Zhen Lu

Natural gas is an environmentally friendly energy source that significantly reduces unburned hydrocarbons and NOx compared to traditional fuel (such as HFO, MGO, etc.) in low speed two-stroke marine engines. However, low-pressure injection dual-fuel engines tend to experience detonation under high load circumstances, thereby restricting the power output of the engine. The factors contributing to detonation in dual-fuel engines are more complex compared to conventional engines. This paper intends to investigate the potential mechanism that leads to the in-cylinder detonation phenomenon in natural gas-diesel dual-fuel marine engines and to search for approaches to improve the in-cylinder pressure oscillations. The numerical simulation outcomes reveal that decreasing the mass of pilot diesel fuel (from 227mg to 75.6mg) and the equivalent ratio (from 0.51 to 0.42) can effectively control the combustion rate and the combustion intensity of the flame front, thus attaining the goal of improving the cylinder pressure fluctuation. Moreover, appropriately enhancing swirl flow can alter the shape of the flame front which can restrain the pressure oscillation within the cylinder. As a consequence, the shape of the flame front is also a crucial factor affecting the pressure oscillation. Eventually, the pressure oscillation can be decreased to lower than 0.1 MPa and it is feasible to increase the power of dual-fuel engine (from 950.16 KW to 1142.85 KW) by raising the natural gas fuel (from 22.1mg to 22.69 mg).

In-cylinder flow characteristics, which are mainly determined by the intake ports, have a decisive influence on the mixture formation and combustion process of internal combustion (IC) engines. To develop the optimum intake ports in a four-valve diesel engine, the full-parametric design method coupled with genetic algorithm (GA) and artificial neural network (ANN) was adopted in this study. Different from previous studies, the full-parametric templates for the tangential and helical ports were established. Then, to improve the precision of the CFD result which was used to train the ANN, the near-wall region affected by viscosity was resolved by the “enhanced wall treatment” during the simulation process. Besides, considering the interaction between the outlets of adjacent intake ports in the four-valve engine, the ANN was divided into three parts to better predict the flow performance of the combined intake ports. Finally, the developed intake ports based on ANN and GA were machined to the flow box and validated by the steady flow test. The low deviation of 4.71% in swirl ratio between the design target and the experiment result confirms that this method can be well applied to the development of the intake ports in a four-valve diesel engine.

The turbulent boundary layer flow in internal combustion (IC) engines has a significant effect on the in-cylinder flow and the wall heat transfer. A detailed analysis of the in-cylinder near-wall flow was carried out on an optical steady flow test bench by using high-resolution particle image velocimetry (PIV) in order to characterize the in-cylinder boundary layer flow in this study. The difference between the in-cylinder boundary layer and the canonical turbulent boundary layer was analyzed. The experimental results show that small-scale vortices with a length scale of about 1–2 mm in the instantaneous flow fields appeared in the wall jet region due to the entrainment of the free jet in the outer region of the wall jet. The viscous sublayer thickness decreased from 0.5 mm to 0.3 mm as the valve lift increased from 2.32 mm to 7.975 mm and the pressure drop from 0.5 kPa to 1 kPa. The dimensionless velocity profile is in good agreement with the law of the wall in the viscous sublayer. However, no obvious logarithmic law distribution region was observed in the logarithmic layer. The distribution of the Reynolds stress and the turbulent kinetic energy is similar to that of the canonical turbulent boundary layer. But the Reynolds stress had a much larger magnitude because the turbulent velocity measured in this boundary layer included not only the turbulence generated by wall shear but also the large-scale turbulent vortices caused by the wall jet.

Emission and fuel consumption in swirl-supported diesel engines strongly depend on the in-cylinder turbulent flows. But the physical effects of squish flow on the tangential flow and turbulence production are still far from well understood. To identify the effects of squish flow, Particle image velocimetry (PIV) experiments are performed in a motored optical diesel engine equipped with different bowls. By comparing and associating the large-scale flow and turbulent kinetic energy (k), the main effects of the squish flow are clarified. The effect of squish flow on the turbulence production in the r−θ plane lies in the axial-asymmetry of the annular distribution of radial flow and the deviation between the ensemble-averaged swirl field and rigid body swirl field. Larger squish flow could promote the swirl center to move to the cylinder axis and reduce the deformation of swirl center, which could decrease the axial-asymmetry of annular distribution of radial flow, further, that results in a lower turbulence production of the shear stress. Moreover, larger squish flow increases the radial fluctuation velocity which makes a similar contribution to k with the tangential component. The understanding of the squish flow and its correlations with tangential flow and turbulence obtained in this study is beneficial to design and optimize the in-cylinder turbulent flow.