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High Power Density Integrated ElectroHydraulic Energy Converter for Heavy Hybrid Off-Highway Working Vehicles Pavel Ponomarev, Rafael Åman*, Heikki Handroos*, Paula Immonen, Juha Pyrhönen, Lasse Laurila Laboratory of Electrical Drives Technology and (*) Laboratory of Intelligent Machines, Lappeenranta University of Technology, Skinnarilankatu 34, 53850 Lappeenranta , Finland, e-mail:
[email protected]
Abstract — Novel high-power-density integrated electro-hydraulic machine for electrical hybrid working vehicles is presented. The machine is dedicated to be used as a mechanical power source for hydraulic actuators with possibility of electrical energy recuperation. Advantages of using such a machine in mobile environments are discussed. The efficiency of the device is studied and compared to traditional valve controlled systems.
Index Terms—Actuators, energy efficiency, heavy working vehicle, hybrid mobile working machines, hybrid vehicles, power conversion, recuperation. I. INTRODUCTION
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EAVY working and constructional vehicles are big consumers of fossil fuels. Fuel efficiency and low emissions of such machines are important factors for manufacturers and constructors to be competitive on the market.
With hybrid technology it is possible to increase the efficiency of the vehicle power transmission system. It is often also possible to maintain the performance of the vehicle even when the original internal combustion engine is replaced by a remarkably smaller one [1]. Improvement of the efficiency of the working hydraulics has an important role when the target is to reduce working machine’s energy consumption [2]. Many heavy working machines use load sensing hydraulic system. In multiple actuator systems the load sensing hydraulic systems suffer from poor efficiency, especially, if in the system different pressure levels and flow rates are required [3]. Development of electrical hybrid mobile working vehicles requires introduction of new compact and powerful components. These components should be electrical, having good controllability and being capable of converting kinetic or potential energy into electricity to be stored in batteries or super capacitors. The force density of present day electro-magnetic mechanical actuators is low and there is a need for electro-hydraulic components which can supply the required power to hydraulic actuators in the mobile environment of hybrid working vehicles. In this paper a new electro-hydraulic device is introduced. This machine has a high power density and allows transformation of electrical energy into hydraulic energy and vice versa. The applicability and efficiency of such a machine are studied. The advantages of using such a machine in a mobile environment instead of traditional valve controlled hydraulic system are discussed.
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II. CONCEPT AND EXAMPLES OF TOPOLOGIES
The two main topologies - serial and parallel hybrid - are illustrated in Fig. 1. In addition to these two, one common topology in mobile machines is the power-split topology but it is more suited for driveline power transmission.
Fig. 1. Architectures of basic hybrid topologies. Top: serial hybrid; bottom: parallel hybrid.
In Fig. 1 on top the drive transmission line is illustrated according to the serial hybrid topology, while on the bottom the parallel hybrid topology is presented. In both cases the hydraulic power circuit for working actuators is also presented. This circuit is often independent parallel system to the drive transmission. It can be noted that the actuator system itself follows the serial hybrid topology. In Fig. 1 block “Acc.” contains normal accessories for the pump-controlled fluid power system, accessories for compensating the difference in volume flows of differential cylinder and safety valves as well. The main benefits of the hybrid topology for a working vehicle are often brought with downsized diesel engine. The downsizing is possible due to the typically low average power of the working vehicles. A typical load cycle often consists of periods of high loading alternated by idle periods. The power of a diesel engine can be decreased to the level of average load [4]. During the peak loadings the additional power can be withdrawn from the energy storage. During idle periods the diesel engine charges the electrical storage – battery or supercapacitors. The engine can be downsized to even lower than 40% in comparison to the conventional hydrostatic transmission system [5]. The fuel consumption can be significantly decreased when the engine is downsized. It brings significant money savings due to the fuel cost and also in CO2 emissions. The main power consumers in a working off-highway or constructional vehicle are traction drive and working actuators. In cranes, excavators, and different stackers – the main actuator is often a boom driven by hydraulic cylinders and gripper or other actuator in the end of the boom. Traditionally, this end actuator is supplied by long hydraulic flexible power transmission line. This line produces additional losses during the operation. Fig. 2 shows a typical off-highway working vehicle with massive container gripper.
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Fig. 2. An off-highway working vehicle with a telescopic boom and a massive stacking gripper with its actuators. [15]
Instead of long hydraulic power lines, the end actuator can be supplied by an integrated electro-hydraulic energy converter (IEHEC) placed directly to the end of the boom. This IEHEC together with the end actuator constitute an electro-hydraulic actuator system. Fig. 3 shows the main power system of a working vehicle with an integrated electro-hydraulic actuator system.
Fig. 3. Example of elimination of long hydraulic pipes – power by wire. Electro-hydraulic actuator system integrated to the main power system with diesel engine (ICE), generator, energy storage and traction motors.
The electro-hydraulic hybrid actuator system is specially developed for compact assembly. In this assembly all the components are located close to each other to avoid long fluid power transmission lines which quite commonly appear in heavy mobile working machines. Normally, in this type of machinery the transmission lines are flexible hoses and pipes of small diameter (for easy bending during e.g. telescope operation) which cause remarkable hydraulic power losses. The actuator design
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proposed allows the replacement of long pipe lines and hoses by electrical cables. In the cables the losses due to their internal resistances are apparent but negligible in comparison with the losses apparent in the hydraulic transmission lines. [6]
Fig. 4. Electro-hydraulic hybrid power transmission system [7].
Fig. 4 shows the circuit diagram of an electro-hydraulic hybrid power transmission system. The system in question is a pumpcontrolled fluid power system. The hydraulic pump is directly driven by the electrical motor as introduced. The pump-controlled fluid power systems have many advantages of which the biggest is the reduction of hydraulic losses in comparison to the conventional directional valve operated systems. But, unfortunately, these systems have also disadvantages which have limited the commissioning of the pump-controlled fluid power systems. Asymmetric differential cylinder is a typical component in producing the force and movements of mechanisms. When it is used together with the pump-controlled fluid power system it can cause problems due to the differential volumes in cylinder chambers. The pump-controlled system is commonly operated in closed loop where it is essential to maintain the even volume flow in input and output ports of the pump. In case of the differential cylinder the difference in volume flows must be equalized, e.g. by using pressure accumulators. The system shown in Fig. 4 consist of an IEHEC, an asymmetric differential cylinder, a pressure accumulator, a separate pump unit for the cooling of the integrated electrical machine and charging the pressure accumulator and pressure relief and check valves. The arrows in Fig. 4 show the operation principle of the system at the operation point at which the actuator is performing the extension movement. At the time less oil flows out of the cylinder piston rod side than it is pumped in due to the different piston effective areas. In closed-loop fluid power circuits this inequality in the oil volumes needs to be compensated. The actuating pressure on the piston side forces the check valve to open, which enables the pressure accumulator to bleed out to lower pressure side. In reversed function the excess oil from the piston side would flow to the accumulator after the check valve is opened due the higher pressure on the piston rod side.
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III. MACHINE DESIGN
The integrated electro-hydraulic machine consist of a hydraulic machine capable to work in both pumping and motoring modes without significant charging inlet pressure, and an integrated tooth coil permanent magnet synchronous machine (TCPMSM) directly operating on the shaft of the hydraulic machine. The TC-PMSM is controlled by an electrical frequency converter. Fig. 5 illustrates a cut-away view of the IEHEC.
Fig. 5. Integrated electro-hydraulic energy converter.
Table I lists the main parameters of the first prototype of IEHEC. TABLE I
Parameters of IEHEC Parameter
Value
Nominal power Flow rate Pressure Voltage Nominal current Coolant flow
26 kW up to 180 l/min up to 400 bar 400 V 44 A 4 l/min @ 70°C
The integrated design allows significant space savings in the mobile environment (see Fig. 6). High power density can be achieved by integration and by usage of direct immersive liquid cooling in the electrical machine.
Fig. 6. Dimension comparison of the IEHEC prototype and traditonal motor-pump in-line coupling.
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One of the essential aspects in designing an energy recovery system in the form of electrical energy is efficient electrical generator. The compact design of the integrated electrical machine imposes the use of efficient liquid cooling. The cooling of electrical machine requires a container for coolant and heat exchangers which would increase the weight and dimensions of the power unit. This is undesired in cases where the power unit will be placed near the actuator far away from the vehicle’s main hydraulic systems. For this reason the electrical machine is specially designed to use the working hydraulic fluid as the cooling media. There is no need to make a new separate cooling circuit for electrical motors in off-highway working vehicles. The working hydraulic fluid can be used as a coolant. A suitable container and a pump for the cooling circuit can be found in the vicinity. Pump-controlled systems are closed loop systems which require a boost pump to maintain the supply pressure at the inlet and also to compensate the leakages that occur in the hydraulic components. Therefore, the electrical machine cooling flow can be obtained from the boost circuit. The oil is then circulated through the machine as the need arises but by turns with the boosting function. Heat exchanger could also be connected to this circuit if the conditions would require. In practice, the work cycle of an actuator system in a mobile machine is very short and the typical allowed working temperature of a PMSM (100°C) is much higher than the hydraulic circuit’s working temperature (50-60°C). Thus, it could be predicted that a heat exchanger is not necessarily needed. The leakage compensation and cooling circuit is involved in the Fig. 4.
A. Hydraulic Machine
Axial piston machine can be operated at high pressure levels and high flow rates. High pressure levels are difficult to obtain with gear pumps. In principle, hydraulic machines work both as motors and pumps, but in practice only few machines are fully ready to be operated in both modes. Bent-axis piston machines are often applicable in both modes and offer high efficiency in both pumping and motoring modes. The hydraulic pump-motor selected for this study is commonly available bent-axis piston machine which is ready to operate as a pump without pre-pressure in the pump inlet and which can be loaded on the inlet port with sufficiently high pressures. [6]
B. Electrical Machine
The electrical machine is designed to achieve maximum torque density. The TC-winding is selected as it has low axial length [17]. High-remanence NdFeB PMs (μPM = 1.17 T @ 100°C) are used in the rotor. Magnets are segmented in order to decrease the eddy-current losses [8]. High grade electrical steel M270-50A is used for the stator and rotor lamination in order to decrease iron losses. Stator has 18 slots. Rotor has 16 poles. High number of poles reduces dimensions of the stator and rotor yokes [18]. The supply frequency is 200 HZ at nominal speed 1500 rpm. In order to decrease AC losses at such a high frequency, the winding is wound with 1mm-diameter wire strands connected in parallel. Extremely efficient cooling allows increasing significantly the current density in the coil windings. At nominal operating point the current density is 8 A/mm2. Short stack length (50mm) ensures the heat removal from the slot conductors via flushed end-windings. This measure increases the current linkage of the machine, which, together with utilization of high flux density permanent magnets in the rotor, results in a high-torque-density and high-power-density machine. The power density of the electrical machine without frame is 750 W/kg, and with frame is 270 W/kg. The measured torque density at nominal point is 4.4
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Nm/kg, and at 300 rpm and 500 Nm is 15 Nm/kg without frame. Selected direct immersion oil cooling is extremely effective. The heat is removed directly from the windings and from the rotor [16]. Fig. 7 shows the temperature rise of the machine on the surface of winding coils at 170 % load and at 250 % load when machine was cooled by 8 l/min oil flow at 50 °C.
Fig. 7. Winding surface temperature rise of the integrated TC-PMSM at 170 % (280 Nm) and at 250 % (400 Nm) load.
The increase in temperature increases Joule losses in the copper windings. However, at the same time the oil-drag losses in the air gap decrease as at higher temperatures the coolant fluid has lower viscosity [8]. Along with outstanding cooling the oil immersion requires additional power to compensate the oil drag losses. In order to decrease the oil drag, the height of the air gap was increased to 2.7 mm which gives at maximum speed and at 100 °C of coolant 1.2 kW of oil drag losses. This amount of losses constitutes only 4 % of the nominal power of the prototype. The point of using PM machine is that it offers great freedom of selecting the machine geometry in comparison to e.g. the induction machine (IM). In IM it is always required to use certain dimensions to produce certain torque. With permanent magnets the dimensions of the rotor might be selected with greater freedom which enables the component integration. The IPM construction of the rotor was chosen in order to withstand the mechanical stress due to the high oil drag in the air gap [8]. The tooth-coil topology of the stator windings allows decreasing the axial length of the machine due to the short end windings [17]. High synchronous inductance of TC-PMSMs allows machine to operate also in wide constant power speed range [9]. Such operation might be needed also in fluid power applications in cases of no-load and high-speed movements.
IV. EFFICIENCY MEASUREMENTS
The efficiency of power conversion is nowadays a more and more important factor. The components of an electrical hybrid off-highway working vehicle should have high efficiency in order to decrease the fuel consumption. The efficiency measurements should include measurements at various operating points as typical workcycles include partial loading conditions.
A. Hydraulic Machine
Fig. 8 illustrates the schematic of the test setup used for the efficiency measurements of the hydraulic machine in pumping mode. The setup consists of a conventional induction machine that as a motor drives the pump via a torque sensor that is
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equipped with a rotational speed sensor. The volume flow produced by the pump is directed through a throttle back to tank. The pressure sensors in up- and down-streams indicate the primary and secondary pressures. The flow sensor function is based on pulse calculation and offers high enough accuracy. The constant temperature is essential to make the measurements comparable. The temperature is controlled using an oil-water heat exchanger in the setup. As measurement procedure, first the rotational speed is set constant and the volume flow at low torque is measured. Then the torque is increased by throttling and the flow is measured. This sequence is repeated with five different rotational speed levels. Volumetric efficiency is calculated by dividing the real flow by theoretical flow at each measured point. Then the torque is set constant when the rotational speed is low and the real axis input torque is measured. Then the rotational speed is increased and the real value measured while the torque is kept constant. This is also repeated with five different torque levels. Consequently, the mechanical efficiency can be calculated by dividing the theoretical torque by the real torque.
Fig. 8. Schematic of efficiency measurement in pumping mode.
Fig. 9 illustrates the schematic of the test setup used for the efficiency measurements of hydraulic machine in motoring mode. The setup consists, again, of the same conventional induction machine that is now driven as generator by the hydraulic motor via torque sensor that is equipped with rotational speed sensor. The generator is controlled by a recuperative grid-connected converter. The volume flow produced by the external hydraulic power unit is directed through the hydraulic motor to the tank. The pressure sensors in up- and down-streams indicate the primary and secondary pressures. The flow sensor function is based on the pulse calculation and offers high enough accuracy. The constant temperature is essential to make the measurements comparable. The temperature is controlled using an oil-water heat exchanger of the external power source. First, the volume flow is set constant and the torque and rotational speed at low pressure are measured. Then the pressure is increased and the torque and rotational speed are measured. This is repeated with six different pressure and volume flow levels. Then the torque is set constant when rotational speed is low and the volume flow and pressure are measured. Then the volume flow is increased and the pressure and rotational speed are measured. This is also repeated with several different torque and volume flow levels.
Fig. 9. Schematic of efficiency measurements in motoring mode.
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Fig. 10 shows the efficiency map interpolated from the measurement points of hydraulic machine in pumping (left) and motoring (right) modes. The best efficiency in pumping mode of 97% can be found in the vicinity of 800 rpm in range from 100Nm to 225 Nm. The best efficiency in motoring mode lies in the region of low speeds and torques
Fig. 10. Measured hydraulic machine efficiency in pumping mode (left) and in motoring mode (right).
B. Electrical Machine
The electrical machine efficiency was measured in the motoring mode. Test setup consists of direct-immersion-oil-cooled TC-PMSM connected to an IM via the torque and speed sensor. Both machines were driven by electrical frequency converters. The 3-phase currents and voltages supplied by electrical frequency converter to TC-PMSM were measured by high precision power analyzer. The input power of the TC-PMSM was measured by that power analyzer. The output power was measured by a torque sensor. Fig. 11 shows the TC-PMSM at measurement bench.
Fig. 11. Direct-immersion-oil-cooled TC-PMSM on efficiency measurement bench.
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Fig. 12 shows the measured interpolated efficiency map of the TC-PMSM. The highest efficiency is 94% at 150 Nm and 1000 rpm. The machine efficiency was measured when cooling was used; hence, the efficiency map shows the total electrical machine efficiency taking into account the drag of the oil coolant in the air gap. At high speeds the efficiency is lower than efficiency of typical electrical machine due to the increased cooling-oil drag.
Fig. 12. Measured efficiency map of electrical machine.
It is sufficient to have only one mode measured efficiency. Of course, the efficiency in generating and motoring modes are slightly different. But this difference is very small and lies below the measuring accuracy of the measurement bench.
C. Integrated Machine
In order to get the total efficiency maps of IEHEC the efficiency maps of TC-PMSM and hydraulic machine were combined. The total efficiency maps in both generating and pumping modes are shown in Fig. 13.
Fig. 13. IEHEC efficiency map in pumping mode (left) and in generating mode (right).
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The maximum efficiency in pumping mode is 90% and in generating mode is 87%. These are the efficiency maps of IEHEC taking into account the coolant drag losses. When taking into account the overall efficiency of the energy conversion, the inverter efficiency should be considered as well. The typical inverter efficiency is about 97%. Hence, the total efficiency of energy conversion in generating mode is 84% at 1000 rpm and 170 Nm. At this speed the hydraulic fluid flows through the hydraulic machine with flow rate of 65 l/min @ 165 bars. In pumping mode the peak efficiency is 87% at 800 rpm and 215 Nm. These speed and torque values impose oil flow of 46 l/min at 205 bars. V. DISCUSSION AND COMPARISON
This novel integrated unit has some similarities with commercial products by Bosch [10], Liebherr [11], Moog [12] and Voith [13]. Products in [11], [12] and [13] feature product tailoring in form of solving the problem with unequal volumes of differential cylinder and the application in [10] deal only one-way operation. Therefore, they are suited for pump-controlled systems. These products have in common that they are not developed for the energy recuperation. Electro-hydraulic pump control, however, offers a very high recuperation potential, especially, in single acting boom cylinder applications. Typically up to 70 % of the potential energy can be converted back to electricity which, of course, results in a significant energy efficiency improvement. [14] VI. CONCLUSION
Electro-hydraulic components for working vehicles are the enabling technology for industry on the way to more efficient and environmentally friendly production. The prototype IEHEC shows good potential to become an interesting alternative in the field of hydrostatic power transmission systems directly operated in pump control. The IEHEC prototype already offers good performance with its best efficiencies of 84 % and 87 % for generating and pumping, but could, of course, still be tailored to be even more efficient and compact. Such an integrated unit could, of course, be built in different sizes to offer driving potential for different kinds of applications. One of the most interesting possibilities is to use an IEHEC in applications having nowadays very long and highly lossy hydrostatic power transmission pipelines producing extremely low efficiencies, replacing them by an electric cable and an IEHEC. In cases of single acting boom cylinders it is possible to recover typically up to 70 % of the potential energy by an electro-hydraulic system. VII. REFERENCES
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