Investigations of the discharging of metal hydride beds for hydrogen-gasoline mixture operation of SI-engines

0360-3199/85 $3.00 + 0.00 Pergamon Press Ltd. © 1985 International Association for Hydrogen Energy.

Int. r. Hydrogen Energy, Vol. 10, No. 5, pp. 305--315, 1985 Printed in Great Britain.

INVESTIGATIONS OF THE DISCHARGING OF METAL HYDRIDE BEDS FOR HYDROGEN-GASOLINE MIXTURE OPERATION OF SI-ENGINES B. LUXENBURGERand W. MULLER University of Kaiserslautern, D-6750 Kaiserslautern, Federal Republic of Germany (Received 11 December 1984) Abstract--A mathematical simulation model is presented describing the discharge process of metal hydrides. The model is applied to FeTi-single tube and multiple tube beds and the results are compared directly to the results of experimental investigations. Furthermore, the most important engine parameters, with respect to the operation with metal hydride stored hydrogen, were determined for the nonsteady FTP 72-driving cycle. Minimum hydride quantities, necessary to supply the engine with hydrogen during the warm-up phase, were calculated for different starting temperatures and initial hydrogen contents of the bed.

INTRODUCTION Considerable time great efforts have been undertaken to investigate fuels for the motor vehicle traffic which are able to substitute the mineral oil products. In this connection the suitability of hydrogen as a non-polluting fuel for combustion engines often has been demonstrated. However, the use of hydrogen-operated engines requires the storage of hydrogen on board the motor vehicle. Besides the storage of liquid and gaseous hydrogen the bounding in metal hydrides is a useful method, with respect to safety aspects. All metals and alloys may be used which are able to store hydrogen reversibly. Basically it has to be distinguished between lowtemperature hydrides having hydrogen pressures of more than 1 bar at ambient temperature and high temperature hydrides needing temperatures of 250--300°C. The storage capacity of low temperature hydrides is up to 2.3 mass-% and between 3 and 7% for hightemperature hydrides. Hydride formation reactions are exothermal whereas additional heat is needed for desorption of hydrogen from the hydride [1, 2, 3]. Until now only few investigations about the dynamical behaviour of metal hydrides and the interaction of hydride behaviour and engine requirements have been published. The results described here may be a contribution to a better understanding of these mechanisms [41.

reproduce the properties of multiple tube beds. The investigations regard the iron titanium hydride FeTiH2 having storage capacities up to 1.8 mass-%. The engine investigations were executed for the concept of a hydrogen-gasoline mixture operation [5-8]. All important data were determined first for stationary engine operation conditions. Experimental and theoretical results enable general statements about the use of different hydride-types in the total range of engine operation. The parameters of unsteady driving conditions such as acceleration, deceleration, cold start, etc. were taken on the basis of vehicle test cycles, e.g. the FTP 72 test. Then it was possible to combine measuring results and the simulation model to investigate how the discharging of the hydride bed is able to supply the engine with hydrogen. Results for stationary engine operation may be taken from ref. [4]. CALCULATION MODEL The discharge process of metal hydrides generally depends on the following parameters: (1) properties of the hydrides, e.g. equilibrium pressure, heat of formation, reaction rate of dehydriding, heat conductivity, etc.; (2) the geometry of the hydride vessel; (3) the heating method, temperature and velocity of the heating fluid; (4) the hydrogen mass flow.

R E V I E W OF T H E INVESTIGATIONS A general review of the investigations is given in Fig. 1. The two components hydride bed and engine firstly have been treated separately. A calculation model describing the discharging of hydride beds was developed and applied to a single tube bed. From the comparison of calculated and experimental results the usefulness of the simulation can be checked. In the same manner the mathematical model has been expanded to

During the discharging some of these parameters are not constant, with respect to the time and position coordinates in the bed. The geometrical conditions are shown in Fig. 2. The hydride appearing as loose or compressed powder is enclosed between two concentric tubes. The heating fluid is flowing around the outer tube and the desorbed hydrogen is collected in the inner

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tube, designed as a porous filter. Consequently the main directions of heat t~ and hydrogen ai HE both are radial. For axial removal of hydrogen the inner tube may be dropped. In this case the main directions of the hydrogen flow and the heat flow are perpendicular to each other. The total condition of the hydride can be described by the local and temporal courses of temperature and charge condition and additionally by the actual hydride pressure.

I.

Radial hydrogen volume flow

II. Axial h~drogen volume flow

Fig. 2. Geometrical conditions of the calculationmodel.

Temperature field

From the following equation [14] OT Pb ' % b " T i

/02T

1 3T

O'T~

+ @(r, z, t)

(1)

describing the instationary conduction of heat in axial and radial direction the temperature field in the hydride bed can be calculated. Pb means the mass density and cpb the heat capacity of the hydride bed. The bed is considered as a homogeneous substance and consequently it is not distinguished between the heat conduction in the hydride particles and the interspace. Additional equations describe the heat conduction from the heating fluid to the hydride bed including the heat flow into the tube and the conduction in the tube wall. The effective heat conductivity Ab of the hydride bed is dependent on the hydrogen pressure corresponding to the equation ;% = fx" Ab.o(PH2), (2) f~ being a factor empirically determined as a function of the temperature level, the hydrogen volume flow and the properties of the hydride bed, e.g. the kind of hydride, its granulation distribution or additives of conduction-improving substances. The pressure-dependent parameter Zb,o was considered on the basis of measurements of Reilly et al. [10] and Toepler et al. [11].

307

INVESTIGATIONS OF THE DISCHARGING OF METAL HYDRIDE BEDS

The procedure is described in detail in ref. [4, 22].. from that the equilibrium pressure is strongly dependent The time and position dependent dissipation flow ~p on the hydrogen concentration of the hydride. Furthermore, the CPI's show a hysteresis, meaning higher is represented as an inverse heat source: pressures during hydrogen absorption and lower during ~ r , z, t) = - a l l ( x ) . ,h o(r, z, t). (3) desorption. In another method of representation called the Van't Hoff isochors, the equilibrium pressure is In this equation AH represents the enthalpy of for- plotted as a function of the reciprocal temperature for mation of the hydride at the charge condition x(r, z, t). constant hydrogen concentration x. As can be seen in the right part of Fig. 3 the results are straight lines which may be expressed by a firstKinetics of the desorption process order equation with the constants A(x) and B(x), const. The reaction rate of hydrogen desorption nio(r, z, t) A is the slope of the lines: is proportional to the hydrogen mass mn2 (r, z, t) stored A(x) in a volume unit of the hydride, the difference between lnpD(X, T) ~ - - + B(x). (5) the equilibrium pressure pv(r, z, t) and the hydrogen pressure Pn2(t) outside of the hydride particles and The absolute values of the concentrations-pressure isofurthermore proportional to an expression derived from therms are strongly dependent on the composition of the Arrhenius equation [1, 9] the alloy, its homogeneousness and possible impurities. An exact mathematical modelling requires the reliP D - - PH2 able knowledge of the pressure--temperature conrno(r, z, t) = rnH2 - ko Po centration behaviour of the hydride obtained in this X e x p ( - E a / R " r). (4) paper from experimental data. The constants A and B of equation (5) are determined The constants ko(x) and EA in equation (4) were intro- experimentally from the Van't Hoff isochors for difduced from ref. [12]. ferent charging conditions. Figure 4 shows as example the concentration pressure Concentration-pressure isotherms and Van't Hoff isotherms for desorption for temperatures of 24, 46 and 67°C of FeTi-hydride. The values were determined from isochors measurements in the single tube bed. The procedure is Figure 3 shows the relation between the equilibrium described in detail in ref. [4]. pressure and the charge concentration defined as hydrogen molfraction in a MeHx compound (Me = metal). Usually the equilibrium pressure is plotted logarithmically. These plots are called concentration-pressure isoloo therms (CPI). The curves show a distinct pressure plabar p 67°C ; teau at which the pressure doesn't change with changing hydrogen concentrations. Near the concentration value 50 x2 there is a second less marked pressure plateau. Apart

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308

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Enthalpy of formation and charge condtion The enthalpy of formation A H can be determined from measurements of the pressure-temperature behaviour of the hydride because it is proportional to the slope of the Van't Hoff-isochors A(x) in equation (5)

[4]

AH(x) = R . A(x),

model is able to calculate temporal and local courses of the temperature and the charge condition in the bed and additionally the time-dependent course of the hydrogen pressure until complete dehydriding. A detailed description of the physical equations and the mathematical solution is given in refs [4, 22].

(6)

R being the gas constant. The actual local charge condition x of the hydride MeHx defined as mole quantity of hydrogen per mole metal may be calculated by use of the following expression

Single tube bed

In order to test the mathematical model mentioned briefly in this paper, a comparision with experimental values is necessary. This was done firstly on the basis of a single tube bed for which the random conditions at both the water and the hydride side are correctly 2 definable. Figure 5 presents the schematic construction x(r, z, t) = Xo - 0.018. PMe ' r M e with the locations of the temperature measurement points. Along the axis of the pressure tube containing the × rhD(r, Z, r) dr, (7) hydride a porous metal filter is located, which collects where x0 means the initial charge condition, PM¢ the the hydrogen flowing radially to it. The 24 temperature density and rM¢ the volume fraction of hydride in the measuring points are located in four planes within the bed. Furthermore, a mass content of 1.8% hydrogen is charge of 18.1 kg FeTi-hydride. Resulting temperature profiles at the center plane of the tube are presented considered stored reversibly in a FeTiH2 hydride. later. The heat is removed during charging of the bed and Hydrogen pressure in the hydride bed supplied during discharging by a water flow system Assuming the flow in the hydride has no pressure surrounding the tube. Additional temperature sensors drop and defining a mean temperature for the hydrogen are located there. A schematic presentation of the whole test stand is between the hydride particles of given in Fig. 6. The hydrogen volume flow and the heating power are independently selectable within cerT,,(t) = ~ T(r, z, t) " dV, (8) tain limits. The hydride containing tube is connected with a systhe hydrogen pressure PIle of the hydride bed can be tem which enables either the activation of the hydride expressed using the equation of ideal gas behaviour [4] or its charging or discharging by the use of three-way taps. The hydrogen volume flow is adjusted by a pH2(t + At) micrometer valve (7) and its mass is determined by the use of a gas-meter (6), a manometer (4) and a pressureF ' p H 2 ( t ) + v ~b r h D ( r , z , t ) d V - r h n 2 ( t + At) reducing valve (5). At the hydrogen outlet the pressure = (9) is measured by two manometers with different effective 1 ~ rhD(r,z,t) d V ranges. During this discharging process the water circuit is temperature-controlled using electrical heating (10, 11) with and the heat evolved during charging is removed by rL a heat exchanger (13). The temperatures (15) at all F= (10) measuring positions are plotted by a multiple channel RH2 " T m " A t ' recorder (14). where rhH~ is the hydrogen mass per unit of time taken At this test stand discharging tests were executed at from the unit volume of the bed, rL is the volume constant values of water temperature and hydrogen between the hydride particles related to the total volume volume flow. Further experiments with variation of Vb, RH2 means the gas constant of hydrogen and At the temperature and hydrogen volume flow were necessary in order to complete the results of the previous meastime interval. By the aid of the basic equations (1) to (9) it is possible urements. Additionally model calculations were exto describe the dehydriding process of hydride beds ecuted and compared with the experimental results. As an example Fig. 7 shows the time dependent regarding both stationary and instationary conditions. The hydrogen mass flow rh H2(t) and the temperature of measured and calculated pressure and temperature the heating fluid may be defined for a bed of given courses with cyclic variation of the heating water temgeometry and properties of the hydride, e.g. on the perature between 25 and 50°C. The measured values basis of experimental data. Beginning with initial con- are marked as points and the corresponding calculation ditions of charge x0 and temperature To the simulation results as lines.

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INVESTIGATIONS OF THE D I S C H A R G I N G O F M E T A L H Y D R I D E BEDS

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First of all the hydrogen volume flow was held constant until its pressure dropped to a value of 2 bar. At this time the volume flow was reduced to half the initial value. As can be expected from the known dependence of the equilibrium pressure of hydrides on temperature approximately corresponding maxima occur for both. Additionally the temperature curves at different locations within the hydride are plotted, point 10 being near the surface of the outer tube and point 14 near the axis. Starting condition is a uniform temperature of approximately 25°C in the hydride. Immediately after starting the discharge process a time-variable temperature profile is founded in the hydride bed. The timedependent variation of the water temperature produces corresponding oscillations superposing the stationary courses. The calculation reproduces this dynamic behaviour satisfactorily.

Comparison with results of other authors Some results of simulation models have been published previously describing the mechanism of metal hydride discharging [9, 14-19]. However, only Fisher and Watson [19] correlate the time-dependent courses of measured and calculated results both of the hydrogen pressure and of temperature profiles within the hydride charge. In this paper the experimental data are taken

from Strickland [17, 18] who used their single tube hydride bed with 37 kg FeTi. Reliable calculations depend strongly on the exact knowledge of the pressure-concentrations temperature behaviour of the hydride. These data however are not published for the hydride charge used by Strickland. Hence comparable data were taken from refs [20] and [21] and were slightly modified to fit the measurements. For example, the hydrogen pressure determined experimentally was used for the initial conditions of temperature and charge. The courses of the concentration-pressure isotherms and of the Van't Hoff isochors are given in ref. [4]. On this basis the discharging experiments described in ref. [18] with hydrogen volume flows of 10, 15, 20, 25, 30 and 40 slpm were simulated by the calculation model. The results were in good agreement with experimental values. As an example in Fig. 8 a comparison of the calculation results of Fisher/Watson and those of the work presented here is shown. The hydrogen volume flow was constant 10 slpm and the inlet temperature of the water was 50 °C. The measured pressure course is best fitted by the simulation model of this work whereas both models agree largely regarding the temperature in the hydride near the tube surface at a radius of 76 mm. But the temperatures near the axis of the tube at a radius of 8 mm again are more precisely simulated by the calculations of this work.

311

INVESTIGATIONS OF THE DISCHARGING OF METAL HYDRIDE BEDS

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As can be seen in spite of the constant hydrogen volume flow and heating water temperature the temperature profile within the hydride is changing during the discharge process.

Multiple tube bed Single tube beds are less suitable for hydrogen storage in vehicles because of long distances for heat conduction in the metal hydride and hence a non-acceptable dynamic behaviour. For this in most of the practical cases multiple tube beds are used. The investigations presented in the following were executed at a waterheated FeTi bed with 23 tubes, as shown in Fig. 9. The hydride containing tubes of 40 mm diameter, are arranged irregular in the housing. The water for heating or cooling of the hydride is flowing parallel to the tube axis. All the single tubes lead to a hydrogen collecting tube. Hence the flow direction of the hydrogen is axial too. The charge material is precompressed FeTi-hydride mixed with 5% aluminum. This treatment increases the hydride density and its heat conductivity. The total hydride mass is 68 kg and its experimental determined

concentration-pressure isotherms are presented in ref. [41. With the assumption that all tubes of the bed contribute equally to the total hydrogen flow the simulation model can be restricted to one representative tube. This is of importance because of calculated time and computer storage space limitations. The comparison of measured and calculated results however showed that this simplification of the model was only possible if for this multiple tube bed an empirically determined factor

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B. LUXENBURGER AND W. MULLER

312

was introduced changing the inlet temperature of the heating water. This factor depends on the charge condition of the hydride and the water temperature itself. The discharging experiments were executed with either constant or variable hydrogen volume flow and water temperature. Some of the experimental and calculated results with cyclic changing hydrogen volume flow between 10.5 and 3 mg H21~-1 • s-1 constant in each case for 10 min are shown in Fig. 10. The water inlet temperature was held nearly constant at 53°C. Beginning with the start conditions with a hydrogen charge, mole concentration of x0 = 1.61 or 80.5% of the maxiumum charge , a hydride temperature of 54.5°C and a hydrogen pressure of P0 = 36 bar, the pressure drops quickly during the first phase due to the high hydrogen volume flow. Every time this volume flow is reduced, the slope of the pressure curve decreases and later on even a slight increase of the pressure occurs. As can be seen the calculated values of the simulation model are in good agreement with the measurements. On addition the water temperature differences between water inlet and outlet being primarily dependent on the hydrogen volume flow were fitted satisfactorily by the calculation. These results demonstrate the efficiency of the simulation model in calculating stationary and instationary discharging of multiple tube metal hydride beds.

to take the energy for discharging the hydrogen from the exhaust gas and/or from cooling water. But generally the heat contents of exhaust gas and cooling water are dependent on the engine operation condition as well as the hydrogen consumption. The hydride bed has to be planned for sufficient mass flow of hydrogen and sufficient pressure too for all operating conditions of the engine. Therefore, it is necessary to know the energy contents of exhaust gas and cooling water and furthermore the hydrogen consumption. A scheme of the test stand needed for these investigations is given in Fig. 11. The vehicle is equipped with a 6-cylinder Otto-engine of 2.8 litre stroke volume modified for hydrogen-gasoline mixture operation (1). The engine is operated exclusively with hydrogen when idle and at low loading, and with hydrogen-gasoline mixtures at part load. At full load the engine is provided exclusively with gasoline [5-8]. The engine is controlled by an electronic control unit adjusting the injection time intervals ti.2 and tiG of the hydrogen and gasoline nozzles and the spark timing IP dependent on the position of the accelerator pedal (9) and the engine speed n. The energy removed by the cooling water (5) is determined from the temperatures at radiator inlet Tcw,i and outlet Tcw,o and from the cooling water volume flow V~w. Additionally, the exhaust gas temperature T~xhand the engine speed n are recorded. The total hydrogen consumption of any test cycle is taken from a gas meter

Engine conditions of hydrogen-gasoline mixture (3). operation The enormous number of measuring data is collected

and stored by a computer (12) which also executes the further calculations. The graphical presentation of the mefisured and calculated results is done by the aid of a plotter (13). 30 12 The desired velocity course of the individual test cycle I-'l is given optically and followed by the driver of the test bar vehicle on the roller test stand. The measuring values S'$ i] Fare collected by the data aquisition system of the com,I II I, 7- puter in intervals of 0.5 s each time. On the basis of II ~ i t I I 6 2O t , ~L_ i L_. , ~__ 1 cold and hot starting conditions, respectively, the usual 3 ! J I I ! i i ' i I I I cycles of the Europe-test, the FTP 72-test and the HighI ! ! I ' i ' | ! J L J L_J L___ way-test were considered. Figure 12 presents the results for the example of the FTP 72-test. The vehicle is pre-conditioned for a time P =36 bar o 10 of at least 12 h at a temperature of 20°C. Apart from To= 54.5°C the vehicle velocity o the courses of the hydrogen conXo=l~61 sumption rriH: the energy flows of exhaust gas • m experiment Eexh,20, based qn a reference temperature of 20°C, and -calculatiOn cooling water E cw and the exhaust gas temperature Texh 0 are plotted. 4 6O A In contrast to the exhaust gas energy available •~ oc immediately after the cold start of the engine, the cool~, 50 • 2 ing water is capable of transferring heat until the driving distance exceeds 6.4 km (10.25 rain). o ~ 40 Hence the hydride bed can be supplied with exhaust 0 50 100 min 150 gas heat at once whereas on the basis of cooling water ttme Fig. 10. Hydrogen pressure and water temperature difference heating in a previous phase the hydride bed must be during dehydriding of the multiple tube bed at a temperature able to deliver the hydrogen needed from the engine without any heating. This time interval depends on the of 50°C and cyclic changing hydrogen mass flow. If hydrides are used for engine purposes it is suitable

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INVESTIGATIONS OF THE DISCHARGING OF METAL HYDRIDE BEDS . . . . . . . . . .

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8 Cooling fan l Engine operating with 9 Accelerator recorder hydrogen-gasoline mixtures IO Dual channel recorder 2 Roller test stand II Multiple channel recorder 3 Gas meter 12 Computer with data aquisition 4 Water-air heat exchanger 13 Plotter 5 Engine cooling water c i r 14 Magnetic disk cuit 6 Water meter with analogeous output 7 Thermocouples with temperature standard and amplifier Fig. 11. Test stand design for the determination of the engine conditions during vehicle test cycles.

starting temperature and the kind of the test-cycle. The figure shows distinct variations of both the hydrogen consumption and the exhaust gas and cooling water energies. By the aid of the engine conditions determined from these experimental data it was possible to derive criterions for the construction of hydride beds from the simulation model. Figure 13 shows a diagram demonstrating the possible interactions between the engine and the hydride bed. Considering a multiple tube bed the mathematical model was applied to the two cases of exhaust gas and cooling water heating on the basis of the test cycles mentioned before. The following random conditions were considered: tube diameter 40mm, length 800mm, void fraction 0.25, volume VRESof the water circuit 101, water volume flow Vw = 0.23 1 s -1, efficiency e of the heat exchanger with exhaust gas operation 0.5 and with cooling water operation 0.8. The concentration-pressure isotherms were taken from Fig. 4. During the calculations it was found out that due to

the low temperature of the bed the time after the cold start represents a critical phase of the test. Minimum hydride mass requirements being necessary to overcome this primary phase of a FTP 72-test are plotted in Fig. 14 dependent on the initial temperature, the initial hydrogen charge of the hydride x0 an on the kind of heating. The minimum hydride mass means that during the discharging process of this bed at every time the calculated hydrogen pressure requirements of 2.5 bar are realized. The results were calculated using an iterative procedure initially assuming a hydride mass. This mass was changed until the minimal hydrogen pressure during the discharging of the bed was just 2.5 bar. As expected more hydride mass is necessary if the bed is heated by cooling water in contrast to exhaust gas. For example, a hydride mass of 100 kg assumed as charged half with hydrogen will overcome the critical cold phase with exhaust gas heating beginning with an initial temperature of 16°C, whereas on the basis of cooling water a minimum initial temperature of 36°C is necessary. Besides the behaviour of the hydride bed during the

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cold phase of the engine the total radius of action of the vehicle is an additional important criterion of judgement.

beds intended for use with combustion engines the two components of bed and engine were first regarded heating by:

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INVESTIGATIONS OF THE DISCHARGING OF METAL HYDRIDE BEDS separately. A calculation model was developed describing the discharge mechanism of the hydride. The results were compared to measurements at a FeTi single tube hydride bed. Furthermore, calculations and experimental investigations were expanded to multiple tube beds, being of special interest for engine applications. The pressure-temperature concentration behavior and the heat conductivity of the hydride are of most important influence on the results of quantitative simulation of dehydriding processes of hydride beds. Variations of parameters describing the kinetic of dehydriding resulted in neglectible differences of the calculation results presented in this paper. Some engine conditions, e.g. hydrogen consumption, energy contents and temperatures of exhaust gas and cooling water were determined. Stationary tests were executed on an engine test stand and in (stationary) driving modes on a roller test stand. Combining the calculation model for the multiple tube hydride bed and the experimental data of the motor vehicle, minimum hydride masses were determined required to overcome the critical warm-up phase of the engine. The investigation presented in this paper considered some different FeTi-hydrides. The behaviour of other hydrides may be simulated in the same manner if the data needed for calculation are known. From the mathematical model it is basically possible to predict the major influences of constructive and hydride-dependent design of hydride beds without any detailed experimental investigation.

Acknowledgements--The authors thank Professor Dr.-Ing. habil. Hans May, University of Kaiserslautern, and the Daimler-Benz AG, Stuttgart, for the kind support of the investigations presented in this paper. REFERENCES 1. Bundesministerium fOr Forschung und Teclinologie, Neuen Kraftstoffen auf der Spur. Verlag T~V Rheinland, Bonn (1974). 2. Bundesministerium fOr Forschung trod Technologie, Auf dem Wege zu neuen Energiesystemen. Tell III: Wasserstoff und andere niehtfossile Energietriiger, Bonn (1975). 3. H. Buchner, Energiespecicherung in MetaUhydriden. Springer, Wien (1982). 4. B. Luxenburger, ExperimenteUe und theoretische Untersuchungen der Entladung yon Metallhydridspeichem far Verbrennungsmotoren bei Wasserstoff-Benzin-Mischbetrieb. Dissertation, Universitiit Kaiserslautem (1983). 5. H. May, F. Sch~ifer, U. Hattingen and B. Luxenburger, Abgasemissionen und Kraftstoffverbrauch eines 6Zylinder Otto-motors bei Wasserstoff-Benzin-Mischbertrieb. Statusseminar Kraftfahrzeugtechnik und Stra13enverkehr' des Bundesministers for Forschung und Tech-

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nologie, Bad Diirrheim, Verlag TUV Rheiniand, pp. 366374 (1979). 6. H. May and D. Gwinner, Possibilities of improving exhaust emissions and energy consumption in mixed hydrogengasoline operation. Int. J. Hydrogen Energy 8, 121-129 (1983). 7. W. Jordan, The influence of hydrogen addition to the air-fuel mixture on Otto-engine combustion. SAE-Paper 790678. 8. H. May, U. Hattingen, B. Luxenburger, D. Gwinner and E. Holl, New results of engine- and driving-tests of a 6cylinder Otto-engine in mixed hydrogen-gasoline operation. Proc. 5th World Hydrogen Energy Conference, pp. 1607-1619. Toronto, Canada (July 1984). 9. W. S. Yu, E, Sunberg and C. Waide, Modeling studies of fixed-bed metal-hydride storage systems. Proc. Hydrogen Economy Miami Conf. (THEME), pp. 621--643, Miami Beach, (1974). 10. J. J. Reilly and R. H. Wiswall, Hydrogen storage and purification systems--II. August 1972-June 1974, Brookhaven National Laboratory (1 August 1974). 11. J. Toepler, O. Bemaner and H. Buchner, The use of hydrides in motor vehicles. J. Less-Common Metals 74, 385-399 (1980). 12. U J. Swartzendruber et al., Numerical physical property data for metal hydrides utilized for hydrogen storage. Proc. 2nd World Hydrogen Energy Conf., pp. 1973-2011. Zurich (1978). 13. N. N., VDI-W~irmeatlas. Berechnungsbl~itter fOr den W~irmeiibergang. VDI-Verlag, Diisseldoff (1974). 14. D. L. Cummings and G. J. Powers, The storage of hydrogen as metal hydride. Ind. Eng. Chem., Process Des. Develop. 13, 182-192 (1974). 15. G. G. Lucas and W. L. Richards, Mathematical modeling of hydrogen storage systems. Proc. 4th World Hydrogen Energy Conf., pp. 1275-1288. Pasadena, California (1982). 16. D. B. Mackay, Automotive hydride tank design. Proc. Ist World Hydrogen Energy Congress, 7C/13-23. Miami Beach, Florida (1976). 17. G. Strickland, J. Milau and W. S. Yu, The behaviour of iron titanium hydride test beds: long term effects, rate studies and modeling. Int. J. Hydrogen Energy 2, 309-327 (1977). 18. G. Strickland and W. S. Yu, Some rate and modeling studies on the use of iron-titanium hydride as an energy storage medium for electric utility companies. Research publication of the Brookhaven National Laboratory (BNL) Nr. 50667 (April 1977). 19. P. W. Fisher and J. S. Watson, Modeling and evaluation of designs for solid hydrogen storage beds. Int. J. Hydrogen Energy g, 109-119 (1983). 20. J. J. Reilly and R. H. Wiswall, Formations and properties of iron titanium hydride. Inorg. Chem. 13,218-222 (1974). 21. N. N., 3. Meileusteinbericht der Daimler-Benz AG zum Forschungsvorhaben 'Entwickiung von Metallhydriden hoher Speicherkapazitiit' Nr. 7507, Stuttgart (1976). 22. B. Luxenburger, Experientelle und theoretische Untersuchungen der instation~ren W~irmeleitung in MetaU hydridschiittungen. Will be published in near future in 'Wiirme- und Stofffibertragung'.