Riassunto analitico
The current work has been realized in collaboration with Porsche AG, and both the internship and the thesis have been carried out at the company’s development center in Weissach, Germany. The work concerned the investigation of mixture stratification and combustion calibration of e-Fuels in a high-performance engine.
Synthetic fuels are a possible solution to make transportation more sustainable. They are obtained through chemical reformulation of carbon dioxide (CO2), captured from the air, and hydrogen (H), extracted from water. Their purpose is to be burnt in conventional internal combustion engines and emit back into the atmosphere the same amount of CO2 that was previously absorbed to produce them. The need for this work arose from unexpected combustion behaviours of these fuels that were measured at the Single Cylinder Engine testbench. Notably, the second half of the combustion lasted approximately twice with respect to the first one, which is unusual compared to the durations usually experienced with standard gasoline. A 3D-CFD simulations campaign have already been performed, but the combustion phasing did not match the experiments: an underestimation for the second half was noted. Mixture distribution was thought to be the reason behind the mismatch of the results, and, to improve the analysis, a more accurate fuel description was required to better mimic the evaporation of the fuels.
With the available hydrocarbon analysis, a surrogate based on the six most abundant components was defined to represent the fuels and an extensive research in scientific literature was started to collect the thermo-physical properties of each component. To describe the fuels with the highest accuracy, two definitions were proposed: a single-component description, characterized, for each property, by the average of the components’ properties and a multi-component description, representing each injected droplet as composed of the six components. To evaluate the single-component saturation pressure, the UNIFAC model was used, calculating the activity coefficients during the evaporation of the droplets and weighing each component’s saturation pressure through the modified Raoult’s law.
The injection simulations with the two fuel definitions provided an overall mixture stratification at the spark time that was comparable. Deepening into the examination of the local lambda distribution evaluated with the multi-component fuel, it was noted that different mixture compositions were present due to the various evaporation behaviour of the components.
However, the higher computational effort required for the multi-component simulation and the lack of evidence for a real benefit to the overall mixture stratification led to the decision to proceed towards combustion calibration with the single-component definition.
With the results achieved on the fuel description side, it was possible to calibrate the combustion of the fuels at different operating points, ensuring that both the first and second halves of the combustion durations were matched.
The idea behind the parameters used to calibrate the combustion was to minimise the variability of the values for each parameter between the different fuels. Successfully, the same spark delay was used at 6000 rpm to account for the time difference between the electric signal and the actual energy released by the spark plug, while the other calibration parameters, ECFM-3Z α and the laminar flame speed correction factor for the periphery of the spark plug, were adjusted slightly to account for the different fuel characteristics.
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