• Energy Research

Abstract This paper presents a linearization of the dynamic equation for a free-piston engine generator (FPEG), and simplifies it to a one-degree forced vibration system with viscous damping. The analogy between a mass-spring damper and a FPEG system is expressed, and the solution to the vibration system is solved. The model was successfully validated with respect to experimental data obtained from a prototype. The simulated piston displacement during steady operation showed similar trends with the test results and the error of the displacement amplitude was controlled within 3%. The state-space equations and the transfer function of the system are obtain using the fast response numerical model. An example of model application in the real FEPG control system was provided. Compared to the previous numerical model with differential approaches, the solving time of the proposed fast response model can be significantly reduced. The simplicity and flexibility of the proposed model make it feasible to be implemented to several computing software, i . e . Matlab, AMESim, Labview, Dymola et al. It can be easily implemented to real-time Hardware-in-the-Loop (HIL) simulation model for the future piston dynamic control system development. In addition, since it reveals how an FPEG operates in a resonant principle, the model is useful for parameter selection in the FPEG design process.

Abstract The free-piston linear generator (FPLG) has more merits than the traditional reciprocating engines (TRE), and has been under extensive investigation. Researchers mainly investigated on the starting process and the stable generating process of FPLG, while there has not been any report on the intermediate process from the engine cold start-up to stable operation process. Therefore, this paper investigated the intermediate process of the FPLG in terms of switching strategy and switching position based on simulation results and test results. Results showed that when the motor force of the linear electric machine (LEM) declined gradually from 100% to 0% with an interval of 50%, and then to a resistance force in the opposite direction of piston velocity (generator mode), the operation parameters of the FPLG showed minimal changes. Meanwhile, the engine operated more smoothly when the LEM switched its working mode from a motor to a generator at the piston dead center, compared with that at the middle stroke or a random switching time. More importantly, after the intermediate process, the operation parameters of FPLG were smaller than that before the intermediate process. As a result, a gradual motor/generator switching strategy was recommended and the LEM was suggested to switch its working mode when the piston arrived its dead center in order to achieve smooth engine operation.

Abstract The piston motion of a free-piston diesel linear generator (FPDLG) is different from the traditional reciprocating engine (TRE). Here we focused on a numerical simulation for the research on the combustion process of an FPDLG by adopting coupled models of zero-dimensional dynamics and multi-dimensional computational fluid dynamics (CFD). Piston dynamics model and CFD model to simulate the combustion process were set up based on the calculated results of free-piston motion, which were validated with tested data from a running FPDLG prototype. According to the coupled parameters of these two models, we studied the exothermic properties of the FPDLG during the combustion process through iterative computation, and compared the simulation results of a TRE to a FPDLG with comparable and similar structural parameters. The results indicated that, combustion in the FPDLG lasted for a longer time compared with that in the TRE. While the heat release before top dead center (TDC), the isochoric heat release, and the heat release during the rapid combustion period were low, the post-combustion became more intense. Furthermore, the average temperature in the cylinder was generally lower, while became higher in the end of expansion stroke. In addition, the maximum combustion pressure was lower and lasted for a shorter time.

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.

AbstractAs an alternative to conventional engines, free-piston engine generator (FPEG) is a promising power generation system due to its simplicity and high thermal efficiency. One crucial technical challenge in the FPEG operation is the initial process of overcoming the compression force to achieve a certain speed which allows a stable and continuous operation, i.e. starting process. This paper proposes a novel method to start the engine by mechanical resonance. A closed-loop control model was developed and implemented in a prototype FPEG which was driven by a linear machine with a constant driving force. Both numerical and experimental investigation was carried out. The results show that once the linear motor force have overcome the initial friction force, both the in-cylinder peak pressure and the amplitude of the piston motion would increase gradually by resonance and quickly achieve the target for ignition. With a fixed motor force of 110N, within 0.8 second, the maximum in-cylinder pressure can achieve 12 bars, the compression ratio can reach 9:1, and the engine is ready for ignition. The results demonstrated that it is feasible to start the FPEG by mechanical resonance in a constant motor force in the direction of the natural bouncing motion.

Abstract Friction work in free-piston engines is expected to be lower than in crankshaft engines due to the elimination of the crank mechanism. In this paper, friction mechanisms were reviewed and compared between a free-piston and crankshaft engine of similar size. The main friction mechanisms were identified to be the piston assembly including piston rings and piston skirt, valve train system, the crank and bearing system for the CSE, and the linear electric generator for the FPE. The frictional loss of each friction mechanism was estimated and discussed. A Stribeck diagram was used to simulate the piston ring friction during hydrodynamic lubrication, mixed lubrication, and boundary condition. It is found that the FPE doesn’t show advantage on piston ring friction force over the CSE, and the frictional loss from the piston ring is even higher. While the elimination of the crankshaft system reduces the frictional loss of the FPE, and the total friction loss of the FPE is nearly half of the CSE.

Abstract Because of the late intake valve closure (LIVC), Miller cycle is kind of low temperature cycle which means it has the ability to refrain the knocking and produce higher thermal efficiency effectively in engines. As kind of clean energy and whose combustion products are perfectly environmental-friendly, ethanol has been considering as an ideal fuel substitution for a long time. Therefore in order to reduce NOx and other particulates emissions from engine, this paper presented the technical route which applied Miller cycle and ethanol to a turbocharged diesel engine. The simulation results shown that, Miller cycle did bring considerable improvements on reducing NOx emission in a certain extent. Comparing with the conventional Diesel cycle NOx emission value has been reduced in the range of 8.5–12.9% by applying Miller cycle. After applying turbocharger into Miller cycle engine model, NOx emission was slightly raised mostly back to the same figure as Diesel cycle produced. Moreover, taking ethanol as fuel also produced large reduction on NOx emission comparing with the conventional engine model which taking diesel as fuel, and the range of reduction was 5.2–8.5% which could be considered as a considerable improvement. However, when turbocharger added under the same situation the figure of the range of reduction was 4.53–5.16% which is slightly lower than without turbocharger. As for particulate emission in the engine, the situation which Miller cycle and turbocharger caused was opposite to the result of NOx emission that both Miller cycle and turbo-charged Miller cycle caused a larger amount of particulate emission probably due to the higher burning temperature.

In this paper, the influence of the gas exchange process on the diesel engine thermal overload is provided. Main components involved in the gas exchange process are discussed. The ambient conditions, the turbocharger performance, and the valve timing that affect the gas exchange process have been investigated. Experiments were conducted to simulate ambient conditions at different geographical locations and demonstrated a decrease in oxygen concentration in the exhaust as the humidity level in the air increased. Additionally, the effect of an inefficient turbocharger on an engine operating at part-load was also investigated. It was observed that an overly lean air/fuel mixture caused inefficient scavenging and the corresponding level of residual gas trapped in the cylinder increased. This resulted in partial combustion which could be observed as white smoke from the engine exhaust stack, therefore indicating the presence of unburnt fuel. Exhaust valve timing measurements showed that the cylinder with the highest wear rate had its valve closure timing 10 crank angle degrees after the cylinder with least wear rate. The exhaust valves were closed earlier than the designed condition which impaired the scavenging process and increased the level of residual gas trapped in the cylinder. This resulted in a reduction of the actual air-to-fuel ratio and high exhaust gas temperatures.