Development of Ports of Four Stroke Diesel Engines
Fresh air is breathed in and products of combustion are exhaled by the inlet and exhaust ports. The efficiency of air flow is given by the flow coefficient. The flow is turbulent for most of the period and avoiding recirculation zones improves the flow. The energy for the flow is imparted by the piston during the intake stroke and only partly in the exhaust stroke as blowdown is a significant contributor during exhaust process. The energy during intake is partitioned between components contributing to flow and to swirl, which is important to support combustion in a direct injection engine. Of all the types of intake ports producing swirl in the cylinder, helical port is amenable to theoretical treatment with less empiricism; also, a helical port is stable in production and highly efficient. Helical port design considers free vortex with a correction for friction at cylinder liner surfaces. The optimum helical port is one which minimises the variation of exit velocity about the periphery of the valve seat. The exhaust port is designed to accelerate the flow at zones where there is a tendency to separate and then, a diffuser is designed to gain pressure. The definitions for the flow coefficient and swirl number vary from institution to institution. The definitions followed in this chapter were pioneered by AVL; these are widely used in many countries as de facto standard. The engine swirl can be reproduced at steady state rig for benchmarking or tuning the ports designed based on theory.
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