A Series Hydraulic Hybrid Drive Train

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					                         A Series Hydraulic Hybrid Drive Train
                                                For Off-Road Vehicles

                                                                              With heavily fluctuating fuel
                                                                           prices, the total cost of own-
                                                                           ership of loaders, excavators,
                                                                           and other off-road machines is
                                                                           nowadays strongly influenced
                                                                           by the fuel costs. Moreover, there
                                                                           is growing concern about CO2-
                                                                           emissions caused by the burning
                                                                           of fossil fuels as well as about the
                                                                           long-term availability of these
                                                                           fuels. The fuel economy and ef-
                                                                           ficiencies of the drive train and
                                                                           the hydraulic implements have
                                                                           therefore become extremely im-
                                                                           portant parameters in the design
                                                                           of future off-road machines.
                                                                              Hybrid transmissions are nor-
                                                                           mally not considered to be a
                                                                           solution for off-road machines.
                                                                           Hybrid drive trains are foremost
                                                                           developed for passenger cars
                                                                           where they can benefit from the
                                                                           recuperation of brake energy. But
                                                                           for many off-road vehicles, brake
                                                                           energy recuperation is not an op-
                                                                           tion. Furthermore, hybrid electric
                                                                           vehicles need sophisticated elec-
                                                                           tric transmissions with delicate
                                                                           and expensive inverters, con-
                                                                           verters, and batteries. Taking the
                                                                                  extreme power transients
                                                                                   in mobile machinery and
                                                                                   the rough operational
                                                                                  conditions of off-road
                                                                                drive trains into account, it
                                                                               is questionable whether the
                                                                           delicate hybrid electric drive
                                                                           trains can be considered a viable,
                                                                           inexpensive, and robust option
                                                                           for off-road applications.
                                                                              Yet, there is still the alterna-
                                                                           tive of the hydraulic hybrid drive
                                                                           train. These are for some years
                                                                           developed for buses, refuse, and
                                                                           delivery trucks [1-6]. This paper
                                                                           describes a hybrid drive train

8 | Off-Highway Directory 2009                            |
                                                                                                     By Peter A.J. Achten, Innas BV
                                                                                                                   and motors. Current piston
  Fig. 1: Hybrid electric swing                                                                                    pumps and motors have a peak
drive for an excavator [16]                                                                                        efficiency of around 93%. A
                                                                                                                   pump-motor-combination could
  Fig. 2: Measured total effi-                                                                                     therefore – in theory – have an
ciency of a 28 cc floating cup                                                                                     efficiency of maximum 86%. In
pump/motor                                                                                                         reality, the part load operation
                                                                                                                   of the hydrostatic machines re-
  Fig. 3: Low speed torque rel-                                                                                    duces the efficiency of both the
ative to the maximum theo-                                                                                         pump and the motor. If, for in-
retical torque                                                                                                     stance, an average efficiency of
                                                                                                                   80% for both the pump and mo-
  Fig 4: Operating points of the                                                                                   tor is assumed, the total aver-
diesel engine of a loader [38]                                                                                     age efficiency of the hydrostatic
                                                                                                                   drive is limited to 64%.
                                                                                                         3            The new Floating Cup Prin-
                                                                                                                   ciple [19-25] strongly improves
    1                                                                                                              the efficiency of hydraulic
                                                                                                                   pumps and motors. Recent
                                                                                                                   tests have proven a maximum
                                                                                                                   total efficiency of up to 98%.
                                                                                                                   The floating cup principle is a
                                                                                                                   multi piston pump, having 24
                                                                                                                   pistons. This creates an almost
                                                                                                                   constant torque output. In ad-
                                                                                                                   dition, the floating cup principle
                                                                                                                   has extremely low friction loss-
                                                                                                                   es at low rotational speeds. The
                                                                                                                   smooth torque and the low fric-
                                                                                                                   tion result in a start-up torque,
                                                                                                                   which is nearly equal to the
                                                                                                                   maximum theoretical output
                                                                                                                   torque of the hydrostatic motor.
                                                                                                         2            Figure 3 shows the torque
                                                                                                                   efficiency of a floating cup mo-
                                                                                                                   tor compared to two other mo-
                                                                                                                   tor principles, measured at low
for off-road machines. The drive      presented an excavator, in which        of the Gryphin, at least for the     speed (< 1 rpm). The floating
train –coined the ‘Hydrid’ [7,        the swing movement of the up-           drive train part of the system. It   cup principle offers a starting
8]– is a full series hybrid system    per carriage is realized by means       has an in-wheel hydrostatic ma-      torque, which is about twice as
with an in-wheel motor in each        of a hybrid electric drive ([16], see   chine in each wheel, hydraulic       high as the radial piston motor.
wheel and hydraulic transform-        Figure 1). Because of the hybrid        transformers instead of electric     For the same output torque at
ers for efficient power control.      system, a smaller internal com-         power controllers, and hydraulic     start-up, the floating cup motor
                                      bustion engine could be applied.        accumulators instead of batter-      can therefore be half the size
HYBRID ELECTRIC DEVELOPMENTS          The fuel consumption is reduced         ies. Added to this, the hydraulic    of a radial piston motor. This is
   There is a common consen-          by 40%, according to Kobelco.           hybrid system also includes re-      not only important for reduc-
sus that hybrid drive trains can         In Europe, Deutz and Atlas           cuperation modes for the hy-         ing the size and weight of the
strongly increase the efficiency      Weyhausen have shown a hy-              draulic cylinders.                   hydraulic components, but also
of a drive train. Especially in the   brid electric wheel loader, in                                               for increasing the drive train
passenger car market segment,         which the flywheel of the in-           FLOATING CUP AND HYDRAULIC           efficiency. If, due to a low start-
the Toyota Prius has proven the       ternal combustion engine is             TRANSFORMERS                         ing torque, the motor size needs
viability of the concept. How-        replaced by an electric motor/            A full hydrostatic drive re-       to be increased, then the same
ever, the increased efficiency        generator [17]. According to the        quires highly efficient pumps        motor will have to run in strong
does not by itself result in a re-    developers, the fuel consump-
duction of the fuel consumption       tion is reduced by 20%.
or the cost of ownership. The            Finally, also Volvo Construc-                                                                        4
electric drive train components       tion Equipment (Volvo CE) has
result in an increased vehicle        announced a concept of a hybrid
weight, which increases the fuel      electric loader called the Gry-
consumption of the vehicles.          phin [17, 18]. Unlike the develop-
Moreover, the added cost for the      ments of Kobelco and Weyhau-
electric components gives doubt       sen, the mechanical drive train
about the cost- benefit-relation-     in the Gryphin is completely re-
ship, especially of the full hybrid   placed by an electric drive, hav-
drive train concepts [9-15].          ing in-wheel electric motors in
   Nevertheless, the trend of hy-     all four wheels.
brid electric transmissions has          The hydraulic hybrid system,
also come to the off-road market.     which is described in this pa-
Kobelco and New Holland have          per, is the hydraulic equivalent |                                                                           Off-Highway Directory 2009 | 9
part load conditions during nor-       engine can be twice as high as in    ed to a constant displacement        loads, and a simple grid for en-
mal operation, which often re-         the optimum or sweet point).         pump. If this would for instance     ergy and power distribution. As
sults in a reduced efficiency.            In the proposed hydraulic hy-     be a 28 cc pump, then the pres-      with electricity production, more
   The Floating Cup Principle is       brid (or Hydrid) transmission        sure range of the accumulator        then one power plant can be
also applied in the design of hy-      (figure 5), the engine is supply-    would result in an engine torque,    connected to the Common Pres-
draulic transformers. A hydrau-                                                                                  sure Rail. The combined power of
lic transformer is the hydraulic                                                                                 all power plants will be chosen
equivalent of a mechanical CVT.                                                                                  as such that it can deliver the
Where a CVT converts torque                                                                                      maximum power that the ve-
and speed, a hydraulic trans-                                                                                    hicle needs for extended periods.
former converts pressure and                                                                                     This can be lower than the maxi-
flow, without principal energy                                                                                   mum power demanded in a con-
losses. In the hydraulic trans-                                                                                  ventional drive train, since the
former developed by Innas (the                                                                                   increased efficiency also reduces
Innas Hydraulic Transformer or                                                                                   the maximum power demand. In
IHT [26-37]), the control of the                                                                                 addition, the accumulators can
transformer is realized by rotat-                                                                                also deliver extra power to the
ing the port plate. Whereas the                                                                                  system, thereby acting as peak
port plate in hydrostatic pumps                                                                                  shaving devices. In most operat-
and motors only has two ports,                                                                                   ing points, the power demand of
the port plate in the IHT has                                                                                    the vehicle is much lower than
three ports: one connected to the                                                                                the maximum power demand.
load, one connected to the high-                                                                                 Having two power plants, one of
pressure supply side, and one to                                                                                 the power plants can be switched
the low-pressure supply side.                                                                                    off, and the power management
   There are also hydraulic                                                                                      can be optimized better for aver-
transformers developed for hy-                                                                                   age operating points.
drostatic drive applications, al-        Fig. 5:
lowing 4-quadrant operation of           Hydraulic                                                               HYDRAULIC IMPLEMENTS
the wheel motors (i.e. forward           circuit of the                                                             Like the electricity grid, there
propulsion, forward breaking,            hydraulic hy-                                                           can be any number of loads
reverse propulsion, and reverse          brid (‘Hydrid’)                                                         connected to the power grid, in-
braking). These transformers                                                                                     cluding hydraulic cylinders. The
have four ports: two for the sup-                                                                                accumulators will avoid load
ply side and two for the load side.                                                                              disturbance reactions from one
   Unlike     valves,   hydraulic
                                                                                       Fig. 6: Hydraulic         load to the other and also allow
                                                                           6a.         circuit for the con-
transformers are based on a                                                                                      the recuperation of energy. The
positive displacement principle,
                                                                                       trol of a hydraulic       hydraulic transformers convert
and therefore have no principle
                                                                                       cylinder by means         the pressure level offered by the
throttle losses. Being a non-
                                                                                       of a hydraulic trans-     high-pressure rail to the pres-
dissipative principle, it is also
                                                                                       former (6a: rod side      sure level required by the load,
possible to convert hydraulic
                                                                                       always pressurized;       thereby avoiding the throttle
power to a higher-pressure lev-                                                                                  losses that would occur with
el. This creates the opportunity                                                                                 valve control.
for recuperating energy from                                                                                        Figure 6 shows two configu-
any hydraulic load, both wheels                                                                                  rations for the control of a dou-
and hydraulic cylinders. For in-                                                                                 ble-acting hydraulic cylinder
stance, in a forklift truck, it is     6b: rod side can be                                                       by means of a single hydraulic
possible to recuperate the break       switched).                                                                transformer. In the second con-
energy of the vehicle as well as                                                                                 figuration, the rod side can be
the energy from the lift cylin-                                                                                  switched between the low-pres-
ders while lowering a load at the                                                                                sure rail and the high-pressure
fork boom [28].                                                                                       6b.        side of the CPR-system. In the
                                                                                                                 upper design (figure 6a), the rod
HYDROSTATIC HYBRID DRIVE                                                                                         side is always connected to the
   In a wheel loader, about half of                                                                              high-pressure side and the hy-
the fuel consumption is used for       ing all its power to a so- called    which can only vary between 89       draulic transformer must am-
the hydraulic cylinders. The oth-      common pressure rail (CPR). The      Nm (at 200 bar) and 156 Nm (at       plify the pressure at the bottom
er half goes to the wheels for the     pressure at the high- and low-       350 bar). As a result, the engine    side of the cylinder in order to
propulsion of the vehicle [38, 39].    pressure side of the CPR-system      will always be operated at a me-     compensate for the hydrostatic
   Figure 4 shows the operating        is defined by the accumulators.      dium to high load, and the oper-     force from the rod side. More de-
points of the diesel engine of         Due to the design of the accu-       ating points with a low efficiency   tails about the control of hydrau-
loader for a typical cycle [38]. For   mulators, the pressure differ-       of the engine (and the pump) will    lic cylinders by means of hydrau-
a large part, the engine is oper-      ence between the high- and low-      be avoided completely.               lic transformers can be found in
ated at a load of less then 50% of     pressure side can only be varied         The new driveline is similar     the literature [28-37].
the maximum load. In this area,        in a limited pressure range, for     to the design of the stationary         Not all hydraulic cylinders
the engine efficiency strongly de-     instance between 200 and 350         electricity production and dis-      need to be controlled by means
teriorates (at a load of 20% the       bar. In the Hydrid, the internal     tribution, having a separation       of a hydraulic transformer. If cyl-
specific fuel consumption of an        combustion engine is connect-        between the power plants and         inders are only used occasion-

10 | Off-Highway Directory 2009                                                                 |
                                                                                                         By Peter A.J. Achten, Innas BV
        ally and don’t consume a large      duces the complex design and
        amount of energy, a simple and      maintenance of current load-
        less expensive throttle valve is    sensing systems. In principle, the
        perfectly acceptable for control-   CPR-system treats the hydraulic
        ling the cylinder. Figure 7 shows   loads (and the power plants) as
        two hydraulic diagrams for a        modules, which can be attached
        forklift truck. The upper diagram   or detached from the system as
        is the conventional system with     long as there is enough power
        a hydrostatic drive and a load-     capacity available.
        sensing system for the cylinders.
        The second diagram shows the        ENERGY RECUPERATION
        CPR- alternative with hydraulic        The possibilities for brake
        transformers for the hydrostatic    energy recuperation in loaders,
        wheel motors as well as for the     excavators, and other mobile              Fig. 8: (Normalized) engine power for a wheel loader
        lift cylinder. The other cylin-     machineries are often less than           during a typical loading cycle [38]
        ders for tilting and clamping are   in on-road applications like gar-
        controlled directly with valves,    bage trucks and city buses. This
        thereby accepting the (limited)     is for a part due to the fact that   sults in a much higher rolling           Another function of the ac-
        throttle losses.                    loaders and other machines           resistance. Moreover, a loader        cumulators is to avoid load de-
             The CPR-system offers a        don’t drive on asphalt but on        needs the kinetic energy of the       pendency. In a hydraulic system,
        simple layout, and strongly re-     sand, mud, and dirt, which re-       vehicle to dig into a pile of dirt.   the oil flow has to be distributed
                                                                                     Still, there are many applica-    across the various hydraulic
                                                                                 tions in which brake energy re-       loads. Thereby, the oil always fol-
                                                                                 cuperation can be of importance.      lows the path of least resistance
                                                                                 Examples are forklift trucks,         and a load change on one of the
                                                                                 dump trucks, and articulated          hydraulic motors or cylinders
                                                                                 haulers, but also for the swing       can cause a change in the oper-
                                                                                 movement of the upper carriage        ating speed of all the other mo-
                                                                                 of an excavator, energy recuper-      tors and cylinders. Load sensing
                                                                                 ation could be a significant con-     systems compensate for these
                                                                                 tributor to a better fuel economy     load disturbance reactions. In
                                                                                 of the vehicle. Depending of the      the CPR-system, the loads are
                                                                                 size, the accumulators in the         controlled at the motors and
                                                                                 CPR- system can store at least        cylinders (often referred to as
                                                                                 part of the recuperated energy.       secondary control). The accumu-
                                                                                 But, first and for all, the accu-     lators act as system capacitance
                                                                                 mulators are important for al-        separating the individual loads.
                                                                                 lowing power management and              It is to be expected that the
                                                                                 load separation. The diagram of       accumulators can play an im-
                                                                                 Figure 8 shows the variation of       portant role in recuperating
                                                                                 the engine power of a loader dur-     energy from the hydraulic cyl-
                                                                                 ing a typical loading cycle. The      inders. Hydraulic cylinders have
                                                                                 graph is split for the drive power    by definition a limited stroke
  7a.                                                                            and the power needed for the hy-      and hence a limited amount of
                                                                                 draulic implements.                   energy. On the other hand, the
                                                                                     The diagram clearly shows         power transients generated by
                                                                                 the strong transient power de-        the hydraulic cylinders can be
                                                                                 mand of both the drive and the        very high, too high to handle
                                                                                 implement functions of the load-      for electric batteries but excel-
                                                                                 er. Hydraulic accumulators are        lent for hydraulic accumulators.
                                                                                 ideal storage devices for power       The hydraulic transformer is the
                                                                                 management in these vehicles.         key enabling technology for dy-
                                                                                 They are simple and robust, and       namically managing the power
                                                                                 they have a high power density,       stream between the accumula-
                                                                                 much higher than electric bat-        tors and the hydraulic cylinders,
                                                                                 teries. Also their efficiency can     without loss of productivity.
                                                                                 be higher than of batteries.
                                                                                    In the Hydrid, batteries are       IMPROVED FUEL ECONOMY
                                                                                 however not excluded as an op-          The hydraulic hybrid im-
                                                                                 tion for storing larger amounts       proves the fuel economy in sev-
                                                                                 of energy. By combining a hy-         eral ways:
                                                                                 draulic and an electric ma-             • Idle losses of the engine are
                                                                                 chine, energy from the CPR-           avoided.
                                                                                 system can be exchanged with            • Losses of the hydrodynamic
                                                                                 electric batteries. Because the       torque converter are avoided.
                                                                                 hydraulic machine will always           • The engine is always oper-
Fig. 7: Comparison of hydraulic circuits for a fork lift truck [28] (7a:         operate at a high pressure and        ated around the sweet point.
Conventional load sensing and hydrostatic drive system; 7b: Common               load, the efficiency of this unit       • Throttle losses in the control of
pressure rail (CPR) system)                                                      can be high.                          hydraulic cylinders are minimized.
 |                                                                       Off-Highway Directory 2009 | 11
  • Energy recuperation can be                                                   emissions form passenger cars, fi-        – the commercial value of the Innas
maximized.                             REFERENCES                                nal report, TNO (the Netherlands)         Hydraulic Transformer, Proc. SICFP’99,
  • Auxiliaries like steering sys-      1. Wendel G.R. (2000) Regenerative       report nr. 06.OR.PT.040.2/RSM             May 26-28, 1999, Tampere, Finland
tems and cooling fans can be           hydraulic systems for increased              14. Greene D.L. et al (2004) Fu-          28. G.E.M. Vael, P.A.J. Achten
decoupled from the engine and          efficiency, Proc. int. exposition for     ture potential of hybrid and diesel       (1998) The Innas Fork Lift Truck -
optimized from an efficiency           power transmission and technical          powertrains in the U.S. light-duty        Working under constant pressure,
point of view.                         conf., NFPA                               vehicle market, ORNL (USA) report         Proc. of 1. IFK, Aachen, Germany,
  It will depend on the kind              2. Wu, B. et al (2002) Optimiza-       nr.: ORNL/TM-2004/181                     Vol 1, March 17-18, 1998
of application how much all            tion of power management strate-             15. Rousseau, A, et al (2005),            29. Werndin R. et al (1999) Ef-
of these factors will contrib-         gies for a hydraulic hybrid medium        Trade-off between fuel economy            ficiency performance and control
ute to an improvement of the           truck, Proc. of the 2002 advanced         and cost for advanced vehicle con-        aspects of a hydraulic transformer,
fuel economy. Eriksson [40] has        vehicle control conf., Hiroshima, Ja-     figurations, 20th Int. Electric Vehicle   Proc. of the Sixth Scandinavian Int.
calculated that a load sensing         pan, Sept. 2002                           Symposium (EVS20), Monaco, (April         Conf. on Fluid Power May 26-28
control of a double-acting hy-            3. Drozdz (2005) Hybrid refuse         2-6 2005)                                 1999 Tampere, Finland
draulic cylinder has an efficiency     truck feasibility study, Transportation      16. Fenoglio, F. (2006) Built             30. Werndin, R. et al (1999) Ef-
of around 37%. By means of a           Development Centre (TDC) of Trans-        around you, http://media.corpora-         ficiency performance and control
hydraulic transformer, most of         port Canada, Report nr. TP 14431E         aspects of a Hydraulic Transformer,
                                                                                 fenoglio.pdf                              Proc. SICFP’99, May 26-28, 1999,
                                                                                    17. Lang, T. (2007) Hydraulik          Tampere, Finland
                                                                                                                              31. Werndin, R. (1999) Efficiency
   For more information, contact                                                 in Baumaschinen, Ölhydraulik +
                                                                                 Pneumatik, nr. 7, p. 404-413.             performance and control aspects
         Peter Achten at:                                                           18. O’Sullivan, B. (2007) Hit or       of a Hydraulic Transformer, Thesis
                                                                                 Myth?, IVT International, Sept.           Linköping University                                                        2007, p. 62-66                               32. Werndin, R. (2001) Controller
                                                                                    19. Achten, P.A.J. et al (2003) De-    design for a hydraulic transformer,
                                                                                 sign and testing of an axial piston       Fifth Int. conf. on fluid power trans-
                                                                                 pump based on the floating cup            mission and control (ICFP2001), 3-5
these losses could be avoided. As         4. Okoye, V.N. et al (2006) Energy     principle, Proc. of the 8th Scandi-       April, 2001 Hangzhou, China
for the drive train, the total effi-   recovery and management in pres-          navian International Conference              33. Xu Bing et al (2005) The CPR
ciency of a pump, a transformer,       sure coupled hydraulic hybrid bus         on Fluid Power, SICFP’03, Tampere,        System adopting a new hydraulic
and the hydraulic in-wheel mo-         using new hydraulic transformer           Finland, May 7-9, 2003                    transformer to drive loads and its
tors will not be higher than the       and clean diesel combustion tech-            20. Achten, P.A.J. (2003) Design-      design, Sixth Int. Conf. on Fluid Pow-
current mechanical drive train,        nology, Proc. 5.IFK, March 20-22,         ing the impossible pump, Proc.            er Transmission and Control (ICFP
but it will certainly improve the      Aachen Germany                            Hydraulikdagarna 2003, Linköping,         2005), Hangzhou, 2005 REFERENCES
average cycle efficiency of the           5. Kovach, J. (2007) Hydraulic hy-     Sweden, June 3-4, 2003                       34. Vael, G.E.M et al (2000) The In-
engine. The effects on the fuel        brid vehicle drive technology, http://       21. Achten, P., van den Brink, T.,     nas Hydraulic Transformer - The key
consumption will be similar to         Schellekens M., Design of a vari-         to the hydrostatic common pressure
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CONCLUSION                             ogy/ 420f06054.pdf                           22. Achten, P.A.J., 2005, Volumet-     SICFP’03 - Tampere Finland
   Off-road machines are work-            7. Achten, P.A.J. (2007) Chang-        ric Losses of a Multi Piston Floating        36. Achten, P.A.J., 2002. “Dedicat-
horses. Productivity is the most       ing the paradigm, Proc. of the 10th       Cup Pump, NFPA/IFPE 2005, Las Ve-         ed design of the Hydraulic Trans-
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economy however has become             ence on Fluid Power (SICFP’07), May          23. Achten, P.A.J., 2004, Power        Aachen, pp. 233-248
equally important. The pro-            21-23, 2007, Tampere Finland              Density of the Floating Cup Axial            37. Van Malsen R. et al (2002)
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new generation of off-road ve-            9. Graham R. (2001) Comparing          and Exposition, November 13-20,           hydraulic grid, Proc. Int. Exp. for
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productivity. The Hydrid com-             10. Alson (2005) Interim report:       Efficiency and Low Speed Behav-           19-21 March 2002
pletely eliminates the mechani-        New powertrain technologies and           ior of the Floating Cup Pump, SAE            38. Filla, R. (2005) Dynamic simula-
cal drive train, thereby creating      their projected costs, EPA (USA) re-      2004-01- 2653                             tion of construction machinery - to-
extra degrees of freedom for the       port nr. EPA420-R-05-012                     25. Post, W., 2004, Determination      wards an operator model, Proc. Int.
suspension and traction con-              11. Heywood, J.B. et al (2004), The    of steady-state performance of In-        Fluid Power Exhibition (IFPE) 2005
trol of the vehicle. The hydrau-       performance of future ICE and fuel        nas Floating Cup type of axial piston     Technical Conference, pp. 429-438.
lic Common Pressure Rail is a          cell powered vehicles and their           pumps (ISO 4409), DCT-report 2004-           39. Filla, R. (2005) Operator and
clear power grid to which power        potential fleet impact, SAE-paper         127, Eindhoven Technical University       machine models for dynamic simu-
plants, loads, and energy storage      2004-01-1011                                 26. Achten, P.A.J., Fu, Z., Vael,      lation of construction machinery,
devices (including electric bat-          12. Weiss, M.A. et al (2000) On        G.E.M. (1997) Transforming future         Thesis Linköping University Institute
teries) can easily be connected,       the road in 2020, MIT (USA). Energy       hydraulics: a new design of a hy-         of Technology, Linköping Sweden
without needing a complete re-         Laboratory Report nr. MIT EL 00-003       draulic transformer, Proc of the Fifth       40. Eriksson, B. (2006) Study on
engineering of the whole system.          13. Smokers R. et al (2006) Re-        Scandinavian Int. Conf. on Fluid          individual pressure control in en-
The floating cup principle and         view and analysis of the reduction        Power, SICFP ‘97, Linköping, Sweden       ergy efficient cylinder drives, Proc.
the hydraulic transformers are         potential and costs of technological         27. Achten, P.A.J., Palmberg, J-O.     of the 4th FPNI-PhD Symposium-
the key to the proposed system.        and other measures to reduce CO2-         (1999) What a difference a hole makes     Sarasota, June 13- 17, 2006

12 | Off-Highway Directory 2009                                                                          |

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