Optimization of the E-TEC combustion system for direct-injected, two-stroke engines toward 3-Star emissions

摘要

This paper describes important aspects of the development process for meeting CARB's "Ultra-Low" 3-Star emissions with engines using the new E-TEC direct injection system. In-house research and analysis of data from other state-of-the-art engines were used to determine achievable emission levels and to set the development targets. A detailed mode-point-specific analysis of the emissions potential of the FICHT direct injection system revealed excellent system capability in homogeneous operation and limited potential for stratified operation. Based on these results, the development work was focused on the reduction of stratified hydrocarbon emissions. Wall impingement of the fuel spray onto the piston surface was identified as a major source of hydrocarbon emissions during stratified operation. A zero-dimensional simulation of various parameters affecting wall impingement indicates that droplet size, in-cylinder temperature, and penetration velocity are the three major factors. Droplet size was reduced drastically through the use of the new E-TEC voice-coil injectors with newly developed outwardly opening swirl nozzles. The in-cylinder temperature is increased with optimized air-flow and engine temperature management. The management of the fuel spray wall impingement was improved significantly by optimizing the splash bowl geometry in the piston surface. KIVA spray simulations with different cylinder head geometries indicate that the design of the near nozzle area and especially the distance of the cylinder wall from the fuel spray are critical for optimal fuel spray development and mixture preparation. The near nozzle geometry was also found to have an effect on penetration velocity. An optimized combustion chamber location that maximizes the distance of the fuel spray from the walls reduced stratified emissions by up to 40% and resulted in an ICOMIA cycle emissions reduction of 18%. The combination of the new E-TEC direct injection system with an optimized combustion chamber and improvements in near-nozzle geometry, injector targeting, piston splash bowl design, air-flow management, and mapping resulted in an overall emissions reduction of over 50% compared to the FICHT direct injection system.