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Cat diesel with two-ring pistons
Page 38
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• A compact piston with two rings is an important feature of a new Caterpillar automotive range of engines — the Cat 1100 Series — available with power outputs from 150 bhp to 225 bhp, the weight of the units being about 12001b (544kg).
Reducing the number of rings from three to two enabled a low-height piston to be developed which was essential to increasing engine displacement without increasing height or width, the saving in piston height from using two rings being 0.928in. (24mm). The piston is claimed to give compression and oil-control characteristics comparable to, if not better than, those of a conventional piston. All the units of the series have a bore of 4.5in. (114mm) and a stroke of 4.1in. (104mm), 4.5in. or 5.0in. (127mm).
Reducing the top-land and ring-belt area by the use of a two-ring pack enabled an ample skirt area to be provided in the limited height. The top ring is designed as a compression/oil-control ring. The face of the ring has a slight taper and when installed the ring-groove insert is twisted. During compression and power strokes, cylinder pressure causes the ring to flatten against the cylinder wall so that the entire ring face comes into contact with the wall. When the pressure is reduced on the intake and exhaust strokes, only the bottom edge of the ring makes contact with the wall and acts as an oil-wiper ring. And friction is reduced.
Of molybdenum-coated ductile iron, the top ring is installed very close to the top of the piston but on extensive field tests it has operated without scuffing or sticking. The oil-control ring is of the double-rail type and is chromium faced. It is claimed that both rails maintain close contact with the cylinder wall and that this gives good oil control.
In tests of a Model 1150, 200 bhp engine, the vehicle logged 126,000 miles on contract haulage and returned an average fuel consumption of 9 mpg. Over the period of the test, oil consumption increased from 1420 mpg to 883 mpg.
The two-ring piston has been applied to diesel engines operating up to 150 psi bmep and to gas engines with a bmep of 180 psi_ It is fitted to the 0343 natural-gas engine that develops 350 bhp at 1800 rpm.