Piston Trajectory Research Articles

Robust Finite-Time Convergence Control Mechanism for High-Precision Tracking in a Hybrid Fluid Power Actuator

In this study, a novel multiphase converged control structure with a switching mechanism is established based on the fundamentals of the fast-terminal sliding mode. This control structure achieves an excellent performance in a nonlinear dynamic system, and its primary objective is to control the piston trajectory in an innovative electrohydraulic, pneumatic, and mechanical hybrid system. This work focuses on abrupt gain-scheduled acceleration, which has been rarely studied in the literature but has increasingly diverse high-technology applications across various industries. The greatest challenge lies in the sensitivity and instability of the system during a sudden actuator acceleration. Moreover, the system parameter uncertainties and external disturbances strongly influence the degree of control of the system. By inheriting the robustness and fast convergence properties of sliding-mode algorithms, the proposed control law not only guarantees a finite-time convergence of tracking errors to their origin but also reduces the impact of composite disturbances in an extremely rapid experimental process. The effectiveness of the proposed structure is analyzed via numerical simulations and industrial implementation.

Self-similar Piston-Shock and CME

Based on a dimensional analysis, we discuss the existence conditions for one-dimensional self-similar solutions to the problem of a piston extending into an inhomogeneous plasma with magnetic field. In these solutions, the Mach number of a shock wave ahead of the piston is constant. As an application, we consider the SOL2010-06-13 solar eruptive event, where this property was found for a coronal wave at the formation stage of a coronal mass ejection (CME). Using a known wave trajectory in the distance–time plane, it is possible to follow the trajectory of an effective (virtual) piston that is a driver of the wave. The trajectory of the virtual piston originates in an eruptive filament during its impulsive acceleration and terminates at a rim bounding an apparent CME bubble in extreme-ultraviolet images, when the rim appears. The velocity of the rim at that time is approximately equal to the fast-mode speed at this place before the expansion onset, and the shock-front has a speed corresponding to a Mach number of about 1.5. We also discuss an analogy between the shock-wave propagation caused by the eruption of a solar magnetic flux-rope and the inverse-pinch effect as known from laboratory plasma physics.

Design and Control of a Controlled Trajectory Rapid Compression and Expansion Machine

This paper presents the design of a controlled trajectory rapid compression and expansion machine (CT-RCEM) that offers the unique capability of real-time piston trajectory control. While in a conventional rapid compression machine/rapid compression and expansion machine (RCEM), changing the operating parameters such as compression ratio and compression time requires mechanical alteration of the hardware, the same can be achieved in the CT-RCEM merely by changing the reference trajectory fed electronically to the controller. With this capability, a CT-RCEM offers higher operational flexibility, wider range of operation, and better repeatability compared to the current state-of-the-art RCEM. More interestingly, a CT-RCEM is naturally suited to investigate the coupling between the piston trajectory and the combustion kinetics—a phenomenon that is extremely difficult to investigate using conventional combustion research facilities. However, high-speed operation and high-force requirement for piston motion present a major challenge for design and control of a suitable actuation system that can provide precise motion control. For the design of the CT-RCEM, first, a dynamic model is developed to understand the impact of various design parameters on the system performance and to identify suitable control strategy for the actuation system. The design parameters in the dynamic model and the controller are tuned in-tandem using simulation studies to obtain the system specifications, which are subsequently translated into a mechanical design. Experimental results obtained after commissioning of the designed CT-RCEM demonstrate that the desired functionality of real-time piston trajectory control has been achieved.

A Controlled Trajectory Rapid Compression and Expansion Machine (CT-RCEM) for Chemical Kinetic Investigations

ABSTRACT In this work, we present a controlled trajectory rapid compression and expansion machine (CT-RCEM) for chemical kinetic investigations of fuels, with the unique capability of precise motion control of the piston in the combustion chamber. This capability allows the CT-RCEM to achieve a wider operating range and extreme flexibility of operation as changes in the piston trajectory are made electronically, unlike the conventional rapid compression machines (RCM) that require mechanical intervention. Also, precise motion control of the piston ensures high run-to-run repeatability of the piston trajectory, which in turn, naturally enables the CT-RCEM to achieve highly repeatable pressure and temperature profiles for compression and expansion over the entire operating range. The key novelty of CT-RCEM, however, lies in the new paradigm of experimental investigation that it provides for chemical kinetic studies. The thermodynamic path of the fuel mixture in the combustion chamber during an RCM investigation (essentially pressure and temperature history) is a function of the heat transfer characteristics of the chamber assembly as well as the piston trajectory. Hence, by suitable selection of the piston trajectory, the CT-RCEM allows not only the capability to set the end of compression thermodynamic state (compressed pressure and temperature), but also the flexibility to tailor the entire thermodynamic path, during and after the compression. In this paper, we demonstrate three novel research capabilities realized by tailoring the thermodynamic path – first, ability to systematically investigate the effect of changing the thermodynamic path of compression on the ignition delay of fuels for same end of compression pressure and temperature; second, ability to quench chemical reactions in the entire combustion chamber at any desired stage; and third, ability to investigate ignition delay of fuels for isobaric post-compression conditions by compensating the potential pressure drop due to heat loss with a unique creeping piston trajectory. We conclude with a discussion of the significance of these new research capabilities for chemical kinetic studies, especially autoignition investigations.

Optimal Stroke Path for Reciprocating Heat Engines

By testing piston motion in reciprocating heat engines as a control variable, one could find piston trajectories, different from the conventional near sinusoidal motion that should increase power production. This results from minimizing frictional losses. The purpose of this study is to determine piston trajectories that are optimal for noncombustion strokes in reciprocating engines, in the sense of minimizing frictional dissipation and hence maximizing efficiency and power. The optimal piston traces for noncombustion strokes are determined by using a combination of optimal control theory and models for the thermodynamic irreversibilities. Hence, the results are germane to external combustion engines and to the noncombustion strokes of internal combustion engines. The optimal piston traces or trajectories obtained here can be viewed as some of the building blocks from which optimal overall cycles can be constructed.

Use of acceleration and optical waypoint measurements to estimate piston trajectory in an impulse facility

This paper describes a prototype device to estimate the trajectory of the piston in a free-piston driver. The device, which screws to the front face of the piston, has an internal data acquisition system which triggers upon piston launch and records the acceleration history across the complete piston stroke. A key feature is the incorporation of an optical waypoint detector. This comprises a laser which is directed onto the compression tube wall, and a photodiode which measures the reflected intensity. A series of evenly spaced high-contrast lines are marked on the tube surface, which appear as a pattern of intensity changes in the photodiode signal and define a set of exact position–time coordinates along the trajectory. Optical pattern recognition has previously been shown to be effective to identify piston traversal past a fixed detector; in this application, the order is reversed so that the detector is on the piston, and the pattern is at the fixed location. Waypoint detection permits computation of a precise in situ calibration of the accelerometer and correction of the acceleration integration along the piston stroke. The key benefits of this concept are that it provides a complete, fully time-resolved, accurate estimate of piston position, is self-contained and portable, and can be readily utilised on any free-piston driver without significant modification to the facility.

A multizone model of the combustion chamber dynamics in a controlled trajectory rapid compression and expansion machine (CT-RCEM)

A novel, controlled trajectory rapid compression and expansion machine (CT-RCEM) with the unique capability of precise motion control of the piston has been developed. The compression and expansion profile can be varied, in the CT-RCEM, by digitally changing the piston reference trajectory assigned to the active motion controller. Besides the ease of operation, and wider selection of operating parameters with higher resolution, this capability lowers the turnaround time between experiments by eliminating the need for any hardware intervention. More importantly, this capability allows us to tailor the thermodynamic path of compression (and expansion) – essentially the pressure and temperature profile – by appropriate selection of the piston trajectory. However, to effectively leverage these features of the CT-RCEM, a numerical model is essential – for selection of appropriate thermodynamic path for chemical-kinetic investigations, as well as for the interpretation of the corresponding experimental results.In this work, we present a computationally efficient, physics based multi-zone model of the combustion dynamics of the CT-RCEM, which accounts for the heat loss and piston crevice flows. We show that the concurrent use of the proposed model with the CT-RCEM allows, for the first time, a systematic investigation of the effect of changing piston trajectory – and consequently – the thermodynamic path of compression on the auto-ignition characteristics of fuels. The effect of changing piston trajectory on autoignition characteristics of dimethyl-ether (DME) observed in CT-RCEM experiments has been explained using the proposed model. The study clearly shows that changing the piston trajectory can significantly affect the measured ignition delay due to the resulting change in the thermodynamic path. Also, a shorter compression time for a given compression ratio does not necessarily guarantee smaller reaction progress during compression. The paper concludes by summarizing the unique insight obtained from this study —the use of CT-RCEM for auto-ignition investigation involving thermodynamic paths with similar pressure and temperature conditions at the end of compression but different intermediate species buildup can potentially provide additional information vital for further understanding of the chemical kinetics.

A Control-Oriented Model for Trajectory-Based HCCI Combustion Control

Previously, the authors have proposed the concept of piston trajectory-based homogeneous charge compression ignition (HCCI) combustion control enabled by a free piston engine (FPE) and shown its benefits on both engine thermal efficiency and emissions by implementing various piston trajectories. In order to realize the HCCI trajectory-based combustion control in practical applications, a control-oriented model with sufficient chemical kinetics information has to be developed. In this paper, such a model is proposed and its performance, in terms of computational speed and model fidelity, is compared to three existing models: a simplified model using a one-step global reaction, a reduced-order model using Jones–Lindstedt mechanism, and a complex physics-based model including detailed chemical reaction mechanisms. A unique phase separation method is proposed to significantly reduce the computational time and guarantee the prediction accuracy simultaneously. In addition, the paper also shows that the high fidelity of the proposed model is sustained at multiple working conditions, including different air-fuel ratios (AFR), various compression ratios (CR), and distinct piston motion patterns between the two end positions. Finally, an example is presented showing how the control-oriented model enables real-time optimization of the HCCI combustion phasing by varying the trajectories. The simulation results show that the combustion phasing can be adjusted quickly as desired, which further demonstrates the effectiveness of the piston trajectory-based combustion control.

Dynamics of Large-Scale Solar-Wind Streams Obtained by the Double Superposed Epoch Analysis: 2. Comparisons of CIRs vs. Sheaths and MCs vs. Ejecta

This work is a continuation of our previous paper [Yermolaevetal2015] which describes the average temporal profiles of interplanetary plasma and field parameters in large-scale solar-wind (SW) streams: CIR, ICME (both MC and Ejecta) and Sheath as well as the interplanetary shock (IS). Like in the previous work we use data of OMNI database, our catalog of large-scale solar-wind phenomena during 1976--2000 [Yermolaevetal2009] and the double superposed epoch analysis (DSEA) method [Yermolaevetal2010]: re-scaling the duration of interval for all types in such a manner that, respectively, beginning and end for all intervals of selected type coincide. We present new detailed results of comparison of two pair phenomena: (1) both types of compression regions (CIR.vs.Sheath) and (2) both types of ICMEs (MC.vs.Ejecta). Obtained data allows us to suggest that the formation of all types of compression regions has the same physical mechanism irrespective of piston (High-Speed Stream (HSS) or ICME) type and differences are connected with geometry (angle between speed gradient in front of piston and satellite trajectory) and full jumps of speed in edges of compression regions. One of consequences of this hypothesis is the conclusion that one of the reasons of observed distinctions of parameters in MC and Ejecta can be fact that at measurements of Ejecta the satellite passes further from the nose area of ICME, than at measurements of MC. We also discuss the impact of Sheath in magnetospheric activity and its contribution in estimation of Sun's open magnetic flux.

Transient motion control for a free-piston engine

The free-piston engine has a unique internal-combustion engine architecture which features unconstrained piston movement because the crankshaft has been removed. The engine design is thus more compact, modular and efficient. One of the technical challenges that hinder the development of free-piston engine technology is the lack of precise control of the piston motion. Previously, a robust repetitive controller was designed and implemented to form a virtual crankshaft to provide stable and precise control of the piston motion. The virtual crankshaft mechanism ensures stable and repeatable free-piston engine operation and opens up an opportunity for controlling the in-cylinder gas pressure–temperature trajectories by altering the piston trajectory. The experimental data from engine motoring tests with the virtual crankshaft demonstrate the effectiveness of the controller. However, the presence of a transient period after combustion prevents the engine from continuous firing. Thus, a transient control scheme is presented in this paper, which consists of a detection and shifting algorithm to remove the transient period, so that continuous engine firing operation can be achieved.

Trajectory-based combustion control for renewable fuels in free piston engines

Previously, the authors have developed an advanced combustion control, namely the trajectory-based combustion control, to further leverage the flexibility of free piston engine (FPE). With the assistance of this control method, the FPE enables optimization of both engine efficiency and emissions by implementing optimal piston trajectories. Extensive simulations have been conducted to prove the effectiveness of this combustion control on fossil fuels. In this paper, the investigation is extended to renewable fuels. Seven renewable fuels are considered herein including hydrogen, biogas, syngas, ethanol, dimethyl ether (DME), biodiesel, and Fischer-Tropsch fuel. The influences of both compression ratio (CR) and piston motion pattern between the two dead centers on the combustion process are considered in the study, which demonstrates the ultimate fuel flexibility and large tolerance of fuel impurity possessed by the FPE. In addition, the simulation results show that at a fixed CR, the thermal efficiency of the FPE can still be enhanced (5% in DME case) by varying the piston motion patterns alone. Furthermore, specific asymmetric piston trajectories are synthesized to further improve the engine thermal efficiency (8% in hydrogen case) and reduce the NOx emission simultaneously (around 70% reduction in hydrogen case). In other words, due to its ultimate fuel flexibility, large tolerance of fuel impurity, and controllable piston trajectory, the FPE, with the trajectory-based combustion control, enables a co-optimization of renewable fuels and engine operation.

Maximum cycle work output optimization for generalized radiative law Otto cycle engines

An Otto cycle internal combustion engine which includes thermal and friction losses is investigated by finite-time thermodynamics, and the optimization objective is the maximum cycle work output. The thermal energy transfer from the working substance to the cylinder inner wall follows the generalized radiative law ( $q\propto\Delta (T^{n})$ ). Under the condition that all of the fuel consumption, the compression ratio and the cycle period are given, the optimal piston trajectories for both the examples with unlimited and limited accelerations on every stroke are determined, and the cycle-period distribution among all strokes is also optimized. Numerical calculation results for the case of radiative law are provided and compared with those obtained for the cases of Newtonian law and linear phenomenological law. The results indicate that the optimal piston trajectory on each stroke contains three sections, which consist of an original maximum-acceleration and a terminal maximum-deceleration parts; for the case of radiative law, optimizing the piston motion path can achieve an improvement of more than 20% in both the cycle-work output and the second-law efficiency of the Otto cycle compared with the conventional near-sinusoidal operation, and heat transfer mechanisms have both qualitative and quantitative influences on the optimal paths of piston movements.

Maximum Work of Free-Piston Stirling Engine Generators

Abstract Using the method of adjoint equations described in Ref. [1], we have calculated the maximum thermal efficiencies that are theoretically attainable by free-piston Stirling and Carnot engine generators by considering the work loss due to friction and Joule heat. The net work done by the Carnot cycle is negative even when the duration of heat addition is optimized to give the maximum amount of heat addition, which is the same situation for the Brayton cycle described in our previous paper. For the Stirling cycle, the net work done is positive, and the thermal efficiency is greater than that of the Otto cycle described in our previous paper by a factor of about 2.7–1.4 for compression ratios of 5–30. The Stirling cycle is much better than the Otto, Brayton, and Carnot cycles. We have found that the optimized piston trajectories of the isothermal, isobaric, and adiabatic processes are the same when the compression ratio and the maximum volume of the same working fluid of the three processes are the same, which has facilitated the present analysis because the optimized piston trajectories of the Carnot and Stirling cycles are the same as those of the Brayton and Otto cycles, respectively.

Using variable piston trajectory to reduce engine-out emissions

Previously, the authors have proposed the concept of piston trajectory-based combustion control enabled by free piston engines (FPEs). With this novel method, the FPE realizes in-cycle adjustment of combustion phase and real-time control of in-cylinder temperature and pressure through variable piston trajectories. As a result, higher indicated thermal efficiency, compared to conventional internal combustion engines (ICEs), is achieved. In this paper, the effects of this new combustion control on engine-out emissions are investigated. First, a comprehensive model is developed that includes different piston trajectories in the FPE, a convective heat loss model and a reduced n-heptane reaction mechanism with major emissions species. Afterwards, the chemical kinetics of CO and NOx emissions are described in details that reveal the feasibility of reducing engine-out emissions by employing novel piston trajectories. At last, analyses of the corresponding simulation results and comparisons of emissions and thermal efficiencies between the FPE and conventional ICEs are presented, which further shows the advantages of the trajectory-based combustion control.

Modeling of piston trajectory-based HCCI combustion enabled by a free piston engine

Previously, the authors have designed and implemented an active motion control “virtual crankshaft” for a free piston engine, which enables precise piston tracking of desired trajectories. With this mechanism, the volume of the combustion chamber can be regulated, and therefore the pressure, temperature and species concentrations of in-cylinder gas can be adjusted in real-time which affect the combustion process directly. This new degree of freedom enables us to conduct piston trajectory-based combustion control. In this paper, a model of the free piston engine running homogeneous charge compression ignition combustion under various piston trajectories is presented. The various piston trajectories have the ability to change the compression ratio and accommodate different piston motion patterns between the top dead center and the bottom dead center. Six reaction mechanisms are employed in the model in order to demonstrate the multi-fuel combustion ability of the free piston engine, and to describe the chemical kinetics of different fuels under various piston trajectories. Analysis of the simulation results is then presented which reveals the piston trajectory effects on the combustion in terms of in-cylinder gas temperature trace, indicated output work, heat loss and radical species accumulation process.

Precise piston trajectory control for a free piston engine

A free piston engine removes the mechanical constraint on the piston motion by eliminating the crankshaft. The extra degree of freedom offers many advantages for reducing fuel consumption and emissions. Nevertheless, stability and robustness of the engine operation has been affected in the meantime. To ensure smooth engine operation, an active motion controller, which utilizes robust repetitive control, was developed previously to regulate the piston motion of a hydraulic free piston engine to track pre-defined trajectories. However, the long piston stroke length, high operating frequency and system nonlinearity impose challenges to precise piston motion control. Therefore, feedforward controllers are investigated in this paper to complement the repetitive control to further improve the tracking performance. The first feedforward design involves the inversion of a linear plant model that describes the dynamics of the engine operation, and the second design is based on the flatness approach, which involves the inversion of a nonlinear model of the system. The two feedforward controllers are designed and implemented on the free piston engine. The experimental and simulation results demonstrate the effectiveness of the proposed control under various operating conditions and reference piston trajectories.

Modeling and Analysis on Mean Effective Torque of Rotors in Twin-Rotor Cylinder-Embedded Piston Engine

Torque, as an important indicator of the engine, is the reflection of performance of cylinder-embedded piston engine. In power stroke, torque varies at different moments, so if the impact of structural parameters on torque needs to be analyzed, the values of torque in power stroke need to be added together and divide lasting time. The average effective torque is the average of the torque generated by explosive force during the entire power stroke, which overcomes the disadvantages of the instantaneous torque which can not reflect the changes in power process. Therefore, the influence of each structure size on the force transmission performance can be analyzed with maximizing the average effective torque. It is concluded that for increasing the average effective torque, the radius of the hinge center trajectory of cap R should be increased, and the length from rotating center O to the trajectory of piston H should be decreased. The length of rod L has little influence on the average effective torque. However, if there is a too large D-value between R and H, the rod may deflect seriously, so that the piston side force increases, causing the piston wear and tear. So the values of R and H should be selected with specified conditions in the design.

The physical properties of the blast wave produced by a stoichiometric propane/oxygen explosion

The trajectory of the primary shock produced by the explosion of a nominal 18.14 t (20 tn) hemispherical propane/oxygen charge was analysed previously to provide the physical properties immediately behind the shock, but gave no information about the time-resolved properties throughout the blast wave. The present study maps all the physical properties of the wave throughout and beyond the positive durations for a range of distances from about 1.6–18 m scaled to a 1 kg charge at NTP. The physical properties were calculated using a hydro-code to simulate the flow field produced by a spherical piston moving with a specific trajectory. This technique has been used extensively to determine the physical properties of blast waves from a variety of sources for which the piston path was determined by high-speed photography of smoke tracers established close to the charges immediately before detonation. In the case of the propane/oxygen explosion, smoke tracer data were not available to determine the trajectory of the spherical piston. An arbitrary piston path was used and its trajectory iteratively adjusted until it produced a blast wave with a primary shock whose trajectory exactly matched the measured trajectory from the propane/oxygen explosion. Throughout the studied flow field the time histories of hydrostatic pressure, density and particle velocity are well described by fits to the modified Friedlander equation. The properties are presented as functions of scaled radius and are compared with the properties of the blast wave from a 1 kg TNT surface burst explosion, and with other measurements of the same explosion.

Optimal motion trajectory for the four-stroke free-piston engine with irreversible Miller cycle via a Gauss pseudospectral method

Abstract A four-stroke free-piston engine with internal and external irreversibilities of finite combustion rate of the fuel, heat transfer and friction is investigated in this paper. Under the condition of the fixed fuel consumption per cycle, the optimal piston motion trajectory for maximizing the net work output of different cases is derived by applying optimal control theory. Considering the path constraints and boundary conditions of this free-piston Miller cycle, a Gauss pseudospectral method (GPM) is presented for solving optimal motion trajectory. The results show that the optimal piston trajectory improves the efficiency by more than 10% and also suitably reduces the heat transfer losses compared to the conventional piston motion. By optimizing the piston motion around the top dead center (TDC), the in-cylinder gas pressure and temperature have a remarkable improvement while the heat transfer losses have a suitable reduction. The effects of other parameters, such as combustion duration and the frictional coefficient on the piston trajectory, are also investigated. It is shown that optimizing the piston motion trajectory is a good approach for the free-piston engine to improve the efficiency and it can provide guidelines for the optimal control of the practical free-piston engine.

Analysis on structural characteristics of rotors in twin-rotor cylinder-embedded piston engine

Twin-rotor cylinder-embedded piston engine is proposed for dealing with the sealing problems of rotors in twin-rotor piston engine where the existent mature sealing technologies for traditional reciprocating engine can be applied. The quantity and forms of its sealing surfaces are reduced and simplified, and what’s more, the advantages of twin-rotor piston engine are inherited, such as high power density and no valve mechanism. Given the motion law of two rotors, its kinematic model is established, and the general expression for some parameters related to engine performance, such as the trajectory, displacement, velocity and acceleration of the piston and centroid trajectory, angular displacement, velocity and acceleration of the rod are presented. By selecting different variation patterns of relative angle of two rotors, the relevant variables are compared. It can be concluded that by designing the relative angle function of two rotors, the volume variation of working chamber can be changed. However, a comprehensive consideration for friction and vibration is necessary because velocity and acceleration are quite different in the different functions, the swing magnitude of rod is proportional to link ratio λ, and the position of rod swing center is controlled by eccentricity e. In order to reduce the lateral force, a smaller value of λ should be selected in the case of the structure, and the value of e should be near 0.95. There is no relationship between the piston stroke and the variation process of relative angle of two rotors, the former is only proportional to the amplitude of relative angle of two rotors.