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

Abstract

<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>

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

“…The Clark group of West Virginia University investigated the operation characteristics and the piston and engine performance of a single-cylinder-type FPELG and a dualcylinder-type FPELG coupled with a mechanical spring on the basis of simulation results and experimental results [8][9][10]. It was found that the operation frequency of the piston and the power of the free piston engine are positively correlated with the spring stiffness, while the efficiency decreased with the increase in spring stiffness [11].…”

“…Moreover, the combustion characteristics of the FPELG coupling with the mechanical spring were also simulated, and the results demonstrated that the combustion starting timing advanced, the adverse work and heat transfer both increased, while the overall efficiency declined [15]. Yingcong Zhou from Stony Brook University researched The Clark group of West Virginia University investigated the operation characteristics and the piston and engine performance of a single-cylinder-type FPELG and a dual-cylindertype FPELG coupled with a mechanical spring on the basis of simulation results and experimental results [8][9][10]. It was found that the operation frequency of the piston and the power of the free piston engine are positively correlated with the spring stiffness, while the efficiency decreased with the increase in spring stiffness [11].…”

Numerical model of variable valve timing distribution for a supercharged diesel engine

“…Mahrous et al [41] studied the effects of the valve overlap (VO) angle on engine performances on the same engine type. They arrived to fix the IV closure and EV opening times and modify the IV opening and EV closing times [45]. The VO angle was reported to influence the airfuel mixture significantly, and the perfect air-fuel mixture was obtained at high VO angles.…”