The piston or gudgeon pin is hardened and ground. The ends fit into straight bearings in the bosses, the piston being fastened by set-screws which are locked by smaller set-screws. The lubricating oil for the pin is deposited in a trough at the front edge of the piston, from whence it flows through a passage to the end of the pin. The pin is drilled on its upper surface to permit the oil to issue onto the bearing surface.
McEwen Diesel Piston and Pin.—McEwen Bros. use a conical head piston of one-piece construction, Fig. 55. The head, after being cast, is annealed for the purpose of removing alI casting strains. Six rings are employed to hold the compres sion while the wiper ring at the bottom has been dispensed with and a series of grooves made to replace this ring. The pin is held by a set-screw.
Snow Diesel Engine Piston.—The Snow Engine has a piston that is a radical departure from the usual design with four-stroke cycle engines. The piston, as seen in Fig. 56, is a single barrel casting with a separate steel head which is concave on its surface. The front end is bolted to the crosshead yoke. The crosshead is provided with a single shoe and has a wrist pin of extra large dimensions. There is a decided advantage in the crosshead de sign since the pin size is not restricted as it is in the trunk piston.
In removing the piston from the cylinder, it is not necessary to dismantle the entire head and valve rigging. The connecting rod can be unbolted from the crank pin, and the piston with drawn through the frame. It is very essential with the crosshead piston that the shoe be properly adjusted. The engineer should measure the thickness of the shoe when first installed and en deavor to maintain this dimension by the insertion of shims.
The Standard Fuel Oil Engine Piston.—The Standard Fuel Oil Engine is of the two-cycle design and has a stepped piston, outlined in Fig. 63. The main or power piston, Fig. 57, is a two-piece barrel casting. The head is conical and extends down over the piston body forming a water-cooling compartment. The water lines are passages cored in the main piston casting and are connected to telescopic tubes at the front end. The power piston is bolted to the enlarged scavenging piston. The scavenging piston, in the 60 h.p. engine, is 30 inches in diameter and is strongly ribbed. The piston or gudgeon pin is bolted to this piston, consequently the latter acts as the cross-head and receives the transverse thrust of the piston.
Since the crank-case does not act as the return lubricating oil receiver, leaks in the water tubes are not so serious as with the vertical engines. The water stuffing-box glands must be kept tight or the seeping water will destroy the lubrication on the walls of the scavenging piston. The cooling water discharge must be kept below 120°, and the line should be vented to avoid steam pocketing at the lower edge of the piston head, which receives a great amount of heat as the exhaust gases pass out through the ports.
Seized Pistons.—Even with the modern types of pistons now in use, many plants have experienced trouble with seized pistons. This difficulty is directly traceable to either of two conditions. One of these is insufficient clearance between the piston and liner. This applies especially to the clearance around the piston immediately below the head. The temperature of the piston head must run very high to establish a heat balance wherein the heat thrown off equals the heat absorbed by the piston. This heat condition causes the head to expand diamet rically. If- there is insufficient clearance between the head and cylinder walls, seizing will occur. To obviate this the piston can be slightly tapered, with a decreasing clearance downward toward the piston pin. The maximum clearance can be as great as inch, while the clearance in the neighborhood of the first piston ring should be approximately .007 inch. Another solution of head expansion is found in the conical piston head. This conical surface possesses ample side clearance for expansion. Incidentally, the conical head assists in the thorough interming ling of the air and atomized fuel as they leave the injection valve. The same relief from seizing is attained by the dished or concave head. This design of head also serves as an aid in Mixing the air and fuel.
It is apparent that piston seizing when it is due to lack of clearance can be eliminated by taking a small taper cut off the piston immediately below the head. The taper may well start between the top pair of rings. Certain cast irons continue to grow even after the clearance has been increased by such a cut. It then becomes necessary to watch this piston and repeat the tapering process as required.