EXPERIMENTAL AND NUMERICAL ANALYSIS OF THE EFFECTS OF WATER INJECTION IN A TURBOCHARGED SPARK-IGNITION ENGINE AT PARTIAL AND FULL LOAD OPERATION

The water injection (WI) technique represents a viable way to improve the performance of spark ignition (SI) engines. The main benefit of using this technique is to take advantage of the high heat of vaporization of water. If introduced into the engine, its vaporization cools the air-fuel mixture entering the combustion chamber. This could lead to greater spark advances, with optimal combustion phasing, and to a reduction of the combustion temperature with consequent lower heat losses. Moreover, due to the vaporization cooling effect, the heat capacity ratio of the in-cylinder gases increases. These three main factors allow improving the thermal efficiency of the engine. In this work, the effects of both direct and port water injection in a downsized PFI spark-ignition engine have been analyzed. For the experimental analysis, low-pressure water injectors have been installed in the intake ports of the engine under study, upstream of the fuel injectors. Experimental tests have been carried out at various operating points. In particular, a performance improvement has been observed at high load conditions. WI allowed a stoichiometric combustion and a BSFC decrease of 5.1% with respect to dry operation in the analyzed case. Furthermore, engine operation with port water injection has been simulated by means of the AVL Fire 3D code. The numerical analysis has been also used in order to simulate the direct water injection in the same engine. A 6-hole high-pressure water injector has been implemented within the 3D model of the engine combustion chamber. Water spray has been validated against available literature data. Medium load engine operating points with in-cylinder water injection have been simulated. CFD analysis allowed to deeply investigate the evolution of water spray and its cooling effect on the air-fuel mixture for both direct and indirect water injection configurations. Then, a 1D approach has been utilized in order to identify the areas of the engine map in which advantages by using port water injection can be obtained, quantifying its effects in terms of performance improvement. Calculations have been carried out using a 1D model validated with respect to the experimental data of the analyzed engine. A knock model has been implemented in the 1D model in order to identify the knock-limited parameters in engine optimization. Threshold values for knock index and exhaust turbine inlet temperature have been imposed as optimization constraints. The model has been subsequently modified by adding water injection into the intake ports. Various operating points have been analyzed. The engine control parameters have been optimized in order to obtain the minimum specific fuel consumption. The most remarkable results show that, at medium and high load operation, it is possible to obtain an improvement in terms of both power and specific fuel consumption. In particular, at full load, WI allowed a brake specific fuel consumption mean improvement of 18% in the engine speed range. Output power mean increase was about 13%. The obtained results show water injection technique could be a useful tool in performance improving of turbocharged spark-ignition engines.