RADIAL DIESEL FROM AMERICA
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nETAILS have become available 'about the development by the Thiokol Chemical Corp., of New Jersey, U.S.A., of an engine of unconventional layout. The engine is called the Dyna-Star and the basic design consists of pairs of cylinders arranged radially, the cylinders of each pair having a common combustion chamber.
The Dyna-Star operates on the twostroke cycle and the design provides uniflow scavenging and charging with unsymmetrical port timing which gives the advantages of opposed-piston-type engines in a simpler, more compact unit than existing engines of this type.
The pistons of each pair of cylinders are connected to a frame by individual connecting rods. This frame is carried on the crankpins of two single-throw crankshafts and it travels in a true circular orbit as the frame links the crankshafts together so that they rotate in phase—any point on the frame describes a circle with a radius equal to the crankthrow. The inboard ends of the connecting rods are connected to the frame by conventional knuckle pins with orbital centres on their respective cylinder centre lines.
Any number of pairs of cylinders can be used but it has been found that an eight-cylinder (four pairs) design gives the best results. Prototype diesel engines with this layout have been built and are now in the course of development, one unit having been installed in a light van. The prototypes produce 100 b.h.p. at an output shaft speed of 2,700 r.p.m.—the crankshafts are geared to the output shaft —they each weigh 400 lb. and have outside dimensions of 2 ft. 2 in. by 2 ft. 4 in. by 1 ft. 9 in. Bore and stroke are 3 in. and 3'5 in, respectively. A petrol engine of the same configuration produces 100 b.h.p., weighs 125 lb. and with a bore of 2-75 in. and stroke of 2-125 in. has overall dimensions of 1 ft. 5 in. by 1 ft. 5 in. by 1 ft. 6 in.
One way in which the scavenging, charging and timing characteristics are obtained is by positioning the cylinders in each pair at a particular angle to each other. This gives a desirable geometrically phased exhaust port opening compared to inlet port opening—the exhaust port is in one cylinder, the inlet port in the other. Additional phase difference can be provided by varying the distance between the knuckle-pin location of each cylinder pair On the frame. Further port timing control can be obtained, of course, by positioning the timing edges of the ports differently along the bOres in the
conventional manner. "
A typical valve port timing layout gives the exhaust port opening 18° before the inlet ports and exhaust closing 2° before the inlet port. A wide selection of timing arrangements is possible by various combinations of cylinder angle, knuckle pin position and port height as described. The current engines have the exhaust piston of each cylinder leading the intake piston by about 10°—given by the angle of the bores and the pin positions—and the location of inlet and exhaust ports provides an additional 10° lead.
Many advantages are claimed for the design, apart from low weight. These include uniform loading of the structure, low vibration level and simple construction. The first engine went on test at the end of 1960 and, although no details of manufacturing plans are available, it is estimated that engines may be available in about two years' time.