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Browsing by Author "Verhelst, S."
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Literature Review on Dual-Fuel Combustion Modelling 2019, Article in monograph or in proceedings (SAE Technical Paper 2019-24-0120)In the search for low greenhouse gas propulsion, the dual fuel engine provides a solution to use low carbon fuel at diesel-like high efficiency. Also a lower emission of NOx and particles can be achieved by replacing a substantial part of the diesel fuel by for example natural gas. Limitations can be found in excessively high heat release rate (combustion-knock), and high methane emissions. These limitations are strongly influenced by operating parameters and properties of the used (bio)-gas. To find the dominant relations between fuel properties, operating parameters and the heat release rate and methane emissions, a combustion model is beneficial. Such a model can be used for optimizing the process, or can even be used in real time control. As precursor for such a model, the current state of art of dual fuel combustion modelling is investigated in this work. The focus is on high speed dual fuel engines for heavy duty and marine applications, with a varying gas/diesel ratio. Modelling is limited to the closed part of the 4-stroke engine cycle. A methodology part is included, describing the origin of the treated work. Modelling of the dual fuel process can be done in various ways. In this Literature Review Paper a structured overview is given of the various modelling approaches used nowadays, to simulate the dual fuel combustion. The covered models include 0D, multi-zone and 3D CFD approaches. All intermediate steps for each approach are explained, and their strong and weak points are mentioned. The modelling techniques are rated on their precision and predictive capabilities in relation to their computational cost. The majority of the models was able to give a good description of the heat release rate, although not always predictive. A good match with experimental results is seen by Wiebe and double-Wiebe functions, but prediction is limited. By including a detailed description of the combustion process, a better predictive heat release rate can be created. Also combinations of a Wiebe model and detailed combustion models are seen. A good prediction of NOx emissions is achieved by models that include the oxidation reactions of nitrogen in their reaction scheme, or make use of the Zeldovich mechanism. A good description of local temperature is needed. This is achieved by 3D CFD models, but also multi-zone models have shown reasonable results here. Although often mentioned as a significant source for CH4 emissions in a dual fuel engine, crevices were hardly included in the simulation work. The 3D models that did include the volume above the piston rings, confirmed the large amount of methane emission originating from this source. When prediction of uncombusted methane is a goal of simulation, it seems this aspect is not to be neglected. The precise spatial description and detailed reaction schemes produce useful results, but come at the cost of high computational effort. Simplified models can be fast, but lack the output of detailed predictive information. This creates an interesting outlook for further development of an intermediate class of models, with enough precision at a calculation effort feasible for control purposes. -
Cylinder to cylinder variation related to gas injection timing on a dual-fuel engine 2019, Article in monograph or in proceedings (SAE Technical Paper 2019-01-1162)The natural gas/diesel dual-fuel engine is an interesting technique to reduce greenhouse gas emission. A limitation of this concept is the emission of un-combusted methane. In this study we analyzed the influence of PFI gas-injection timing on cylinder to cylinder gas-distribution, and the resulting methane emissions. This was done on a 6 cylinder HD engine test bench and in a GT-power simulation of the same engine. The main variable in all tests was the timing of the intake port gas injection, placed either before, after, or during the intake stroke. It showed that injecting outside of the intake window resulted in significant variation of the amount of trapped gaseous fuel over the 6 cylinders, having a strong impact on methane emissions. Injecting outside of the intake stroke results in gas awaiting in the intake port. Both testing and simulation made clear that as a result of this, cylinder 1 leans out and cylinder 6 enriches. The simulation showed how this is caused by the airflow into the manifold, which enters the manifold close to cylinder 1. This flow picks up gas residing before the first cylinders, and distributes it over the cylinders further downstream. The richer cylinders have a higher absolute methane emission. By calculating the ratio between trapped and emitted methane, we found operating points where the enrichment caused by unequal gas distribution led to lower relative methane emission. This can be attributed to the better combustibility of methane under richer conditions. Although the injection timing was placed outside the intake window on purpose in our experiments, the same distribution problem occurs in situations where the gas injection is longer than the intake stroke. This is a common situation with currently available dual-fuel systems. The solution can be found in a gas injection system with significantly higher capacity.
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