Emergency Bnsking ofatrec Piston Energy Converter WEST Maitin

Emergency Breaking for Free Piston Energy Converters.

Martin West, Steve Long, Jiabin Wang, Chris Bingham and David HoweTHE UNIVERSITY OF SHEFFWELD

Departinent of Electronic ad Electrical EngineeringMappin Street, Sheffield, United Kingdom

Tel.: +44 / (0) - 114.2225195Fax: +441/ (0) -114.2225196E-Mail: mJ.west(dshef.ac.uk

AcknowledgementsThe authors wish to acknowledge the European Commission (EC) for support of the project 'Free pistonEnergy Converter (EPEC)', Contract N°: G3RD-CT-2002-00814, and thank the project partners (VolvoTechnology Corporation, Sweden; NOAX B. V., Netherlands; IINNAS B. V., Netherlands; IFP, France;ABB Corporate Research, Sweden; Chalner University of Technology, Sweden; and Royal Institute ofTechnology, Sweden) for permission to publish this work.

Keywords<Energy converter for HEY>, zautomotive application>>, fault handlngstrategy>, <(permanent magnetmoton>, (drnear drive>»

AbstractFree piston energy converters are a potential technology for future hybrid vehicles, as well as stationarypower generation applications. A candidate 2-stroke system comprises of two opposing combustionchanbers with a common piston rod, and integrated with a tubular permment magnet electrical machinefor the conversion of mechanical to electrical energy. A key issue for the ultimate adoption of suchsystems, however, is their robustness in the event of a fault to enable a safe shutdown, with minimal

mechanical or electrical damage. The paper considers system braking issues and the iLmportance of earlyfault detection. Results are presented to demonstrate the effectiveness of passive and active brakingtechniques for a range of dc-link supply voltage and operating output powers.

IntroductionIn a free piston energy converter (FPEC), chemical energy is converted directly to electrical energy,offering itself primarily as a power source in serial hybrid configurations for automotive applications [1-3]. The FPEC is a 2-stroke system comprising of two opposing combustion chambers [4-5] having acommon piston rod on which is mounted a linear permanent magnet translator, Fig. 1. Thus, motion isachieved by way of combustion in the two chambers occurring in anti-phase. This eliminates thecrankshaft, such that the piston movement is free, being controlled by a linear permanent magnet machine

via a power electronic converter, such that variable compression ratios may be achieved.

The proposed system utilises the latest technology in combustion research, viz. a Homogeneous ChargeCompression Ignition (HCCI) combustion system, running on diesel fuel. Ignition of the air-fuel mixturein a HCCI combustion system occurs when the gas pressure and temperature have reached a certain valuein or after the compression stroke. This is a highly unstable process, since, in order to achieve HCCI early

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fuel injection is required to ensure a homogenous distribution of fuel, with the risk that combustion couldoccur well before top dead centre (TDC), necessitating (state-of-the-art) control technologies.

The combination of HCCI combustion, the elimination of the crankshaft and the integration of a highpower dense permanent magnet electrical machine, aims to provide a very clean and highly efficient newtechnology for vehicle propulsion. The target of such a system is to provide a power density in excess of0.6kW/kg (excluding battery weight), and to have the potential of meeting Euro V emission limits whenscaled up.

This paper describes the tubular electrical machine and outlines the FPEC system, focusing on thedynamic braking requirement, which is one of the many issues tat need to be addressed throughout thedevelopment of such a system. The worst-case scenario, for which a high dynamic braking ability isrequired, occurs when the exhaust valve of the combustion chamber tat opposes the chamber undergoingcombustion, does not close. Hence, the translator is traveling at high velocity with no gas to compress, andhence no medium to absorb its high level of kinetic energy, and the energy of the pressure differencebetween the two chambers. The braking requirement is both a machine-design issue and a control issue,and more importantly, the ability to sense such a failure early enough to provide the largest range over

which to apply the braking force.

Inlet Teeth Coils Backiron

Piston /Port L

' \ ~~~~~~~~ExhaustShaft Permanent Valve

MagnetFig. 1: Free piston energy converter (FPEC)

Braking Requirements

In the event of a failure that necessitates the translator to be stopped to prevent damage to the FPECsystem, the electrical machine needs to provide sufficient braking ability. However, the machine has beenoptimised for a specified generating profile, which, to a certain extent compromises its braking ability.One common method of emergency braking employed with synchronous machines is resistive braking, [6-8], which also has the added benefit of being able to operate in the event that the failure includes the lossof the inverter/controller. The other means of braking, active braking, assumes that the machine controlleris still operational during the fault condition, since it utilises appropriate control of the synchronousmachine.

Wit an average velocity of 1 lm/s, the velocity and force profile of the FPEC system are as shown in Fig.2, and given that the translator mass is 9kg, the kinetic energy of the translator may be approximated as550J. In addition to the translator kinetic energy, there also exists the energy owing to the pressuredifference between the two combustion chambers. Thus, a total energy in excess of TkJ will need to beextracted under fault conditions to halt the translator.

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Fig. 2: Typical velocity profile of the free piston Fig. 3 Equivalent per phase circuit diagram underenergy converter resistive braking

Resistive BrakingFig. 3 shows the equivalent "per phase" circuit diagram of the linear permanent magnet machine underresistive braking. The electromagnetic force that can be produced by the machine under resistive brakingconditions is dependent on the load resistance RL, and is given by:

F=- k2V(R+RL)(R+RL ) + (eL)2

(1)

where v is the linear velocity, k the rms-flux linkage, R the phase resistance, L the synchronous inductanceand lo, the electrical frequency which is related to the linear velocity v by:

)e = VC -

Z'j(2)

Fig. 4 shows the braling force as a finction of linear velocity and load resistance. As is evident from Fig.4, for a given velocity v, there exists an optimal value of RL which yields maxiLmum braking force. Thisoptimal value can be found by differentiating F with respect to RL and solving for:

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As can be sn, if the load resiance RL varies with the linear velocty v^ accoding to (4) during raking,the maim possible baking firce Is constat and independent of v. Hene, as the speed is reduced, inorder to maintain the xmum raking force, the load resitance nds to be reuced proportonally, ashighlighted in Fig.5. It should be noted that when the linear velocty is below -1l0 m/s, the opdmal valueof the load reisance beonmes negafive, but in practice ca only be zero.

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Ass ing the mean pisto velocity and tat a fault is detcted at the point at which the exhaust valveshuld close, with 67% of the stroke remaining, then, if the resistnce is switched approprately such thatthe a lm b a force of 1 O per unit is available at a1l speeds, the acbievable ng as a unfionof linear velocity is shown in Fig. 6, an only -450J of the ttal traslator energy may be extractd priorto impact of lhe piston with the ylinder head at a velocity of 7A1 ml; thus reulting in catastrphicsystem failure. Althfough switching a load resistae bak agmanitthe tnsl velocity aod to (4) ispossibl;, it is much easy to use a fixed-value load resistance. Fig. 7 shows the variation of impact energywith fixed value of load resistance over 0.1 m available ain distance. As wil be seen, if a Died loadresistance is used, the opimal value is 0227 n which yields the inimum pact energy of 232 1Copared with the variable load resistae aking, uilising fixed RL does not sigifcantly compromisebraking ability. In any ease, however, it is aparn tat some form of active in alo required forthis fault scenaio.

Variation of braking force over velocity profile

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Active BrakingA dynamic FPEC model is employed to demonstrate the active braldng capability of the electricalmachine, Fig. 8. It comprises of the FPEC combustion system, the tubular permanent magnet machineand the associated controllers, and is created in Simulink. The PLEXIM'0 Sulink toolbox is used tomodel the electrical machine as a dqO non-linear model with a vector controller. In the event of an

emergency braldng, the velocity command is set to zero, and the velocity controller outputs a baking force(or q-axis current) demand to the electrical machine. This force demand is, however, limited to 2.7 timesof the rate force, which is the maximum force that can be applied during braking [9]. The two combustionunits compute the pressure and force due to combustion process for the given power demand. This forcetogether with the frictional force calculated from a dedicated fiction model is fed into the mechanicalmodel which govems the movement ofthe piston and the translator of electrical machine.

velocity

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To simulate the effects of a fault an exhaust valve failing to close, the simulation model assumes the faultis detected immediately, as the valve is ordered to close, and that no retarding force other than frictionexists. In practice, the pressure in the 'opened' cylinder would actually present a relatively small but non-zero retarding force due to the choked flow of compressible gas through the opening of the exhaust, butthis has been omitted in the following study, for simplicity.

Fig. 9 shows the electromagnetic force profiles that are required of the electrical machine over a numberof generating strokes for a constant power generation of 1.0 per unit, and the emergency braldng force thatmay be actively obtained in the event of a detected fault. The profiles are given for various converter de-link supply voltages. It can be seen that at high speed, due to the high back-EMF which is generated in thewindings of the tubular machine, the developed electromagnetic force is limited when operated from lowdc-link voltages. At maximum force, which can be obtained at almost all velocities for dc link voltage >400V, the machine is operated at its current limit (the vector controller maintaining the phase currents attwice of the rated value)

Fig. 10 shows the emergency braking force of the electrical machine when operating at various outputpowers for a dc-link voltage of 350V. Whilst some limitation in the braking force at high speeds isevident, this is negligible compared to other factors, such as the retarding force due to 'pumping losses'and prospect for detecting the fault conditions slightly earlier.

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The position of the Strlator, plottd against velocity, is shown in Fig. 11 for dc-link voltages of 20OV,350V and 500V, and a power of 1.0 per unit. It can be seen that in all tbree cases, if braking is appliedimmediately after the fault being detected, the tranlator will be stopped before reaching TDC. It is alsoeident that limited benefit is gained with regard to the braking of the ranslator for dc-link voltages >350V; which again highlights the fact that early fault detection has a major influece on the brakingcapability.

70

60

50

40

30

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Fig. 12 Prototype ofa free-piston energy converter

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Prototype ofFPEC

A prototype of FPEC, comprising a linear electrical mahine and two combustion units as shown in Fig.12, has been constructed. The system is currently under testing and its braling perfonnance will bereported in the future.

Conclusion

The paper has identified a number of potentially catastrophic fault mechanisms tat can arise during theoperation of FPECs, and describes an investigation into passive and active electrical braling mechanismsto accommodate such fault conditions. A candidate FPEC with an integrated tubular PM synchronousgenerator has been used to demonstrate that either passive or active braking, or a combination ofboth mayultimately be necessary, and to highlight the fact that due to the very high combustion forces which aregenerated and the floating TDC characteristic of FPECs, early detection of a fault condition is essential ifcatastrophic damage is to be avoided. It has also been shown that the electrical machine impedance andthe dc-link voltage have sigpificant impact on the ability to brake the translator. Ulimately, this impactson the size and rating of the electrical energy storage medium (capacitor, battery, flywheel etc) requiredfor an FPEC system.

References[1] P. Van Blarigan, N. Paradiso, and S. S. Goldsborough, Homcogelneous charge compression igltifion with a free

piston: a new approach to ideal Otto cycle performance, SAVE Paper 982484 (1998), pp. 89-106[2] P. Van Blarigan, "Advanced internal combustion electrical generator", Proceedings of the 2002 DOE Hydrogen

Program Review: NRELJCP-610-32405, 2002.[3] D. Carter, E. Wechner, "The free piston power pack: sustainable power for lhybrid electric vehicles," SAE

Initernational, 2003, paper 2003-01-3277.[4] J. Fredriksson, I. Denbiatt, "Simulation of a two-st-oke free piston engine,", SAE Paper 2004-01-1871.[5] A. P. Kleemann, J-C. Dabadie, S. Heriot, "Conputiational design studies for a high-efficiency and low-

emission free piston eigine prototype, SAE Paper 2004-01-2928.[6] J. Wang, G. Jewell and D. Howe, "A geneml framework for the analysis and design of tubular linear permnanent

magnet maclines," IEEE Transactions on Magnetics, Vol.35, No.3, May 1999, pp. 1986-2000.[7] N. M. Rash, E. Spooner, D. Howe, "Pennanent mnagnet alternator for use as an electrodynamic rilway brake,"

Proc. IEE, E/ectrica/ Machines - Design and Applications, 254, 1985, pp. 208-212.[8] H. Yamaguchi,Hii. Osawa, T. Watanabe, H. Yamada, "Brake control characteristics of a linear synchronous

motor for ropeless elevator," P-oc. IEEE AMC'96-MIE 1996, pp. 441-446.[9] J. Wang, D. Howe, "A linear permanelnt magnet generator for a free piston energy converter", Proceedings of

Ilnterlatiolnal Electrical Machines ald Drives Conference (tEMDC2005), San Alntonio, TX, USA, 2005

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