A numerical study on the soot and combustion performance of a diesel engine with pip shape

Journal of Mechanical Science and Technology - The shape of the combustion chamber plays an important role in the formation of the air-fuel mixture in the chamber, which has a great influence on...     

Positioning control of an underwater robot with tilting thrusters via decomposition of thrust vector

Positioning control of an underwater robot is a challenging problem due to the high disturbances of ocean flow. To overcome the high disturbance, a new underwater robot with tilting thrusters was proposed previously, which can compensate for disturbance by focusing the thrusting force in the direction of the disturbance. However, the tilting motion of the thrusters makes the system nonlinear, and the limited tilting speed sometimes makes the robot unstable. Therefore, an optimized controller is necessary. A new positioning controller is proposed for this robot using a vector decomposition method. Based on the dynamic model, the nonlinear force input term of the tilting thrusters is decomposed in the horizontal and vertical directions. Based on the decomposition, the solution is determined by a pseudo-inverse and null-space solution. Using the characteristics of the decomposed input matrix, the final solution can be found by solving a simple second-order algebraic equation to overcome the limitations of the tilting speed. The positioning was simulated to validate the proposed controller by comparing the results with a switching-based controller. Tracking results are also presented. In future work, a high-level control strategy will be developed to take advantage of the tilting thrusters by focusing the forcing direction toward the disturbance with a limited stability margin.

     

Comparison of the effects of multiple injection strategy on the emissions between moderate and heavy EGR rate conditions: part 2-post injections

To draw a comparison of the effect of multiple injection strategy on the engine-out emissions under two different EGR rate conditions, the effect of pilot injection on emissions and combustion was evaluated and discussed in part 1. Thus, in the second research as part 2, the effects of post injection on the engine-out emissions were systemically evaluated for two different EGR rate conditions (30 % and 60 %). Since the behavior of diesel combustion is significantly different as EGR rate is changed, the characteristics of post injection was different between two EGR rate conditions. This research was investigated as varying injection parameters such as the timing and quantity of the post injection. The results show that the close post injection with injection interval as 10 degree has the potential to reduce PM emission, regardless of EGR rate. However, the reason of reduction of PM emission is different for each case. For a moderate EGR rate condition, close post injection with interval 10 degree enhances the fuel at bottom of bowl. Thus, the distribution of fuel can be improved. On the other hand, for a heavy EGR rate condition, close post injection with interval 10 degree has the charge cooling effect to prolong the ignition delay, rather than well-matched injection targeting. Especially, there is an effect to oxidize PM emission under moderate EGR rate condition as post injection is applied. However, post injection for late cycle of combustion under heavy EGR rate condition does not oxidize PM emission due to low oxygen concentration (～ 10%).     

Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine

The effects of in-cylinder EGR stratification on combustion and emission characteristics are investigated in a single cylinder direct injection diesel engine. To achieve in-cylinder EGR stratification, external EGR rates of two intake ports are varied by supplying EGR asymmetrically using a separated intake runner. The EGR stratification pattern is improved using a 2-step bowl piston and an offset chamfer at the tangential intake port. When high EGR gas is supplied to the left (tangential) port, a high EGR region is formed at the central upper region of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and PM is reduced significantly. When high EGR gas is supplied to the right (helical) port, a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NO_x is reduced without PM penalty. Stratified EGR potentially reduces NO_x by maximum 45%, without penalties of performance and other emissions. A proper in-cylinder swirl with stratified EGR maximizes the effects and achieves simultaneous reduction of NO_x by 7% and PM by 23%. Moreover, the robustness of stratified EGR is evaluated under various operating conditions and injection strategies.     

Void growth and coalescence in f.c.c. single crystals

In this study, we investigate deformation behavior of f.c.c. single crystals containing microvoids by using three-dimensional finite element methods. The unit cell analysis has been conducted to study the effect of stress triaxialities, crystallographic orientations and initial void volume fractions on the growth and coalescence of voids in f.c.c. single crystals. The locally homogeneous constitutive model for the rate-dependent single crystal plasticity is implemented into a finite element program (ABAQUS) by means of the user-defined subroutine (UMAT). To identify the effect of stress triaxiality and the crystallographic orientation on the void evolution, the stress triaxiality was kept constant during deformation. The numerical results showed that the stress triaxiality and the deformation mode specified by the crystallographic orientation have a competitive effect on the evolution of voids. For the low level of stress triaxiality, the deformation mode is mainly determined by the crystallographic orientation. For high stress triaxiality, however, the deviation from the specific deformation mode is large even for incipient void growth and the void growth rate is mainly determined by stress triaxiality and the initial void volume fraction. For the small initial void volume fraction, the growth rate of a void is rapid compared with the large one and the effect of the initial crystallographic orientation is significant.     

Bioinspired Geometry‐Switchable Janus Nanofibers for Eye‐Readable H2 Sensors

Nanoscale architectures found in nature have unique functionalities and their discovery has led to significant advancements in various fields including optics, wetting, and adhesion. The sensilla of arthropods, comprised of unique hierarchical structures, are a representative example which inspired the development of various bioinspired systems, owing to their hypersensitive and ultrafast responsivity to mechanical and chemical stimuli. This report presents a geometry‐switchable and highly H₂‐reactive Janus nanofiber (H‐NF) array inspired by the structural features of the arthropod sensilla. The H‐NF array (400 nm diameter, 4 µm height, 1.2 µm spacing distance, and hexagonal array) exhibits reversible structural deformation when exposed to a flammable concentration of hydrogen gas (4 vol% H₂ in N₂) with fast response times (5.1 s). The structural change can be detected with the bare eye, which is a result of change in the optical transmittance due to the structural deformation of the H‐NF array. Based on these results, an eye‐readable H₂‐sensor that requires no additional electrical apparatus is demonstrated, including wetting‐controllable H₂‐selective smart surfaces and H₂‐responsive fasteners.     

3-D Simulation of the combustion process for di-methyl ether-fueled diesel engine

The characteristics of spray, auto-ignition, and the combustion process of di-methyl ether(DME) were investigated using a 3-D simulation of a combustion vessel under various engine conditions, including high temperature and pressure. Spray impingement and nonpremixed combustion models were incorporated into the computational fluid dynamics code, called STAR-CD. A Peng-Robinson equation of state was introduced to calculate the evaporation rate of DME droplets that were exposed to high-pressure conditions. A Laminar flamelet concept was used to simulate non-premixed combustion. A skeletal chemical kinetics mechanism consisting of 28 species and 45 reactions was used as the chemical mechanism for DME. Compared with the experimental data, the flamelet concept combustion model predicted the essential features of the combustion process and auto-ignition characteristics of the DME spray reasonably well for various initial conditions. The combustion process of a direct injection engine fueled by DME was also simulated and verified through experimental data.