59RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics

Boosting Efficiency through the Use ofHydrostatics

1. Hydrostatic transmission forwheeled loaders

In the power range up to 60 kW, the drive conceptof a hydrostatic transmission has become generallyestablished, and particularly in Europe, on accountof the known advantages of hydrostatictransmission over hydrodynamic transmission(Fig. 1). The hydrostatic transmission has eminentlyproved its value in tough everyday conditions andis characterized by its high cost-efficiency inpractical use, among other things:– Reduced fuel consumption

Higher traction effort at low drive speedsensures more efficient utilization of theinstalled engine power.

– Reduced brake wearHydrostatic deceleration greatly reduces wearon the travel brake.

– Less tyre wearPrecision control of the travel drive helps toprevent the drive wheels slipping.

– More convenience for the driverSimple, straightforward operation relieves thedriver and makes the machine easier tooperate.

So that these advantages of a hydrostatictransmission can also be made available towheeled loaders in the middle and upper powerclasses, it is necessary to develop drive conceptswith which the desired effect can also be achievedin these power ranges.

• Large torque conversion range,therefore few or no steps whenusing gear shifts

• High traction at low drive speeds

• Torque can be transmitted in bothdirections, thus permitting simplereversing and hydrostaticdeceleration

Fig. 1: Advantages of hydrostatic versus hydrodynamic transmission

Advantages of hydrostatic versushydrodynamic transmission

• Demand-based power division

• Easy operation

• High starting efficiency

• Little heat generated in the blockpoint

• Resolved design

Peter Dschida / VMT/EBrueninghaus Hydromatik GmbHElchingen plantPhone +49 (0) 73 08 / 82 24 34Fax +49 (0) 73 08 / 53 38E-Mail: peter.dschida@bru-hyd.com

60 RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics

2. Hydrostatic drive conceptsfor wheeled loaders> 60 kW

The larger torque conversion range required inwheeled loaders of higher power classes is coveredby a mechanical gearshift system downstream ofthe hydrostatic drive. Depending on the powerclass required, this system involves up to three gearshifts. The gears are shifted via electronicallymonitored switching points in order to ensure thatthe shift is effected at the right moment to protectthe gear components. With this drive concept,however, the power flow must be interruptedduring the gear shifting process, thus resulting invery hard shift jolts in the vehicle.Two-motor systems have been developed in orderto obtain a drive concept covering the loaders’ fullworking and road travel ranges withoutinterrupting the traction effort. These systems willbe discussed in more detail here (Fig. 2).

2.1 Two-motor drive withsummation gearbox

Concepts with two variable-speed motors withsummation gearbox have already been used inseries with great success in the power class up to100 kW (Figs. 3 and 4). Two variable-speed motorsof different size are used here. The advantage ofsuch a system is that a larger output torque isavailable for the working range, while the roadtravel range is covered by the higher permissiblespeed of the smaller motor, since the larger motorreduces its displacement to V = 0 cm3 beforereaching its maximum speed. The motive power isalso reduced when the displacement V of the largermotor equals 0 cm3, thus permitting a higher speedwith improved overall efficiency.A summation gearbox with integrated clutch canbe used to increase the difference in size and thusthe control range of the system. The advantage ofthis solution is that the larger motor is disengagedmechanically upon reaching its maximumpermissible speed and therefore no longer has anyeffect on the road travel range.It is also possible to use a constant displacementmotor with lower displacement for road travel. Thisimproves the overall system efficiency, since themotor always operates at the maximum angle.Here too, the motor with the larger displacementis disengaged mechanically.

Fig. 2: Hydrostatic drive concept for wheeled loaders

A6VM

A4VG

A6VM

A2FM

Fig. 3: Two-motor drive with summation gearbox

with

out t

ract

ion

effo

rt in

terru

ptio

nw

ith tr

actio

n ef

fort

inte

rrupt

ion

60 120 250Drive rating in kW

2-speed powershift gearbox 3-speed power shift gearbox

Summation gearbox Switchable summation gearbox

i1, i2, i3

i1, i2, i3

i1, i2

A2FM

A6VM

Fig. 4

Vehicle speed

Trac

tion

effo

rt

Overall efficiency η

61RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics

BB-3

A4VG

CAN-Bus

A6VM

Fig. 5: Summation gearbox with three gear-shift clutches

Gearselection

Direction oftravel

Drivepedal

Inchingpedal

Diagnostics,parameterization,process monitoring

Engine with electronicinjection pump

Rpm speed

BB-3 BODEMsoftware

Load limiting control- Automotive driving- Gearbox management- Inching function- Diagnostics

2.2 Two-motor drive withswitchable summationgearbox (2+2 gearbox)

The two-motor drive with combined switchablesummation gearbox (Figs. 5 and 6) has beendeveloped for vehicles in the power class from 100kW to approx. 250 kW. With this drive concept, theadvantages of a hydrostatic transmission are nowalso available to this power class. Twohydrostatically parallel motors deliver their torquevia two separate input shafts, each of them hasone or two gear ratios. The total torque istransmitted to the drive axle by a central shaft.The advantage of such an arrangement is that thegear shifts required for the control range areeffected without interrupting the power flow andtherefore without shift jolts and withoutinterruption of the traction effort. This is possiblebecause one hydrostatic motor delivers its torque

to the drive train via the central shaft while theother motor is set to zero. Its torque is consequentlyzero and the gears can be shifted without jolts.The entire system is controlled by a microprocessor,since the gear shifting process depends on anumber of variables, such as vehicle speed, loadstate of the engine, position of the drive pedal, etc.and the purpose of the drive was to ensure thesimple operation already familiar from hydrostaticsystems, as well as a completely automatic gearshifting process. The complete gearboxmanagement and control of the hydrostaticcomponents, including such functions as loadlimiting control, automotive driving, inching andhydrostatic braking, is taken over by theprogrammable control system.

Microcontroller+

Software

62 RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics

3. Boosting efficiency throughthe use of hydrostatics ascompared with hydrodynamics

The power consumption of the various systems isillustrated in Fig. 7. It not only presents the idealcurve of traction over speed for a wheeled loader,with power as the product of traction times speedremaining constant over the entire curve profile, butalso shows the limiting curves for a 4-speed powershift gearbox with reversing gearbox and torqueconverter. The growing divergence from the idealcurve is clearly evident. Compared with the profilefor a 2-motor drive with switchable summationgearbox, it can be seen that the power is utilizedmore effectively, particularly in the working range,thus yielding a higher ground speed withcorresponding traction effort. Another advantageof the hydrostatic drive is that it adapts to the profileof the ideal curve automatically, i.e. withoutintervention by the driver.

Power class in kW

A4VG 71

90

125

180

A6VM 55

80

107

140

Gear ratio

1st gear

2nd gear

3rd gear

100 150 180 250

Motor 1 (forward/reverse) Motor 2 (forward/reverse)

6.10 : 1

Ñ

1.34 : 1

3.03 : 1

3.03 : 1

Ñ

4020

100 %

50 %

Fig. 7: Comparison, hydrostatic vs. hydrodynamic

Traction F in kN

Ideal power curve(traction x speed=constant)

Speed v in km/h4-speed power shiftgearbox with torqueconverter

Hydrostatic transmissionand switchablesummation gearbox

63RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics

PM in kW

nmax

100%

P

M

3

2

1

Fig. 9: Comparison, hydrostatic vs. hydrodynamic: engine characteristics

Hydrodynamic transmission

Hydrostatic transmission – DA control

Hydrostatic transmission – electronic control + CAN Bus

1: Engine drag torque

2: Actual engine torque

3: Max. possible engine torque

Specific consumption be in g/kWh

The situation becomes even clearer if we considerthe response of the drives when additionallyswitching auxiliary equipment, such assimultaneous actuation of the steering andimplement hydraulics (see Fig. 8). Since only 50%of the input torque will be available to thehydrodynamic drive when 50% of the power arediverted to the auxiliary equipment, its outputtorque will consequently also be reduced by half.Moreover, the diesel speed will be reduced byroughly 20%, resulting in a reduction in speed forboth the travel drive and the equipment. In the caseof a hydrostatic drive on the other hand, themaximum traction effort remains unchanged, sinceload limiting control of the hydrostatic systemmerely reduces the pump delivery, but not themaximum working pressure. As a result, the fullfeed power is still output despite actuation of theimplement hydraulics. For this reason, 50% drivepower means 50% speed, but still with maximumtraction effort. Intervention by the driver is notnecessary, thus relieving him in this particularsituation.

Reducing the speed of the drive motor reduces thelevel of noise emitted, extends the motor life andsaves fuel. The converter torque that can betransmitted declines as the square of the speed:for this reason, a converter drive will require themaximum diesel speed in order to deliver themaximum transmittable torque (Fig. 9). With ahydrostatic drive, on the other hand, the torque canbe matched to the diesel speed via the pumpcharacteristic, thus permitting utilization of a morefuel-efficient lower speed. Further optimization ispossible if use is additionally made of anelectronically controlled system communicating viathe CAN bus interface with the electronicallycontrolled fuel injection system of the diesel engine.Since the actual torque, maximum possible torqueand drag torque over speed are signalled by theengine electronics, the variable characteristic curveof a hydrostatic drive can be optimally adapted tothe power available. This not only yields theadvantage of operating at lower drive speeds, butalso of working in the most advantageousconsumption range of the diesel engine in eachcase.

nM in rpm

4020

100 %

50 %

Fig. 8: Comparison, hydrostatic vs. hydrodynamic – 50% decline in power due to auxiliary equipment

Ideal power curve

Traction F in kN

Speed v in km/hHydrodynamic system

Hydrostatic system

64 RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics

4. Electronic loader controlThe more widely electronic systems are used inmotor vehicles, the more important it is to usecomponents which have been specially developedfor mobile applications, particularly in conjunctionwith hydrostatic drives (Fig. 10). A special profileof requirements applies in the case ofmicrocontrollers and the software used and mustmeet the specific demands imposed on themachines with regard to:– Functional operation– More efficient use of the power available– Greater fuel efficiency– Lower noise levels– High operational safety– Easy maintenanceIn addition to the basic functions of a hydrostatictransmission, other supplementary functions canalso be realized through the electronics in orderto improve the efficiency of the system (Fig. 11).Incorporation of the CAN bus interface in particularmakes it possible for data to be exchanged withthe electronic control system of modern dieselengines and also with the electronic control systemof the implement hydraulics.

Fig. 10: Electronic loader control with Rexroth components

Microcontroller

Diagnostic system: BODEM

External electronics

2+2 gearbox Transm. motor A6VM

Implement pump A10VO

Transmission pump A4VG

Diesel engine control

DisplayM-control joystick

Implement hydraulic valves

ActuationCommunication via CAN Bus

– Traction limitationThe maximum traction of the machine can belimited in accordance with the type of groundbeing travelled on so that the wheels areprevented from slipping and thus avoidingunnecessary tyre wear, as well as ensuringmaximum traction.

– Speed limitationThe maximum speed can be infinitely variedso that the maximum working speed canremain variable while at the same timespecifying a crawling speed. This is particularlyadvantageous when working with suchsupplementary equipment as a sweeper orsnow plough, etc.

– Eco modeParticularly when travelling on the road, it isoften found that the diesel engine generatesmore power than is required by the traveldrive for the momentary speed. Since the loadstatus is detected by the travel electronics onthe basis of the engine straining speed, theengine speed can be reduced to the point atwhich the permissible straining value is

obtained. The hydraulic travel motor issimilarly swivelled to a correspondinglysmaller angle in order to maintain therequired vehicle speed.

– Constant speed driveThis function maintains a constant vehiclespeed regardless of any increase in the drivespeed due to switching additional auxiliaryequipment. It is superimposed on the loadlimiting control in order to protect the dieselengine against overloading. In this context, itis advantageous to include the implementhydraulics in the electronic management. Inview of the innumerable solenoids actuated,it is best to control them via a separatemicrocontroller which can exchange therequired data with the travel drive control viathe CAN bus interface.

– Gearbox managementIf a two-motor drive with switchablesummation gearbox is used, the entire gearshifting strategy is managed by the travelelectronics (refer also to section 2.2).

65RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics

Fig. 11: Electronic loader control

Diagnostics

Over-speed safety

Gearbox management

Constant speed drive

Joystick control

Variablecharacteristics

Eco mode

CAN Bus

Diesel engine control Implement hydraulics

Automotive driving

Selection of directionof travel

Load limiting control

Traction limitation

Speed limitation

Inching/Braking

5. ProspectsNew drive concepts combined with electroniccontrol and the new generation of diesel engineswith electronically controlled fuel injection pumpsopen up new possibilities for boosting theefficiency and operating convenience of wheeledloaders, particularly in the higher power classes.These concepts provide the basis for extension ofthe hydrostatic system, as well as possibilities foroptimization of the construction machines and willhave a growing influence on the overall design ofthe machines.

66 RE 00 207/10.00

Boosting Efficiency through the Use of Hydrostatics