Florian Kock

The Free Piston Linear Generator - Development of an Innovative, Compact, Highly Efficient Range-Extender Module

The present paper introduces the Free Piston Linear Generator (FPLG) - a compact electricity generation unit, which is being developed at the German Aerospace Center (DLR). It is designed as a free-piston combustion engine with integrated linear generator. This combination allows for highly efficient conversion of the chemical energy stored in a fuel to electrical energy. By combining a two stroke combustion chamber, a linear alternator and an adjustable gas spring the engine design results in a compact package. In comparison to conventional combustion engines, additional degrees of freedom are available for controlling the combustion process. In this context efficiency advantages are expected due to the missing mechanical link to a crank which leads to flexibility in terms of stroke and compression ratio. Applied as a range-extender-unit, the system provides additional electric energy to electric vehicles in case of discharged batteries. In addition to this automotive application, the FPLG can be used as an auxiliary power unit for example in aircrafts as well as in decentralized combined heat and power plants (CHP). A step-by-step development strategy leading to the first overall FPLG system has been pursued. In the past years, the DLR has developed and experimentally verified several demonstrator prototypes for all necessary subsystems. Development systems comprising of several of the subsystems and (in some cases) of an auxiliary hydraulic actuator have been investigated. Free piston specific challenges have to be addressed, e.g. increased requirements concerning steadiness of the combustion process. In addition to the hardware development, active control of piston motion is a major technological challenge. Model based algorithms for this purpose have been developed and implemented on a real time system. Applied to one of the development systems, their performance is evaluated in experiments.

Flatness-Based High Frequency Control of a Hydraulic Actuator

This paper presents the design and implementation of a nonlinear feedforward control algorithm for a hydraulic actuator driven by a multistage servo valve. Combined with a conventional feedback control algorithm, high frequencies can be achieved even for large-scale strokes. In addition to the desired trajectory, the feedforward controller accepts the predicted dynamic load on the hydraulic actuator as an input. The performance of the control concept as well as the advantages of the load input are verified in simulations and experiments. Being exemplarily used for realizing a crankshaft-less test stand for free piston engines, the control algorithm is potentially suitable for further applications using hydraulic actuators in high frequency domain.

Experimental study of the in-engine variability of a free piston linear alternator

Mit der Verbreitung der Elektromobilitat, werden Hubkolbenmotoren kunftig haufiger als reine Range Extender, also zur Wandlung von chemische in elektrische Energie, verwendet. Dabei ist es fur die Betriebsstrategie des Hybridfahrzeugs vorteilhaft, mehrere Betriebspunkte nutzen zu konnen. Es ist das Bestreben verschiedener Forschungs- und Entwicklungsprojekte, diese unterschiedlichen Betriebspunkte mit jeweils hohem Wirkungsgrad darzustellen. Um in der Verbrennung moglichst variabel zu bleiben, wird am DLR ein Freikolbenkonzept verfolgt. Der Freikolbenlineargenerator kombiniert eine Verbrennungseinheit mit einer Linearmaschine, die die elektrische Energie auskoppelt, und einer Gasfeder, die als Energiespeicher, unter anderem fur die zur Verdichtungsbewegung benotigten Energie, eingesetzt wird. Ein solches System wurde in Form eines Funktionsdemonstrators aufgebaut und in Betrieb genommen. Der bezuglich Hub und Verdichtung variable Prufling verwendet eine innere Verbrennung im Zwei-Takt-Otto-Prozess, einen elektromagnetischen Ventiltrieb sowie eine Benzin-Direkteinspritzung. Im Paper werden die Ergebnisse von Messungen dargestellt, in denen elementare Betriebsparameter der Verbrennung variiert werden, darunter auch Verdichtungsverhaltnis und Ventilsteuerzeiten.