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Three-boss Piston Could Benefit Diesels
Page 58
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THE five-bearing three-boss piston was obviously applicable to diesel engines. claimed Mr. C. D. Brewer of the Ford Motor Co. during a discussion on "The Design and Development of Pistons for Automobile Engines" presented by Mr. E. J. Robinson of Hepworth and Grandage Ltd. at a meeting of the Institution of Mechanical Engineers in Coventry recently.
In his paper, Mr. Robinson emphasized that increased gas pressures were likely to cause piston failure in the form of fatigue cracks initiated by stress concentration in the bosses and webs. These resulted from gudgeon-pin bending, and replacement of the conventional threebearing configuration of piston-pin assembly by a five-bearing type using a forked connecting rod would greatly reduce piston-boss stresses and skirt distortion. In the three-boss pistoo described by Mr. Robinson a centre boss is integral with the crown and is braced by a steel restrictor plate arranged centrally on a dimensional plane, which is made possible by the forked rod.
Referring to spark-ignition engines, Mr. Brewer stated that compression ratios would not, at any time in the future, approach the ratios of diesel engines. Incorporating the combustion chamber in the piston crown reduced combustion temperatures and detonation. It also enabled the piston rings to be employed more efficiently to dissipate heat. Replying to Mr. Brewer, Mr. Robinson said that the three-boss piston had not been considered for compression-ignition units because diesel engine users did not require quiet operation of the pistons, but the possibility would be reviewed. In his opinion, there would be further increases in the compression ratios of petrol engines, the designers of which were following diesel practice, notably in the rise of combustion chambers in the crown and of fuel injection.