Investigation of Partially Premixed Combustion in an Optical Engine : In-Cylinder Flow and Combustion Characterization

Sammanfattning: The diesel engine is one of the most economic power sources and is therefore used in a range of technical solutions. The low fuel consumption combined with high engine performance makes it attractive for transportation vehicles. They are relatively unproblematic in terms of emissions of carbon-monoxide and hydrocarbons but, on the other hand, they are associated with high emissions of nitrogen oxides and soot. In the atmosphere, these emissions constitute a serious concern for both human health and the environment. For this reason, the recent research and development efforts of many automakers and research institutes have shifted focus to new technologies such as advanced combustion concepts. Partially premixed combustion (PPC) is one of the novel combustion concepts that aim to yield low $NO_{x}$ and soot emissions while maintaining high engine efficiency. This combustion process belongs to a class of technologies called low temperature combustion concepts. In conventional diesel combustion, the combustion is promptly initiated by auto-ignition as fuel is injected into the cylinder, vaporizes, mixes, and reacts with hot air. In PPC, by contrast, all the fuel is injected before it auto-ignites, which is made possible by the distribution of mixture strengths (fuel stratification) prior to combustion. The PPC process combined with high exhaust-gas recirculation (EGR) rates gives lower combustion temperatures and thus reduces the $NO_{x}$ emissions and soot formation.All the combustion occurs in premixed mode, which mainly means in mixture packets that have mixed with oxygen to different degrees during the ignition delay. The ignition delay is controlled by temperature, charge composition, fuel type and fuel injection timing. The work presented in this thesis is experimental investigations to understand the in-cylinder combustion and fuel–air mixing process under gasoline-PPC conditions. The first part describes a new approach to determine the in-cylinder combustion stratification for various injections strategies. The second part is devoted to the in-cylinder fluid motion which is important for the fuel-air mixing, heat transfer, combustion, and emission formation processes. For these two parts, an optical engine is used to carry out the experimental investigations. The last part consists of multi-cylinder experiments where the engine is fuelled with a blend between gasoline and ethanol and combustion is studied through an endoscope. Different strategies were used to maintain stable combustion and relatively low emissions of soot under PPC conditions.