On the Direct Injection of Supercritical and Superheated H
O into ICEs: The Role of the Injector Geometry
The transition to electric vehicles in the transportation sector still faces multiple technological challenges and large investments as regards both vehicle design and vehicle charging infrastructure. Therefore, internal combustion engines still play a role in such a sector, making the engine improvements, in terms of pollutant emissions and efficiency, essential to mitigate the impact of human activities on the environment. One of the possible approaches to improve the efficiency of internal combustion engines is the recovery of the engine exhaust heat, from both the hot exhaust gases and the engine cooling system.
In recent years, among the energy recovery strategies, the use of direct injection of H2O under supercritical and superheated thermodynamic states has been explored. Such a technique uses pressurized water recovered from the exhaust gases, heated to high temperature by using the engine exhaust heat and re-injected into the engine combustion chamber. This results in higher in-chamber pressure, which increases the engine work and efficiency.
The injector geometry is a key component of the process, as it determines the structure of the resulting under-expanded jet and the in-chamber flow field, thus affecting the jet interaction with combustion. In this work, three different injector geometries, namely an axial, an open-nozzle and a 4-holes injector, and two injected fluids, i.e. supercritical water and superheated steam, have been considered in order to highlight the advantages and drawbacks of each of them. To this end, a CFD model of a 4-stroke spark ignition internal combustion engine has been used. The results show that the injected supercritical water penetrates faster compared to superheated steam for all three injector geometries. The 4-holes and the open-nozzle injectors present the shortest and the longest penetration time, respectively, with both injected fluids. Besides, the 4-holes injector has given the highest TKE increase, followed by the axial injector and the open nozzle injector. The TKE is, for all cases, three orders of magnitude higher than the case without injection.