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Observations of the scavenging flow field have been made in a modified poppet-valved two-stroke engine with a transparent cylinder. Four kinds of cylinder heads with different port configuration were created to analyze their effects on the scavenging flow and develop new scavenging concepts. A mineral oil fog discharge system was used to visualize the air flow during the scavenging process. All of the images were recorded by a high speed video camera which show the development of the scavenging processes and clearly indicate the scavenging jet structure, the tumble pattern and the location of re-circulation regions. The analyses allow us to judge the quality of the scavenging processes. The small changes in port geometry could significantly affect the scavenging flow. Tumble as well as swirl should be considered as main means to organize the scavenging flow in order to avoid short-circuiting losses and create condition favorable to combustion.

Three-dimensional computations are performed to investigate the scavenging characteristics of poppet-valved two-stroke engines. The new subroutines are developed to handle various valve shrouds. Visualization of scavenging flow on a modified two-stroke transparent cylinder engine is used for validation and compared well in flow pattern. The flows in engines with different configurations are simulated at a speed of 5000 r/min. The stroke bore/ratio and valve arrangements were considered to investigate their effects on the scavenging flows. Additionally, valve timing and boost pressure have also been studied. The results show that stroke/bore ratio of 0.4 to 0.6, shroud angle of 69° to 108°, turn angle of shroud of 18° to 28°, average tumble ratio of 1.2-1.6 were found to be the optimum range for effective scavenging.

This paper presents the effect of application of scavenging-port injection to a two-stroke two-cylinder boat engine fueled with a compressed natural gas (CNG). Experiments were carried out at a constant speed of 3000 rpm and WOT condition with varying excess air ratio in lean region. A CNG injector for a production automobile engine was utilized and installed into the scavenging ports. Comparison was made with homogeneous charge operation using a gas mixer. By applying the scavenging-port injection, unburned-fuel in the exhaust gas was reduced close to half which must be due to the reduction of fuel short-cutting during the gas exchange process. The lean-burn limit was extended from the excess air ratio of 1.21 to 1.57. The maximum decrease of BSFC reached to 25 %. This suggests that stratified-combustion could be realized in its lean-burn region.