Advancing the understanding of pilot ignition in dual fuel engines : Experimental and conceptual studies

Sammanfattning: The energy transition is looking for new technologies to reduce CO2 emissions. One of the promising options is the dual fuel engine. In this concept, based on a diesel engine, a small pilot diesel injection is used to ignite a second fuel. This second fuel can be a low carbon or renewable high-octane fuel, that otherwise could not be applied in a diesel engine. Good results on emissions and efficiency are reported for this concept, but there is still room for improvement. This study worked on advancing the understanding of the dual fuel engineResearch on a dual fuel truck engine showed what could be achieved, and where limitations are seen. One of the limitations, cylinder to cylinder variation, was investigated. It could be explained making use of experiments and 1D simulations. The cause was found in exchange of injected fuel in the intake port, through the manifold. This resulted in enrichment of downstream cylinders, with fuel from the cylinders at the begin of the intake manifold. In a next stage, optical research was performed on a dual fuel marine engine. Here it was shown how the combustion could run in two different modes, Conventional Dual Fuel (CDF) and Reactivity Controlled Compression Ignition (RCCI). A fundamental difference in control mechanism was found between the two modes, where advancing the pilot results in earlier combustion in CDF mode, and in later combustion in RCCI mode. Initiated by a different pilot injection timing, a significant difference in dilution of the pilot fuel was seen in RCCI. This results in a more graduate heat release, which is known to create lower NOx emission. Since the optical campaign did not allow emission measurements, a second campaign was performed on a standard configuration. This indeed showed that with the RCCI combustion, created by a very early pilot timing, extremely low NOx emissions could be achieved. However, this reduction of polluting emissions came with an increase in the emission of uncombusted methane, which is a strong greenhouse gas. An optimum was found at late RCCI combustion, close to the transition to CDF, combining low emissions with a high efficiency. The position of the found optimum, and the limited availability of explanation of the operational differences between the combustion modes led to the creation of a conceptual model. It describes how the start of combustion is related to the pilot injection timing. It explains how in CDF mode the combustion can start after a short ignition delay, in which the liquid pilot fuel is diluted just enough to be ignitable. This gives an almost constant relation between the injection timing and the start of combustion. In RCCI mode the pilot fuel is injected much earlier, resulting in a much higher dilution. As a consequence, a higher temperature and pressure are required to start combustion. This explains why advancing the injection in this mode results in a later start of combustion and vice-versa. The higher dilution created by earlier injection requires ignition conditions that occur later in the engine cycle.All in all, the found optima and insights should help to run engines effectively in dual fuel mode. This allows the usage of renewable high-octane fuels in widely available diesel engines. In this way the transition towards sustainable transport can be strongly accelerated, without the need for high investments in electric infrastructure, depleting the world’s rare minerals and replacing the complete engine ecosystem.