A New Piston Design for a Cross-Scavenged Two-Stroke Cycle Engine with Improved Scavenging and Combustion Characteristics

This paper describes a unique design of deflector piston for a cross-scavenged two-stroke cycle engine which incorporates the advantages of good scavenging, rapid combustion and reduced thermal loading on the piston. Test results are presented to confirm this statement from two small capacity outboard marine engines and comparisons are made between the experimental test results from the modified and standard power units; of significance is the reduced fuel consumption rate of the modified engines in both cases. A high bmep 400 cm3 single cylinder engine is designed, constructed and tested so as to determine the extent of deflector burning under conditions of high thermal loading. On all three engines the ignition timing for best power is shown to be in the 21-24° btdc region, by comparison with 32-38° btdc conventionally. The spark plug seat temperatures are reduced to 150 C maximum at peak power by comparison with 250-280 °C normally. Startability is unaffected by the new piston design whilst the idle and low speed engine behaviour could be superior to a conventional deflector piston type engine which means it is considerably better than a loop-scavenged engine. Photographs of the scavenge flow are presented from experimental tests which would indicate that the scavenge characteristics of this new piston design could be very good.