Fig. 74. Principles of Operation, Sterling Crankless Opposed-Piston Diesel Engine. Courtesy of The Sterling Engine Company, Buffalo, New York.
1801 52nd Dr., NE, Marysville, Washington 98721
Thought our readers would be interested in this section of the book, Diesel and Other Internal Combustion Engines by Howard E. Degler, published by the American Technical Society in 1937.
Eliminating crankshaft, connecting rods, cylinder heads, gaskets, valves, and camshafts, the Sterling crankless high-speed engine may forecast a revolutionary trend in design. The compactness of the engine and the principles of operation are shown by Fig. 74. The engine has four horizontal cylinders, each containing two opposed reciprocating pistons. Thus the engine is equivalent to the usual two-cycle 8-cylinder diesel engine of corresponding bore and stroke. The cylinders are arranged about a straight drive shaft which carries an inclined disk (also called wabble plate) at each end. The disks are virtually flywheels. The pistons act directly upon the circular disks set angularly upon the shaft, so that the gas pressure transmitted by the pistons forces the disks and shaft to rotate.
The mechanical bearing units which transfer the power from pistons to drive shafts utilize the same principle as the Michell and Kingsbury thrust bearings: that is, from the piston base, a U-shaped bridge passes over the rim of the disk and carries two bearing surfaces.
The thrust surface is a slipper of rectangular shape with babbitt bearing surface, mounted on a half sphere; the other bearing (which serves only to position the piston) is a bronze-faced half sphere. This provides a universal mounting which permits the bridge to adapt itself to any motion of the inclined disk. The bridge is guided by short rods extending through guide holes, Fig. 74.
The engine follows conventional 2-cycle design in that circumferential ports replace valves. Scavenging air is provided by a pressure pump. However, instead of the usual rotary or centrifugal blower, piston type pumps are used, and the piston is mounted directly on the end of the piston bridge rod (left side in Fig. 74). Air is distributed by a simple rotary valve at the left end of the shaft.
There are separate rows of intake and exhaust ports at opposite ends of the cylinder, and extending completely around it. Thus one piston uncovers the exhaust ports, the other the intake ports. Exhaust ports are uncovered first. The charge of fresh air is given a spiral or swirling motion to clean out the combustion chamber. The air charge is compressed between the pistons, and the fuel oil is injected under a pressure of about 2000 pounds per square inch. Slightly concave piston surfaces prevent spray impingement on the top of the pistons. Air enters at one end of the cylinder and leaves at the other, providing uniflow scavenging and charging. Out-of-phase pistons provide a longer combustion interval, at approximately constant pressure.
The high speed (1200 to 1800 r.p.m.) of the engine and the 2-stroke cycle combine to provide uniform average cylinder temperatures and greater combustion flexibility. The heat economy resulting from the proper combination of combustion space and port area, together with the absence of water-cooled cylinder heads, results in high cylinder capacity and excellent power output. Horizontal opposed movement of the pistons provides smoothness of operation, and the absence of a crankshaft avoids piston side thrust on the cylinder walls; this lessens piston and ring wear. Torsional vibration is practically eliminated, as is direct structural loading of the frame. Electric starting is possible because of the light weight of the engine: it weighs but 13 to 20 pounds per horsepower, depending upon size and service.
Lubrication of inclined disks and piston shoes is accomplished by spraying oil through nozzles directly upon the disks. The pistons, being exposed for a considerable portion of their length to the oil thrown from the disks by centrifugal action, are satisfactorily lubricated and cooled. This compact type of engine is available in the following 4-cylinder units: 3 x 4 inches, 125 horsepower at 1800 r.p.m.; 5 x 6 inches, 300 horsepower at 1500 r.p.m.; 6 x 8 inches,. 500 horsepower at 1500 r.p.m.; and 7 x 9 inches, 600 horsepower at 1200 r.p.m. This engine is particularly adaptable to marine service, portable and semi-portable applications, direct connection to electric generators, air compressors, etc.