The paper describes preliminary results of a research activity finalized to the development of a new scavenging concept for the reduction of the HC emitted by a small-size two-stroke carbureted crankcase-scavenged SI engine. Further developments of a well-established model (1Dime code) are presented, with particular emphasis on combustion and scavenging processes simulation. The rate of heat release is computed through a two-zone model, based on a “fractal” representation of the turbulent flame front. A CAD procedure evaluating, at each crank-angle and flame radius, the intersections between the flame surface and the actual combustion chamber walls, has been developed.
Scavenging is modeled through an original two-zone approach which accounts for mixing and short-circuiting processes. The latter are directly related to the in-cylinder turbulent flow regime, inlet and exhaust flow velocities, and engine speed. The main objective is a reduction of the number and the influence of the so-called “tuning parameters”, always necessarily present and crucial in any scavenging model.
The simulation of the in-cylinder phenomena is coupled to the computation of the 1D flow in the external ducts, which is based on the employment of a finite volume, total variation diminishing (TVD) technique. The whole engine is described, starting from the air-cleaner box up to the exhaust tailpipe. The model also includes a procedure for the simulation of the reed valve dynamics.
A number of comparisons with experimental data are presented with reference to instantaneous pressure data within the crankcase, the cylinder and the exhaust pipe in the whole engine operating range, including part-load conditions. The performance and emissions characteristics of the engine are also compared with overall test-bench data.
The results presented demonstrate that this kind of model is competitive with more complex 1D-3D approaches.