For small moderate powers a single-cylinder engine possesses the advantages of the simplest possible construction, inexpensive to manufacture and maintain, and more economical in the use of fuel. Along with its advantages, however, it possesses two inherent defects, particularly from the standpoint of its use in commercial cars, for which reason it is new seldom employed. In discussing the cycle of operations it was stated the power impulses were irregular, due to the fact that a power stroke occurs every other stroke, or every fourth stroke and that the gases which are compressed in the cylinders require a certain portion of power to accomplish.
Therefore, in order to keep the engine running at a fairly uniform speed against a nearly constant resistance it is necessary to employ a heavy fly-wheel in which some of the energy liberated on the power stroke can be stored, to be given out again during the idle strokes.
A single-cylinder motor is sadly lacking in mechanical balance and the vibrations are great. These vibrations are due to the reactions of the explosion and inertia forces. In a single-cylinder motor the entire reciprocating mass (i.e., the piston with its parts, the connecting rod, bearing and crank pin), is in a single unit and the reaction of the inertia force of the parts produces a strong vibrating effect, while in multi-cylinder motors the reciprocating masses can be divided into several units and so arranged as to move in opposite directions, thereby neutralizing the inertia effects.
The two-cylinder motors present some advantages over the single-cylinder type. However, their use is also somewhat limited, and it is only a question of time until all commercial cars will be equipped with four-cylinder motors. The two-cylinder type presents an advantage over the single cylinder in that there are two reciprocating masses, so arranged that one effects the force of the other. However, perfect balance has not been reached and vibrations are still noticeable to a considerable degree.
In the four-cylinder motors now extensively used in commercial cars, the two inside reciprocating masses work together and the two outside work together. However, they are so arranged that the two inner ones always set against the two outer ones. Although they are equal in weight, they operate in opposite directions.
The turning movement of a four-cylinder motor is also much more uniform than that of a one or two-cylinder motor, hence the torque reaction and vibration due to it are much smaller. This four-cylinder type when properly constructed meets the requirements of vibrationless running quite satisfactorily.
From what has been previously said of the piston strokes and crank shaft revolution it can readily be understood that the piston travels up and down within the cylinder, while the crank shaft rotates in its bearings, in order to impart a turning effort to the rear or driving wheels of the vehicle through suitable power transmitting units. This conversion of reciprocating into rotary motion will be depicted below.
The construction of a gasoline engine is quite simple in its elementary form, being comprised of a cylinder, a piston provided with rings operating within the cylinder, a pair of valves or ports which permit at the proper time the entrance and escape of the gases, a connecting rod which connects the piston with the crank shaft, a case which supports the crank shaft and carries the valve operating mechanism, this valve mechanism being comprised of a shaft with separate or integral cams, suitable shaft bearings and a train of gears driven from the crank shaft; also a set of push rods which act as intermediate members between the valve stems and the cams.
The cylinders are usually castings made from close-grained iron provided with either water jackets for water cooling or fins, sometimes called ribs for air cooling. They are open at one end, while the other end is closed, forming the combustion chamber, in which the valves are located. The walls of this cylinder are made very smooth and are generally brought to a high polish by grinding, so that the piston with its rings can slide freely within the cylinder. This cylinder, with its parts, is bolted to the crank case, which carries the various other parts. Fig. 11 depicts a cylinder in section showing its various parts.
The pistons of gasoline engines are of the trunk type, as illustrated in Fig. 12, Wing somewhat longer than the diameter of the cylinder. Near the center of the piston bosses are formed, which receive the piston pin or gudion pin as it is sometimes termed. Near the top three or four grooves are turned into the piston which receive the eccentric piston rings, while near the lower end oil grooves are turned for distributing and collecting the surplus lubricating oil on the cylinder walls. These pistons are made a trifle smaller in diameter than the cylinder to permit of the expansion of the metal on the power stroke, while rings, curried on the piston, penult of flexibility, so that the gases cannot escape by the piston.
The crank shaft is a horizontal steel shaft carried in journals, or linings, inside of the crank case, while offsets corresponding with the number of cylinders are provided. These offsets are termed the crank pins and carry the large end of the connecting rod and its hearings: it is also provided with a taper or flanged end to which the flywheel is attached by means of a key or nut or bolts.
The connecting rod forms an intermediate link between the piston and the crank shaft. It is usually made a drop forging, the small end carrying a bearing for the piston pin, while the large end is divided and carries the crank pin bushing.