Piston Dynamics Research Articles

Accelerated pump tube concept for hypersonic ground testing

This paper examines a new hypersonic wind tunnel concept which is referred to here as an ‘accelerated pump tube’ (APT). The facility can be considered a hybrid between a gun tunnel and an expansion tube. Like a gun tunnel, the test gas is isentropically compressed with a piston to raise its pressure and temperature; like an expansion tube, the compressed test gas is subject to an unsteady expansion to further increase its enthalpy and stagnation pressure. State-to-state analytical calculations incorporating high temperature equilibrium chemistry are presented which demonstrate that the APT is theoretically able to simulate a wide hypersonic terrestrial flight envelope from Mach 5–14, with significant capacity for pressure-length scaling. Potential practical benefits of this concept include relatively simple internal flow processes, removal of the need for a film secondary diaphragm and helium as a driver gas, and a reduction in experimental uncertainty through better defined internal boundary conditions and relatively low mechanical complexity. To assess the feasibility of the APT concept in a practical context, the paper considers how an APT based around a 25 m long, 600 mm diameter pump tube should be configured for a Mach 10 test flow. One-dimensional Lagrangian CFD analysis is used to compute piston dynamics for three different pump tube compression ratios and the associated test flows are characterised. The CFD results suggest that the concept should be viable in practice, and also validate the Mach 10 state-to-state calculations.

Effects of the cross angle on output characteristics and tilting behaviour for 750 mL/r axial piston pumps

Large displacement axial piston pumps (>500 mL/r) are the core power components of large construction machineries' hydraulic system. The demand for higher power in large construction machinery is increasing. High pressure and large displacement are the primary areas of focus for enhancing the power of piston pumps. However, this may exacerbate issues with the pumps' output characteristics, such as flow or pressure pulsation and leakage. This paper uses a cross angle of the swash plate to optimize the pulsation and leakage for the 750 mL/r axial piston pumps. Piston and cylinder dynamics affect flow and pressure pulsations, and leakage is related to cylinder tilting behaviour. The pumps’ flow distribution, kinematics and dynamics of the piston and cylinder, output flow and pressure are analysed. A virtual prototype is built based on the rigid-flexible-hydraulic coupling model. The oil film flexible body of the valve plate pair is added. The maximum principal stress and strain of the oil film flexible body are used as indicators of tilting behaviour. Effects of the cross angle on output characteristics and tilting behaviour are investigated through the numerical solution. Results show that the cross angle of −0.5° can effectively reduce the pressure pulsation rate by 0.89% and the flow pulsation rate by 3.77% while optimizing the tilting behaviour of the cylinder. In addition, the cylinder’s tilting behaviour is verified by the wear detection on the valve plate surface.

Quantum control of classical motion: piston dynamics in a Rabi-coupled Bose–Einstein condensate

We develop a model and explore the dynamics of a hybrid classical-quantum system consisting of a classical piston and a self-interacting pseudospin 1/2 Bose–Einstein condensate with a time-dependent Rabi coupling. We investigate the mechanical work produced by the piston moving as a result of the quantum pressure of the condensate. The time-dependent Rabi field redistributes the condensate density between the spin components and, as a result, causes a time-dependent pressure acting on the piston. Correspondingly, the motion of the piston produces quantum evolution of the condensate mass- and spin density profiles. We show how by optimised design of the time-dependent direction of the Rabi field based on a quasi-stationary quantum pressure approximation, one can control both the position and velocity of the piston.

Research on Piston Dynamics and Engine Performances of a Free-Piston Engine Linear Generator Coupling with Various Rebound Devices

Free-piston engine linear generators (FPELGs) are an innovative linear power device that exhibits the distinctive dynamics of pistons and performance of free-piston engines. Furthermore, the single-cylinder/single-piston FPELG structure type has more advantages than other FPELG structure types, including a straightforward structure and ease of control. However, when coupled with various rebound devices, the operational characteristics and piston and engine performance of single-cylinder/single-piston FPELGs are quite different. Therefore, this paper aims to quantitatively compare the dynamics of the piston and engine performance of single-cylinder/single-piston FPELGs coupled with various types of rebound device. The results indicate that when the equivalent stiffness of the gas spring is greater than that of the mechanical spring, the operating frequency of the piston of the FPELG coupled with a gas spring will be higher than that when coupled with a mechanical spring. During the compression stroke, the piston velocity of a FPELG coupled with a mechanical spring changes linearly, while the piston velocity of a FPELG coupled with a gas spring changes nonlinearly. FPELGs coupled with gas springs have shorter compression and expansion durations compared to those coupled with mechanical springs. In addition, the indicated powers of FPELGs coupled with ideal gas springs and mechanical springs are 1.5 kW and 1.3 kW, respectively. However, due to leakage, the thermal efficiency of a FPELG coupled with an actual gas spring is reduced by approximately 2.5% compared with the FPELG coupled with the ideal gas spring. Furthermore, the operation frequency of the piston is positively correlated with the stiffness of the mechanical spring. In addition, as the stiffness of the mechanical spring increases, the combustion process of the engine becomes close to an isovolumetric process. The changes in piston dynamics and engine performance when increasing the initial gas pressure of the gas spring are similar to those when increasing the stiffness of the mechanical spring.

Effect of damping coefficients on a free-piston Stirling engine under a constant power condition based on a thermodynamic-dynamic coupled model

The Free Piston Stirling Engine (FPSE) is a complex system, in which the transient behaviors of its thermal and dynamic characteristics are coupled. To examine the strong coupling system under constant heating power and evaluate the effect of damping coefficients on engine performance, prediction of the aforementioned characteristics is essential. To this end, this study develops an FPSE system model based on quasi-static thermodynamics and piston dynamics to determine the thermal performance of the working fluid and the dynamic behavior of the displacer and piston, and the model is validated on the Re-1000. Then, the model is applied to examine the influence of the damping coefficients of two pistons on reactor core heater temperature, power, efficiency, amplitude, frequency, and phase angle under constant power condition. The results indicate that the model can well capture the transient behaviors of thermal and dynamic characteristics at Re-1000. After 0.7 s, the engine reaches a steady-state operation. Under the constant heating power of 3000 W, the displacer and piston damping significantly affect the engine's performance. Four displacement damping coefficient points are specified, and the piston damping coefficient changes in the range of Cp = 500 ∼ 1600Ns/m. As a result, the heater's stable working-point temperature changes from 590 K to 1060 K leading to an increase in indicated power from 1000 W to approximately 1500 W. The displacer and piston amplitudes show an increasing and decreasing trend respectively, with a decreasing trend in frequency and phase angle, but enhanced heat transfer performance. The dynamic-thermodynamic coupling model provides an opportunity to instantly calculate the open-loop response of the FPSE, which is a reference for future FPSE power-load matching designs.

Design and characterization of the Sandia free-piston reflected shock tunnel

A new reflected shock tunnel capable of generating hypersonic environments at realistic flight enthalpies has been commissioned at Sandia. The tunnel uses an existing free-piston driver and shock tube coupled to a conical nozzle to accelerate the flow to approximately Mach 9. The facility design process is outlined and compared to other ground test facilities. A representative flight-enthalpy condition is designed using an in-house state-to-state solver and piston dynamics model and evaluated using quasi-1D modeling with the University of Queensland L1d code. This condition is demonstrated using canonical models and a calibration rake. A 25-cm core flow with 4.6-MJ/kg total enthalpy is achieved over an approximately 1-ms test time. The condition was refined using analysis and a heavier piston, leading to an increase in test time. A novel high-speed molecular tagging velocimetry method is applied using in situ nitric oxide to measure the freestream velocity of approximately 3016 m/s. Companion simulation data show good agreement in exit velocity, pitot pressure, and core flow size.

A comparison of main bearings load of two-cylinder “V” and boxer motorcycle engines

The paper deals with determining the load of the crankshaft main bearings in two-cylinder motorcycle engines. Two cylinder arrangements, used in two-cylinder motorcycle engines, are considered - the boxer-engine and V-engine. The aim of the research is to determine the influence of cylinder arrangement on the magnitude and circumferential distribution of the bearing load of the engines considered. For this purpose, models of a two-cylinder boxer engine and a corresponding V-engine crankshaft drives were developed using multibody dynamics simulation software. Previously, by using a one-dimensional gas analysis software the engine cycles were simulated in order to determine the cylinder pressure required to simulate piston dynamics. As a result of the dynamics simulations, bearing load diagrams of the boxer- and V-engine were obtained, which were then compared and analyzed.

ВЛИЯНИЯ ИНЕРЦИИ РАБОЧЕЙ ЖИДКОСТИ НА ЭКСПЛУАТАЦИОННЫЕ ПАРАМЕТРЫ ГИДРОМОЛОТА

The study objective is to justify the parameters that ensure efficient energy conversion of the hydrohammer. Research methods include review, analysis and generalization of research results and experience in designing hydraulic devices; mathematical modeling based on laws of kinematics and dynamics of solid, liquid and gas; theoretical research based on numerical experiments. The novelty of the work is in the development of a mathematical model of a hydraulic hammer describing the cooperation of the percussive mechanism, distributor, hydraulic pneumatic accumulators and hydraulic drive. The study results include a mathematical model of a hydraulic hammer that takes into account the dynamics of its body and reflects the features of the working cycle of a hydrokinematic circuit with a controlled chamber of the working and reverse stroke; the numeric results of the mathematical model and experimental studies; dynamic analysis of the piston pin during the reverse stroke. Conclusions: the hydrohammer operation is influenced by accidental or purposeful changes in certain parameters of the hydraulic drive, which must be taken into account at the design stage of its subsystems in order to find a rational area of operation; one of these parameters is the inertia of the working fluid; formed during transition periods, inertial forces have a significant impact on the main parameters of the hydrohammer piston pin movement and dynamics of the power pulse hydraulic system; the time of filling the working cavity of the hydraulic cylinder of the striker is determined, the degree of influence of the working fluid inertial component on the time of the piston pin movement on the magnitude of the working stroke is defined.

Sliding Mode Analysis of a Counterbalance Valve Induced Instability in an Electrohydraulic Drive

Abstract This paper analyzes the dynamic effects of an electrohydraulic drive that uses a counterbalance valve for rod volume compensation. It shows that local stability analysis is not sufficient in this particular case to get general statements of the system's chattering properties. A reduced-order switched system is proposed to gain deeper insights in system dynamics with saturation effects such as the end-stop of a valve poppet and solutions are compared numerically to the full-system dynamics which incorporates pressure built-up, piston, and valve dynamics as well as motor dynamics. It is shown that in cases of, e.g., fast valves with small cracking pressures undesirable chattering of the full system exists which can be easily understood in terms of the reduced-order system in form of sliding mode solutions. The paper also describes under which conditions such sliding modes exist, how they behave, and how they can be interpreted in terms of the full system.

Improved Mathematical Approach for Modeling Sport Differential Mechanism

Improved mathematical and simulation modes of the active differential mechanism (DM) with controllable torque redistribution would better contribute to developing intelligent vehicle transmissions. The issue is caused by actualizing the precise steerability control using advanced automated transmissions, allowing torque vectoring for all-wheel-drive vehicles and ensuring an option for correcting the vehicle trajectory. This paper presents an alternative mathematical method for obtaining differential equations for modeling vehicle transmission components and its implementation for simulating the Audi sport DM. First, the steerability issues of sport DM technology are discussed, and the sport DM design is described in detail. Then, a mathematical approach is proposed that includes three types of equation systems: generalized dynamics equations, kinematic constraint equations, and gearing condition equations. The approach also considers the flexibility of the clutch’s frictional pack, friction torque, lockup condition, and piston dynamics. Finally, a Simulink model that reflects the DM operation and calculation procedures is developed. A series of simulations of the sport DM operation with forcible torque distribution is carried out. The results show that the proposed mathematical model is universal, efficient, and accurate.

Piston-Driven Pneumatically-Actuated Soft Robots: Modeling and Backstepping Control

Actuators’ dynamics have been so far mostly neglected when devising feedback controllers for continuum soft robots since the problem under the direct actuation hypothesis is already quite hard to solve. Directly considering actuation would have made the challenge too complex. However, these effects are, in practice, far from being negligible. The present work focuses on model-based control of piston-driven pneumatically-actuated soft robots. We propose a model of the relationship between the robot’s state, the acting fluidic pressure, and the piston dynamics, which is agnostic to the chosen model for the soft system dynamics. We show that backstepping is applicable even if the feedback coupling of the outer on the inner subsystem is not linear. Thus, we introduce a general model-based control strategy based on backstepping for soft robots actuated by fluidic drive. As an example, we derive a specialized version for a robot with piecewise constant curvature.

Mechanism of Proton Pumping in Complex I of the Mitochondrial Respiratory Chain

We propose a physical mechanism of conformation-induced proton pumping in mitochondrial Complex I. The structural conformations of this protein are modeled as the motion of a piston having positive charges on both sides. A negatively charged electron attracts the piston, moving the other end away from the proton site, thereby reducing its energy and allowing a proton to populate the site. When the electron escapes, elastic forces assist the return of the piston, increasing proton site energy and facilitating proton transfer. We derive the Heisenberg equations of motion for electron and proton operators and rewrite them in the form of rate equations coupled to the phenomenological Langevin equation describing piston dynamics. This set of coupled equations is solved numerically. We show that proton pumping can be achieved within this model for a reasonable set of parameters. The dependencies of proton current on geometry, temperature, and other parameters are examined.

Investigations for a Trajectory Variation to Improve the Energy Conversion for a Four-Stroke Free-Piston Engine

Internal combustion engines with a crankshaft have been successfully developed for many years. They are lacking in the fact that the piston trajectory, i.e., position as a function of time, is limited by the crankshaft motion law. Position-controlled electric linear machines directly coupled to the piston allow to realize free-piston engines. Unlike the crankshaft-based engines, they allow for a higher degree of freedom in shaping the piston trajectory, including adaptive compression ratios, which enables optimal operation with alternative fuels. The possibility of adapting the stroke course results in new degrees of freedom with which the combustion process can be optimized. In this work, four-stroke trajectories with different amplitudes and piston dynamics have been proposed and analyzed regarding efficiency. A simulation model was created based on experimental measurements for testing the proposed trajectories. It could be proved that the variation of the trajectory resulted in an improvement of the overall efficiency. The trajectories were described analytically so that they can be used for a prototype in a future work.

Research on the Operating Characteristics of Hydraulic Free-Piston Engines: A Systematic Review and Meta-Analysis

The hydraulic free-piston engine (HFPE) is a kind of hybrid-powered machine which combines the reciprocating piston-type internal combustion engine and the plunger pump as a whole. In recent years, the HFPE has been investigated by a number of research groups worldwide due to its potential advantages of high efficiency, energy savings, reduced emissions and multi-fuel operation. Therefore, our study aimed to assess the operating characteristics, core questions and research progress of HFPEs via a systematic review and meta-analysis. We included operational control, starting characteristics, misfire characteristics, in-cylinder working processes and operating stability. We conducted the literature search using electronic databases. The research on HFPEs has mainly concentrated on four kinds of free-piston engine, according to piston arrangement form: single piston, dual pistons, opposed pistons and four-cylinder complex configuration. HFPE research in China is mainly conducted in Zhejiang University, Tianjin University, Jilin University and the Beijing Institute of Technology. In addition, in China, research has mainly focused on the in-cylinder combustion process while a piston is free by considering in-cylinder combustion machinery and piston dynamics. Regarding future research, it is very important that we solve the instabilities brought about by chance fluctuations in the combustion process, which will involve the hydraulic system’s efficiency, the cyclical variation, the method of predicting instability and the recovery after instability.

Influence of piston displacement profiles on the performance of a novel dual piston linear compressor

Abstract A novel dual piston linear compressor has been proposed in this paper, which is suitable for various applications such as refrigeration and gas compression owing to capacity modulation by stroke control. A numerical model for the proposed dual piston linear compressor was built to investigate the influence of different piston displacement profiles on the performance of the linear compressor. Sub-models on the piston dynamics, cylinder gas thermodynamics, motor force and frictional force were presented using MATLAB/Simulink. The piston was controlled to oscillate with the 5 typical piston displacement profiles. From the simulation results, it is found that the displacement profile of a triangle curve shows the highest compression efficiency and highest electrical efficiency. Moreover, the piston velocity and the current in the coil of the linear motor are considered to be necessary for feedback signals in the real-time control system to adjust the motor force and to reduce the effect of the back electromagnetic force.

Kinetic simulations of piston-driven collisionless shock formation in magnetized laboratory plasmas

Laboratory laser experiments offer a novel approach to studying magnetized collisionless shocks, and a common method in recent experiments is to drive shocks using a laser-ablated piston plasma. However, current experimental capabilities are still limited to spatiotemporal scales on the order of shock formation, making it challenging to distinguish piston and shock dynamics. We present quasi-1D particle-in-cell simulations of piston-driven, magnetized collisionless shock formation using the code PSC, which includes a model of laser-driven plasmas that can be well-matched to experimental conditions. The simulations cover a range of upstream and ablation parameters and yield several robust signatures of shock formation, which can provide a reference for experimental results.

Modeling of gas parameters in the cylinder of the automatic gun during firing

The change of gas parameters in the cylinder of the automatic gun has a great influence on the dynamics of the gun. Many factors have an influence on the gas parameters in the gas cylinder such as the diameter and position of the gas port, the initial volume of the gas cylinder, the gap between the piston and the cylinder, initial temperature of the cylinder parts, etc. The analytical model was made to analyze the parameters on which depends gas piston dynamics. The numerical simulation with CFD software ANSYS Fluent was performed to analyze the change of the thermodynamic properties of the gases in the cylinder, temperature change of the cylinder parts and dynamics of the gas piston. The experiment was performed to provide pressures in the barrel and the gas cylinder and velocity of the gas piston. The comprehensive comparisons between results obtained by the analytical model, the numerical simulation and experiments have been performed and good agreements were observed.

Feasibility of Multiple Piston Motion Control Approaches in a Free Piston Engine Generator

<div class="section abstract"><div class="htmlview paragraph">The control and design optimization of a Free Piston Engine Generator (FPEG) has been found to be difficult as each independent variable changes the piston dynamics with respect to time. These dynamics, in turn, alter the generator and engine response to other governing variables. As a result, the FPEG system requires an energy balance control algorithm such that the cumulative energy delivered by the engine is equal to the cumulative energy taken by the generator for stable operation. The main objective of this control algorithm is to match the power generated by the engine to the power demanded by the generator. In a conventional crankshaft engine, this energy balance control is similar to the use of a governor and a flywheel to control the rotational speed. In general, if the generator consumes more energy in a cycle than the engine provides, the system moves towards a stall. If the generator consumes less energy, then the effective stroke, compression ratio and maximum translator velocity must rise steadily from cycle-to-cycle until the heat transfer losses stop the increase. Moreover, when stiff springs are added to the FPEG system, the dynamics becomes more sinusoidal and more consistent with increasing spring stiffness. To understand the behavior of proposed control and cycle-to-cycle variations, a comprehensive FPEG numerical model with a 1 kW target electric power was developed in MATLAB<sup>®</sup>/Simulink. An FPEG system corresponding to that numerical model has been operated in the laboratory. This MATLAB<sup>®</sup>/Simulink numerical model has been used to examine the sensitivity of FPEG dynamics and performance parameters to the changes in design and operating inputs. A difficulty during the modeling is associated with the cycle-to-cycle energy balance, and this difficulty is also reflected in the real-world FPEG control. Therefore, the authors have devised a control strategy similar to the real world intended control methodology. In this numerical model, two different feedback control methodologies were implemented and investigated. These control methodologies were applied to regulate the generator load with selected control or input variables, namely peak pressure, mid-stroke piston velocity, trapped compression ratio and dead center set points. The controllers with optimized coefficients demonstrated the feasibility of energy balance management during the transient operation. Based on the simulation results, the controllers with compression ratio, peak pressure and dead center clearance set points as control variables demonstrated stable FPEG operation whereas the mid-stroke velocity failed to achieve the steady-state operation due to deviation in the piston dynamics. The simulation results from this study will be used as the pathway for improving and optimizing the experimental FPEG design.</div></div>

Theoretical analysis of dynamic characteristics in linear compressors

Linear compressor technology is characterized by the absence of a crank mechanism, which has gained increasing attention in vapor compression systems due to its compactness and lower friction losses in comparison to conventional reciprocating compressors. Limited work was found in the open literature related to the development and in-depth validation of a comprehensive linear compressor dynamic model that couples thermodynamic aspects with both mechanical and electrical sub-models. This paper presents a comprehensive and generalized simulation model that is used to simulate the dynamic performance of a linear compressor. The model is based on mass and energy balance equations applied to open control volumes. The overall model is composed of several sub-models including the piston dynamics, electrical motor, valve dynamics, and leakage flows. The thermodynamic model and the sub-models are integrated into a compressor overall energy balance that describes the different heat flows and losses. The linear compressor model is able to predict both transient and steady-state behaviors of the piston movement, internal pressure and temperatures as well as the overall performance. Comparisons of predicted and measured mass flow rates as a function of operating frequency are also presented within this work.

Numerical investigation into the fuel evaporation and mixture formation characteristics of a free-piston diesel engine

The free-piston engine generator becomes a new-type potential substitute for the conventional crankshaft combustion engine. This article presents a simulation to study the fuel spray and mixing characteristics of a diesel free-piston engine generator by comparing a corresponding crankshaft combustion engine. A full-cycle model which couples with piston dynamics, combustion, and gas exchange is developed to simulate the fuel spray, atomization, and mixing in the free-piston engine generator. The result indicates that compared with the crankshaft combustion engine, the free-piston engine generator provides a higher temperature and pressure for fuel spray and mixing during the ignition delay, but its ignition delay lasts shorter. The free-piston engine generator shows a shorter spray penetration and more fuel impingement due to its smaller combustion chamber volume during the injection process. The free-piston engine generator exhibits a lower level of air utilization and worse uniformity of fuel–air mixture in combustion chamber. In addition, the shorter ignition delay of free-piston engine generator makes the time of atomization, evaporation, and mixing of fuel shorter, and the mixing effect of free-piston engine generator is worse, resulting in less combustible mixture formed during the ignition delay. In addition, some guiding suggestions have been proposed to improve the fuel spray and fuel–air mixing characteristics of free-piston engine generator.