Analysis of the reasons for the formation of under-collar cracks in the bushing of medium-speed combustion engines

Вибрация цилиндровой втулки, вызванная перекладкой поршня, развивает колебания вдоль по длине втулки от бурта до основания и акустические колебания внутри втулки, которые приводят к возникновению растягивающих напряжений и деформаций в поверхностных слоях металла; при взаимодействии с водой создаются условия для диффузии водорода во втулку. Диффузия водорода повышает внутреннее давление, что вызывает растрескивание структуры металла под действием напряжений. Происходит деградация металла – снижение прочностных и пластических свойств. Причиной появления подбуртовых трещин и кавитационные разрушения цилиндровой втулки является усталость деградированного металла от «водородного растрескивания под напряжением» и действия циклических растягивающих напряжений. Для повышения долговечности втулок по подбуртовым трещинам необходимо: 1. Увеличение износостойкости скользящей поверхности втулки для стабилизации величины теплового зазора; 2. Повышение жесткости втулки за счет увеличения толщины втулки в подбуртовой зоне; 3. Применение метала втулки с малой чувствительностью к «водородному растрескиванию под напряжением» (замена чугуна с пластинчатой формой графита на сферическую). The vibration of the cylinder bushing caused by the piston displacement develops the vibrations along the length of the bushing from the collar to the base and acoustic vibrations inside the collar, which lead to tensile stresses and deformation in the surface layers of the metal; when interacting with water, conditions for the diffusion of hydrogen into the bushing are created. The diffusion of hydrogen increases internal pressure, which causes the cracking of the metal structure under stress. The degradation of the metal that is the decrease in strength and plastic properties occurs. The reason for the formation of under – collar cracks and cavitation destruction of the cylinder bushing is the fatigue of the degraded metal from "hydrogen stress cracking" and the action of cyclic tensile stresses. To increase the durability of the bushings along the under – collar cracks, it is necessary to: 1. Increase the wear resistance of the sliding surface of the bushing to stabilize the value of the thermal gap; 2. Increase the stiffening effect of the bushing by increasing the thickness of the bushing in the under – collar zone; 3. Apply the metal of the bushings with low sensitivity to "hydrogen stress cracking" (replace the cast iron with lamellar graphite for the spherical one).

Stimulation efficiency of an actuator driven piston at the biological interface to the inner ear

Direct acoustic cochlear stimulation uses piston motion to substitute for stapes footplate (SFP) motion. The ratio of piston to stapes footplate motion amplitude, to generate the same loudness percept, is an indicator of stimulation efficiency. We determined the relationship between piston displacement to perceived loudness, the achieved maximum power output and investigated stapes fixation and obliteration as confounding factors. The electro-mechanical transfer function of the actuator was determined preoperatively on the bench and intraoperatively by laser Doppler vibrometry. Clinically, perceived loudness as a function of actuator input voltage was calculated from bone conduction thresholds and direct thresholds via the implant. The displacement of a 0.4 mm diameter piston required for a perception equivalent to 94 dB SPL at the tympanic membrane compared to normal SFP piston displacement was 27.6–35.9 dB larger, consistent with the hypothesis that the ratio between areas is responsible for stimulation efficiency. Actuator output was 110 ± 10 eq dB SPLFF @1Vrms ≤ 3 kHz and decreased to 100 eq dB SPLFF at 10 kHz. Output was significantly higher for mobile SFPs but independent from obliteration. Our findings from clinical data strongly support the assumption of a geometrical dependency on piston diameter at the biological interface to the cochlea.

Development of a Testing Station for Empirical Verification of the Algebraic Model of Dry Ice Piston Extrusion

Efficient use of resources is a very important consideration for every production process, especially where waste materials are used as raw materials. One example of these kinds of processes is dry ice extrusion. Based on the subject literature, it can be observed that the machines available in the market that are used to compress dry ice are characterized by high working force value. This leads to low efficiency of resource consumption, in regards to both electrical energy and carbon dioxide. This paper presents a proposed design of a test stand used for measuring compression force as a function of piston displacement in the course of the dry ice extrusion. The first part of the article presents the testing methodology and test stand design. The second part presents the results of measurement of compression force as a function of piston displacement with three different die types. The results of the study allowed to establish the difference between the values of the measured limit force and the values calculated with an analytical model. The test stand design and the results presented in this paper are important for further research and development works in the area of efficient extrusion and compaction of dry ice.

The effect of channel aspect ratio on air entrapment during imbibition in soil-on-a-chip micromodels with 2D and 2.5D pore structures

“Soil-on-a-chip” micromodels designed with X-ray CT images were fabricated by tabletop CNC machining and dry adhesive bonding. The competition between film flow and piston displacement causes the air entrapment differences in 2D and 2.5D micromodels.

Large Eddy Simulation of Heat Transfer Within a Multi-Perforation Synthetic Jets Configuration

Synthetic jets are produced by devices that enable a suction phase followed by an ejection phase. The resulting mean mass budget is hence null and no addition of mass in the system is required. These particular jets have especially been considered for some years for flow control applications. They also display features that can become of interest to enhance heat exchanges, for example, for wall cooling issues. Synthetic jets can be generated through different mechanisms, such as acoustics by making use of a Helmholtz resonator or through the motion of a piston as in an experience mounted at Institut Pprime in France. The objective of this specific experiment is to understand how synthetic jets can enhance heat transfer in a multi-perforated configuration. As a complement to this experimental setup, large-eddy simulations are produced and analyzed in the present document to investigate the flow behavior as well as the impact of the synthetic jets on wall heat transfer. The experimental system considered here consists in a perforated heated plate, each perforation being above a cavity where a piston is used to control the synthetic jets. Placed in a wind tunnel test section, the device can be studied with a grazing flow and multiple operating points are available. The one considered here implies a grazing flow velocity of 12.8 m s−1, corresponding to a Mach number around 0.04, and a piston displacement of 22 mm peak-to-peak at a frequency of 12.8 Hz. These two latter parameters lead to a jet Reynolds number of about 830. A good agreement is found between numerical results and experimental data. The simulations are then used to provide a detailed understanding of the flow. Two main behaviors are found, depending on the considered mid-period. During the ejection phase, the flow transitions to turbulence and the formation of characteristic structures are observed; the plate is efficiently cooled. During the suction phase, the main flow is stabilized; the heat enhancement is particularly efficient in the hole wakes but not between them, leading to a heterogeneous temperature field.

The impact of camshaft control on combustion processes in a compression ignition VCR engine

The article contains an analysis of thermodynamic indicators of the CI engine with direct fuel injection. The tested unit was an engine with an unconventional crank-piston configuration, in which at a constant compression ratio, different engine crankshaft control variants were compared. In this article the time ranges in which the piston was located in the TDC were analyzed. Additionally, tests were carried out on a conventional unit for comparative purposes. For the purposes of the work, a piston displacement characteristic was generated in dynamic analysis in SolidWorks Motion. Then, this characteristic was used in the simulations of the combustion process in the AVL Fire program, where the most important thermodynamic indicators of the engine cycle were analyzed.