Effect of the tailpipe entry geometry on a two-stroke engine's performance prediction

Abstract

It is standard practice in one-dimensional 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 effects 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 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. An experimental set up was designed and constructed to measure mass flows for a variety of tail pipe entry geometries at a range of pressures covering the pressure ratios encountered in a real engine. From the mass flow results the coefficients of discharge for a range of pressure and area ratios and reverse cone angles could be calculated and arranged into matrix form to define Cd-maps. The Cd-maps were incorporated into the simulation software and tested to ensure that it functioned correctly. <p<Finally, the simulation results with and without the Cd-maps 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. This is the part of the power curve after maximum power and very important in the development of high performance two-stroke engines. These maps can be used for all future simulations on any engine size that uses the same tail pipe geometry.