Increasing HCCI power density through thermal stratification : impact of water direct injection

In the path towards a 100% renewable electricity production, long-term storage is needed to match the energy production and consumption. Power-to-Fuel is one of the most promising systems able to accomplish this role. To restitute electricity from various fuels, the Homogeneous Charge Compression Ignition engine is an interesting candidate. HCCI combustion is a Low Temperature Combustion mode which enables low nitrogen oxides and particulate matter emissions while at the same time showing Diesel-like efficiencies. However, the very high pressure rise rates associated with HCCI combustion imposes the use of lean mixtures. This is why these types of engines are characterized by a relatively low power density. For HCCI engines to reach viability at global scale, it is crucial to increase their power density. In this thesis is explored a technique which could allow to burn richer mixtures and therefore produce more mechanical work : Water Direct Injection. The local evaporation of a water spray, by creating thermal gradients inside the cylinder, could allow to stagger the combustion, hence reducing the pressure rise rate. The objectives of this work can be divided in two steps. The first step of this thesis has been to design and calibrate a reliable injection system. After the mounting of the injection system onto the HCCI monocylinder test bench of the UCLouvain, the second step of this work has been carried out. This step consists in realizing the experiments on the engine test bench. Even if Water Direct Injection still needs further investigations, this technique has shown very encouraging results. Indeed, the fuel-air equivalence ratio could be increased from 0.2 to 0.5, leading to a 215% boost of the power density.