Established practice in connecting-rod design is almost all in favor of the common H-section rod, usually with two bolts to attach the cap. In some cases four bolts are used, since with four bolts a flaw or crack in one is less likely to cause damage than is the case when only two are used. The old scheme of hinging the cap at one side is now practically obsolete, having been discarded because of the fact that it made accurate adjustment of the bearing surfaces almost impossible.
Tubular rods, in place of the H-section, are giving good service in several of the long-stroke foreign motors, and it is difficult to see why this form is not superior to that in common use. The question of cost, however, is a consideration, since it is necessary to bore the hole through the inside of the rod, whereas a forged rod of H-section requires no machining except at the end.
The wonderful progress in welding, however, has made it possible to construct a tubular connecting rod at a very low expense, and, due to its many advantages, this is finding much favor for small motors. The two ends are machined and a section of tubing welded to them.
One advantage of the tubular rod, in addition to its superiority for withstanding the compression load to which a rod is chiefly subject, is that it can be used as a pipe to convey oil from the big end to the piston-pin bearing.
In Fig. 7 is illustrated an example of a very light-weight, high-quality, aviation-motor connecting rod, machined out of a solid bar of alloy steel, and provided with four bolts in the cap.
The greatest variations in automobile crankshaft design, aside from those permitted or made necessary by differences in the quality of material, are due to the conditions involved in the different combinations of cylinders that can be utilized. Thus the number of crank throws, as well as their position, varies with the type of motor.
The duty of a crankshaft is of so severe a character, involving the practical equivalent of thousands upon thousands of heavy blows, that for any but very heavy, slow-running motors, the crankshaft should be made of nothing but the finest alloy steels obtainable.
In the valves and valve mechanisms of modern gasoline engines there have been and are impending more interesting changes than seem in prospect in any other portion of the mechanism of the modern automobile. Particularly is this the case with reference to the present tendency to discard the poppet valve with its many objectionable features.
Even where there is no tendency toward the use of a sleeve-valve or slide-valve form of motor, much experimenting has been done with increasing the number and changing the position of the valves. As an example of the former, many of the cars in the last Grand Prix race, in France, had four valves for each cylinder, two inlets and two exhausts. As an example of the latter, the same race showed all but two makes of car with the valves in the head, either vertical or inclined, except in one case, in which they were horizontal.
Though the very first internal-combustion engines ever made were operated with slide valves, the poppet valve was introduced very early in the history of this art, and has reigned supreme in practically all types of gas and gasoline engines.
The chief advantage of the poppet valve is its capacity for continuing operative at excessively high temperatures, but since the cooling of engines has progressed to the status of high reliability this advantage is of less importance than formerly. And the disadvantages of poppet valves - the small openings that they afford, the noisy and hammering action they involve, their tendency to leak and in other ways give out, and the necessity for frequently regrinding them - are objections so serious that it is not to be wondered at that the prospect of their elimination is so widely welcomed.
About the only recent improvements that have been made in poppet valves are in the quality of material used in them - the best valves now being those with cast-iron and nickel heads, which offer maximum resistance to warping from the heat to which they are subjected, and with carbon-steel stems, which are superior in their wearing qualities. Much use has been made recently of tungsten as a material for valves. Steel containing this is even harder than nickel steel, and experiments have shown that it does not warp as much. In practice, the objection found to cast-iron heads was that the fastenings to the carbon-steel stem were not sufficiently strong to withstand the constant pulling and pushing to which a valve was subjected. As a result they separated, causing trouble.
In the operation of poppet valves, the cams become an important factor. These are the parts, which, in revolving, raise the valves so that they open at the proper time. In addition, they are so shaped as to hold them open for just the right length of time, and allow them to close, through the medium of the valve spring pressure, at the proper point in the cycle. The importance of this can be seen, if we consider that opening the slightest fraction of a second too late will reduce the amount of the charge very much, and thus lessen the power developed by the motor.
Fig 8. Fiat Marina Motor with Encased Valve Action This Photograph Protected by International Copyright.
The use of casings to enclose the valve stems, springs, and push rods, so as to keep these elements from exposure to dirt, while at the same time silencing in large degree the noise they otherwise make, is also becoming usual.
An excellent example of this may be seen by referring back to Fig. 5, in which it will be noted that the whole aide of the motor where the valve mechanism is located is covered with a long, removable plate, keeping in noise and lubricant, and keeping out dirt. Usually, however, on a six-cylinder motor the valve enclosure is made in two parts, one-half enclosing the mechanism of the valves in the first three cylinders, the other, those in the last three. This is, of course, the preferred construction on those six-cylinder engines which have the cylinders cast in threes, instead of in a block, as the one referred to. On some motors where this construction has not found favor, the designers have followed the plan of enclosing the individual valve mechanisms. While more expensive, this method is equally as efficient. On the other hand, it adds to the parts, and the whole modern tendency has been to reduce the number of parts.
Fig. 9. Carbureter Bids of Moline-Knight 50-Horsepower Motor Courtesy of Moline Automobile Company, East Moline, Illinois.
A characteristic example of present methods of casing in poppet valves is shown in Fig. 8, which is an example of a Fiat marine motor, with the valve stem pit in the side of the motor covered by a readily removable aluminum plate.