PISTONS AND PISTON PINS pistons of all engines properly fall into two general classes—Crosshead and Trunk Pistons. The crosshead piston is usually shorter than the trunk piston and is provided with a crosshead which receives the side thrust due to the an gularity of the connecting-rod. Figure 49 illustrates this type. With the trunk piston the upper end of the connecting-rod is supported by the piston pin, which is fastened in the piston. Consequently the piston receives the side thrust which is taken up by the crosshead in the former type. The engine shown in Fig. 7 employs the trunk design of piston.
The American manufacturers of small and medium powered engines, up to 200 h.p. per cylinder, have with few excep tions designed their engines with trunk pistons. Owing to the high cylinder pressures of the Diesel engine, the side thrust of the piston is of serious consequence, although in a 200 h.p. cylinder the piston can be constructed of a length sufficient to bring the side pressure within reasonable limits.
Trunk Pistons.—On units with a rating beyond 200 h.p. per cylinder practically all builders use a crosshead design of piston; in fact, the first Diesel built, even though of 25 h.p., employed a crosshead, and the present-day trunk piston is actually an adaptation of gas engine practice. The trunk piston possesses certain features that make it attractive to the average operator. Since the side pressure is taken by the piston, there is no crosshead shoe to adjust. This adjustment, on a Diesel, must be made with a degree of knowledge possessed by none save experienced engineers. A guide clearance that would be quite satisfactory on a high-grade steam engine will prove entirely too liberal with the oil engine. This nicety of running fit necessitates constant adjusting of the crosshead shoes. The operator should understand that, beyond the pound that it occasions, a loose crosshead will allow the piston to bind in the cylinder, producing heavy scoring.
The trunk piston presents a problem in lubrication that does not exist with the crosshead type. The side thrust is borne by
the trunk piston along its entire length, but this bearing surface extends over only a portion of the circumference. This rubbing area must be positively and copiously lubricated. The problem of lubricating a surface periodically exposed to hot cylinder gases is difficult. If the cylinder and piston are not oiled, either the piston or cylinder liner will cut.
Piston Clearance.—Since the transverse pressure throws the piston against the bearing side of the cylinder, the clearance between the piston and cylinder must be less on the trunk than on the crosshead design. This is evident since, with the trunk piston, the entire clearance exists on the piston opposite to the wearing side. On the crosshead type the clearance is fairly well distributed around the piston; consequently a clearance between piston and cylinder of .007 inch is actually a clearance of .015 when the engine is firing. The crosshead piston can, then, be allowed a greater clearance than can the trunk type. This ob viates danger of piston seizing when the engine is stopped after a run.
Crosshead Piston.—The crosshead design eliminates the heat difficulties of the piston-pin brass which are so often present with the trunk piston. Opportunity is also afforded for a heavier reinforced piston head. There is also less likelihood of the piston fracturing since it is not confined at the pin bosses. The cross 5 head and rod design admits of an oil guard at the front end of the cylinder thereby preventing the throwing of lubricating oil into the cylinder with the consequent carbonization. The drip ping of dirty cylinder oil or unconsumed fuel oil into the crank case, where it renders unusable the bearing oil that is held there, is also eliminated by this design. To this can be ascribed the lower lubricating oil consumption of the crosshead design of en gine. These manifest advantages are, it is the feeling of the majority of engine builders, offset by its greater complication of parts and the greater necessity for intelligent adjustments.