Comparison of Stratified and Homogeneous Combustion in a Direct-injected Two-stroke Engine for Snowmobile Applications

The University of Idaho has been developing gasoline direct injection two-stroke engine technology for use in snowmobile applications as part of the Clean Snowmobile Challenge organized by the Society of Automotive Engineers. Applying GDI to a twostroke engine significantly reduces emissions of unburned hydrocarbons and improves fuel economy by reducing or eliminating the short circuiting of fuel that occurs in conventional carbureted two-stroke engines. The University of Idaho’s GDI design allows for two different modes of combustion, stratified and homogeneous. Stratified combustion is used during idle and light to moderate loads at low engine speeds while homogeneous combustion used at moderate to high loads and medium to high engine speeds. During calibration work there has been debate on where the transition from stratified to homogeneous combustion should occur and which mode of combustion provides better fuel economy during cruise point operation of the engine. This thesis presents the process and results of determining which mode of combustion provides better fuel economy during cruise point operation, and where the transition from stratified to homogeneous combustion should occur. Furthermore, results are presented regarding the effects of increased squish velocity on the brake specific fuel consumption and torque output during stratified operation. Additionally, this thesis is intended to provide a basic understanding and background of two-stroke engine theory, GDI systems, and stratified and homogeneous combustion to new students on the Clean Snowmobile team.