Engine stability : A study of the events occurring prior to thecombustion in a small two-stroke engine

Abstract: This thesis is a study conducted in collaboration with the engine performance group atHusqvarna AB. The study focuses on engine stability of smaller two stroke handheld enginesrunning on E10 (10% ethanol mixture in gasoline). The reason for the study is the new EUproposition that by 2020 all fuel must have 10 % renewable fuel content. To meet thisproposition Husqvarna has evaluated E10 and found that the engine stability of smaller twostroke engines are affected in a negative way by the fuel.The study focuses on events occurring prior to the combustion and mainly the carburetor. Theobjective for the thesis is to seek what contribution the events occurring prior to thecombustion have to the engine stability and find simple and implantable solution to improvethe stability with regards to the carburetor.The study has been conducted in three different work packages, system understanding to buildknowledge of how the carburetor operates, fault finding to seek potential attributes that canaffect the stability and fault mode analysis to seek why the attributes affect the stability.Furthermore, all the attributes found has been tested and validated on the engine to seek theircontribution to the stability.The conclusion made of the thesis is that with simple and implementable improvements of thecarburetor the engine stability could be increased with 40 %. A total of five differentattributes were found to affect the stability of the engine. Furthermore, a very detailedexplanation of how the carburetor operates and components inside the carburetor has beenestablished during the thesis.