Effect of reinforcement volume fraction and size distribution on the tribological behavior of Al-composite/brake pad tribo-couple

Tribological behavior of aluminium matrix composite (AMC)/brake pad tribo-couple under dry sliding conditions was studied using Pin-on-Disc machine. Brake pad material was used as pins while the AMC formed the rotating disc. Series of experiments were performed to characterize the tribological nature of the tribo-couple. Load and sliding speeds were varied over a range to represent actual braking conditions in passenger cars. Effect of volume fraction and size distribution of reinforcement on wear and friction coefficient has been studied. It was observed that a heterogeneous tribo-layer was formed over the worn surfaces during the wear tests. Presence of tribo-layer was believed to cause two effects: acting as a lubricant layer and acting as a source of wear debris. Morphology and topography of worn surfaces and debris were studied using scanning electron microscope (SEM), electron probe micro analyzer (EPMA), and X-ray diffraction (XRD) techniques. When the reinforcement in the matrix has wide size distribution, wear rate and friction coefficients are found to be higher compared to composite containing mono-size reinforcement.     

双绕组电动机能耗制动接线方式、直流磁势分析

通过对双绕组电动机能耗制动与普通三相异步电动机能耗制动的对比,在定子绕组中通入相同数值的直流电流条件下,得出结论：在通入大小相等的直流电流,双绕组电动机产生的直流磁通势是普通三相异步电动机直流磁通的两倍。介绍了双绕组电动机能耗制动两套绕组的4种不同的电气接线方式,每种电气接线的直流磁通势大少不同,两套绕组的等效电阻也不同。生产实践中可以根据生产工艺对制动转矩大小和制动时间长短的不同要求,选择一种理想的电气接线。     

数控回转工作台摩擦片式刹车结构研究

分析数控回转工作台摩擦片式刹车结构的工作原理,利用悬臂梁在集中载荷下的挠曲模型推导摩擦片弹性系数和刹车扭矩的计算公式,并应用于实例计算。利用有限元方法校验摩擦片的弹性系数并计算回转工作台的扭转刚度。设计数控回转工作台刹车扭矩的检测试验,测试回转工作台在许用载荷和过载载荷条件下扭矩和形变的关系,验证回转工作台的刹车扭矩和扭矩刚度的计算结果。研究结果为数控回转工作台刹车结构的设计和机床整机调试提供依据。     

液压缓冲无杆牵引车刹车工况下的仿真研究

在飞机地面牵引移动的过程中,无杆式飞机牵引车因遭遇突发事件等因素造成单独刹车时,通常会产生过大的牵引载荷,又因常规无杆式飞机牵引车与飞机连接所组成的牵引系统在纵向上只有飞机前起轮胎是柔性体,不能够起到良好的缓冲作用。因此,提出一种具有抗冲击的动刚度液压缓冲无杆式飞机牵引车方案,以A320-200为例,基于Adams建立“无杆牵引车(车架-缓冲器-夹持举升机构)-飞机”多体系统模型,牵引车单独制动进行仿真,对常规无杆式飞机牵引车和液压缓冲无杆牵引车在最大刹车力及相同刹车距离下的牵引载荷变化情况对比。结果表明,在以上两种情况下,选择合适的缓冲距离,液压缓冲无杆牵引车都能够降低牵引载荷到限制的范围内。     

Kinetic modeling and experimental study on the combustion,performance and emission characteristics of a PCCI engine fueled with ethanol-diesel blends

The combustion process in the Premixed Charge Compression Ignition (PCCI) engine is basically restricted by the in cylinder charged mixture components. Also, the homogeneity of the charged mixture is determining the quality and process of the chemical reaction during the first stage of combustion which establish the auto-ignition process. In the present work, the engine experimental setup is equipped with a new suggested modification on the original fuel system device in order to produce a perfect commixture of diesel/ethanol at different blends ratio with the charged air. The obtained laboratory results are used to validate the simulation's data of the PCCI engine ignition. The prediction is performed using a detailed kinetic reaction mechanism. The simulation study has been achieved to predict the auto-ignition timing and the combustion characteristics of the PCCI engine fueled with different blends of ethanol and diesel at different volume percentage. The obtained results show that the premixed ratio of the ethanol in the ethanol/diesel fuel blends can be used to control the auto-ignition timing and the combustion characteristics at different engine air/fuel ratios. Also, the main pathway of this work is to establish the influence of the engine operating parameters which including the premixed ratio, fuel–air equivalence ratio on the engine performance, combustion and emission characteristics of the PCCI engine. These effects are studied and traced through the simulation result data of the in-cylinder pressure, temperature, and gas phase heat release at different a premixed ratio of ethanol-diesel fuels blends of 0, 10, 20, 30,40 and 50% (by volume).     

BRAKE TEST OF SiCp/A356 BRAKE DISK AND INTERPRETATION OF EXPERIMENTAL RESULTS

Material properties are obvious different between aluminum matrix composites and iron and steel materials. After the brake disk braked at the same speed, the average temperature of the aluminum brake disk is 1.5 times as high as one of iron and steel brake disk, the thermal expansion value of the aluminum brake disk is 2 times as big as one of iron and steel brake disk. Mechanical property of the material decreases with the temperature increasing generally during braking, on the other hand, the big thermal stress in the brake disk happens because the material expansion is constrained. Firstly, the reasons of the thermal stress generation and the fracture failure of brake disks during braking are analyzed qualitatively by virtue of three-bar stress frame and sandwich deformation principles in physic, and then the five constraints which cause the thermal stress are summarized. On the base of the experimental results on the 1:1 emergency brake test, the thermal stress and temperature fields are simulated; The behavior of the fracture failure is interpreted semi-quantitatively by finite element analysis. There is the coincident forecast for the fraction position in term of the two methods. In the end, in the light of the analysis and calculation results, it is the general principles observed by the structure design and assembly of the brake disk that are summarized.     

煤矿带式输送机运行中的问题研究

分析研究了煤矿带式输送机运行中存在的问题,并指出如何采取相应的措施,以保证带式输送机的正常运行.     

Comparative analysis of driver’s brake perception-reaction time at signalized intersections with and without countdown timer using parametric duration models

Countdown timers display the time left on the current signal, which makes drivers be more ready to react to the phase change. However, previous related studies have rarely explored the effects of countdown timer on driver’s brake perception-reaction time (BPRT) to yellow light. The goal of this study was therefore to characterize and model driver’s BPRT to yellow signal at signalized intersections with and without countdown timer. BPRT data for “first-to-stop” vehicles after yellow onset within the transitional zone were collected through on-site observation at six signalized intersections in Harbin, China. Statistical analysis showed that the observed 15th, 50th, and 85th percentile BPRTs without countdown timer were 0.52, 0.84, and 1.26 s, respectively. The observed 15th, 50th, and 85th percentile BPRTs with countdown timer were 0.32, 1.20, and 2.52 s, respectively. Log-logistic distribution appeared to best fit the BPRT without countdown timer, while Weibull distribution seemed to best fit the BPRT with countdown timer. After that, a Log-logistic accelerated failure time (AFT) duration model was developed to model driver’s BPRT without countdown timer, whereas a Weibull AFT duration model was established to model driver’s BPRT with countdown timer. Three significant factors affecting the BPRT identified in both AFT models included yellow-onset distance from the stop line, yellow-onset approach speed, and deceleration rate. No matter whether the presence of countdown timer or not, BPRT increased as yellow-onset distance to the stop line or deceleration rate increased, but decreased as yellow-onset speed increased. The impairment of driver’s BPRT due to countdown timer appeared to increase with yellow-onset distance to the stop line or deceleration rate, but decrease with yellow-onset speed. An increase in driver’s BPRT because of countdown timer may induce risky driving behaviors (i.e., stop abruptly, or even violate traffic signal), revealing a weakness of countdown timer in traffic safety aspect.     

The effect of braking energy on the fatigue crack propagation in railway brake discs

Thermal fatigue cracks can often be found on the friction surface of brake discs used in railway vehicles after a period of usage and include crackle, radial and circumferential patterns. These cracks typically exhibit different initiation and propagation behavior under different braking conditions. In this paper, the effect of braking energy on fatigue crack evolution is analyzed by using experimental testing and numerical simulations. Macro observations show that a significant number of radial cracks appear on the surface of brake discs which operate at 300 km/h, while crackles typically appear after repeated emergency braking (EB) at 200 km/h. No crack growth was observed on disc surfaces after routine braking. The cyclic load that leads to the fatigue crack propagation consists of compressive stress during braking and residual tensile stress after cooling. Simulation results show that the depth of cracks correlates well with the residual tensile stress distribution in brake discs. Breaking tests exposed that the fracture surface of fatigue cracks which were covered by oxides shows nearly elliptic-type. Higher braking energy leads to a hardened layer on the friction surface and oxide generation near the crack edges, which are also important factors that contribute to accelerating crack propagation.     

Pollutant emissions,performance and combustion behaviour assessment of an Si gas engine fuelled with Lcv gas containing carbon monoxide and hydrogen diluted by nitrogen

This paper includes the experimental test data of an SI engine fuelled with simulated LCV gas (Low Calorific Value), which resembles synthesis gas in composition. The LCV gas was simulated by a mixture of carbon monoxide, hydrogen and nitrogen. During the experiment, the lower heating value of the LCV gas was altered by dilution with nitrogen. A single-cylinder Honda GX270 engine was adopted in the experiment to assess the impact of LCV gas on the system performance. This engine is typically used to power various machines and for electrical energy production in small generator sets. A modified engine was connected to an electric generator, which was loaded with an electric resistor. Engine operation was controlled using a microprocessor controller. All tests were performed at constant engine speed (3000 rpm). The engine was working at wide-open throttle for all mixtures. All mixtures were burned at stoichiometric conditions and with fixed value of ignition timing (30 deg bTDC). The indicated performance of the SI engine was evaluated based on the in-cylinder pressure measurements. No significant impact on the main internal parameters of the tested SI engine fuelled with simulated LCV gas diluted by nitrogen was observed. The experimental tests showed that the combustion duration increased for the mixtures with higher content of inert gas. Increase in the LHV raised the specific emissions of NO_x and decreased specific emissions of CO and HC.