Investigation of PPC in an Optical Engine : With focus on Fuel Distribution and Combustion Characterization

Abstract: The global society is dependent on transportation to function and a big part of these transports are driven by an internal combustion engine. Heavy-duty vehicles utilizes conventional diesel combustion is a combustion concept where the fuel is often ignited during the injection event and while it gives a high combustion efficiency, the combustion mode is also characterized by high emission of NOx and soot. The transports also contributes to greenhouse gas emissions and to emissions of substances harmful to humans. These negative effects have driven research in advanced combustion concepts that combine high efficiency with low emissions. A number of advanced combustion concepts have been developed utilizing low temperature combustion and partially premixed combustion is one of them. Partially premixed combustion utilizes dilution of the charge to lower the combustion temperature, which means that the ignition delay is longer.The ignition delay for PPC can be further delayed by adding EGR. The advantages with keeping a low combustion temperature is reduced thermal NOx-emissions and lower heat transfer. The increased ignition delay will lead to reduced the soot emissions. Another concept that utilizes low temperature combustion is HCCI, where a homogeneous mixture is created before ignition. Since the charge is homogeneous the combustion will progress rapidly through a series of auto-ignition events. The limitations with HCCI is that this rapid heat release limits the possibility of reaching higher loads and at lower loads the emissions of HC and CO increases which limits the efficiency. The PPC concept is a way to combine the benefits, and to limit the disadvantages, of HCCI combustion and conventional diesel combustion. PPC is thus an intermediate concept and the focus of this thesis is to investigate the transition from HCCI-like combustion to more stratified combustion. The studies are mainly carried out in an optical engine with optical diagnostics, including laser-based diagnostics. This includes investigation the effects of the piston geometry and spray angle on the combustion. The thesis links the combustion characteristics with the fuel distribution and the mixing process for a sweep of injection timings. The studies are carried out with fuel having a high octane rating in order to increase the ignition delay. In most of the studies, the PPC transition was carried out with primary reference fuels, but the thesis also includes characterization of methanol combustion in PPC mode for a variety of injection strategies. The methanol study has shown that by injecting a small amount of methanol as a pilot can trigger the combustion of the main injection. The study also shows that by adopting a double injection strategy the inlet temperature required to ignite the fuel can be reduced significantly