cylinder heads to transfer this up-and-down motion to the valves which are set in the center of the cylinder heads and are thus parallel to the push rods.
Fig. 20. Overhead Valve Motion with Followers Working Directly on valve Steams and Having no Push Rods.
Courtesy of Chalmers Motor Company, Detroit, Michigan.
Fig. 21. Section through Briscoe Eight, Showing Camshaft Arrangement.
In the majority of V-type motors, both eights and twelves, the valves are in side pockets, the cylinders being of the L-type, and thus there is no radical innovation except the inclined push rods and valve systems. In a few of these motors, however, a follower is used between the cams and the push rods because of other structural reasons.
When any kind of a cam follower is used differing from the usual direct-lift push rod, this may or may not affect the shape of the cam. Usually it does not, so that the shape does not have to be taken into account. Ordinarily these followers are used to prevent side thrust on the push-rod guide, the follower itself taking all the thrust and being so designed as to be readily removable or adjustable, to take care of this. In cases where this does not obtain, the object usually sought is the removal of noise. The two objects may be combined as in the case shown in Fig. 22. This represents an enlarged view of the cam mechanism of the famous one-cylinder French car, Peugeot. It will be clear that the action is that of one cam operating both the exhaust and the inlet valves through the medium of a pair of levers, upon which the cam works alternately. Difficulties in Making Cams. There was a time when the production of a good, accurate camshaft was a big job in any machine shop, well-equipped or otherwise, and represented the expenditure of much money in jigs, tools, and fixtures. Now, however, the machine-tool builder has come to the rescue of the automobile manufacturer, and special tools have made the work easy. So it was with the production of the shaft with integral cams; this used to be a big undertaking but, today, special machinery has made it an easy matter. The illustrations, Figs. 23 and 24, show some of the product of a cam milling machine. This is now the favored way of putting out engines, for the integral cams and shaft have the advantage of much lower first cost and, with proper hardening, will last fully as long as those made by cutting the cams separately and assembling them in their proper position on the shaft.
Fig. 22. Cam Mechanism of Peugeot Single-Cylinder Engine.
Fig. 23. Cams Integral with Shaft - Milling Machine Job.
Fig. 24. Another Camshaft with Integral Cams.
An even later improvement in the way of a machine for producing cams on an integral shaft is the grinding machine which has been developed for this purpose. This works to what is called a master camshaft - that is, a larger-sized shaft which has been very accurately finished. This master shaft is placed in the grinding machine, the construction of which is such that the grinding wheel follows the contour of the very accurate master shaft and produces a duplicate of it, only reduced in size, a reducing motion being used between master shaft and grinder-wheel shaft.
The result of this arrangement is a machine which is almost human in its action, for it moves outward for the high points on the cams and inward for the low spots on the shaft. Moreover, it has this further advantage that all shafts turned out are absolutely alike and thus accurately interchangeable. It allows also of another arrangement of the work, the drop-forging of the shafts within a few thousandths of an inch of size; the surface of skin is easily ground off in one operation, then the hardening is done, and the final grinding to size is quickly accomplished. In this way, the shafts may be produced more cheaply than was formerly the case, and have in addition the merits brought out above, namely, greater accuracy, superior interchangeability, and quicker production.
The same process is applicable to, and is used for, other parts of the modern motorcar; thus crankshafts are ground, pump and magneto shafts are finished by grinding, and many other applications of this process are utilized. The process can be extended indefinitely, the only drawback being that a master shaft is very expensive.
With the old method of making the cams and shaft separate, the amount of inspection work was very great and represented a large total expense in the cost of the car. Thus, it was necessary to prove up every cam separately, as well as every shaft, and, later, the cams and shaft assembled. One of the forms of gages used for inspecting cams is shown in Fig. 25. It is in two pieces, dovetailed together. This allows of the testing of many shapes of cam with but one base piece and a number of upper or profile pieces equal to the number of different cams to be tested. To test, the cam is slipped into the opening, and if small, the set screw forces it up into the formed part of the gage, showing its deficiencies; while if large, it will not enter the form.
Fig. 15. English Cam Gage.