• Journal of the Korean Society for Precision Engineering
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• v.33 no.9
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• pp.723-730
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• 2016

This paper presents a cable-driven hydraulic actuator named Cable Rod Hydraulic Actuator (CRHA). The cable actuating system is attractive for designing a compact joint in robotic applications since it can be installed remotely from the joint. Recently, cable rods have been used in pneumatic area for inertia reduction. However, designing cable rods in hydraulics is challenging because it is difficult to achieve flexibility and endurance simultaneously under high pressure conditions. In this paper, the cable rod, which consists of a cable and jacket, is proposed to meet both requirements. To design the CRHA, we determined the design parameters, such as cylinder size, and selected the cable rod's material by friction and leakage test. Finally, comparisons experiments about step and frequency responses with conventional hydraulic actuators were performed to assess feasibility for robotic joints, and the results show that the proposed system has good bandwidth and fast response as robotic joints.

• Journal of Drive and Control
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• v.13 no.2
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• pp.1-9
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• 2016

In this paper, an elementary force fighting problem was investigated. The problem is encountered when a double-rod type EHA(electro-hydrostatic actuator) is combined with a single-rod type EHA to build a redundant actuator system with synchronized motion. When the rod-side chambers of the two different types of EHAs have the same effective piston areas and are simultaneously pressurized by an external load, the two EHAs behave identically, sharing the external load equally. However, when the piston head-side chamber of the single rod type EHA, having a larger effective area than the rod-side chamber, is pressurized by the external load, an abnormal force fighting between the two EHAs occurs, unless their pump speeds are properly decoupled. In this study, the output drive forces of each EHA were obtained from the cylinder pressure signals and applied to the position control for each EHA to maintain the balance between their pump speeds. Adding minor force difference feedback loops to the position control, the force fighting phenomena could be eliminated and steady state synchronization errors were reduced. The power consumption of the pumps also could be remarkably reduced, avoiding unnecessarily high load pressures to the pumps.

• Journal of the Korea Concrete Institute
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• v.15 no.5
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• pp.726-736
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• 2003

Concrete columns confined with high-strength fiber composites can enhance its strength as well as maximum strain. In recent years, several equations have been developed to predict the behavior of the concrete columns confined with fiber composites. While the developed equations can predict the compressive strength of the confined columns with reasonable agreement, these equations are not successful in predicting the observed maximum strain of the columns. In this paper, a total of 61 test results is analysed to propose an equation to predict both compressive strength and maximum strain of concrete cylinders. The proposed equation takes into account the effects of confining pressure and cylinder size. Furthermore, in order to verify the proposed stress-strain curve for concrete cylinders, six cylindrical specimens were tested. Comparisons between the observed and calculated stress-strain curves of the tested cylinders showed reasonable agreement.

• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.2
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• pp.8-14
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• 2007

Compression ignition of homogeneous charges in IC engines indicates possibilities of achieving the high efficiency of DI diesel engines with low level of NOx and particulate emissions. The objectives of this study are to further understand the characteristics of the HCCI(Homogeneous charge compression ignition) combustion and to find ways of extending the rich HCCI operation limit in an engine-like environment. DME fuel is supplied either in the form of premixture with air or directly injected in the combustion chamber of a rapid compression and expansion machine under the conditions of various equivalence ratio and injection timing. The cylinder pressure is measured and the rate of heat release is computed from the measured pressure for the analysis of the combustion characteristics. The experimental data show that the RCEM can operate without knock on mixtures of higher equivalence ratio, when DME is directly injected in the combustion chamber than introduced as a fraction of a perfect or nearly perfect premixture. Very early fuel injection timings usually employed in HCCI operation are seen to have only insignificant effects in control of ignition timing.

• Journal of ILASS-Korea
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• v.21 no.2
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• pp.95-103
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• 2016

The effect of pilot injection quantity on the combustion and emissions characteristics of a compression ignition engine with a biodiesel-compressed natural gas (CNG) dual fuel combustion (DFC) system is studied in this work. Biodiesel is used as a pilot injection fuel to ignite the main fuel, CNG of DFC. The pilot injection quantity is controlled to investigate the characteristics of combustion and exhaust emissions in a single cylinder diesel engine. The injection pressure and injection timing of pilot fuel are maintained at approximately 120 MPa and BTDC 17 crank angle, respectively. Results show that the indicated mean effective pressure (IMEP) of biodiesel-CNG DFC mode is similar to that of diesel-CNG DFC mode at all load conditions. Combustion stability of biodiesel-CNG DFC mode decreased with increase of engine load, but no notable trend of cycle-to-cycle variations with increase of pilot injection quantity is discovered. The combustion of biodiesel-CNG begins at a retarded crank angle compared to that of diesel-CNG at low load, but it is advanced at high loads. Smoke and NOx of biodiesel-CNG are simultaneously increased with the increase of pilot fuel quantity. Compared to the diesel-CNG DFC, however, smoke and NOx emissions are slightly reduced over all operating conditions. Biodiesel-CNG DFC yields higher $CO_2$ emissions compared to diesel-CNG DFC over all engine conditions. CO and HC emissions for biodiesel-CNG DFC is decreased with the increase of pilot injection quantity.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.8
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• pp.1575-1582
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• 1992

This study describes the response performances of actual engine speed, turbocharger speed, air mass flow rate through engine, boost pressure ratio, exhaust temperature and combustion efficiency for a six-cylinder four-stroke turbocharged diesel engine during the change in operating conditions by using the computer simulation with test bed. In order to obtain the transient conditions, a suddenly large load was applied to the simulation engine with the several kinds of inertia moment in turbocharger and engine, and engine set speed. From the results of this study, the following conclusions were summarized The inferior response performances was mainly caused by turbocharger lag, and air mass flow rate and boost pressure ratio were closely related to the turbocharger speed. A reduced moment of turbocharger inertia resulted in less transient speed drop and much faster recovery to the steady state of the engine. The increase of moment of engine inertia reduced cyclic variation of engine speed. When a large load was applied to the engine at high speed, the engine could be fastly recovered. However, when the same load was applied to the engine at low speed, the engine was stalled.

• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.3
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• pp.54-62
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• 2007

The knock characteristics in an engine were investigated under homogeneous charge compression ignition (HCCI) operation. Liquefied petroleum gas (LPG)and gasoline were used as fuels and injected at the intake port using port fuel injection equipment. Di-methyl ether (DME) was used as an ignition promoter and was injected directly into the cylinder near compression top dead center (TDC). A commercial variable valve timing device was used to control the volumetric efficiency and the amount of internal residual gas. Different intake valve timingsand fuel injection amounts were tested to verify the knock characteristics of the HCCI engine. The ringing intensity (RI) was used to define the intensity of knock according to the operating conditions. The RI of the LPG HCCI engine was lower than that of the gasoline HCCI engine at every experimental condition. The indicated mean effective pressure (IMEP) dropped when the RI was over 0.5 MW/m2and the maximum combustion pressure was over 6.5MPa. There was no significant relationship between RI and fuel type. The RI can be predicted by the crank angle degree (CAD) at 50 CA. Carbon monoxide (CO) and hydrocarbon (HC) emissions were minimized at high RI conditions. The shortest burn duration under low RI was effective in achieving low HC and CO emissions.

• Tribology and Lubricants
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• v.31 no.4
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• pp.141-147
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• 2015

Gas foil thrust bearings (GFTBs) have attractive advantages over rolling element bearings and oil film thrust bearings, such as oil-free operation, high speed stability, and high-temperature operation. However, GFTBs have lower load carrying capacity than the other two types of bearings owing to the inherent low gas viscosity. The load carrying capacity of GFTBs depends mainly on the compliance of the foil structure and the formed hydrodynamic wedge, where the gas pressure field is generated between the top foil and the thrust runner. The load carrying capacity of the GFTBs is very important for the suitable design of oil-free turbomachinery with high performance. The aim of the present study is to identify the characteristics of the load carrying performance of GFTBs. A new test rig for the experimental measurements is designed to provide static loads up to 800 N using a pneumatic cylinder. The maximum operating speed of the driving motor is 30,000 rpm. A series of experimental tests—lift-off test, static load performance test, and maximum load capacity test—estimate the performance of a six-pad GFTB, in terms of the static load, driving torque, and temperature. The maximum load capacity is determined by increasing the static load until the driving torque rises suddenly with a sharp peak. The test results show that the torque and temperature increase linearly with the static load. The estimated maximum load capacity per unit area is approximately 80.5 kPa at a rotor speed of 25,000 rpm. The test results can be used as a design guideline for GFTBs for realizing oil-free turbomachinery.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.1
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• pp.23-32
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• 2016

Various studies have been conducted for drag reduction of a high-speed vehicle by injecting counterflow jet from its nose cone. In this study, in order to obtain baseline data and key parameters for drag reduction method, the counterflow jet study of the USA is reviewed and summarized. The nose cone shapes of each study are hemisphere cylinder, truncated cone, and reentry capsule, and their test conditions are summarized accordingly. Key parameters for drag reduction are jet mach number, mass flow rate, and pressure ratio. Even though drag reduction effects show various results according to given test conditions, it is found that the drag reduction effect reaches up to 40~50%.

• Structural Engineering and Mechanics
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• v.91 no.1
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• pp.75-86
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• 2024

In this paper, the porous metal PM-35 is proposed as the filler material of filled thin-walled tubes (FTTs), and a series of experimental study is conducted to investigate the dynamic behavior and energy absorption performance of PM-35 filled thin-walled tubes under impact loading. Firstly, cylinder solid specimens of PM-35 steel are tested to investigate the impact mechanical behavior by using the Split Hopkinson pressure bar set (SHP); Secondly, the filled thin-walled tube specimens with different geometric parameters are designed and tested to investigate the feasibility of PM-35 steel applied in FTTs by the orthogonal test. According to the results of this research, it is concluded that PM-35 steel is with the excellent characteristics of high energy absorption capacity and low yield strength, which make it a potential filler material for FTTs. The micron-sizes pore structure of PM-35 is the main reason for the macroscopic mechanical behavior of PM-35 steel under impact loading, which makes the material to exhibit greater deformation when subjected to external forces and obviously improve the toughness of the material. In addition, PM-35 steel core-filled thin-wall tube has excellent energy absorption ability under high-speed impact, which shows great application potential in the anti-collision structure facilities of high-speed railway and maglev train. The parameter V0 is most sensitive to the energy absorption of FTT specimens under impact loading, and the sensitivity order of different variations to the energy absorption is loading speed V0>D/t>D/L. The loading efficiency of the FTT is affected by its different geometry, which is mainly determined by the sleeve material and the filling material, which are not sensitive to changes in loading speed V0, D/t and D/L parameters.