Challenges and Advantages of Stratified Combustion in Gasoline Direct-Injected Engines

Doctoral thesis, 2017

The modern world is based on an extensive transport network in which passenger vehicles play a major role. Although passenger vehicles have improved significantly in recent decades, they still contribute to the pollution of our environment and global warming. Consequently, new ways of reducing their emissions are needed. Moreover, most modern passenger vehicles are propelled by a combustion engine that operates on non-renewable fuels such as gasoline or diesel produced from crude oil. However, such fossil fuels are limited resources, so there is also a need to reduce the fuel consumption of passenger vehicles. As such, improvements in engine technology will play a central role in the development of more efficient and cleaner passenger vehicles. In recent years, alternatives to the internal combustion engine such as electric motors and fuel cells have attracted increasing attention. While these technologies are undergoing rapid development and electric vehicles in particular are gaining market share, the combustion engine remains the dominant power source for passenger vehicles. The most common type of combustion engine used in passenger vehicles is the gasoline engine. Several advanced combustion concepts have been developed to make gasoline engines cleaner and more efficient. One such concept is stratified combustion, which is discussed in this thesis. In gasoline engines, stratified combustion increases fuel efficiency. However, it also tends to produce high particulate emissions and can reduce combustion stability. These problems mainly occur because stratified combustion involves a complex mixing process that generates a heterogeneous air/fuel mixture with both rich and lean regions. This thesis describes work undertaken to minimize or eliminate these drawbacks, particularly the increased particulate emissions, while maintaining low fuel consumption. Most of the studies presented herein were performed with metal and optical single-cylinder engines, but a four-cylinder production engine was also used in some cases. Tests were performed in steady state mode at various engine operating points. All engines utilized in the studies were fitted with a Spray-Guided Direct-Injected (SGDI) system and multi-hole solenoid-actuated fuel injectors. Depending on scope of the study, the engines were equipped with different measurement devices such as instruments for measuring pressures and temperatures or emissions of HC, NOx, CO, CO2, O2 and particulates (both mass and number). The results obtained during this thesis work have been presented in five publications. The first of these publications describes a study on the relationship between particulate emissions during stratified combustion and generic combustion variables such as the fuel injection pressure, as well as injection and ignition timings. This was done to identify variables that could be manipulated to reduce particulate emissions. The later publications describe how measured particulate emissions are affected by forced induction, increased fuel injection pressure, the use of novel ignition systems, air movements, and the use of different sampling systems. Key objectives of these studies were to find ways of reducing particulate emissions and increasing combustion stability. It was found that stratified combustion in SGDI gasoline-fueled engines fitted with a solenoid multi-hole injector can increase fuel efficiency but does not alleviate the problem of high particulate emissions. In addition, a positive correlation between the extent of non-premixed flames and particulate mass and number emissions was identified. The use of novel ignition systems was shown to expand the ignition window, while boosting and increasing the fuel injection pressure were found to reduce soot levels. Finally, the internal relationship between the ignition and injection timings was found to strongly affect combustion stability and soot levels, primarily because it influenced the dilution of the air-fuel mixture and the risk of the fuel spray striking the piston top. The thesis concludes with some suggestions for ways of improving stratified combustion and directions for future research.