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Experimental methods are needed to understand unsteady three-dimensional flows. They also become an important tool where geometrical details or the complexity of the applied model would require an over-proportional numerical effort to investigate a scavenging flow. The experimental method presented in this paper applies visualization techniques to analyze the scavenging flow of small two-stroke cylinders. The principle at work is based on similarity laws, which transfer the dimensions of an actual engine into an enlarged model. The modular set-up allows fast modifications of the port geometry and detailed visualization of the flow coming out of individual transfer ports of multiple port cylinders by using different dyes. Results are presented as movies, which give a very graphic impression of the unsteady three-dimensional flow inside the cylinder.

In order to meet the new stringent emission limits for small handheld power tools technologies like methods to improve the scavenging process and after treatment devices are deployed. Additionally, professional machines such as chain saws add particular requirements to avoid mechanical failures. By precise control of the carburetion it is possible to yield the necessary reduction of the emission level yet to maintain the performance of the engine. Thus, the development of an Electronically Controlled Carburetion System to reduce the exhaust emissions of small IC engines for handheld power tools was initiated at the WHZ, Germany. Key requirements of such a system are: low cost, high reliability, low electrical power consumption, high accuracy in metering the fuel demand of the engine. First results are promising. A correlation between exhaust gas temperature and the fuel demand of the engine was confirmed for both, wide open throttle and part load operation.

Two-stroke engines are widely used in handheld power tools due to the demand in high power-weight-ratio and reliability. Particularly, two-stroke engines remain to be the ultimate power source for handheld chain saws and continuous efforts resulted in an improved engine design with increased power output and reduced weight. Besides these continuing efforts engine manufactures must make their products compatible to exhaust emission standards, which have been implemented in most of the important markets worldwide [1], [2]. One particular area of interest is the scavenging process of naturally aspirated two-stroke engines, which affects power and emission output, both. Advanced scavenging methods offer the possibility to design two-stroke engines which achieve the lowest present emission levels i.e. hydrocarbons plus nitrogen-oxides: 50 g/kWh without compromising the power output at given engine parameters such as displacement or rated speed.

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.