Piston Ring Construction
As all pistons must be free to move up and down in the cylinder with minimum friction, they must be less in diameter than the bore of the cylinder. The amount of freedom or clearance provided varies with the construction of the engine and the material the piston is made of, as well as its size, but it is usual to provide from.005 to.010 of an inch to compensate for the expansion of the piston due to heat and also to leave sufficient clearance for the introduction of lubricant between the working surfaces. Obviously, if the piston were not provided with packing rings, this amount of clearance would enable a portion of the gases evolved when the charge is exploded to escape by it into the engine crank-case. The packing members or piston rings, as they are called, are split rings of cast iron, which are sprung into suitable grooves machined on the exterior of the piston, three or four of these being the usual number supplied. These have sufficient elasticity so that they bear tightly against the cylinder wall and thus make a gas-tight joint. Owing to the limited amount of surface in contact with the cylinder wall and the elasticity of the split rings the amount of friction resulting from the contact of properly fitted rings and the cylinder is not of enough moment to cause any damage and the piston is free to slide up and down in the cylinder bore.
These rings are made in two forms, as outlined at Fig. 123. The design shown at A is termed a “concentric ring,” because the inner circle is concentric with the outer one and the ring is of uniform thickness at all points. The ring shown at B is called an “eccentric ring,” and it is thicker at one part than at others. It has theoretical advantages in that it will make a tighter joint than the other form, as it is claimed its expansion due to heat is more uniform. The piston rings must be split in order that they may be sprung in place in the grooves, and also to insure that they will have sufficient elasticity to take the form of the cylinder at the different points in their travel. If the cylinder bore varies by small amounts the rings will spring out at the points where the bore is larger than standard, and spring in at those portions where it is smaller than standard.
It is important that the joint should be as nearly gas-tight as possible, because if it were not a portion of the gases would escape through the slots in the piston rings. The joint shown at C is termed a “lap joint,” because the ends of the ring are cut in such a manner that they overlap. This is the approved joint. The butt joint shown at D is seldom used and is a very poor form, the only advantage being its cheapness. The diagonal cut shown at E is a compromise between the very good form shown at C and the poor joint depicted at D. It is also widely used, though most constructors prefer the lap joint, because it does not permit the leakage of gas as much as the other two types.
There seems to be some difference of opinion relative to the best piston ring type—some favoring the eccentric pattern, others the concentric form. The concentric ring has advantages from the lubricating engineer’s point of view; as stated by the Platt & Washburn Company in their text-book on engine lubrication, the smaller clearance behind the ring possible with the ring of uniform section is advantageous.
Fig. 124, A, shows a concentric piston ring in its groove. Since the ring itself is concentric with the groove, very small clearance between the back of the ring and the bottom of its groove may be allowed. Small clearance leaves less space for the accumulation of oil and carbon deposits. The gasket effect of this ring is uniform throughout the entire length of its edges, which is its marked advantage over the eccentric ring. This type of piston ring rarely burns fast in its groove. There are a large number of different concentric rings manufactured of different designs and of different efficiency.
Figs. 124, B and 124, C show eccentric rings assembled in the ring groove. It will be noted that there is a large space between the thin ends of this ring and the bottom of the groove. This empty space fills up with oil which in the case of the upper ring frequently is carbonized, restricting the action of the ring and nullifying its usefulness. The edges of the thin ends are not sufficiently wide to prevent rapid escape of gases past them. In a practical way this leakage means loss of compression and noticeable drop in power. When new and properly fitted, very little difference can be noted between the tightness of eccentric and concentric rings. Nevertheless, after several months’ use, a more rapid leakage will always occur past the eccentric than past the concentric. If continuous trouble with the carbonization of cylinders, smoking and sooting of spark-plugs is experienced, it is a sure indication that mechanical defects exist in the engine, assuming of course, that a suitable oil has been used. Such trouble can be greatly lessened, if not entirely eliminated, by the application of concentric rings (lap joint), of any good make, properly fitted into the grooves of the piston. Too much emphasis cannot be put upon this point. If the oil used in the engine is of the correct viscosity, and serious carbon deposit, smoking, etc., still result, the only certain remedy then is to have the cylinders rebored and fitted with properly designed, oversized pistons and piston rings.