An investigation of topological features of homogeneous isotropic turbulence with an inverse energy cascade is performed by using a direct numerical simulation (DNS). The inverse energy cascade is induced by reversing the velocity field (i.e., under the transformation from u_i to -u_i) of a freely decaying isotropic turbulence. In the backward energy transfer process, the joint probability density function (PDF) of Q and R exhibits a novel shape which seems to be symmetric with the well-known teardrop shape about the Q-axis (Q and R are the second and third invariants of the velocity gradients tensor, respectively). The unexpected shape of the joint PDF of Q and R indicates that the turbulence with an inverse energy cascade is dominated by vortex compression and tubelike structures. The PDF of the intermediate eigenvalue of the strain-rate tensor in the inverse energy cascade is negatively skewed, which is opposite to the universal feature of the forward energy cascade. Nevertheless, the preferential alignment of vorticity with the intermediate eigenvector is rather robust, no matter the direction of the energy transfer is forward or backward. In addition, it is universal that the vorticity is mostly perpendicular to the eigenvector which corresponds to a strain-rate eigenvalue with the maximum absolute value among the three eigenvalues. Since the velocity gradient invariants are closely related to the local flow topology, the numerical results reported in this work are expected to shed light on the intrinsic dynamics and mechanisms of inverse energy cascade.

Zero carbon emission is a mainstream trend in the development of internal combustion engines (ICEs) in the future. ICEs need to constantly surpass the existing working mechanism, especially in order to explore the possibility of new combustion methods. Dual-fuel combustion is a good way to reduce carbon emissions and achieve clean combustion. However, the traditional internal combustion engine is limited by its own structure, restricting its performance improvement. The opposed-piston, two-stroke (OP2S) diesel engine is a potential power system with a high degree of structural adjustability. Therefore, this work attempted to apply methanol/diesel dual-fuel to OP2S engines in order to explore efficient and clean combustion modes in the future. In this work, a one-dimensional simulation model of an OP2S diesel engine was established and verified. The effect of the port height to stroke ratio on the performance of the OP2S diesel engine was mainly studied for different methanol blending ratios. The results show that the methanol blending ratio does not affect the port height to stroke ratio where the optimal values of the MIP and scavenging efficiency appear. The optimal methanol blending ratio for the power performance of OP2S diesel engines is 5~15%. There is a trade-off relationship between the MIP/scavenging efficiency and trapping efficiency. For the optimization of an OP2S methanol–diesel engine, priority should be given to ensuring an optimal MIP and scavenging efficiency, and then to the appropriate consideration of the trapping efficiency.

Opposed-Piston, Two-Stroke (OP2S) diesel engine is a potential powertrain to achieve efficient and clean combustion. However, its non-uniformity of mixing and combustion restricts performance improvement. To solve this problem, this work proposed a lateral swirl combustion system (LSCS) for the OP2S diesel engine. Applying CFD simulation, this work studied the effects of different combustion chamber parameters on the mixing and combustion performance, power, and emissions of the LSCS of the OP2S diesel engine. Furthermore, the combustion parameters were optimized by sensitivity analysis and parameter normalization methods. The results show that the LSCS helps improve the non-uniformity of mixing & combustion of the OP2S diesel engine, accelerate burning, improve the power of the OP2S diesel engine and reduce soot emissions. Besides, under certain fuel supply conditions, it is necessary to make good use of the space of the axial and radial sections of the combustion chamber and make the convex edge height realize the split-flow effect. Therefore the performance of the LSCS can be improved effectively in the OP2S diesel engine.

Zero carbon emissions will dominate the future of internal combustion engines (ICEs). Existing technology has pushed the performance of ICEs operating on traditional working principles to almost reach their limit. The new generation of ICEs needs to explore new efficient combustion modes. For new combustion modes to simplify the emission after treatment, the opposed-piston, two-stroke (OP2S) diesel engine is a powertrain with great potential value. Combined with dual-fuel technology, the OP2S diesel engine can effectively reduce carbon emissions to achieve clean combustion. Hence, methanol/diesel dual fuel was burnt in the OP2S engine to create a clean combustion mode for future demands. In the present work, a 1D simulation model of an OP2S diesel engine was established and verified. We investigated the influence of port height to stroke ratio (HSR) on power and emission performances of the OP2S diesel engine under different methanol ratios. The results show that the methanol ratio extremely influences the indicated power (IP) with the HSR of intake ports increasing. The IP decreases by about 1.8–2.0% for every 5% increase in methanol. Correspondingly, the methanol ratio extremely influences the indicated thermal efficiency (ITE), with the HSR of exhaust ports increasing. The ITE increases by about 2.1–3.1% for every 5% increase in methanol. The increasing methanol ratio reduces the HSR of ports for the optimal IP and ITE. To balance power performance and emission performance, the methanol ratio should be kept to 10–15%.

Common rail injector response characteristics depend on the control chamber pressure change rate, the outlet throttle diameter by manufacturing errors, or wear-induced deviations that affect the rate of pressure change in the control chamber, so the accuracy of the outlet throttle diameter directly affects the control valve response consistency. This paper presents a computational fluid dynamics (CFD) simulation of the effect of the deviation of the outlet throttle diameter on the average mass flow rate of the outlet throttle during the opening of the spherical valve in order to reduce this difference and ensure uniform injection characteristics. The results illustrate that with the increase in outlet throttle diameter deviation, the volume of gas phase in the control valve increases and the rate of pressure reduction in the control chamber accelerates, and the sensitivity coefficient of the average mass flow rate of the outlet throttle to the outlet throttle diameter deviation remains unchanged at 24.77. Cavitation occurs in the area of the outlet throttle when the spherical valve lift is 0.045 mm. The increase in rail pressure not only leads to an increase in the low-pressure area on the inner wall of the outlet throttle, an increase in the volume share of the gas phase, and an increase in the hydraulic impact on the sealing wall but also makes the average mass flow rate of the outlet throttle and the rate of change of the control chamber pressure during the opening of the spherical valve more sensitive to the deviation of the outlet throttle diameter.