Crankless Opposed-Piston High-Speed Diesel Engine

By Staff
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Fig. 74. Principles of Operation, Sterling Crankless Opposed-Piston Diesel Engine. Courtesy of The Sterling Engine Company, Buffalo, New York.
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Fig. 73. General View of Sterling Crankless Opposed-Piston Diesel Engine, Two-Cycle, Solid-Injection, Four-Cylinder, 3 by 4 Inches; Develops 125 Horsepower at 1800 r.p.m. 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.

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