The principles of engine design and the methods and details of engine construction are certainly, in interest and importance, second to none of the other factors that combine to produce the complete modern automobile.
How automobile engines operate, the reasons underlying the various details of different designs, and the relative merits of different constructions are all too little understood by the generality of those who have to do in a practical way with the new conveyance.
In all motors, of whatever sort, and of any type whatever other than those in which there is a perfectly continuous development of the power through constantly rotating elements - as in the electric motor and the steam turbine - there must be reciprocating elements that function through indefinitely repeated series of operations. Such a series of operations is termed the cycle of the engine, as is abundantly explained elsewhere herein, so it will suffice here to call attention to some of the merits and demerits of the different cycles that are in practical use.
That type of internal-combustion engine in which every stroke in one direction is a power stroke affords a maximum of power impulses to any given number of engine revolutions, but because of other limitations it is not always possible to make a two-cycle engine run as fast as a four-cycle, so that in the generality of cases the number of explosions in a given period of time, or for a given vehicle speed, is no greater with a two-cycle than with a four-cycle engine.
In addition to this, most two-cycle engines are often difficult to start, apt to be wasteful of fuel, not at all flexible in the matters of speed and pulling power, and in various other respects difficult to apply to automobile service. Their greatest merit is their extreme simplicity.
The four-cycle engine is the type by which nine hundred and ninety-nine out of every thousand of present-day automobiles are propelled. Varied through an immense number of possible forms, and with minor differences in the product of every maker, its fundamental functioning has nevertheless proved so far the most suitable for automobile propulsion.
With the succession of suction, compression, explosion, and exhaust strokes afforded by the four-cycle motor, there is secured a very positive and reliable functioning, and by the expedient of a sufficient cylinder multiplication to afford good mechanical balance and frequent power impulses, its flexibility, durability, and practical quality in every respect can be brought to very high standards in a well-designed and honestly built motor.
Fig. 2. Eight-Cylinder V-Typo Motor of the Latent Cadilac Car Shown Installed in the Chasois.
At the same time, the fact that so much more attention has been paid to the four-cycle motor than to any of its possible competitors for popular favor undoubtedly accounts in some measure for its present pre-eminence, and it is an open question with many engineers as to just what virtues might or might not be realized with other constructions were they as exhaustively experimented with and exploited.
There seems no reasonable limit to the extent to which cylinder multiplication can be carried, in the effort to improve the mechanical balance and to even the torque of gasoline motors, but established practice has, nevertheless, settled upon four-cylinder vertical engines as those most suitable for the propulsion of the average automobile - this being the least number of vertical cylinders with which mechanical and explosion balance can be secured.
The use of six cylinders, with the crank throws 120 degrees apart, and the explosions occurring once for every 120 degrees of crankshaft rotation, affords a smoother-running motor than the four-cylinder.
Still better than the "six" from every standpoint but that of cost, which has prevented its wider application to automobiles, is the V-shaped, eight-cylinder motor, of the type illustrated in Fig. 2, which gives a good view of the unit power plant of an equally well-known American machine. In both of these, there is used a four-throw shaft, similar to the ordinary four-cylinder crankshaft - which is much cheaper to manufacture than a six-cylinder crankshaft - and the two rows of cylinders, each practically constituting a separate four-cylinder engine, are made to work upon the common crankshaft at 90 degrees apart.
Fig. 3. Overhead View of Steams-Knight Eight-Cylinder Sleeve-Valve Motor Courtesy of P. B. Stearns, Company, Cleveland. Ohio.
The most recent tendency in car motors is toward the eight-cylinder V-type, following the marked success of this form in aviation use.
Not only has the V-form been produced in the poppet valve form but also in the Knight sleeve-valve type, an example of which 13 shown in Fig. 3. Furthermore, a considerable number of twelve-cylinder V-type motors have been built, a good example being seen in Fig. 4.
In aviation work, no form of motor has made as great progress as the rotating cylinder type, which has been built usually with an odd number of cylinders, as five, seven, or nine; or when these are paired with an even number, as ten, fourteen, or eighteen. As yet, this type has not been applied to motorcars, but, considering its advantages, it would not be strange to see this done at an early date. These motors have a single throw crankshaft of very light weight; the rotation of the cylinders at a rapid rate allows of their being air-cooled and also very light in weight, eliminating all parts and weight in the cooling system; the large revolving mass does away with the need for a flywheel, while the practical elimination of reciprocating parts reduces vibration to a minimum.
Fig. 4. Side View of National Twelve-Cylinder V-Type Motor Courtesy of Motor Vehicle Company. Indianapolis, Indiana.
In the extreme, motors of the V-type have been constructed with sixteen cylinders, eight in each group. These have been very successful in aeroplanes and motorboats, particularly the latter.