Numerical Analysis of an Air-Head Stratified Scavenging Concept for Emission Reduction of a Two-Stroke Engine 2001-24-0051

Small two-stroke engines enjoy wide-spread acceptance in the field of hand-held outdoor equipment due to performance advantages over competing technologies. The main issue with these engines is their high hydrocarbon emission, which makes it difficult to comply with future emission laws such as the 2005 standards for small handheld engines of the U.S. Environmental Protection Agency (EPA). As a result, considerable research and development work is currently being carried out both in industrial and public research facilities to improve the hydrocarbon emission performance of two-stroke engines. Major emission reductions are expected from stratified scavenging concepts. However, the development and tuning of stratified scavenging engines requires comprehensive knowledge of the flow processes in the ports and inside the engine cylinder. Therefore, the investigation presented in this paper aims at the analysis of the three-dimensional displacement and mixing processes inside the transfer ports and the cylinder of an EPA-Class IV two-port two-stroke engine for research purposes featuring an air-head stratified scavenging concept. The approach applied was a detailed 3D flow simulation based on boundary conditions gained from one-dimensional calculations. A special evaluation method was developed which allows the analysis of the principal hydrocarbon loss mechanisms. The effect of air-head scavenging was investigated by comparison to a calculation without stratification. The numerical calculations performed at three different engine speeds showed good agreement with the experimental results. Starting from the original configuration, further investigations were carried out in order to analyse the potential and limitations of this concept. For this purpose, additional numerical calculations were performed assuming improved stratification and increased air-head mass. The results of these calculations lead to new approaches for further development in order to meet future hydrocarbon emission limits.