Effect of Direct Water Injection on Combustion and Performance of Homogeneous Charge Compression Ignition Engine-A Computational Fluid Dynamics Analysis

摘要

Nowadays, homogeneous charge compression ignition (HCCI) engines are becoming very popular because they all have low emissions of nitrogen oxides (NO_x) and soot. However, they have problems such as (i) narrow operating load range and (ii) irregular combustion phasing. To overcome these problems, various strategies, such as variable compression ratio (VCR), exhaust gas recirculation (EGR), water injection, mixture stratification, etc., are being tried. However, direct water injection is found to be a better strategy for controlling the combustion phasing and improving the performance because it can also reduce knocking. Although the effectiveness of the water injection depends on in-cylinder water evaporation, which in turn depends on the water injector configuration. Therefore, in this article, a computational fluid dynamics (CFD) analysis has been done to study the effect of water injector configuration on the performance of an HCCI engine. For the study, three water injector orientations along with four water spray patterns are considered (a total of twelve cases). The engine is operated at the upper load limit, which occurs at the equivalence ratio (ER) of 0.74 (without water injection), with a compression ratio (CR) of 12.5 and at the engine speed (ES) of 2000 RPM. The CFD models used for the study are validated from the available experimental data in the literature. Here the optimization of water injector orientation and configuration is done based on the heat release rate (HRR) and the maximum rate of pressure rise (MRPR). From the analysis of results, it is found that the amount of water evaporated in the various water injector configurations is about 30%-87% at the time of ignition. Also the water injector orientation of 45° with a hexagonal shape water spray pattern is the optimum one. With this configuration, it is possible to increase the ER up to 0.85. At this ER, it is found that the indicated mean effective pressure (IMEP) of the engine is increased by about 25%, and the NO_x emissions are reduced by about 70.3% than that of the case without water injection (WWI) with a marginal penalty on hydrocarbon (HC) and carbon monoxide (CO) emissions.