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Review of Design Concepts and Performance Evaluation of a Small Symmetrical Crank Case Loop Flow Scavenged Two Stroke Engine in the Light of Latest Developments in Ring Materials
Published December 01, 1993 by Associazione Tecnica Dell'Automobile in Italy
This paper is a critical analysis of important principles and concepts involved in determining the port geometry and their layout in a small two-stroke engine particularly in the light of latest developments which have taken place in recent years i the improvement of ring materials. Various factors involved in designing a cylinder block with particular emphasis on ports design were analytically examined in a chronological order considering availability of rings in high strength ductile material such as KVI etc. and were then applied to already existing small I.C. engines. In order to compare the performance of cylinder block assembly based on analytical design vis-a-vis existing engine, tests for output performance were conducted. Since manufacturing of an altogether new cylinder block was not possible, modifications in the existing cylinder were made. To overcome the problem of machining the ports of cylinder block, a sleeve was fabricated with modified dimensions and was fitted in the parent cylinder block. Rings for use in the piston assembly of both modified cylinder block and existing cylinder block were made from KVI material. In the process of simplification, maximum out-put at dynamometer shaft was taken the criterion for comparison.
It is established that old design engine suffers from a drawback in which an increase in the transfer port mean area is achieved at the cost of effective stroke length. This should be compensated by increasing the width of ports because, with the development of superior ring material, there is no fear of ring breakage due to wider ports. Consequently, a change in the basic concept of determining port areas and their arrangement in a two stroke engine cylinder block has been suggested. This has further opened a vast scope for switching over to designing of more and more cylinder blocks of two stroke engines with increased number of ports, thereby achieving a higher power output to cylinder bore ratio. With the latest developments in casting techniques of low pressure die casting for cylinder blocks and piston manufacturing techniques with which authors have been closely associated, it is hoped that this will bring a revolution in two- stroke engine industry.