The sudden and powerful outward movements of the piston under the pressure from the combustion are transmitted to a crank shaft, which must be of great strength in order to resist the heavy strains under which it operates. It is made of the best steel available for the purpose, and has as many cranks as the engine has cylinders. The cranks are generally made in one piece with the shaft for the sake of strength, and for stiffness there are as many bearings as possible. The number of bearings for the crank shaft of an engine with four or more cylinders depends on the arrangement of the cylinders. If the cylinders are evenly spaced, there will be room for a bearing between each pair of cranks, so that a four-cylinder engine will have five bearings, one at each end, the other three being between the cranks. If the cylinders are in pairs, there will not be room between the cranks of a pair for a bearing, the only space for it being between the pairs; a four-cylinder engine built in this way will thus have but three bearings, one at each end and one in the center. Crank shafts are described by their bearings as three, five, etc., point crank shafts.

The relative positions of the cranks of a crank shaft are expressed in degrees of a circle; if, for instance, the cranks project from opposite sides of the shaft so that they are a half revolution apart, it is called a 180-degree crank shaft.

The outer ends of the crank arms, which correspond to the cranks of a bicycle, support the crank pin, which may be likened to the pedal, and to this the large end of the connecting rod is attached, the small end being connected to the piston. The connecting rod must be of great strength, tough but not brittle, and is made of steel or bronze.

The piston is a trifle smaller than the bore of the cylinder, and its length is usually greater than its diameter. It is hollow, with one end closed, the closed end being that against which the pressure is exerted. A wrist pin passes through it, and through the small end of the connecting rod, to enable the latter to swing from side to side in following the turning of the crank shaft. A tight joint is maintained between it and the cylinder walls by cast-iron piston rings, which are of square or rectangular cross-section, split so that they may spring open, and fitted into grooves cut around the piston. They are of such shape that their tendency to expand keeps them pressed against the cylinder walls, but being split, their elasticity prevents their binding; they fit the grooves snugly, and while they may move freely in them, they hold the pressure from escaping. The number of rings varies with the design of the engine, but the most usual arrangement is three to a piston, placed around the upper end.

The cylinder should be of the highest grade of cast iron, with the smoothest possible surface for the piston to slide against.

The valve openings, or seats, are circular, and are usually made slightly funnel shaped, the disks that cover them being slightly conical to fit. The large end of the funnel is toward the combustion space, so that when the disk is lifted from its seat it moves inward. Valves are held against their seats by coil springs that surround the valve stems, which are rods extending from the center of the disks, and there are two methods by which they are opened. In an automatic valve, the spring that holds the disk against its seat is weak, and the higher pressure outside of the cylinder during the suction stroke forces the disk away from its seat against the pressure of the spring. The valve remains open until the pressure in the combustion space is about equal to that outside, when the spring draws the disk back to its seat, to which it is held as long as the pressure inside is higher than that of the atmosphere. This arrangement is only possible for inlet valves, and is largely used, but exhaust valves, and often inlet valves as well, are mechanically operated; that is, they are opened and held open by a mechanism driven by the crank shaft, in the form of a cam. A cam can best be described as a “wheel with a hump on it,” or, in other words, it is a piece of metal mounted on a shaft, cylindrical in form except for one portion, which projects farther from the shaft than the rest. The cam revolves with the shaft, and the projection, called the nose, will displace anything resting against it. The illustration shows a cam in three positions of its revolution, with the end of a valve stem resting against it—the roller being attached to the stem to reduce the friction. The valve stem is held in guides, so that the only movement it may have is up and down; when the cam revolves, the nose lifts the stem and opens the valve, holds it open as long as the flat end of the cam is under the stem, and when the nose passes from under, the valve is drawn to its seat by the spring.

The moment at which an automatic valve opens is governed partly by the tension of its spring; if it is too strong, greater pressure will be required to open it, and it will close sooner than if the tension is light. Accurate adjustment of this spring is necessary in order that the charge may enter the combustion space without delay, and continue to enter as long as possible. The opening and closing of mechanically operated valves depend on the shape of the cam, and not being affected by the more or less uncertain action of a spring, they are more positive in action.

The cam shaft on which the cam is mounted is driven by the crank shaft, but as the valve opens but once during two revolutions, the cam shaft revolves at half speed, making one revolution while the crank shaft makes two. This is done by means of gears.

If two gears running together, or in mesh, have the same number of teeth, they will make the same number of revolutions, but if one has twice as many teeth as the other, the smaller will revolve twice while the larger revolves once. As the cam shaft must revolve but once while the crank shaft revolves twice, its gear must have twice the number of teeth as the gear on the crank shaft. The cam shaft is also called the secondary, or half-time shaft, and the gears that drive it the two-to-one gears.

In some designs of engines, the nose of the cam bears directly against the valve stem, but it is more usual to place a valve-lifter rod, or push rod, between them, the cam acting on the rod and lifting it, and that in turn lifting the valve stem. When the nose of the cam is not acting on the stem or rod, there must be a small space between them, for if the stem or rod rests firmly against the cam at all times, the valve disk might be prevented from seating firmly. The space is left between the stem and lifter rod, the spring acting only on the stem.

The valves may open into the cylinder in a variety of ways; both may be in one pocket, or one may be in a pocket and the other in the head, or each in a separate pocket, or both in the head. The first two illustrations show automatic inlet valves, and the third and fourth mechanically operated valves; when two mechanically operated valves are in the head, it is necessary to open them by means of rocker arms, for because of their position it would be impossible for the valve-lifter rod to act directly on their stems.

If after the explosion the burned gases were permitted to escape directly into the open air from the cylinder, the effect would be the same as the firing of a gun, and for the same reasons. The pressure in the cylinder being higher than that of the atmosphere, the sudden expansion of the gases would produce a report, and as this would be most undesirable for an automobile, provision is made by which the gases are cooled and permitted to expand gradually, so that when they reach the open air they are at its pressure, or nearly so. This is done in the muffler, or silencer, to which the exhaust pipe conducts the products of combustion. The muffler consists of a series of chambers of different sizes, one inside of the other; the gases pass from the smaller to the larger, expanding as they go, until from the largest they should escape without noise, having lost their heat and pressure.

While the pressure exerted during the power stroke depends on the heat of the gases, and it is necessary to have the engine hot in order that there may be as little loss of heat as possible, the temperature must not be permitted to rise to the point at which the lubricating oil would burn. Lubricating oil for gasoline engines is made to stand high heat, but if heated beyond its limit it will burn, and then, besides the loss of its property of lubrication, a deposit of carbon, hard or gummy, will form, fouling the combustion space or piston rings, and interfering with the operation of the engine. Overheating is prevented either by circulating water through channels surrounding the combustion space, or by directing a blast of air against it.

The channels, called water jackets, provided for the circulation of the water, are usually cast with the cylinder, or formed of sheet metal. Cool water enters at the bottom and escapes at the top, absorbing heat during its passage. Of the two systems of keeping the water in circulation, the most usual consists of a rotary pump, which forces the water through the jackets and then to a cooler, or radiator, which is so placed that it is exposed to the air currents set up as the car moves. In order to cool the water, the radiator must have a large surface exposed to the air, and the water must pass through it in small streams. The early types consisted of coils of small copper tubing, on which were strung disks of copper, the water flowing through the tubing, and the disks absorbing its heat and giving it up to the air, but these are being abandoned in favor of cellular or honeycomb radiators. These types, which are usually placed at the extreme front of the car, are made up of a great number of short lengths of small tubing, in any one of several shapes, placed side by side, and held together either by plates or by soldering their ends.

The heated water enters at the top of the case in which the tubes are contained, and flows to the bottom, finding passages between the tubes, while the air passes through, being assisted by a fan driven by the engine.

The second system of keeping the water in circulation follows the principle that heated water tends to rise, its place being taken by the cooler water that tends to sink. This, called the thermosiphon or gravity system, requires all of the parts and connections to be large and completely filled with water. The water in the jacket rises as it absorbs heat from the cylinder walls, and flows out to the radiator, which it enters at the top. Its place in the jacket is taken by the cooled water from the bottom of the radiator, and this circulation continues, being more rapid as the difference in temperature between the heated and cooled water increases. It is naturally not so rapid as circulation that is forced by a pump, and more liable to become inoperative by the clogging of the pipes, jacket, or radiator.

Of the methods of increasing the surface of a cylinder in order to cool it by a blast of air, the most usual is to cast it with flanges that project from all parts of the combustion space. These become heated as the temperature of the cylinder walls increases, and the air that is blown against them carries off the heat. Other methods consist of setting pins or copper strips into the cylinder walls, of such form that the air current strikes a surface composed of points, by which the heat easily passes to the air. Another system consists of surrounding the combustion chamber with a jacket open at the bottom, air being blown into it from the top by a powerful blower.

Other things that are necessary for successful air cooling are large valves by which the hot gases may be quickly discharged when their period of usefulness is ended, and small cylinders rather than large, as the heat from small quantities of gases may be carried off more quickly than from large.

The lubrication of a gasoline engine must be carefully looked after, as on its thoroughness depends the continued delivery of power. The most usual method of lubricating the piston and cylinder walls is to keep the crank case filled with oil to such a point that the end of the connecting rod dips into it in turning. This spatters the oil to all parts of the crank case, and a portion is caught in a groove cut around the lower end of the piston. The inward movement of the piston spreads the oil on the cylinder walls, and it is distributed around the piston rings, so that they move easily in their grooves. As the oil is used up, it is replaced from a lubricator so that a constant level is maintained, and this operates either by gravity, or by a small force pump driven by the engine, or by the maintaining of pressure in the oil tank.

Mechanically operated or pressure lubricators supply oil to all parts of the engine, and as the quantity passed to each bearing is adjustable, a feed may be maintained that is exactly suited to the requirements.

The bearings of an automobile operate under such widely different conditions that one kind of lubricant will not be suitable for all. The maker of the car has tested the different oils, and it is advisable to follow his instructions on the brand and grade most suitable for each bearing, rather than to try experiments. The Lubrication Table on pages 244 and 245 gives the kind and quantity of lubricant most suitable to the work performed by each bearing.