Minimizing operating costs continues to be a high priority for designers of mobile equipment, and hybrid drives represent a viable solution. No matter which sector a machine used, both electrical and hydraulic series solutions have established themselves on the market in an evolutionary process. In such cases, it is often a downsizing of the diesel engine that results in significant fuel savings with the aid of a hybridization. Depending on the machine and the work process involved, energy recovery functionalities can also make a contribution to energy efficiency.
Operation of an Asphalt Roller
Asphalt compactors with tandem rollers are machines that can benefit from hybridization because a typical operating cycle requires significant performance peaks and valleys for the diesel engine during reversing procedures. Depending on the machine version, dynamic compaction systems produce vibration of the roller drums rotating centrifugal weights powered by hydraulic motors. The necessary compaction power is applied by driving over the hot asphalt that was laid down shortly beforehand with activated vibration.
Figure 2 shows the simplified drive topology of a tandem roller. The traction drive is powered and controlled by a closed hydrostatic circuit with variable-displacement pump and fixed-displacement motors in the drums. The hydrostatic circuit provides both propulsion for the front and the rear roller drums.
2. This simplified schematic shows the layout of a standard drive system for tandem-roller asphalt compactor.
Potential for Downsizing
During a reversing procedure, the machine simultaneously decelerates and stops the vibration. These actions must be closely synchronized to achieve an optimum compaction in the asphalt. When starting up the machine in the opposite direction, the roller must be accelerated by the traction drive and the vibration drive must resume either in parallel or with a slight time offset. This results in considerable load peaks for the diesel engine, shown schematically in Fig. 3.
3. This plot shows a typical load cycle of a diesel engine for powering a tandem roller asphalt compactor.
The power requirement of the internal diesel engine essentially is determined by the load peaks. During the compression process at constant travel speed (perhaps 6 km/hr), the power required of the diesel engine is considerably less than when load peaks occur. Managing these load cycles presents the opportunity for hybridization to allow downsizing the diesel engine.
A smaller-sized engine provides power for the basic operating load, and the hybrid drive provides additional power to accommodate load peaks. Therefore, both fuel consumption and CO2 emissions are reduced. If necessary, the complexity of the devices for exhaust gas aftertreatment can also be reduced considerably, thus achieving a reduced amount of maintenance and lowering operating costs.
An example of potential for large-scale downsizing could be a mid-sized machine in a weight class of about 9 tons. Hamm AG, a manufacturer or heavy construction equipment based in Germany, collaborated with HYDAC International to develop a hybrid drive to allow replacing an 85-kW diesel engine with a 55.4-kW one. Hamm also replaced the machine’s hydraulic fan drive with a more energy-efficient electrical version. Additional improvements to the drive train and a revised steering unit also reduced the power requirement.
Advantages of Hydraulic Hybrid Technology
Of course, hybrid technology can encompass more than just hydraulics. Electrical drives with batteries or double-layer capacitors and flywheel storage systems can also be used. No single technology is suitable for all applications in mobile equipment, and combinations of technologies may often produce the greatest benefits.
In the context of the present application of tandem roller for asphalt construction, hydraulics technology does, however, hold distinct advantages. Hydraulic drives have been state- of-the-art technology for these mobile machines for decades. Therefore, technicians’ familiarity with hydraulic components, circuits, and systems proves advantageous.
The high level of service friendliness was also important to Hamm‘s decision to go with a hydraulic hybrid. The high power density of the hydraulic accumulator and hydrostatic drive is a clear advantage considering the limited installation space available. Two aspects of the hydraulic hybrid are particularly significant: the readily amortizable system assembly and the robustness of the hydraulic accumulators, which—above all— can easily withstand strong, permanent vibrations and high temperatures.
With regard to the hybridization topologies, serial, parallel, or even power-split approaches can be selected. Because the present drive system has two hydraulic main consumers (propulsion drive and vibration drive), a central input-output of energy connected to the diesel engine through a mechanical interface is an advantage. The boost function required for load-peak smoothing was implemented using the hybrid topology presented in without major modifications to the drive concept (Fig. 4).
4. Drive topology of the hybrid tandem rollers in Hamm’s HD+ 90i PH hybrid machine is shown at left, with a photo of the drive shown at right.
The tandem rollers with power-hybrid drive use a transfer gearbox between the 55.4-kW engine and the traction drive and vibration pump. A hybrid motor-pump (HMP) is also connected through the transfer gearbox. The hybrid motor-pump is a variable-displacement axial-piston pump (max displacement of 28 cc) from Bosch-Rexroth. A pair of HYDAC 10-l hydraulic bladder-type accumulators provide supplemental flow to accommodate peak loads. They are loaded and unloaded via a HYDAC control block.
Loading and unloading cycles are implemented with different swivel angles of the hybrid hydrostatic unit. Energy is buffered in pump operation, and the HMP is swiveled into the motor operation to meet peak load demand. With a diesel engine speed of 2,300 rpm, a maximum boost power of approximately 20 kW can be produced temporarily with a pump speed of about. 3,000 rpm, full swivel angle, and a maximum accumulator pressure of 280 bar. The hybrid functions are controlled with a HYDAC HY-TTC 30 controller, which is connected to the machine’s electronic control unit.
Successful Field Testing and Validation
Intensive trial and measurement runs have been carried out for validating the newly developed hybrid drive. Performance of conventional machine rated at 9 tons (85-kW diesel engine) was compared to that of HAMM’s HD+ 90i PH (55.4 kW-diesel engine) hybrid machine. Examples of measured cycle data are shown in Fig. 5, with the same operating phases shown in Fig. 2.
5. Measurement results for comparing a conventional machine to a hybrid version. Signature of engine power is shown at top, with the power signature of the hybrid drive (and its components) under that, and the pressure of a typical work cycle at bottom. Note how the accumulator pressure undergoes a sharp drop from about 4 sec to 6 sec. This indicates the accumulators providing supplemental energy to the drive during peak demand.
The performance peak is smoothed in Phase A2 by the energy feed of the hybrid drive, reducing the load on the diesel engine. During the compaction process at constant travel speed, (Phase B), considerably less power is required in the hybrid drive, even though the accumulators are being charged.
During the development stage of the hydraulic hybrid system (especially in connection with the software and function) the focus for Hamm and HYDAC was centered on the end users. Both the conventional and the hybrid machines were equipped with a field data recording device to obtain an extended data basis. Therefore, fuel consumption for both machines was recorded under real operating conditions over a period of approximately 230 hr and compared to one another. Results confirmed the same fuel savings as in the test cycles. Moreover, machine operators from such companies as Strabag (Germany), NCC (Sweden), and Eiffage (France) all agreed: no difference in handling or performance was noticed in comparison to conventional machines.
Laboratory tests were also conducted to confirm that the compaction result in the asphalt was also correct. As a result, several 9-ton rollers with power-hybrid drive were put into operation at a wide variety of construction sites (inner-city streets, country roads, and highways) throughout Europe.
After hundreds of operating hours, the operators praised the reduced noise and emissions above all else—a result from the smaller diesel engine. Not surprisingly, test results also showed a reduction in fuel consumption of at least 15%. An additional benefit for the machine operators is reduced maintenance. The 55.4 kW diesel engine functions without an SCR catalytic converter, so operators don’t need to add diesel exhaust fluid (AdBlue). Consequently, HAMM now offers the Power Hybrid vibratory roller HD+ 90i PH in six different series versions and with a variety of oscillation and vibration options.
Following EU emission legislation requirements for Tier 5, the manufacturers of mobile equipment have focused on producing machines with internal-combustion engines rated between 19 and 37 kW. Many machines in this category are comparatively severely affected by Tier 5 and now require such technologies as common-rail injection and exhaust gas aftertreatment (particle filters, SCR, or both). Great challenges exist here with respect to costs and installation space. Here, too, hydraulic hybrid systems, in combination with smaller diesel engine (less than 19 kW), may present a worthwhile alternative.
Daniel Feld is a development engineer at HYDAC International GmbH. For more information on HYDAC products and services in North America, visit hydac-na.com.