An experiment of the combustion characteristics with laser-induced spark ignition

The combustion characteristics and minimum ignition energies using laser-induced spark ignition were demonstrated for quiescent methane-air mixtures in an optically-accessible, constant volume combustion chamber. Initial pressure and equivalence ratio as well as spark energy were varied in order to explore the flame behavior with laser-induced spark ignition. Shadowgraphs for the early stages of combustion process showed that the flame kernel becomes separated into two, one of which grows back towards the laser source. Eventually after a short period, the two flame kernels developed into two flame fronts propagating individually, which is unique in laser-induced spark ignition. For a given mixture, lower initial mixture pressure and higher spark energy resulted in shorter flame initiation period and faster flame propagation. The results of minimum ignition energies for laser ignition shows higher values than electric discharge results, however, the difference decreases toward lean and rich flammability limits.     

Effects of turbulent mixing on spray ignition in spray system

When a column of droplets freely falling from an ultrasonic atomizer was ignited behind a reflected shock, no ignition occurred at a temperature below 1100 K, even if the pressure was as high as IMPa. Although, a higher temperature condition ensured ignition, no luminous flame was observable by high-speed photography, and even if a luminous flame lump appeared at an extremely high temperature, it disappeared without spreading over the entire column of droplets in this case. It is known however that, if a fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650 K with a luminous flame spreading over the entire spray. These differences could be caused by the effects of turbulent mixing between fuel droplets and hot air, in fact, turbulence-generating rods were placed on the upstream side of the spray column. Experimental results indicates that the ignition limit was lowered to 840 K, and the ignition delay period was decreased by increasing the intensity of turbulence. Furthermore, the light emission of the flame was intensified, and normal spray combustion was maintained in the low-temperature atmosphere after the shock tube ceased its operation.     

Method for accurate measurement of output ignition curves for combustion engines

The method for accurate measurement of output ignition curves for combustion engines is intended to solve the problems of universal programmable ignition control units. Just that versatility of these units originates during their application in specific type of engine, there is a significant difference between the programmed and actual, output curve. Therefore it is necessary to define a correction curve, whereby the programmed curve will be adjusted to ensure the required output. The principle consists in simulating the engine running by means of a impulse generator. The measuring system consists of a high-speed electric motor with an impulse generator, a speed controller of the electric motor and a stroboscopic lamp. The impulse generator is equipped with a circular protractor, on which a ray of the stroboscopic lamp is incident at the time of simulated mixture ignition. The result is a real curve, which is compared with an input curve by means of computer. Comparing the curves we obtain a required correction one.     

Mechanism-based modeling of friction and wear

Equations for “predicting” friction and wear that product design engineers can readily apply require new methods to develop. These include the following. 1. The planned cooperation of several technical disciplines.

2. The selection of a well studied system.

3. Parametric equations (models) for the observed friction or wear behavior, including all variables known by an interdisciplinary group.

4. Data that cover a wide range (e.g. several decades) of most relevant variables.

5. Methods for adjusting parameters in the models to closely match the data. The results of three research efforts are discussed in terms of the prospects for developing equations either by curve fitting the data or by describing underlying phenomena involved in the sliding process. Finally, possible contributions of several technical disciplines toward modeling the conditions for scuff prevention are suggested.     

On the role of copper in brake friction materials

Copper is a major ingredient in friction materials used for automotive braking. The purpose of this study was to find out how copper contributes to good brake performance properties in addition to providing good thermal conductivity. Microstructural investigations of copper chips at the surfaces of brake pads revealed a zone of severe plastic deformation which provides high hardness, but there is also evidence of recrystallized copper nano-particles which are incorporated into friction layers as soft ingredient once detached from the pad surface. Thus copper seems to play a dual role, firstly as reinforcing element of the brake pad providing primary contact sites, and secondly as solid lubricant by contributing to the formation of a layer of granular material providing velocity accommodation between the rotating disc and fixed pad. Confirmation for this hypothesis was obtained by modelling contact sites on the nanometre scale with the method of movable cellular automata. Results show both, the similarity of steel fibres and copper macro-particles in respect to providing primary contact sites, as well as similar sliding behaviours of friction layers containing either copper or graphite as soft inclusions. Furthermore, it is shown that not only material properties, but also the concentration of solid lubricant particles in the friction layers, determine conditions for friction force stabilization and smooth sliding behaviour.     

汽车制动器摩擦材料的研究现状和发展

介绍汽车制动器摩擦材料的研究现状。探讨半金属基摩擦材料、非石棉有机摩擦材料和粉末冶金摩擦材料的优缺点。提出汽车制动摩擦材料研究的发展趋势。     

电子节气门面向控制建模研究

建立了电子节气门过程模型.列举了近期电子节气门面向控制的建模研究成果.研究的目的是如何建立更为简单和精确的电子节气门系统模型,进而提高控制器设计和仿真计算的效率.     

Effects of ZrSiO4 in non-metallic brake friction materials on friction performance

The effects of ZrSiO₄ (zirconium silicate or zircon) as an abrasive on brake friction performance and friction layers of non-metallic brake friction materials were evaluated. The experimental results indicated that ZrSiO₄ enhances friction coefficient, but depresses wear rate. However, ZrSiO₄ can improve the negative wear rate of the friction materials. The formation and development of friction layers are complex so that the friction layers formed during friction process were carefully characterized using scanning electron microscopy (SEM), light microscopy (LM), and X-ray diffraction (XRD) methods. Following characteristics of friction layers were identified—(1) dynamic behavior: the structure of friction layers changes at the different surfacial positions and across sample's thickness; (2) friction condition dependence: formation of friction layers depends upon temperatures, time, and thermal history such as fade and recovery; and (3) compositional dependence: the compositions of friction surface and bulk differ, nevertheless the bulk's composition determine the friction layers. The phenomena as baryte films, altered layers, iron patches, and zircon loose areas formed on the friction surfaces were observed. Baryte films were detected on the friction surfaces of Zr-0 (sample without zircon). Baryte films have positive effect on wear property, but the films disappear in the presence of ZrSiO₄. The amount of carbonaceous materials decreases with the increase in ZrSiO₄. Only negligible thickness of altered layers was found on the friction surfaces of Zr-0 sample, while samples containing zircon show out relatively thick altered layers. Both iron-patches and zircon loose areas increase with the ZrSiO₄ contents. The relationships among formulation, friction performance, and friction surfaces were summarized.     

An experimental and mathematical study on the effects of ignition energy and system on the flame kernel development

A constant volume combustion chamber is used to investigate the flame kernel development of gasoline air mixtures under various ignition systems, ignition energies and spark plugs. Three kinds of ignition systems are designed and assembled, and the ignition energy is controlled by the variation of the dwell time. Several kinds of spark plugs are also tested. The velocity of flame propagation is measured by a laser deflection method, and the combustion pressure is analyzed by the heat release rate and the mass fraction burnt. The results represent that as the ignition energy is increased by enlarging either dwell time or spark plug gap, the heat release rate and the mass fraction burnt are increased. The electrodes materials and shapes influence the flame kernel development by changing he transfer efficiency of electrical energy to chemical energy. The diameter of electrodes also influences the heat release rate and the burnt mass fraction.     

Elevated-temperature tribology of metallic materials

The wear of metals and alloys takes place in many forms, and the type of wear that dominates in each instance is influenced by the mechanics of contact, material properties, the interfacial temperature, and the surrounding environment. The control of elevated-temperature friction and wear is important for applications like internal combustion engines, aerospace propulsion systems, and metalworking equipment. The progression of interacting, often synergistic processes produces surface deformation, subsurface damage accumulation, the formation of tribo-layers, and the creation of free particles. Reaction products, particularly oxides, play a primary role in debris formation and microstructural evolution. Chemical reactions are known to be influenced by the energetic state of the exposed surfaces, and that surface energy is in turn affected by localized deformation and fracture. At relatively low temperatures, work-hardening can occur beneath tribo-contacts, but exposure to high temperatures can modify the resultant defect density and grain structure to affect the mechanisms of re-oxidation. As research by others has shown, the rate of wear at elevated temperatures can either be enhanced or reduced, depending on contact conditions and nature of oxide layer formation. Furthermore, the thermodynamic driving force for certain chemical reactions, the kinetics of those reactions, and the microstructure can all affect the response. The role of deformation, oxidation, and tribo-corrosion in the elevated-temperature tribology of metallic alloys will be exemplified by three examples involving sliding wear, single-point abrasion, and repetitive impact plus slip.     

Friction and wear of friction materials containing two different phenolic resins reinforced with aramid pulp

Friction and wear characteristics of automotive friction materials containing two different phenolic resins (a straight novolac resin and a modified novolac resin) were investigated using a pad-on-disk type friction tester. Six different friction materials with different relative amounts of the phenolic resins and aramid pulp were manufactured and tested. Two different test modes were employed to examine the friction characteristics concerning accumulated thermal history (a constant initial temperature test: test mode I) and friction heat (a constant interval test: test mode II). Friction characteristics such as friction stability and wear resistance were changed significantly according to the type of phenolic resins and to the relative amount of the resin and aramid pulp. Friction materials with the modified novolac resin showed better friction stability than those with the unmodified novolac resin. In particular, the friction materials that were reinforced with 10 vol.% of aramid pulp showed substantial improvement on friction stability regardless of the resin type. However, the friction materials with the modified resin showed significant reduction in wear resistance.     

Analysis of compression ignition combustion in a schnurle-type gasoline engine comparison of performance between direct injection and port injection systems-

A two-stroke Schnurle-type gasoline engine was modified to enable compression-ignition in both the port fuel injection and the in-cylinder direct injection. Using the engine, examinations of compression-ignition operation and engine performance tests were carried out. The amount of the residual gas and the in-cylinder mixture conditions were controlled by varying the valve angle rate of the exhaust valve (VAR) and the injection timing for direct injection conditions. It was found that the direct injection system is superior to the port injection system in terms of exhaust gas emissions and thermal efficiency, and that almost the same operational region of compression-ignition at medium speeds and loads was attained. Some interesting combustion characteristics, such as a shorter combustion period in higher engine speed conditions, and factors for the onset of compression-ignition were also examined.     

Performance and evaluation of eco-friendly brake friction materials

Eco-friendly brake friction materials were formulated without copper, lead, tin, antimony trisulfide, and whisker materials, to minimize their potential negative environmental impacts. A combination of scanning electron microscopy with energy dispersive microanalysis, profilometry, and thermogravimetry allows successful analysis of friction surface and thermal stability of friction materials. Extension evaluation method was applied to rank the friction materials using multi-parameter criteria, including friction, wear, thermal stability, cost of raw materials, and parameters from the brake effectiveness evaluation procedure (BEEP) assessment. The eco-pad (sample E) exhibited the best overall quality, expressed as the weighted average dependent degree in extension evaluation.     

Application of nano powdered rubber in friction materials

Styrene butadiene nano powdered rubber and nitrile-butadiene nano powdered rubber were used for manufacturing clutch facings, disc brake pads and brake linings to replace conventional styrene butadiene rubber and nitrile-butadiene rubber. The results of constant speed friction test and dynamometer test showed that nano powdered rubber can substantially improve properties of friction materials. The friction coefficient of friction materials modified with nano powdered rubber varies steadily with the change of temperature, and the wearing rate of friction materials is relatively low by using nano powdered rubber. These results indicate that nano powdered rubber has ideal application effect in various friction materials and is a kind of novel rubber modifier for friction materials.     

Tribological performance of tin-based overlay plated engine bearing materials

A block-on-ring test setup is employed to investigate tribological performance of Sn-based overlay plated bearing materials under mixed and boundary lubrication condition. Pb-containing bearing material is also studied as a reference. Sn-based and Pb-based overlays have shown similar transition in friction when rotational speed is varied. Under relatively longer test duration, Sn-based overlays exhibit comparable friction and wear properties with that of Pb-based overlay. It takes longer time to obtain steady-state friction for Sn-based overlay than Pb-based overlay. Wear behavior of tested samples are also similar except for tests in the mixed lubrication regime where Sn-based overlays show better wear resistance. In the Sn-based overlays main wear mechanisms are adhesive and abrasive wear leading to exposure of the Ni interlayer.     

Prediction of brake friction materials recovery performance using artificial neural networks

The brake friction materials in an automotive brake system are considered as one of the key components for overall braking performance of a vehicle. The sensitivity of friction material performance and accordingly brake performance, versus different operating regimes, has always been an important aspect of its functioning. In this paper, the influences not only on the brake operation conditions but also on the formulation and manufacturing conditions of friction materials have been investigated regarding friction materials recovery performance by means of artificial neural networks. A new neural network model of friction material recovery performance, trained by the Bayesian Regulation algorithm, has been developed.     

Forced stick-slip oscillations allow the measurement of the friction force: Application to paper materials

Different methods for measuring the friction forces are investigated in this paper. We consider the paper-on-paper contact as an example of application. We first underline several drawbacks for the two main standard methods, namely the inclined and horizontal plane methods. In particular, the horizontal plane test method often involves stick-slip oscillations that make the measurement of the friction force impossible. We then propose a method for characterizing these oscillations and removing their influence on the friction force measurement. The comparison of the proposed method to standards suggests that our proposed method delivers measurements that are much more accurate and repeatable. We finally discuss the validity of averaging the friction force measured during the sliding movement.     

Effect of superconducting transition on tribological properties of materials

The results of the tribological study of metal superconductors (lead, niobium, vanadium, tantalum) and high-temperature superconducting ceramics YBa₂Cu₃O_7−δ at temperatures lower and higher than the critical temperature of the superconducting transition are presented and discussed using background literature data. New information is given on the effect of the superconducting transition on the friction coefficient and the friction surface properties of lead and superconducting ceramics. Some general rules have been determined and the differences in friction behaviour of the materials studied have been observed for the superconducting and normal states. It was shown that it would be advantageous to carry out subsequent studies under similar conditions to reach superconducting and normal states in order to exclude the temperature and environmental effects on the friction behaviour of various superconducting materials.     

Contact and wear of paper-based friction materials for oil-immersed clutches—wear model for composite materials

Paper-based friction materials are widely used for oil-immersed clutches in automatic transmissions for passenger cars. It is known that repetition of engagements of the clutches causes wear of the friction materials leading to running-in with an increase in contact area. Observation of the contact of a typical paper-based friction material has been made by contact microscopy using the reflection of polarized light. The results have shown that contact is made at the top of its particulate and fibrous constituents, and comparison with the results of laser microscopy has revealed that the contact area observed by the contact microscopy is the contour contact area. An analysis has been made to describe this behavior by employing a mechanical model in which spherical and columnar asperities are supported by an elastic halfspace and wear under the loads they support. Analytical results describe the observed change in the contour contact area and demonstrate applicability of the model to wear of composite materials.     

Influence of humidity on the friction of diamond and diamond-like carbon materials

The friction of diamond and diamond-like carbon (DLC) materials was evaluated in reciprocating sliding wear testing under controlled relative humidity. The testing conditions were a displacement stroke of 100 μm, an oscillatory frequency of 8 Hz and a normal load of 2 N. The coefficient of friction of diamond and hydrogen-free DLC (a-C) coatings against a corundum sphere in the steady regime decreased with an increase in relative humidity. A water layer physisorbed at the interface between the mating surfaces played two major roles: acting as a lubricant and increasing the true area of contact. However, it was noticed that the friction coefficient of the hydrogenated DLC (a-C:H) coatings first increased and then decreased with increasing relative humidity in the steady state. There appeared to be a critical relative humidity for the a-C:H coatings, at which the steady-state friction reached the maximum value. The frictional behaviour of the a-C:H coatings also showed dependence on the wear test duration. The interaction between hydrogen and oxygen at the interface between the a-C:H coating and water layer was mainly responsible for such behaviour.