Time Resolved Scavenging Analysis for Two-Stroke Engines

A detailed knowledge of the scavenging process becomes necessary during the development process, when the performance and in particular the emission output of two-stroke engines needs to be qualified and evaluated. This paper presents an experimental approach to describe the composition of the flow at the exhaust port by means of flow visualization and to quantify the relative changes of the scavenging losses imposed by design changes. The experimental set-up has been described in previous SAE papers and has been expanded by a transparent exhaust port, which gives optical access to the flow through the exhaust port. The gases in the cylinder are represented by differently colored water-based fluids. Typically, the burnt gas is clear and the fresh charge is colored to visualize its progress during the scavenging cycle and its distribution inside the cylinder. The flow through the exhaust port is captured with a digital camera together with timed markers to synchronize the movie with the crank angle during the cycle. Hence, the amount of colored liquid in the transfer port indicates the concentration of fresh charge. The gray scale value or the hue angle of the color can be correlated to the concentration by applying the Lambert and Beer's law, which allows a relative quantification of the scavenging loss. Moreover, the movies of the flow do not only indicate the flow characteristics versus time or crank angle but also yield a concentration profile across the port. These data together with the very graphic nature of the movies themselves provide a deep insight of the scavenging flow, particularly in great detail in the vicinity of the exhaust port, which helps the design engineer to understand the effect that changes of the engine design have on the condition of the scavenging process and therefore of the emission performance of his engine. The paper presents recent experimental data to illustrate the capacity of the principle, gives a detailed analysis of the measurement technique and discuses the limits of the application.