The upcoming introduction of Euro VII legislation requires manufacturers to introduce new technologies to comply with the emission limits imposed by EU. For heavy-duty vehicles, a valuable solution could be represented by Recuperated Split-Cycle Engines. In this configuration, the conventional four-stroke cycle is splitted into two cylinders. In the first one, air is inducted and compressed. It is then sent to the recuperator where it is pre-heated by the exhaust gases of the second cylinder. Finally, it is sent to the expander cylinder, where fuel is injected and the combustion, expansion and exhaust phase can be performed. Theoretically, this technology should guarantee lower emissions, as it has a higher efficiency than traditional engines and has the ability to control combustion conditions. In this thesis work, a preliminary study of the air intake process and the consequent mixing between air and fuel that takes place in the expansion cylinder was carried out through a CFD analysis with the OpenFOAM software. The key for emissions reduction is the achievement of a premixed condition in which combustion can be started. Initially, the intensity of the turbulence generated by the motion of the air charge entering the expander cylinder was evaluated, and the potential for the creation of a homogeneous mixture was verified. Hence, fuel injection and subsequent mixing with air was simulated. As mixing problems were encountered, several configurations were evaluated as first proposals to improve the performances. In conclusion, an attempt was made to simulate the combustion process. Since no experimental data are available, the initial conditions in the cylinder were assumed starting from the data found in the literature. Better results are expected by using more accurate geometry and referring to experimental data.
L’imminente introduzione della normativa Euro VII impone ai costruttori di introdurre nuove tecnologie per rispettare i limiti di emissioni imposti. Per i mezzi pesanti, una possibile soluzione può essere rappresentata dai motori Split-Cycle con recupero termico. Si tratta di una configurazione in cui il normale ciclo quattro tempi viene diviso in due cilindri. Nel primo, l’aria viene aspirata e compressa. Dopo essere stata preriscaldata dai gas combusti, viene immessa nel secondo cilidro, dove il combustibile viene iniettato e avvengono le successive fasi di combustione, espansione e scarico. A livello teorico, questa tecnologia dovrebbe garantire emissioni inferiori, in quanto presenta un’efficienza maggiore rispetto ai motori tradizionali e ha la possibilità di controllare le condizioni di combustione. In questa tesi è stato effettuato uno studio preliminare del processo di aspirazione di aria e il conseguente miscelamento tra aria e combustibile che avviene nel cilindro di espansione, tramite un’analisi CFD con il software OpenFOAM. La chiave per l’abbattimento delle emissioni è il raggiungimento di una condizione premiscelata in cui avviare la combustione. Inizialmente è stato valutata l’intensità della turbolenza generata dal moto della carica di aria in ingresso al cilindro, ed è stato verificato il potenziale per la creazione di una miscela omogenea. Successivamente, è stata simulata l’iniezione di combustibile e il conseguente miscelamento con aria. Poiché sono stati riscontrati problemi di miscelamento, diverse configurazioni sono state analizzate come prime proposte per il miglioramento del processo. In conclusione, è stato effettuato un tentativo di simulazione del processo di combustione. Non disponendo di dati sperimentali, le condizioni iniziali nel cilindro sono state ipotizzate a partire dai dati reperiti in letteratura. Migliori risultati sono attesi utilizzando una geometria più accurata e riferendosi a dati sperimentali.
CFD modeling of gas exchange and fuel-air mixing in a recuperated split-cycle engine
Vaghini, Alessandra
2019/2020
Abstract
The upcoming introduction of Euro VII legislation requires manufacturers to introduce new technologies to comply with the emission limits imposed by EU. For heavy-duty vehicles, a valuable solution could be represented by Recuperated Split-Cycle Engines. In this configuration, the conventional four-stroke cycle is splitted into two cylinders. In the first one, air is inducted and compressed. It is then sent to the recuperator where it is pre-heated by the exhaust gases of the second cylinder. Finally, it is sent to the expander cylinder, where fuel is injected and the combustion, expansion and exhaust phase can be performed. Theoretically, this technology should guarantee lower emissions, as it has a higher efficiency than traditional engines and has the ability to control combustion conditions. In this thesis work, a preliminary study of the air intake process and the consequent mixing between air and fuel that takes place in the expansion cylinder was carried out through a CFD analysis with the OpenFOAM software. The key for emissions reduction is the achievement of a premixed condition in which combustion can be started. Initially, the intensity of the turbulence generated by the motion of the air charge entering the expander cylinder was evaluated, and the potential for the creation of a homogeneous mixture was verified. Hence, fuel injection and subsequent mixing with air was simulated. As mixing problems were encountered, several configurations were evaluated as first proposals to improve the performances. In conclusion, an attempt was made to simulate the combustion process. Since no experimental data are available, the initial conditions in the cylinder were assumed starting from the data found in the literature. Better results are expected by using more accurate geometry and referring to experimental data.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/173109