液压自由活塞发动机电控压缩冲程机理与能量回收的基础研究

Hydraulic free piston engine can work at the best combustion or lowest emission condition due to its variable compression ratio. But in the certain condition, the accurate compression ratio is hard to keep when the compression process is driven by the hydraulic system. This leads hydraulic free piston engine work unsteadily. In addition, part of the kinematic energy of the free piston assembly in the expansion stroke is converted into heat because of the throttling action, which reduces the energy usage ratio and output of the system. Based on the aim to make the compression ratio stable and improve the energy usage, this project puts forward a new hydraulic free piston engine structure with motor/generator which couples with the gear-rack to drive the free piston assembly and achieves synchronous operation. This new structure introduces the hydraulic accumulator and super-capacitor to the energy saving system. Multi-body dynamic model including internal combustion engine, hydraulic system, gear and motor/generator is set up. The characteristics of the compression and expansion stroke of the proposed hydraulic free piston engine with the motor/generator controlled compression stroke are studied. The energy flow and energy distribution during the compression and expansion stroke are analyzed. The main parameters which influence the compression ratio and operation characteristics are discussed. Through monitoring the load condition, pressure of the compression accumulator, state of charge of the energy storage unit, the conversion conditions between the hydraulic energy and the electric energy are recognized and established. And a control strategy based on the working condition discrimination and multi-energy storage conversion is put forward to make the compression ration stable and improve the energy usage.

液压自由活塞发动机（HFPE）具有可变压缩比，能根据工况使发动机处于最佳燃烧点或最低排放点。但在特定工况下，基于液压压缩装置的HFPE压缩比的精确控制较难保证，导致HFPE的工作不稳定。另外，膨胀冲程后期由于节流缓冲现象导致部分活塞组件的动能以热能形式损失掉，不利于能量的回收。针对HFPE压缩比的精确控制与能量回收问题，提出一种新型电动/发电机-机械传动机构耦合的动力驱动方案，以液压蓄能器与超级电容作为能量储存单元，建立燃烧腔压力-机械传动机构-电动/发电机-液压腔压力之间的多体动力学模型，研究电动/发电机的运行特性对压缩比和压缩过程稳定性的影响，分析电动/发电机与机械传动机构间的运动耦合方式及解耦控制，探讨压缩/膨胀过程中的能量流动和分配情况，研究液压能与电能存储的比例和相互转换对HFPE运动特性的影响，提出一种基于多能量存储方式转换的控制策略，为保证压缩比稳定和提高能量利用提供参考。

液压自由活塞发动机（HFPE）具有可变压缩比，能根据工况使发动机处于最佳燃烧点或最低排放点。但在特定工况下，基于液压压缩装置的HFPE压缩比的精确控制较难保证，导致HFPE的工作不稳定。另外，膨胀冲程后期由于节流缓冲现象导致部分活塞组件的动能以热能形式损失掉，不利于能量的回收。本项目针对HFPE压缩比的精确控制与能量回收问题，提出一种新型电动/发电机-机械传动机构耦合的动力驱动方案，以液压蓄能器与超级电容作为能量储存单元的能量回收方法。主要研究内容及创新：（1）提出一种电动/发动机-机械传动机构驱动压缩冲程的机理及系统优化方案，对电机驱动下的活塞组件的运动机理、压缩冲程的影响因素及压缩冲程的能量传递和分配进行研究；（2）针对膨胀冲程的特点，提出一种利用液压蓄能器及超级电容复合储能的能量回收方法，研究能量转换效率及能量回收机理，并深入探讨了压缩冲程与膨胀冲程间的耦合，为更好地进行能量回收提供支撑；（3）针对电-液复合存储能量管理系统和控制策略进行研究，探讨压缩/膨胀过程中的能量流动和分配情况，研究液压自由活塞发动机在不同工况下能量储存单元的储能状态及最优控制策略，保证在压缩比基本恒定的前提下，提高能量回收效率。