Title: Effect of the tailpipe entry geometry on a two-stroke engine's performance prediction using a 1–D gasdynamic simulation

Authors: C.G.J. Van Niekerk, D.J. De Kock, J.A. Visser

Addresses: Alvis OMC, 12 Barnley Road, Benoni, 1500, South Africa. ' Department of Mechanical Engineering, University of Pretoria, 0002, Pretoria, South Africa. ' Department of Mechanical Engineering, University of Pretoria, 0002, Pretoria, South Africa

Abstract: It is standard practice in 1–D gasdynamic simulations of high performance two-stroke engines to model the exhaust tail pipe entry as an area change using an algorithm similar to the area change of the reverse cone. In the reverse cone, the area continually steps down, while at the tail pipe entry, it changes from stepping down to constant area. At this point, a vena contracta can form that affects the flow resistance of the tail pipe. In an effort to improve the accuracy of the gasdynamic simulations, the area change algorithm at the tail pipe entry was replaced with a restriction algorithm that incorporates a coefficient of discharge (Cd) and allows an increase in entropy on the expansion side. The coefficient of discharge is defined as the actual measured mass flow divided by the mass flow predicted by the restriction algorithm. The Cd-values were determined from the measured mass flows for a variety of tail pipe entry geometries at a range of pressures covering the pressure ratios encountered in a real engine. Simulation results with and without the Cd-values were compared to measured results and it was shown that incorporating this refinement improves the accuracy of the simulation results on the ||over run|| part of the power curve.

Keywords: 1–D flow modelling; gasdynamics; two-stroke engine; performance prediction; tailpipe geometry; exhaust geometry; simulation; coefficient of discharge.

DOI: 10.1504/IJVD.2004.005244

International Journal of Vehicle Design, 2004 Vol.35 No.4, pp.349 - 364

Published online: 15 Sep 2004 *

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