Effects on Scavenging and Hydrocarbon Emissions: Altering the Combustion Chamber of a Small Two-Stroke Engine

Sammanfattning

This thesis focuses on how changing the combustion chamber geometry in a small two-stroke engine can improve performance. The engine used for this thesis is a Husqvarna AB chainsaw engine with Schn¨urle porting and air-leading stratified scavenging. Four different combustion chambers were subjected to CFD simulations, and from them two were selected for engine testing. In total, five combustion chambers were tested in an engine testing rig. A detachable head design was used, and the different designs were compared to the standard hemispherical combustion chambers found in production engines. The tests were performed with wide open throttle using an electric brake to alter engine speed. Ignition timing and engine speed sweeps were performed. The results from engine testing showed that offsetting the combustion chamber dome towards the intake side increased the fuel trapping efficiency, and increasing the squish area showed a similar trend. An offset oval design, with the wider part of the oval being place in the same direction as the crankshaft, consistently showed the highest fuel trapping efficiency. This design had both an offset and an increased squish area. It also showed the lowest hydrocarbon emissions in all tests. Combustion performance was analyzed and no significant difference in rate of heat release was observed. The difference was seen in the start of combustion, where the higher squish options had a later onset of combustion. The best performer was the decreased squish ratio alternative. Although the fuel trapping efficiency had a bigger impact on the hydrocarbon emissions and fuel efficiency, this information can be used to further optimize future designs. In conclusion, the offset oval performed best in total, but more optimization using the findings from this thesis as well as validation from a solid cylinder-head is needed.