In this work, lean mixed-mode combustion is numerically investigated using computational fluid dynamics (CFD) in a spark-ignition engine. A new E30 fuel surrogate is developed using a neural network model with matched octane numbers. A skeletal mechanism is also developed by automated mechanism reduction and by incorporating a NOx sub-mechanism. A hybrid approach that couples the G-equation model and the well-stirred reactor model is employed for turbulent combustion modeling. The developed CFD model is able to predict pressure and apparent heat release rate (AHRR) traces. Two types of combustion cycles (namely, purely-deflagrative and mixed-mode cycles) are observed. The mixed-mode cycles feature earlier flame propagation and end-gas auto-ignition, leading to two distinctive AHRR peaks. The validated CFD model is then employed to investigate the effects of NOx chemistry. The NOx chemistry is found to promote auto-ignition through residual gas, while the deflagration phase remains largely unaffected. Sensitivity analysis is finally performed to understand effects of fuel properties, including heat of vaporization (HoV) and laminar flame speed (SL). An increased HoV tends to suppress auto-ignition through charge cooling, while the impact of HoV on flame propagation is insignificant. In contrast, an increased SL is found to significantly promote both flame propagation and end-gas auto-ignition. The promoting effect of SL on auto-ignition is not a direct chemical effect; it is rather caused by an advancement of the combustion phasing, which increases compression heating of the end-gas.

In thrust bearings, cavitation may occur at high rotational speeds or low lubricant supply pressures, and it will influence the bearing performances. In this paper, a hydrodynamic tapered land thrust bearing has been studied both experimentally and numerically, with concentration on the cavitation phenomenon and its effects on the bearing performances. Evident cavitation regions have been observed in the experiments at higher rotational speeds. Traditional Reynolds equation and 3D Navier–Stokes equation (3D NSE) with a cavitation model have been used for numerical simulation, and the predicted results are examined against the experimental results. Compared with Reynolds equation, 3D NSE with Rayleigh–Plesset model provides better predictions of both oil–film pressure profile and cavitation area. Furthermore, the effects of the cavitation phenomenon on the thrust bearing performances are studied by parametric studies involving various rotational speeds and oil feeding pressures, using 3D NSE. It is found that the load capacity decreases at higher speeds because of enlargement of the cavitation area. And the negative effects of cavitation can be reduced at smaller film thickness and higher oil supply pressure. Conclusively, the above results show that the cavitation phenomenon has significant influences on the bearing performances at higher speeds, and 3D NSE provides an effective tool for analyzing the cavitation effects in thrust bearings.

The prediction of cavitation in lubricants is of significance to the design and analysis of hydrodynamic bearings. In this paper, a numerical model for the gaseous cavitation in hydrodynamic lubrication problems is derived based on the cavitation mechanism and origination, which is different from the conventional cavitation conditions. The proposed gaseous cavitation model is then used to calculate the performance of several types of bearings, including a two-axially grooved plain journal bearing, a misaligned journal bearing and parallel thrust bearings with surface textures. The results are validated with experimental data and numerical results calculated by the Half-Sommerfeld, Reynolds, and JFO conditions. The advantage of this new model lies in its accuracy and its independence from the cavitation pressure, which is usually difficult to be accurately defined. Thus this gaseous cavitation model can be useful in the accurate prediction of the cavitation phenomenon and performance of hydrodynamic bearings under steady state.

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In the present study, mixed-mode combustion of an E30 fuel in a direct-injection spark-ignition engine is numerically investigated at a fuel-lean operating condition using multidimensional computational fluid dynamics (CFD). A fuel surrogate matching Research Octane Number (RON) and Motor Octane Number (MON) of E30 is first developed using neural network based non-linear regression model. To enable efficient 3D engine simulations, a 164-species skeletal reaction mechanism incorporating NOx chemistry is reduced from a detailed chemical kinetic model. A hybrid approach that incorporates the G-equation model for tracking turbulent flame front, and the multi-zone well-stirred reactor model for predicting auto-ignition in the end gas, is employed to account for turbulent combustion interactions in the engine cylinder. Predicted in-cylinder pressure and heat release rate traces agree well with experimental measurements. The proposed modelling approach also captures moderated cyclic variability. Two different types of combustion cycles, corresponding to purely deflagrative and mixed-mode combustion, are observed. In contrast to the purely deflagrative cycles, mixed-mode combustion cycles feature early flame propagation followed by end-gas auto-ignition, leading to two distinctive peaks in heat release rate traces. The positive correlation between mixed-mode combustion cycles and early flame propagation is well captured by simulations. With the validated numerical setup, effects of NOx chemistry on mixed-mode combustion predictions are investigated. NOx chemistry is found to promote auto-ignition through residual gas recirculation, while the deflagrative flame propagation phase remains largely unaffected. Local sensitivity analysis is then performed to understand effects of physical and chemical properties of the fuel, i.e., heat of evaporation (HoV) and laminar flame speed (SL). An increased HoV tends to suppress end-gas auto-ignition due to increased vaporization cooling, while the impact of HoV on flame propagation is insignificant. In contrast, an increased SL is found to significantly promote both flame propagation and auto-ignition. The promoting effect of SL on auto-ignition is not a direct chemical effect; it is rather caused by an advancement of the combustion phasing, which increases compression heating of the end gas.