Representation of Two-Stroke Engine Scavenging in 1D Models Using 3D Simulations

Paper #:
  • 2018-01-0166

Published:
  • 2018-04-03
Abstract:
The current 1D models of two-stroke engine scavenging use a scavenging curve, which determines dependence of burnt gas fraction in exhaust port on current burnt gas fraction in a cylinder. This dependence eliminates classic dependence of charging efficiency on delivery ratio with the well-known phases of burnt gas perfect expelling, mixing with fresh charge or even short-circuit scavenging, i.e., direct escaping fresh charge in an exhaust port. They can be found using time-consuming 3D simulations of scavenging flow together with scavenging curves. The direct use of charging efficiency dependence is not possible during the integration of differential equations for 1D model, since it would need iteration during every integration step. The three described phases, depending on the amount of of gas delivered through an inlet port, are present, nevertheless, at any two-stroke engine, although with different timing and intensity. The substitution of inlet flow mass by state variable, the mass fraction of burnt gas in a cylinder, is possible if pure fresh charge is delivered. Then, the mass fraction of burnt gas reflects integrated mass of fresh charge. If massive back-flow of burnt gas to inlet system occurs at the start of scavenging or if EGR is applied, the results from 3D simulation are useful only if the backflow or EGR level is the same in the 3D and 1D cases. Since 1D simulation is used to optimize port or valve timing by a fast way, this is not very often a case. Moreover, changing the pressure ratio over an engine occurs in optimization of the whole air-loop with similar results for a backflow. The paper proposes the way of using scavenging curves with reduced value of cylinder burnt-gas contents, so called scavenging progress variable, which makes the use of single result of 3D simulation possible for more variants of inlet timing. The reduction is based on a stepwise integrated burnt gas contents in fresh charge for every 1D simulated case. By this way, it accelerates the optimization process substantially. The results are validated using dedicated 3D cases simulations.
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