Asymmetric vibration characteristics of two-cylinder four-stroke single-piston hydraulic free piston engine

The structure and working principle of a two-cylinder four-stroke single-piston hydraulic free piston engine(HFPE) were introduced. The basic vibration equation of free piston assembly(FPA) was established based upon the energy conversion between the injected fuel and the friction together with the load. Both the theoretical and numerical results show that the vibration system of FPA is a nonlinear conservative autonomous system in one cycle. The FPA vibration is symmetric with constant amplitude when FPA is only driven by the compression pressure in the compression accumulator and that in the combustion chamber. When considering the friction and load, FPA could still achieve a stable vibration after a few cycles' adjustment whether the input energy is equal to the consumed energy or not. The vibration characteristics are different when FPA vibrates in the compression stroke and the expansion stroke, which is the unique feature of the single-piston HFPE.

Research on Coupling Transfer Characteristics of Vibration Energy of Free Piston Linear Generator

In order to clarify the mechanism and main influencing factors of the vibration energy coupling transmission with a dual-piston structure,a thermodynamic and dynamic coupling model of the free piston linear generator(FPLG)was established.The system energy conversion,vibration energy coupling transmission,and influencing factors were studied in detail.The coupling transmission paths and the secondary influence mechanism from in-cylinder combustion on vibration energy transmission were obtained.In addition,the influence of the movement characteristics of the dual-piston on the vibration energy transmission was studied,and the typical parameter variation law was obtained,which provides theoretical guidance for the subsequent vibration reduction design of the FPLG.

Fluid dynamic modeling of a free piston engine with labyrinth seals

Preliminary design and simulation of a free piston engine suitable for small-scale energy production in distributed energy systems is presented in this paper.The properties,particularly the properties of gas seals of the engine are simulated using a simulation program developed for this case,and the results are utilized in preliminary main design parameter selection.The engine simulation program was developed by combining and modifying the source codes of the simulation and calculation programs obtained from Helsinki University of Technology,Tampere University of Technology,and Lappeenranta University of Technology.Because of the contact-free labyrinth seal used in the piston,the efficiency of the motor is lower than the efficiency of a conventional motor with oil lubricated piston rings.On the other hand,the lack of bearing losses,and the lack of losses associated with a crankshaft system and a gearbox,as well as the lack of lubrication oil expenses,compensates this effect.As a net result,this new motor would perform slightly better than the conventional one.Being completely oil-free,it is very environmentally friendly,and its exhaust gases are completely free of oil residuals which are causing problems in normal gas motors.

Flat-type permanent magnet linear alternator:A suitable device for a free piston linear alternator

We proposed the flat-type permanent magnet linear alternator (LA) for free piston linear alternators (FPLAs) instead of the tubular one. Using the finite element method (FEM), we compare these two kinds of LAs. The FEM result shows that the flat-type permanent magnet LA has higher efficiency and larger output specific power than the tubular one, therefore more suitable for FPLAs, and that the alternator design can be optimized with respect to the permanent magnet length as well as the air gap.

Influence of shock attenuation on tailored operation in free piston shock tunnels

The free piston shock tunnel is a type of shock tunnel with high performance. For this type of tunnel, the influence mechanism of shock wave attenuation on tailored operation is explored by numerical simulation and theoretical analysis. By introducing the normalized velocity, the simple constraint equation for shock wave under the tailored operation is deduced. Moreover, the real gas effect is also taken into account in this equation. Based on the equation, the tailored operation of shock tunnels can be predicted with very few calculations. The present study shows that the change rate of the thermodynamic state of the gas behind the shock wave is inconsistent with the attenuation rate of the shock wave, which is the fundamental reason why the wind tunnel achieves tailored operation at a lower Mach number of shock waves. This lower Mach number of shock waves differs from the corresponding ideal value by a factor, which is about the square root of shock attenuation rate.

Sensitivity and effect of key operational parameters on performance of a dual-cylinder free-piston engine generator

The free-piston engine generator(FPEG)is regarded as the next generation of energy conversion system which may replace traditional engines in the future.The effect of key operational parameters like excess air ratio of input mixture and ignition position on the engine performance of a dual-cylinder FPEG was investigated,and their sensitivity was analyzed in this paper.The operating compression ratio of the system is susceptible to changes in excess air ratio and ignition position.At the same time,it decreases from 15.8 to 6.6 when excess air ratio increases from 0.85 to 1.15,but it increases from 6.1 to 13.3 as ignition position increases from 15 mm to 20 mm.The operating frequency and indicated power are more sensitive to changes in excess air ratio than ignition position.But it is the opposite for the indicated thermal efficiency and friction loss.Excess air ratio and ignition position have a quite similar influence on heat transfer.Therefore,from the perspective of system operation and performance,it is preferable to keep excess air coefficient slightly below 1.0.In contrast,when selecting ignition position,it is of great importance to comprehensively consider the risk of structural damage caused by the increase in the compression ratio and in-cylinder gas pressure.

Study on the Control Strategy of Free Piston Expander-Linear Generator Used for Organic Rankine Cycle Waste Heat Recovery

This study presents a free piston expander-linear generator (FPE-LG) for engine waste heat recovery.The control strategies based on displacement (control strategy A) and valve timings (control strategy B) are proposed.The operation and output characteristics with different control strategies are conducted.The results show that control strategy has significant effect on the FPE-LG performance.When FPE-LG operates with control strategy A,the operation frequency is determined by the intake pressure,advanced exhaust duration,fixed position point and external load resistance,whereas it is only related with the intake/exhaust time and advanced exhaust duration (ta) for control strategy B.The external load resistance has slight influence on the stroke length for these two control strategies.For control strategy A,when advanced exhaust duration is more than 40 ms,the stroke length rises with the intake pressure,whereas it first rises and then reduces when advanced exhaust duration is less than 40 ms.There exist the optimal external load resistance and advanced exhaust duration to achieve the maximum power output with control strategy A,whereas the average power output reduces with the increasing advanced exhaust duration when using control strategy B.Stable operation and high power output can be obtained by using different control strategies and adjusting relevant operation parameters.

An experimental study of a single-piston free piston linear generator

Free piston linear generator(FPLG)is a promising range extender for the electrical vehicle with unparallel advantages,such as compact structure,higher system efficiency,and reduced maintenance cost.However,due to the lack of the mechanic crankshaft,the related piston motion control is a challenge for the FPLG which causes problems such as misfire and crash and limits its widespread commercialization.Aimed at resolving the problems as misfire,a single-piston FPLG prototype has been designed and manufactured at Shanghai Jiao Tong University(SJTU).In this paper,the development process and experimental validation of the related control strategies were detailed.From the experimental studies,significant misfires were observed at first,while the FPLG operated in natural-aspiration conditions.The root cause of this misfire was then identified as the poor scavenging process,and a compressed air source was leveraged to enhance the related scavenging pressure.Afterward,optimal control parameters,in terms of scavenging pressure,air-fuel equivalence ratio,and ignition position,were then calibrated in this charged-scavenging condition.Eventually,the FPLG prototype has achieved a continuous stable operation of over 1000 cycles with an ignition rate of 100%and a cycle-to-cycle variation of less than 0.8%,produced an indicated power of 2.8 kW with an indicated thermal efficiency of 26%and an electrical power of 2.5 kW with an overall efficiency of 23.2%.

Numerical Simulation of a Spark Ignited Two-Stroke Free-Piston Engine Generator

A numerical program is built to simulate the performance of a spark ignited two-stroke free-piston engine coupled with a linear generator. The computational model combines a series of dynamic and thermodynamic equations that are solved simultaneously to predict the performances of the engines. The dynamic analysis performed consists of an evaluation of the frictional force and load force introduced by the generator. The thermodynamic analysis used a single zone model to describe the engine＇ s working cycle which includes intake, scavenging, compression, combustion and expansion, and to evaluate the effect of heat transfer based on the first law of thermodynamics and the ideal gas state equation. Because there is no crankshaft, a time based Wiebe equation was used to express the fraction of fuel burned in the combustion. The calculated results were validated by using the experimental data from another research group. The results indicate that the free-piston generator has some advantages over conventional engines.

Performance analysis of a 2-stroke micro free-piston swing engine

The micro free-piston swing engine （MFPSE） is a new structure, free-piston internal combustion engine. The dynamic model integrated MFPSE with a power generator and thermodynamic models in compression, power and scavenge processes based on the open thermodynamic systems were presented. A simulation was executed at given geometric parameters and initial conditions. The results manifest that the working principle of MFPSE is feasible.

Experimental study on compression stroke characteristics of free- piston engine generator

The compression stroke characteristics of free-piston engine generator were studied. The numerical model of the compression stroke was established based on thermodynamics and dynamics equation,and the leak loss,heat loss and friction loss were considered. Through solving numerical equations,the in-cylinder pressure of compression stroke under different compression ratios was calculated,energy transfer and conversion process was analyzed,and the calculated results were experimentally verified. The results showed that the actual effective output of electronic energy and the compression energy stored in the com-pressed gas accounted for about 70%. The compression energy gradually increased with the increasing com-pression ratio. When the compression ratio was more than 7. 5,the actual compression energy increased slowly and the energy error between simulation and test decreased.

空间斯特林热电转换技术在轨试验

为提高空间同位素电源的效率和功率,满足深空探测、载人登月和火星登陆等航天任务对航天器能源的需求,梳理了空间自由活塞斯特林热电转换涉及的关键技术,设计了天宫空间站空间自由活塞斯特林热电转换装置的在轨试验方案,在轨开展了斯特林热电转换试验,验证了空间微重力环境下双活塞自由运动的间隙密封、双活塞相位保持等关键技术,获取了空间环境下双自由活塞精确的运动相位保持及漂移特性、动力学与热力学强耦合特性等关键参数。在轨试验结果表明,试验装置在轨运行性能良好,热电转换效率达到24.72%,为未来空间高效同位素电源等不依赖太阳能的能源技术应用奠定了基础。

自由活塞发动机电动压气机优化及熵产分析

为了使设计工况下匹配的增压器性能更优从而提高系统能量利用率,针对自由活塞发动机匹配的电动压气机,采用遗传算法对原压气机进行了优化,研究了优化前、后压气机内部流场特性,以熵产分析的方法量化了3种不可逆因素导致的流动损失,并将优化前、后压气机熵产和涡度分布对比分析,进一步明确了压气机的优化策略.结果表明:叶轮和扩压器中湍流耗散、传热耗散和黏性耗散的比例近乎为6∶3∶1.不可逆损失主要集中在叶顶间隙和叶片压力面,损失来源为间隙泄漏流与部分主流掺混形成的反向涡团.压壳中湍流耗散占比超过85%,具体表现为局部损失和沿程损失.优化后压气机等熵效率提高了6%,流道内涡团的范围、幅值大幅减小,不可逆流动损失降低.上述工作揭示了压气机流场中不同耗散机制的不可逆因素对能量损失的影响机理,为压气机气动优化设计提供指导.

大尺寸自由活塞激波风洞自由流参数诊断方法

高焓激波风洞自由流具有一定程度的热化学非平衡,使得流场参数的确定存在困难。针对大尺寸自由活塞高焓激波风洞的总温2 700~4 700 K、总压5.6~20.3 MPa条件,采用非接触吸收光谱技术、接触测量技术等测试技术,结合多温度多组分数值模拟方法,共同诊断喷管名义马赫数10出口自由流的参数。结果显示,温度测量值与计算值最大偏差小于7.5%,速度测量值与计算值最大偏差小于5%,NO浓度测量值与计算中最大偏差小于12%。

宇航级自由活塞斯特林发电机回热器优化设计

为提升一台宇航级自由活塞斯特林发电机(FPSG)回热器性能,提出了一种基于响应面法的回热器优化方法。基于Sage平台构建了FPSG仿真模型,研究了回热器长度、孔隙率、丝径对FPSG输出PV功与热功转换效率的影响,得到了回热器的优化参数。结果显示:FPSG输出PV功随回热器长度、孔隙率、丝径的增大而减小;热功转换效率随回热器长度的增大而缓慢增大,随孔隙率的增大而先增大后减小,随丝径的增大而减小;优化后回热器长度为40 mm,孔隙率为85%,丝径为30μm;经实验验证,FPSG输出PV功提升16.9%,热功转换效率提升12.9%。

空间自由活塞斯特林发电机气体轴承研究

自由活塞斯特林发电机具有可靠性高,使用寿命长,可适应多种形式的热源等优点,在深空探测和探月工程领域具有广泛的应用前景。气体轴承技术作为发电机的关键技术之一,可使活塞实现无接触的支撑,并做高频往复运动,从而提高发电机的可靠性。为了研究发电机在实际运行过程中气体轴承的特性,建立了气体轴承耦合间隙密封的三维计算模型,通过CFD数值模拟对流场进行了求解,分析了在交变流动下气体轴承承载力以及耗气量的变化规律,发现随着压缩腔的压力变化,气体轴承提供的承载力呈周期性变化趋势,承载力与供气压力及偏心率呈正相关关系。增大气体轴承节流孔孔径,可使承载力变大,但一个周期下的耗气量将增多。对于所研究的千瓦级发电机,动力活塞采用直径为0.1 mm的节流孔,在交变流动下,气体轴承最小可以产生1200 N/mm的径向支撑刚度,符合设计需求。

基于人工神经网络的自由活塞膨胀机-直线发电机输出性能敏感性分析

为了优化用于余热回收的自由活塞膨胀机–直线发电机的输出性能,提出了一种基于人工神经网络预测的方法。以输出性能为指标,对进气压力、进气温度、进气持续时间、膨胀持续时间和排气持续时间这5个输入参数进行敏感性分析,并利用正交试验的极差分析结果进行对比验证。研究表明,输入因素影响程度由大到小分别为进气压力、进气温度、进气持续时间、膨胀持续时间、排气持续时间;最佳的输入因素参数配置为,进气压力0.3 MPa,进气持续时间40 ms,膨胀持续时间70 ms,排气持续时间70 ms和进气温度30℃,峰值输出电压可达8.96 V,峰值输出功率可达33.74 W。该方法适用于研究参数的敏感性,可为后续的参数优化与系统改进工作提供有价值的参考依据。

水蒸气对微型氢气HCCI自由活塞发动机燃烧和NOx排放特性的研究

为了解决微型均质压燃自由活塞发动机燃烧温度和压力过高、尾气排放氮氧化物浓度高的问题,采用数值模拟的方法,建立微型均质压燃自由活塞发动机的数值模型,并通过与试验结果的对比验证模型的正确性。在此模型基础上,以水蒸气为稀释剂,研究燃料稀释和进气稀释对微型氢气均质压燃自由活塞发动机燃烧和氮氧化物排放特性的影响。研究结果表明:水蒸气的存在会延迟着火时间;在燃料稀释方式下,当稀释率由0增加至0.1时,最高燃烧温度降低145 K,最高压力降低1 MPa,氮氧化物排放浓度降低69%,但同时平均有效指示压力下降了0.11 MPa;而在进气稀释方式下,最高燃烧温度和氮氧化物排放下降的幅度较小,且最高燃烧压力呈现先增加后减少的趋势。因此,在降低燃烧温度、压力和氮氧化物排放方面,燃料稀释的效果较好,但发动机的做功能力会随燃料稀释率的增大而降低。

补油参数对离子液体氢气压缩机的性能影响

为了研究离子液体压缩机的热力性能,建立了离子液体压缩机的能耗分析模型,分析了离子液体压缩机中的能量转化机制,将整个系统的能量损耗分为液压损耗、油缸损耗和气缸损耗,再进一步根据损耗发生部件和原因细分为溢油损耗、补油损耗、管道损耗、缓冲损耗、摩擦损耗、进气阀损耗、排气阀损耗和换热节能等7个部分,对单级离子液体压缩机进行了仿真计算,并对仿真模型进行了试验验证。结果表明,在所有压损中,溢油损耗、补油损耗、缓冲损耗是最主要的3个部分,通过调节补油压力和补油泵流量可改变这3个损耗的大小并影响压缩机的绝热效率;补油压力的变化主要影响溢油损耗值,当补油压力从1 MPa增大到6 MPa,溢油损耗增大了20.6%,绝热效率降低了10.57%;补油泵流量变化主要影响补油损耗值,当补油泵流量从20 L/min增大到120 L/min,补油损耗增大了35.8%,绝热效率降低了21.84%。研究可为离子液体氢气压缩机的高效设计和运行提供理论依据。

多级无油活塞式压缩机多场耦合特性分析研究

为提升无油活塞式压缩机的可靠性,满足轨交车辆对压缩机可靠性要求,通过对新研制的无油活塞式压缩机开展热动力学多场耦合分析研究,明确了吸、排气过程中的压力分布特性、不同曲柄角对应的温度分布特性、压缩机工作过程中的气体速度分布和流量特性,呈现了压力和温度发生较明显变化的区域,展示了气体速度场中的涡流分布情况。结果表明,压力波动主要存在于吸、排气口附近,最大压力约为0.51 MPa,其余部分压力均布;压缩机排气时的内部温度均匀分布,在吸、排气口附近存在最大温度梯度约为50 K;运用分析结果对压缩机的设计、可靠性优化进行理论指导,对试制样机在排气压力在1.1 MPa的工况下进行了1000 h疲劳试验,试验全程中重点部位的振动峰值为9.2 mm/s、温度峰值为148℃,满足了规定工况下保证长时间稳定性能运行的要求。研究结果可为无油活塞式压缩机的优化设计提供参考。