High Piston Velocity Research Articles

EFFECT OF RAPID COOLING FROM THE SEMISOLID OR LIQUID RANGE ON STRUCTURE AND PROPERTIES OF M2 TOOL STEEL

M2 tool steel was used as a feedstock material for casting from the the semisolid range between 1310-1350oC corresponding to 25 - 50 % of the liquid phase. The chemical composition of the investigated steel is following: 0.85%C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. A specially constructed machine allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. After heating of the feedstock using induction heating by the stamping of semi-liquid sample to a graphite mould pre-heated by resistance heating. The material after thixoforming from 1315oC shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness significantly increases up to 780 HV after thixo-forming due to formation of martensite. The other part was cast from the liquid phase at 1450o C using melt spinning method allowing high cooling rate. The microstructure of melt spun ribbon shows a cellular microstructure of austenitic and martensitic structure without the eutectic. The WC and M23C6 carbides were located at the cell’s boundariers. The microhardness near 1300 HV of the ribbon was much higher than that of semisolid formed samples.

Analysis of impact loads in a magnetorheological energy absorber using a Bingham plastic model with refined minor loss factors accounting for turbulent transition

This study presents the improvement of a Bingham-plastic damper model incorporated with refined minor losses (BPM damper model) by considering fluid friction models so as to predict the stroking load of a magnetorheological energy absorber (MREA) during high speed impact. MREAs produce a variable stroking load that adapts to a range of payload mass and/or impact velocity by applying a magnetic field. When used for impact protection, design goals are: (1) to provide optimal stroking load over a wide range of impact velocities without exceeding the maximum allowable force, and (2) maintain a high dynamic range, defined as the ratio of maximum field-on force to field-off force, at high impact speeds. Thus, it is critical to accurately predict the field-off damper force at high piston velocity. In order to more accurately capture the exponential increase in damper stroking load with piston velocity, various velocity squared dependent minor losses were added to the model. The increased roughness of the walls inside the fluid flow orifice produced by the electromagnetic coils was taken into consideration, and the combination of smooth and rough walls was adopted. A piecewise formula for predicting Darcy friction factor for fluid flowing through a rough pipe in the laminar, transition, and fully turbulent regimes was implemented. An MREA was fabricated and drop tested to obtain stroking load data in the field-off and field-on states. The refined damper model was shown to more accurately predict damper force in both the low and high speed ranges than previously tested BPM damper models.

High shear rate flow in a linear stroke magnetorheological energy absorber

To provide adaptive stroking load in the crew seats of ground vehicles to protect crew from blast or impact loads, a magnetorheological energy absorber (MREA) or shock absorber was developed. The MREA provides appropriate levels of controllable stroking load for different occupant weights and peak acceleration because the viscous stroking load generated by the MREA force increases with velocity squared, thereby reducing its controllable range at high piston velocity. Therefore, MREA behavior at high piston velocity is analyzed and validated experimentally in order to investigate the effects of velocity and magnetic field on MREA performance. The analysis used to predict the MREA force as a function of piston velocity squared and applied field is presented. A conical fairing is mounted to the piston head of the MREA in order reduce predicted inlet flow loss by 9% at nominal velocity of 8 m/s, which resulted in a viscous force reduction of nominally 4%. The MREA behavior is experimentally measured using a high speed servo-hydraulic testing system for speeds up to 8 m/s. The measured MREA force is used to validate the analysis, which captures the transient force quite accurately, although the peak force is under-predicted at the peak speed of 8 m/s.

Effect of Piston Motion on Combustion of CNG Free Piston Engine (FPE)

CNG is thought to be one of the most promising alternatives to traditional fuels. The multi-fuel ability is another characteristic of the FPE. The piston motion can be controlled to have beneficial effects on the engine performance. This article investigates the effect of piston motion on combustion of four-stroke CNG FPE using a multidimensional simulation model. It is found that the high piston acceleration and velocity at top dead center increases expansion ratio and fasting combustion, reduces the heat transfer losses and decreased NOx emissions formation. At the same time, the turbulent kinetic energy of the gas during the combustion process is added.

Regulation of CO<sub>2</sub> emissions from temperate streams and reservoirs

Abstract. It has become more and more evident that CO2 emission (FCO2) from freshwater systems is an important part of the global carbon cycle. To date, only a few studies have addressed the different mechanisms that regulate FCO2 in lotic and lentic systems. In a comparative study we investigated how different biogeochemical and physical factors can affect FCO2 values in streams and reservoirs. We examined the seasonal variability in CO2 concentrations and emissions from four streams and two pre-dams of a large drinking water reservoir located in the same catchment, and compared them with environmental factors that were measured concurrently. All the streams were generally supersaturated with CO2 throughout the year, while both reservoirs functioned to a small degree as CO2 sinks during summer stratification and CO2 sources after circulation had set in. FCO2 from streams ranged from 23 to 355 mmol m−2 d−1 and exceeded the fluxes recorded for the reservoirs (−8.9 to 161.1 mmol m−2 d−1). Both the generally high piston velocity (k) and the CO2 oversaturation contributed to the higher FCO2 from streams in comparison to lakes. In both streams and reservoirs FCO2 was mainly governed by the CO2 concentration (r = 0.92, p < 0.001 for dams; r = 0.90, p < 0.001 for streams), which was in turn affected by metabolic processes and nutrients in both systems and also by lateral inflow in the streams. Besides CO2 concentration, physical factors also influence FCO2 in lakes and streams. During stratification, FCO2 in both pre-dams was regulated by primary production in the epilimnion, which led to a decrease of FCO2. During circulation, when CO2 from the hypolimnion was mixed with the epilimnion, FCO2 increased on account of the CO2 input from the hypolimnion. The CO2 from the hypolimnion originates from the mineralisation of organic matter. FCO2 from streams was mainly influenced by geomorphological and hydrological factors affecting k, which is less relevant in low-wind lakes. Under high-wind conditions, however, k regulates FCO2 from lotic systems as well. We developed a theoretical framework describing the role of the different regulation mechanisms for FCO2 from streams and lakes. In summary, the dominant factor affecting FCO2 is the concentration of CO2 in the surface water. Lake stratification has a very important regulatory effect on FCO2 from lakes on account of its influence on CO2 concentrations and metabolic processes. Nevertheless, FCO2 values in heterotrophic streams are generally higher. The higher k values are responsible for the comparatively high degree of FCO2. On a Central European scale, CO2 emission from streams is probably of greater importance than the CO2 flux from standing waters.

Magnetorheological Damper Utilizing an Inner Bypass for Ground Vehicle Suspensions

This study presents the design, fabrication, and test of a magnetorheological (MR) damper utilizing an inner bypass that can simultaneously produce large dynamic range (i.e., ratio of field-on to field-off stroking load) and low field-off stroking load at high piston velocity. These two damper properties, large dynamic range and low field-off stroking load, are critical to achieving high performance in ground vehicle suspensions. The MR damper is comprised of a pair of concentric tubes, a movable piston-shaft arrangement, and an annular MR fluid flow gap between the concentric tubes. The inner tube serves as the piston guide, the inner surface of the annular flow gap, and the bobbin for the five electromagnetic coils used in this design. The outer tube serves as the flux return and the outer surface of the annular flow gap. The annular flow gap is an inner bypass annular valve where the rheology of the MR fluids, and hence the stroking load of the damper, is controlled. The MR damper is analyzed using a Bingham-plastic nonlinear fluid mechanics model. To experimentally validate the analysis of the MR damper with the inner bypass, and to illustrate its advantages over the MR damper with the conventional annular orifice, the prototype damper is tested in terms of controllable damping force or stroking load, dynamic range, and equivalent damping as a function of shaft/piston velocity.

An experimental study of the hydraulic free piston engine

A prototype of hydraulic free piston engine has been developed to achieve efficient energy conversion directly from chemical energy of fuel to hydraulic energy. In stable running condition, in order to verify the features of this kind of engine, the piston dynamics, combustion process and hydraulic characteristics are investigated through series of tests of the prototype. The experimental results show that the majority of fuel burns in the rapid combustion phase which is due to the high piston velocity in the final part of the compression stroke. The combustion is characterized by a constant volume process. Furthermore, the indicated thermal efficiency and the indicated mean effective pressure of the prototype presented in this study are 41% and 5.2bar respectively.

Thixoforming of spray formed M2 tool steel

Using SW7M (M2) tool steel as a feedstock material, several thixoforming tests were performed at various starting temperatures in the semisolid range between 1310-1350°C corresponding to 25 - 50 % of the liquid phase as indicated by the results of DSC studies. The chemical composition of the investigated steel is following: 0.85 %C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. Specially constructed apparatus allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. Fast heating of the feedstock was achieved using induction heating through a high frequency generator, followed by the stamping of semi liquid sample to a graphite mould pre-heated by resistance heating. Observations of the microstructure before and after semisolid processing were performed using optical, scanning and transmission electron microscopy. The initial material has a very fine grain size of ∼2-5 μm. This material after thixoforming at 1315°C shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness of spray formed steel is around 304 HV significantly increases up to 780 HV after thixo-forming due to formation of martensite. At the end of paper the basic results of numerical simulations were shown.

Thixoforming technology of high carbon X210CrW12 steel

Several samples were cast using thixoforming method. The feedstock used was a hot rolled commercial bar of the steel X210CrW12 with the following chemical composition: 2,11%C, 10,5%Cr, 0,7%W, 0,4%Si. An apparatus was constructed allowing the thixoforming of steel parts in a protective argon atmosphere and at a high piston velocity powered by a high pressure air. It was designed and built in such a way to allow first pumping air out and argon in, fast heating of a feedstock using high frequency generator, stamping of semi liquid sample at a high piston rate of a few m/s with in earlier resistance heated mould and simultaneous control of sample and mould temperature to ±1°C. The liquid fraction was determined using Differential Scanning Calorimetry (DSC). Observation of the microstructure was performed using optical and transmission electron microscopy. It showed a globular structure in the material, when pre-heated to the temperature of 1230°C at a liquid fraction of 30%. After several experiments with various sample and mould temperatures, a desired globular microstructure after a thixoforming process was obtained. Basic technological parameters which permit the complete filling of the die and a useful microstructure, such as piston velocity, temperature of the die, were elaborated. The microstructure after the thixoforming process showed globular structures with a size of 50 to 200 µm of the oval particles of the austenite within an eutectic matrix. The crystal structure of globules was mainly austenite and that of the eutectic ferrite and M7C3 carbide.

A Magnetorheological Damper with Bifold Valves for Shock and Vibration Mitigation

This study presents the design and fabrication of a flow-mode bifold magnetorheological (MR) damper for shock and vibration mitigation for high piston velocity (15 mph or 6.75 m/s) as well as an evaluation of its performance at low speed. Based on a Bingham-plastic (BP) model, as well as a BP model coupled with a low speed hysteresis model, two theoretical MR damper models for flow-mode MR dampers are constructed. Using the design strategy associated with the Bingham-model based damper model, two MR damper designs for achieving the performance requirement with a limited space are considered: first, the conventional MR damper that has an MR valve inside the piston head and second, the bifold MR damper that has MR valves at each end of the damper. After numerically comparing the damping performances of the two MR damper designs, the bifold MR damper has been chosen because its dynamic range is better at high speed. The bifold MR damper was tested at a relatively low piston velocity using an MTS testing machine under sinusoidal loading. Experimental data compare well with the results predicted by the theoretical models.

非対称シリンダを有するサーボ試験機での岩石試験の制御性

In the previous papers, control performance of servo-controlled testing machines with a symmetrical cylinder was discussed analytically and numerically. An asymmetrical cylinder is widely used to reduce the cost and size of testing machines. However the asymmetrical cylinder has nonlinear characteristics, and the control performance of it is inferior to the symmetrical cylinder. In this paper, the control performance of a servo-controlled testing machine with asymmetrical cylinder is discussed analytically on basis of the experimental results. Uniaxial compression tests using Sanjome-andesite, Akiyoshi-marble and Inada-granite were conducted in order to clarify the control characteristics. Actually, pressures of upper and lower chambers within the cylinder were measured. It was found that the pressures changed depending on the direction of piston movement. The pressure decreased in case of moving to the loading direction, and the amount of pressure decrease was large at high piston velocity. The pressures in the cylinder were analytically examined with aid of a simple model which consisted of a cylinder and a servo-valve. These theoretical results coincided with the experimental results. Sometimes in the post-failure region, the piston tended to move reversely. At that time, the asymmetrical cylinder showed inferior control performance caused by the followings ; 1) Larger amount of oil inflow is necessary. 2) Feedback gain changes depending on the direction of piston movement. The control performance of asymmetrical cylinder can be possibly improved by adjusting the followings; 1) large oil-leakage at the cylinder, 2) variable flow-coefficient at orifice, 3) variable feedback gain.