Processes in Optical Diesel Engines - Emissions Formation and Heat Release

Abstract: This project deals with questions related to generic diesel combustion research and optically accessible engines are used to study the combustion process. Images of the combustion are analyzed together with in-cylinder pressure and exhaust gas emission measurements. The results can be divided into two main categories: one about evaluations of optically accessible engines and one about studies of emissions and heat release. The focus is on the link between the combustion event and engine-out emissions. The reason for putting efforts on the first category is to get more reliable information from the second one. First, the mechanical behaviour during operation of optical engines is discussed. Optical engines may suffer from distortion of the in-cylinder volume trace due to mechanical deformation from mass, pressure and thermal forces. This distortion causes errors in heat release calculations. A method to account for the errors is therefore developed which makes the calculations insensitive to mechanical deformations. The optical access also affects the engines heat transfer properties which can cause differences in heat release and engine-out emissions compared to all-metal engines. Fortunately, it is possible to compensate for the differences and achieve realistic engine-out emissions and combustion phasing by adjusting the charge temperature. The engine-out soot emissions are the result of a complex series of events including fuel air mixing, premixed combustion, mixing controlled combustion, and late soot oxidation. The first part of the emission section in this thesis deals with factors that are known to be important for the soot formation and estimates how they affect the engine-out soot emissions. The goal is to identify characteristics of the soot formation that are important for the engine-out smoke level and thereby the soot particle mass. Two studies are included in this chapter, one about air entrainment and one about early soot formation. It is indicated that the rate of soot-formation during the quasi-steady jet-phase has a rather weak relation to the level of engine-out soot. This is despite the usage of few, small, nozzle holes leading to a long injection duration with a large portion of the combustion taking place during the fuel injection period. The observation is explained as follows: First, a large fraction of the soot is formed in the transition between the premixed and spray-driven combustion which weighs down the importance of the quasi-steady jet-phase. Second, factors that enhance the soot formation during the jet phase in some cases also enhances the soot oxidation during this phase. Third, correlations to emissions of CO and UHC further point to the importance of the oxidation process and characteristics of the heat release indicates that this is partly related to the late cycle. The second part of the emission section deals with the sources of CO and UHC during low temperature combustion. It is suggested that the squish volume is crucial for the engine-out CO and UHC levels during low load. Two combustion concepts with different injection strategies are investigated, one with very early start of injection (SOI) and one with SOI close to TDC. The squish volume is indicated to be the major source of not fully oxidized products in both cases which shows that the importance of this source is not easily erased.