piezoelectric injection systems

13

Piezoelectric Injection Systems

R. Mock and K. Lubitz

13.1 Technical Motivation for Direct Injection Systems

The origin of direct injection can be doubtlessly attributed to Rudolf Dieselwho used air assisted injection for fuel atomisation in his first self-ignitionengine. Although it became apparent already at that time that direct injectionleads to reduced specific fuel consumption compared to other methods of fuelinjection, it was not used in passenger cars for the moment because of itsdisadvantageous noise generation as the requirements with regard to comfortwere seen as more important than a reduced specific consumption.

Especially the cold start Diesel knock proved to be unacceptable for a longtime. Although one knew at least on a theoretical basis that pilot injectionswould be the appropriate countermeasure, one was not able to realise themwith the available injectors. This situation changed fundamentally in 1989when Audi launched the first TDI engine with Diesel direct injection in whichthe problem of pilot injection was solved. Nonetheless, the combustion noisecould not yet be reduced to the level of a gasoline engine. The reason for thiswas that using the pump-injector principle, the ratio of the dosed fuel amountas well as the timing between pilot and main injection could not be adjustedwith the required flexibility.

It was not before Diesel high pressure pumps became available that theway was paved for the “common rail” technology. In 1997, the high volumeproduction of “common rail” injectors was launched using electromagneticDiesel injectors, in 2000 the first vehicles left the production plant whichwere equipped with piezoelectric “common rail” injectors [1–4] (see Fig. 13.1).Above all, it was the high flexibility of fuel dosing which permitted a variabilityof fuel injection in form of several (up to seven) split quantities – somethingwhich had not been considered as possible till then. This technical featurealong with the high dosing precision and spray quality broke new ground fora triumphant success of Diesel power train technology which at least in Europehas by far outnumbered the Gasoline vehicles viewing the number of newlyregistered cars.

300 R. Mock and K. Lubitz

Fig. 13.1. Photographic view of the first generation PCR injector

13.2 Description of Advanced Direct Injection Systems

13.2.1 Diesel: Common Rail Injectors (2,000 Bar)

The first generation of piezoelectric “common rail” Diesel injectors (abbrev.:PCR) which in September 2000 was employed for the first time in a seriesproduction vehicle uses a servo-hydraulic principle instead of a direct driveby the piezo actuator to move the injector’s nozzle section (Fig. 13.2).

For that purpose, the high pressure Diesel fuel (0 < p < 1, 500 bar) actsin two ways on the nozzle section. By directly feeding the Diesel fuel to thebottom part of the nozzle, a constant force is exerted on it. The “controlpiston” which forms the upper part of the nozzle section is pressurised by thefuel which is fed via two throttles into a control chamber which is closed bya servo valve. As long as the servo valve is closed, the pressure in the controlchamber is equal to that acting on the lower end of the nozzle. As the crosssection area of the control piston is slightly larger than that of the lower end ofthe nozzle, a resulting force is generated which presses the nozzle into its seat.

13 Piezoelectric Injection Systems 301

Fig. 13.2. PCR Injector: pressure distribution in the closed state (upper view) andin the open state (lower view) of the servo-hydraulic injector drive

To trigger an injection, an electric voltage is applied to the piezoelectricactuator. Via a tappet the expanding actuator lifts the servo valve off itsseat and a ring-shaped gap opens through which the fuel inside the controlchamber can escape into the low pressure drain (p = 1bar). As a consequence,the pressure above the control piston drops almost instantaneously. As thepressure on the lower end of the nozzle remains constant, the direction of theresulting hydraulic force on the nozzle is reverted so that the nozzle is liftedoff its seat and thereby the nozzle outlets are released through which the fuelis injected into the combustion chamber. In addition it is atomised because ofthe high system pressure of up to 1,500 bar.

A spring between the control piston and the nozzle part on the one handacts as an impact damper, on the other hand it keeps the nozzle in the closedstate as long as no rail pressure is present – as it is, e.g. the case when theengine is turned off.

When the voltage across the piezo actuator is switched off, it contracts.Driven by a spring force, the servo valve is moved back to its seat. The


ring-shaped gap is closed again so that via the two throttles the full sys-tem pressure is built up again inside the control chamber. As soon as the fullpressure is reached, the resulting force acting on the nozzle and control pistonagain points towards the seat. The nozzle moves into its stationary positionand the nozzle outlets are closed.

Via the diameter ratio of the two throttles, the opening and closing veloci-ties of the nozzle can be adapted to the requirements of the respective engine.

That such a servo principle is used instead of, for instance, a direct driveis due to the fact that the mechanical work that has to be applied in or-der to open the nozzle exceeds the capabilities of the actual piezo actuatorby one order of magnitude. This circumstance is discussed in more detail inChap. 13.3.1.

13.2.2 Gasoline High Pressure Direct Injectors (200 Bar)

A fuel atomisation, the quality of which being sufficient for a good combus-tion, is reached in the case of gasoline at pressures of about 200 bar. As aconsequence, it is feasible to realise gasoline high pressure injectors actuatingthe nozzle by a direct drive (Fig. 13.3).

In the case of the injector for which series production has been startedin 2006 [5–8], the functional unit of piezo actuator, the nozzle and hydrauliccompensator is mounted into a double-walled, cylindric housing. The fuel isfed to the nozzle via the gap between the inner and the outer tubes, thusacting as a coolant for the inner injector parts, mainly the actuator unit.

The top cap of the piezo actuator is supported by the hydraulic compen-sator, whereas the bottom cap is in contact with the valve needle. Duringthe mounting procedure, the needle is loaded by a spring which generates aforce of about 200 N by which the needle is pressed into the seat. After theinstallation and the application of the electric contacting to the piezo actu-ator, the hydraulic compensator unit is mounted into the injector housingand loaded by a force of about 100 N against the piezo actuator. Thereby theforce between the needle and the seat is reduced by just this number so that100 N remains as a resulting force which retains the nozzle in its seat againsta maximum fuel pressure of 200 bar.

The fuel enters the gap between the needle and the cartridge at the lowerend of the injector housing across two boreholes in the inner tube of the hous-ing. To prevent the piezoactuator from being wetted by the fuel, a bellowbetween the upper end of the cartridge and the needle acts as a flexible sealin the axial direction. Its effective pressure-loaded cross section area is di-mensioned such that the hydraulic forces acting on the bellow and the needlecompensate in essence.

As the piezo actuator is electrically charged, it expands against the forcesby the compensator and the needle. As the compensator is conceived suchthat it forms a hard bearing for a fast switching motion, the valve lifts off


fuel inlet

piezoactuator

hydrauliccompensator

needlegroup

Fig. 13.3. Photographic view and CAD cross section of a piezoelectric high pressuregasoline injector

due to the electric excitation of the actuator after the closing force has beenovercome. A ring-shaped gap opens between the needle and the seat throughwhich the fuel is injected and atomised.

As soon as the desired fuel quantity is reached, the piezo actuator is elec-trically discharged and as a consequence contracts. The preload spring thenpulls the needle back to the seat. The ring-shaped gap is closed, and theinjection cycle is complete.

13.2.3 Gas: CNG/Hydrogen Injectors (<200 Bar)

Considering the CO2 problem and the global warming, gaseous fuels increas-ingly become the focus of attention for developers. They either exhibit a


greatly improved CO2 balance than gasoline or Diesel, or like hydrogen theycan completely be generated by means of regenerative energy sources andaccordingly are CO2 neutral.

At present, the development focuses on injectors which take into accountthe specific requirements of the injection of gaseous fuels [9]. The fundamen-tal difference to liquid fuels is the completely different flow characteristics ofgases. To dose a gaseous fuel which contains the same amount of energy asliquid fuels, one has to inject a considerably larger volume. At a pressure of,e.g. 20 bar the injector has to deliver a throughput quantity which exceedsthat of gasoline by a factor of fifty in order to obtain the same engine power.The injection timing and the gas pressure, however, are defined by systemrequirements so that the larger volume throughput has to be realised by anincreased cross section of the orifice. As the diameter of the employed nozzlecan be increased only to a limited amount, there remains only an increaseof the needle lift to a value of 300 µm to guarantee the throughput quantity.However, using the present piezo actuators, a stroke of 50 µm cannot be ex-ceeded. Therefore a hydraulic transformer is mounted between the actuatorand the nozzle which at the same time acts as a thermal compensator. Theworking principle of this injector including the hydraulic transformer elementis shown in Fig. 13.4.

gas inlet

piezoactuator

hydraulictransformer

needlegroup

Fig. 13.4. Photographic view and CAD cross section of a piezoelectric CNG injector


13.2.4 Future Perspectives of Piezo Injectors

Unlike their electromagnetic counterparts, piezoelectric injectors provide arather well defined sensor signal. This is simply due to the fact that apiezoelectric material always shows the piezo sensor and the actuator effectat once. Monitoring the time dependent voltage and the current of a piezoactuator, which is employed in an injector, yields a lot of information aboutthe inner states of the injector as well as the surroundings of the injector.Thus using model-based control strategies which treat the injector as a partof a mechatronic control system it is possible to detect important informationabout the switching process such as opening and closing time of the servovalve in the PCR by recreating the inner state variables which describe theeffect under consideration. Moreover, it appears to be possible to even em-ploy a piezoelectric injector to determine quantities related to the combustionprocess just by isolating their impact on the injector function by a model-based control approach. At present, this is work in progress and the firstsubstantial success in improving the existing products with respect to injec-tion precision, environmental friendliness and the reduced system costs provesthat the mechatronic system approach is one which is worth to be pursuedover the next years.

13.3 Technical Specifications of Injection Systems

13.3.1 Diesel: Common Rail Injectors (2,000 Bar)

Due to the different working principles of the three injector concepts presentedbefore, the technical requirements which have to be established for a suitablepiezo actuator greatly differ. Nevertheless, the basic procedure by which themechanical and electric characteristics are specified is the same in all the threecases. For the PCR, this procedure will be explained in some detail.

By the functional principle of the PCR, it becomes obvious that the open-ing and closing event only occurs when the pressure inside the control chamberhas reached its respective end value. Particularly during the opening event,the pressure drop inside that chamber has to be effected sufficiently fast sothat the movement of the control piston and the valve needle begins after aminimum time delay. As this pressure drop is substantially influenced by theflow characteristic of the servo valve, the piezo actuator has to fulfil two tasksin this case:

1. It has to provide sufficient stroke so that the servo valve is fully unthrottledin the opened condition (i.e. only the fixed throttles in the high pressurefeed determine the flow rate)

2. It has to generate enough force to open the servo valve sufficiently fastagainst the pressure in the control chamber


To guarantee the first requirement, the flow cross section area released byit has to be significantly larger than that of the two fixed throttles which actin series with it. In case of the PCR the flow cross section at a stroke of 35 µmis roughly twice as large as that of the largest fixed throttle – therefore justsufficient to fulfil the requirement.

The force on the servo valve built up by the chamber pressure can becalculated from the pressure loaded surface of the valve at a given pressure.Taking into account also the force of the closing spring, the actuator has togenerate a force of about 400 N at maximum system pressure before the servovalve is moved at all. Detailed simulations show that in order to obtain asufficient dynamics of this process one indeed needs about 1,000 N!

The piezo actuator’s maximum force, however, is not available during thecomplete actuation process. Due to the limited stiffness of the mechanicalconnections (weldings, etc.), part of the mechanical energy generated by theactuator is consumed by the mechanical work needed to expand the housingand its components. Provided that the stiffness values of housing and me-chanical connections are optimised, the actuator has to exhibit a blockingforce of about 2,100 N – at a blind lift of 35 µm this corresponds to a springstiffness of 60N µm−1 – to guarantee a net blocking force of 1,100 N for theswitching cycle.

From the electric point of view, the second requirement means that theactuator has to be electrically energised with a rise and fall time of 100 µs tothe maximum field strength of 2 kV mm−1. Regarding technical boundariesand system costs, these electric characteristics have to be realised at moderatevoltages (<200V) and currents (<30A). The consequence of this is that thepiezo actuator has to be conceived as a multilayer stack with an appropriatenumber of piezoelectric layers.

13.4 Material Aspects

Motor injection systems in the past were realised solely with electromagneticdrives. In order to replace them by the completely new concept of a piezo-electric drive, the latter must have comparable production costs, life time andreliability and in addition a remarkably better functionality. It is described inthe following section how a specific material design, i.e. matching of construc-tion form, material composition and fabrication technology can lead to sucha solution.

If we consider the functional principles of Diesel injection valves shownin Sect. 13.2.1, we can summarise the specifications for the piezoelectric driveitself by the following conditions: Drive voltage of up to 200 V DC, drivefrequency of up to 100 Hz, piezoelectric elongation of 40 µm, blocking force ofabout 2,000 N, pulse length without rise and fall times 50–100 µs, operatingtemperatures from −40 to +160◦C, where the maximum temperature is givenby the external motor heating and by self-heating, and finally a minimum lifetime of >109 cycles.


The solution is a piezoceramic prismatic stack with dimensions of 7× 7×30mm3. This stack is manufactured as a monolithic component in multilayertechnology and has an internal structure as shown in Chap. 4.5, p. 123. Onthe base and top of this part, where force and elongation are transferred, al-ways a 1–2 mm thick piezoelectrically inactive zone is incorporated in order toavoid friction and mechanical tensile stresses by piezoelectric transverse con-traction. The active part of the stack consists of some hundreds of 80–100 µmthick ceramic sheets lying in between thin AgPd-electrodes having alternatingpolarity. They are driven by a voltage of 200 V or an electric field strength of2 kV mm−1. In combination with the effective d33∗ the active height of thestack determines the elongation and the cross section the force of the actuator.

During the rise of a pulse the actuator mainly generates force and afterthe opening of the valve, a no-load deflection. The specification values requirean effective d33∗ of 750 pm V−1 at 2 kV mm−1. Such large values are onlyachieved by soft-PZT. A further boundary condition, the maximum opera-tion temperature, in addition demands a material with Curie temperatures>300◦C. In consequence only low doped soft PZT can be used, for examplewith Rare Earths on A-place or Nb on B-place of the perovskite.

Using standard ceramic fabrication technology such materials have sinter-ing temperatures of 1,250–1,300◦C. The manufacturing of multilayer stackswith AgPd-70/30 inner electrodes, however, needs a reduction of the sinteringtemperatures to values <1,130◦C. This can be achieved by the applicationof high energy milling whereby the primary particle size before sintering canbe remarkably reduced [10]. Particularly the sintering temperature can be re-duced by small additions of acceptor elements like K or Ag (see Chap. 4.3 and4.5) by which the soft PZT does not change its properties like low mechani-cal quality factor or high large signal d33. Looking at Figs. 4.22 and 4.23 ofChap. 4.5 we can see that using AgPd inner electrodes the Pd is forming aPbPd interface between PZT and electrode and that Ag may diffuse into theceramic grain volume. In combination with a fine adjustment of the dopingconcentrations, the reduction of the sintering temperature can be achievedwhich is necessary for the application of the multilayer technology.

A first piezoelectric multilayer stack was realised already in 1983 as adriving element of a needle printer, admittedly using a low sintering complexceramic with a Curie temperature of 150◦C [11]. The first multilayer stackfunctioning in an injection valve with AgPd inner electrodes, low doping anda Curie temperature of 320◦C however was presented not until the end of thenineties [12,13].

The stack technology is similar to that described in Chap. 4.5 but requirestwo essential modifications: The first one relates to the polarisation in com-bination with the driving conditions, the second one concerns the method ofcontacting the stack.

If a free stack is driven by an above described pulse sequence, it splitsafter some hundreds of cycles. Although the lowest resonance frequency ofthe actuator is about 50 kHz and electrical pulses of 100 µs imply a quasistatic


driving condition, in this dynamic mode during the falling pulses mechanicaltensile stresses occur which by undercritical and critical crack growth destroythe stack very fast. Therefore a constant compressive stress must act on thestack in such a way that on no account tensile stresses can occur within thesample. That will be realised by tube springs in which the stack is weldedunder a compressive stress of 10 MPa [14].

The polarisation takes place within the tube spring. The superimposedcompressive stress remarkably reduces the remanent relative elongation com-pared with a free poling. The number of domains oriented parallel to the pol-ing field is lowered due to the ferroelastic influence of the mechanical stress.During driving and under an electrical field of 2 kV mm−1 many domainsare switched in field direction and the stack elongation reaches the necessaryamount.

This kind of polarisation has a further beneficial effect. In Fig. 13.5, thethermal expansion coefficients are plotted of unpoled and poled piezoceram-ics parallel and perpendicular to the poling direction [15]. In poling direction,against all normal experience the coefficient is negative. This is due to adecrease of the spontaneous polarisation and of the anisotropic lattice distor-tion with increasing temperature. The stack polarisation under mechanicalcompression stress creates a domain structure similar to that of an unpoledsample. Thus the thermal expansion coefficient is positive and comparable tothat of the surrounding metallic tube spring.

The most important point which at all has made possible the applicationof piezo stacks as injection valve drives and which guarantees life time andreliability, is the application of quite specific electrical contacts. They mustcontrol and eliminate the detrimental influence of in principle originating

20 30 40 50 60 70

α = +7 x10–6 K–1

α = +2 x10–6 K–1

α = –5 x10–6 K–1

temperature [�C]

unpoled

Pr

Pr

–4 x10–4

–3 x10–4

–2 x10–4

–1 x10–4

0

1 x10–4

2 x10–4

3 x10–4

4 x10–4∆I/I

Fig. 13.5. Thermal expansion coefficients of unpoled and poled samples paralleland perpendicular to the remanent polarisation


3

2

1

00 1 2

S [1

0–3]

E [kV/mm]

Crack 2 + 3

Crack 3 at 1140 V/mm

Crack 1 at 960 V/mm

Crack 2 at 1130 V/mm

Crack 1

inactive contact zone

Fig. 13.6. Formation of delamination cracks during poling inside the inactive con-tact areas

cracks. Within the contact areas of the stacks (see Fig. 4.21, Chap. 4.5) evenduring polarisation and beginning operation tensile stresses arise althoughthe active stack area is under compression stress by the tube spring. Thesestresses can be calculated, are highest in the middle of the stack and leadto delamination cracks in the contact area in general between PZT and theelectrode. In Fig. 13.6, the evolution of such cracks during poling is measuredby sound emission analysis and also their position is shown.

These fissures act as mechanical stress relievers so that their number aftershort operation time remains constant, depending on design, technology andespecially on the strength of the interface PZT – inner electrodes. The cracktips stop permanently within the transition region to the active stack areawhich is under compression stress. Also some design variations are knownwhere a series of artificial slits within the contact areas of the stack is incor-porated during the fabrication having the same stress relief function [16, 17].During operation in the region of the relief cracks a large stroke can be ob-served corresponding to the total elongation of the adjacent undamaged stacksegment. Locally relative elongations of more than 100% may occur which,after short operation time, unravel the originally continuous metallic contactstrip. In order to provide all these single stack elements with electrical energy,a multiple contact design is necessary. That can be realised for example by awire harp or by soldered metal wool.

With the sum of the described measures, it succeeds to manufacture reli-able stacks with life times of >109 cycles. Meanwhile, they are in use in carsa millionfold. It is remarkable that the absolute stack length as well as theelongation remain constant within narrow limits during life time. This meansthat both the remanent domain structure and the number and kind of do-mains switched up to saturation during each pulse remain unchanged. Thisalso shows the future potential of piezoceramic stacks in the area of actuatorapplications if material and technology are modified specifically.


References

1. D. Baranowski, W. Kluegl, D. Schoeppe, in Simulation and Design Optimizationof a Common Rail Piezo Injector for Passenger Car DI Diesel Engines FuelInjection System, London, GB, Dec 1–2, 1999 (IMechE Seminar Publication,London, 1999)

2. K. Egger, D. Schoeppe, Diesel Common Rail II – Einspritztechnologie fuer dieHerausforderungen der Zukunft Fortschritt-Berichte VDI Reihe 12, Nr. 348,Band 2

3. R. Mock, P.-C. Eccardt, B. Gottlieb, A. Kappel, K. Niederer, in Modellierung derdynamischen und akustischen Eigenschaften eines piezoelektrischen Common-Rail-Diesel-Hochdruckinjektors mit ANSYS XXVI. FEM-Kongress, Baden-Baden, D, 15–16 Nov. 1999

4. K. Egger, J. Warga, W. Klugl, Motortechnische Zeitschrift. 63(9), 14–17(English), 696–704 (German) (2002)

5. E. Achleitner, S. Berger, H. Frenzel, M. Klepatsch, V. Warnecke, Benzin-Direkteinspritzsystem mit Piezo-Injektor fur strahlgefuhrte BrennverfahrenMTZ (Motortechnische Zeitschrift) 05/2004

6. E. Muller, R. Otte, Potenzial der Doppeleinspritzung beim DI-Ottomotor MTZ(Motortechnische Zeitschrift) 09/2006

7. A.E. Funaioli, E. Achleitner, H. Baecker, in Direct Injection Systems for OttoEngines SAE World Congress and Exhibition, April 2007, Detroit, MI, USA,Session: Direct Injection SI Engine Technology (Part 3 of 3), 2007-01-1416

8. W. Mahrle, C. Luttermann, M. Krauss, N. Klauer, High-Precision Injection inCombination with the New BMW Twin-Turbo Petrol Engine MTZ Worldwide03/2007

9. A. Koch, G. Lugert, E. Magori, G. Bachmaier, S. Eisen, in Key Components forGaseous Fuels: Actual Demands and Future Concepts 1st International Sympo-sium on Hydrogen Internal Combustion Engines, University of Graz (Austria)September 2006

10. K. Lubitz, H. Hellebrand, D. Cramer, I. Probst, in Euro-Ceramics II, Proceed-ings ECerS’91, September 11–14 (1991), Augsburg, 1995

11. S. Takahashi, A. Ochi, M. Yonezawa, T. Yano, T. Hamatsuki, I. Fukui, Ferro-electrics 50, 181 (1983)

12. C. Schuh, K. Lubitz, T. Steinkopff, A. Wolff, in Piezoelectric Materials in Ad-vances in Science, Technology and Applications, ed. by C. Galassi et al. (KluwerAcademic, Dordrecht, 2000), pp. 391–399

13. K. Lubitz, C. Schuh, T. Steinkopff, A. Wolff, in Proceedings of the 7th Interna-tional Conference on Actuators, Actuator 2000, June 19–21, Bremen, Germany,pp. 58–61 (2000)

14. Patent EP 1053568 B1, 200015. K. Lubitz, C. Schuh, T. Steinkopff, A. Wolff, in Piezoelectric Materials in De-

vices, ed. by N. Setter (edited and published, Lausanne, 2002), pp. 183–19416. Patent US 6307306 B1, 200117. Patent EP 0844678 B1, 2002

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