PISTONS, PISTON PINS AND CONNECTING-RODS Types of Pistons.—While the majority of the two-stroke-cycle engines of the low-pressure type use pistons having deflector lips, similar to Fig. 259, there are several other designs coming into vogue. Quite a number adopt the double piston, Fig. 260. This piston, as shown, has part of the head cut away to form a de flector for the air; and, to allow the exhaust to uncover at the proper time, a similar portion is removed from the bottom side of the piston. This simplifies the casting of the piston, but it is likely to prevent free exhaust as the curve will -set up eddy currents, and the scavenging will not be perfect by any means. Still another design is Fig. 261, which is used on the De La Vergne four-stroke-cycle type D.H. engine. The conical head will allow expansion without seizing. Another design which is coming into favor, especially for vertical engines, is the one adopted by the Muncie Co. and outlined in Fig. 262. This has a straight face head, and, if the air ports enter the cylinder at an angle, the elimination of the deflection lip will not prove objectionable. In those engine designs where the scavenging air is compressed in the front end of the cylinder, the common practice is to use a crosshead and piston rod. This necessitates a considerable departure in the piston design. Figure 275, showing the Buckeye Machine Co.'s design, is along the lines of this type piston as ordinarily built. This piston, as regards strength, is of excellent design. The head is strongly ribbed—which prevents distortion of both the head and piston body. See, also, Fig. 255.
Piston Clearance.—There is much difference in the practice of engine builders in regard to the clearance between cylinder walls and piston. On new engines it seems to range anywhere to .005 or .006 inch. The variation is not dependent upon size; the largest clearance encountered has been on pistons below 10 inches in diameter. Apparently the values chosen have depended upon the builder's viewpoint. If he desired absolute freedom from piston seizing, he chose a large value; if good com pression was attractive, a small clearance was used at the risk of piston seizing. In operation, the engineer should see that the clearance does not exceed .008 inch; that is, the difference in cylinder and piston diameter should not exceed .0016 inch, nor be less than .005 inch. This clearance value is considerably greater
than in either the gasolene or Diesel engine. However, owing to the heat conditions in the cylinder of the two-stroke cycle type of engine, the piston expansion is rather extensive. Operating at a fairly high rate of speed, the cooling of the piston is difficult, especially in view of the fact that either the frame or the front end of the cylinder is enclosed, preventing any cooling by air currents which assist materially in the gasolene engine. Since the two-stroke-cycle receives twice as many fuel charges as does the gasolene or Diesel for the same number of revolutions per minute, the amount of heat that the cylinder jacket absorbs is approximately twice as great. The piston receives more heat than it can radiate at ordinary temperatures. The result is that it must reach a higher temperature limit before the heat balance is attained. Being at a higher temperature, it is natural to expect the piston to expand more than does a gasolene engine piston. There must be more clearance to accommodate this expansion. While there must be clearance ample for this, the maximum allowable clearance is limited as well. It seems to be the impression of most engineers that, as long as the rings fit snugly and hold compression, the actual piston clearance does not matter. On the contrary, it is highly important that there be no excessive play. The air ports and exhaust passages are practically in line. If the piston is loose in the cylinder, the air charge will flow around the piston into the exhaust ports, even though the rings are snug enough to prevent the cylinder compression from being lost. A great many instances of poor scavenging are traceable to excessive piston clearance. Another common practice is the dependence engineers place on the piston rings. These should not be expected to hold the cylinder com pression with the piston badly worn. Probably nine out of ten operators, as soon as the piston shows signs of wear, order a new set of rings and feel that these are a cure-all. When' the piston wear exceeds the above-mentioned clearances, and the engine shows signs of loss of power or excessive fuel consumption, it becomes necessary to install a new piston and rebore the cyl inder. Most builders furnish replacement pistons from )i to inch oversize.