Use of electrofuels in Homogeneous Charge Compression Ignition engines : numerical study of oxy-combustion potential

The present master’s thesis is a continuation of previous works on the HCCI technology conducted at UCLouvain. The latest focused on the use of electrofuels in HCCI engines for the electricity restitution phase of Power to Fuel to Power cycles. Research on fuel blending and water direct injection were thus previously performed. In this thesis, the numerical investigation on the use of pure oxygen as oxidiser, i.e. oxy-combustion, in HCCI engines fuelled by methane is conducted. The comparison between conventional air combustion and oxy-combustion is emphasised and the use of carbon dioxide to reduce the oxy-combustion brutality is investigated. The objectives of this work are multiple. First of all, the context in which HCCI oxy-combustion could be used is set. Fundamental concepts of HCCI engine operation and control are explained and the potential of HCCI oxy-combustion is detailed based on a review of the few scientific papers on the subject. A description of the engine test bench followed by the modifications required to perform oxy-combustion are presented. A 0D numerical model is then used to determine the required intake thermodynamic conditions to have a proper auto-ignition timing for both combustion types. Different fuel contents and charge compositions are simulated. Afterwards, 3D axisymmetric simulations are performed to precisely predict the combustion behaviour in terms of timings and pressure rise rates. Using the 3D simulation results, we define the optimal auto-ignition timing to maximise the engine load and indicated efficiency for different equivalence ratio contained in our research engine operating range. The impact of various parameters is discussed and the comparison is made between air and oxy-combustion. Ringing occurs at lower fuel content for pure oxy-combustion due to the more brutal and rapid combustion. In our research engine, with the optimal combustion timing, the limit is reached at FuelMEP = 9 [bar] for an IMEPg of 3.7 [bar]. The addition of CO2 in the intake charge has been proven to allow an increase of more than 40% in engine load by extending the combustion duration thus allowing the use of higher fuel contents.