The effect of different port water injection (PWI) ratios on the knock in gasoline direct injection engines is explored. First, a simulation study is carried out under knock conditions to explore the time and location of the knock and the chemical mechanism that causes it. Second, water injection amount to 10%, 20%, 30%, and 40% are set to explore the effects of different water injection ratios on knocking. The influence of the different water injection ratios on the chemical mechanism of knocking is evaluated and that of the chemical mechanism on the in‐cylinder temperature and the impact of heat release rate (HRR) is explored. The results show that PWI can effectively reduce the occurrence of spontaneous combustion in the cylinder by reducing the flame propagation speed. On one hand, the changes in the low‐temperature reaction substances CH2O and HRR tend to be consistent and the peaks of CH2O and HRR move back with the increase in water injection. So, different ratios of PWI can affect the flame propagation speed by affecting the generation and consumption of CH2O, OH radicals, and HCO radicals. Also, PWI can significantly reduce the generation of in‐cylinder OH radicals, ultimately suppressing the generation of the high‐temperature stage. In addition, PWI can effectively reduce the mean temperature and mean pressure. Compared to other water injection ratios, a 30% water injection ratio is the most effective at inhibiting knocking. At this time, the knocking intensity is the lowest, and the cycle power reaches its maximum of 1120 J.

In this study, two different piston structures (bowl-shaped pit and trapezoidal pit) are constructed; the mixture formation, combustion, and emissions of a gasoline direct injection engine with the two piston types are compared and analyzed by computational fluid dynamics simulation. The results show that piston A (bowl-shaped) can form a combustible mixture near the spark plug at the ignition time and has higher pressure peaks and accumulated heat release than piston B (trapezoidal), which helps improve the combustion efficiency of the internal combustion engine. Furthermore, three pistons with different bowl-shaped pit depths (pistons A1, A2, and A3) are designed based on piston A. The results show that piston A2 (7.7 mm) has advantages in terms of strengthening the turbulence in the cylinders, promoting fuel evaporation, increasing the in-cylinder turbulent kinetic energy and the velocity of the airflow near the spark plug at the ignition time, and accelerating the rapid diffusion of combustion and the rapid increase in in-cylinder temperature and pressure. Also, piston A2 can reduce the CO and soot emissions.

In this study, the research method of numerical simulation is used to explore the inhibition of different water injection pressures on knock combustion of turbocharged direct injection gasoline (GDI) engines by coupling computational fluid dynamics with a chemical-kinetics model. First, the ignition advance angle and compression ratio are increased to induce the GDI engine to knock, and then the influence of the water injection pressure on the in-cylinder, evaporation of water, and the knock of the gasoline engine are analyzed. The simulation results show that, compared with no water injection, the direct injection of water in the cylinder can significantly reduce the knock intensity. When the water injection pressure is greater than 40 bar, the knock intensity is less than 2 and the knocking is completely suppressed. In this work, the effects of different water injection pressures on knocking are explored by analyzing the effects of water injection pressure on water atomization, in-cylinder combustion, and the knocking mechanism. On the one hand, the evaporation rate of water increases with increasing water injection pressure and the quality of the liquid film generally improves. On the other hand, direct water injection can significantly reduce the distribution of CH 2 O in the end mixture, thereby reducing the generation of H 2 O 2 and further suppressing the spontaneous combustion of the end mixture. At the moment of knock, when the water injection pressure is greater than 40 bar, the detonation mechanism of the no. 7 monitoring point does not produce a sudden change in HCO radicals. The water spray can effectively reduce the NO x emission, and the NO x emission under the water spray pressure of 120 bar is the lowest. However, after spraying water, it will increase CO emissions.

This paper explores the effects of six different cases of port water injection on the combustion, knock suppression and emissions of a supercharged gasoline direct injection (GDI) engine through numerical simulation. The six different intake port water injection cases included three vertical distances from cylinder center to water injector and two different injection directions. The results showed that cases 2 and 4 allowed more water and air to enter the cylinder and thus suppressed the knock, so the pressure oscillation was small. Case 2 had the largest turbulent kinetic energy in the center of the cylinder, which in turn facilitated the propagation of flame to the cylinder wall and suppressed the knock. The water injection cases shortened the combustion delay period compared to the no water cases. At the same time, the strong low temperature reaction of the end mixture produced a large amount of CH2O that decomposed into HCO. A high concentration and a large area of HCO distribution can predict the occurrence of a knock. In addition, the water injection cases (except for case 6) reduced the in-cylinder soot, unburned hydrocarbon (UHC) and CO emissions compared to the no water cases, but it increased NOX emissions.

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