Traditionally, hydraulic pumps on utility, front-end and backhoe loaders, excavators, work trucks, cranes, and other mobile equipment have been driven by the machine’s diesel or gasoline engine, generally through a power takeoff (PTO). The constantly-running engine can be a problem on equipment used in residential areas, near hospitals, or other places where noise is a concern. Moreover, an always-running engine adds up to substantial operating and maintenance costs.
Combining a fixed-displacement pump with a variable-speed electric motor can provide efficiency in the 96% range because it enables the pump to run at the speed the application requires at the time instead of running an engine at full speed and using valves or other controls to govern pump output. But until recently, limitations in battery technology and high costs have made it impractical to use variable-speed electric pump drives in mobile equipment, especially with fully-electric vehicles.
In a conventional mobile hydraulics, a diesel or gas engine drives a hydraulic pump via a PTO to power the machine’s hydraulic functions. In a traditional electric-hydraulic system, the vehicle’s engine drives an electric generator that powers an electric motor that drives a hydraulic pump. Now, several manufacturers are going a step further with semi- or fully electric vehicles in which the hydraulic power and often the motive power is provided by variable-speed electric motors driving hydraulic pumps. This method is more energy-efficient by providing power on demand and reducing or eliminating exhaust emissions. In many applications, it also offers additional energy savings when a regenerative circuit recharges the batteries during load-lowering.
Battery Power Options
Most mobile equipment contains some type of battery to store energy, whether for supplemental or full electric power. Although lead-acid batteries are common, relatively inexpensive, and readily available, lithium-ion batteries provide three times the energy for their size. Although their initial cost is considerably higher than lead-acid batteries, they have advantages that include high energy density, relatively low self-discharge, and low maintenance requirements. Nickel-metal hydride (NiMH) batteries are another power-dense alternative, with an energy density approaching that of lithium ion batteries.
Absorbed glass mat (AGM) batteries, used in some mobile equipment, are a sealed lead-acid battery design in which the sulfuric acid is absorbed by a very fine fiberglass mat, making the battery spill-proof. An AGM battery has very low internal resistance, is capable of delivering high electrical current on demand, and offers a relatively long service life, even when operated until almost discharged—called deep cycling.
Whatever type of battery is used, a mobile electro-hydraulic system consists of one or more battery packs, which require a battery management system to control charging and determine how much each battery needs to be charged. Because battery-powered mobile equipment generally is used in urban locations or on shorter, more predictable routes, charging usually can be done in a stationary facility at a shift’s end.
Hydraulic pumps on mobile equipment have been designed and improved over time to be driven by internal-combustion engines, whereas electrically driven pumps have different requirements. For example, an inexpensive pump with high power losses can waste valuable battery capacity, so electric pump drives require high-efficiency pumps. In addition, axial-piston pumps often used in traditional mobile hydraulic systems can generate relatively high noise from vibration caused by pressure pulsations and related and widely fluctuating pump forces. This noise may be barely noticeable when the pump is used with a diesel engine, but noise can be significant when powered by a quiet electric motor. In addition, according to Bucher Hydraulics, Klettgau, Germany, operation of conventional axial-piston pumps in winch drives can be problematic because torque ripple issues can hinder performance, startup efficiency can be low, and high-pressure performance at low rotary speed may suffer.
To address these issues, Bucher Hydraulics developed a 24-piston, fixed-displacement version of its AX pump, which is said to outperform conventional axial-piston pumps in terms of efficiency, starting torque and speed range. Jan Friedrich, Bucher’s head of marketing, explains, “Previously, axial-piston units generally had seven or nine pistons with long strokes. Each side of the new pump contains 12 pistons, with pistons paired to face each other. As a result, the forces are completely compensated in the pump and motor units.”
Bucher Hydraulics’s 24-piston AX pump contains 12 pistons per side to balance reaction forces and provide smoother and more-efficient low-speed operation than traditional piston pumps.
Bucher reports that the AX pumps have mechanical efficiency up to 99%, even at start-up, because of low friction and direct torque transmission between shaft and pistons. Overall efficiency is said to approach 94% due to the short stroke, optimized displacement angle, balanced forces, and hydrostatic bearing. Displacement in AX series pumps and motors ranges from 18 to 76 cm³/rev, with continuous maximum pressure of 450 bar and a peak of 500 bar. Maximum speed is 3,600 rpm.
Another development is the use of high-speed solenoid valves to control hydraulic pump displacement. The Digital Displacement Pump (DDP), developed by Artemis Intelligent Power Ltd., Loanhead, Scotland., uses multiple radial cylinders that are enabled and disabled in real time with ultra-fast mechatronic valves controlled by an embedded computer instead of changing the stroke of the piston with a swashplate mechanism.
The Digital Displacement Pump is shown here installed in an excavator and instrumented with sensors for testing.
This technology makes it practical to replace mechanical gearboxes with hydraulic transmissions. Benefits include lower energy usage (typically less than one-third that of a conventional axial-piston swashplate pump), response times typically 10 times faster, and the complete elimination of high-frequency noise. When applied to an excavator, for example, the technology has demonstrated fuel savings of up to 20%, as well as an increase in productivity of nearly 30%.
The DDP is actually a series of fixed, positive-displacement, reciprocating piston pumps arranged radially around a cam ring. The pumps can be turned on and off individually, and each has its own control system, which consists of a solenoid-operated poppet valve, a check valve, and a piston position sensor
Driving the Pump
DC motors traditionally have been used to drive pumps because of their variable-speed capability (especially at low speeds), their simple control system, high starting torque, and good transient response. While brushed, wound-field DC motors have been the primary pump drive choice for many years, permanent magnet (PMDC) and brushless DC motors have become more popular. This is because of their simple and compact design, high efficiency and power density, a wide range of available frame sizes, and their need for less maintenance, according to Ohio Electric Motors.
The speed of a dc motor can be controlled easily by several different methods according to MET Motors. Possibilities include using differing tap arrangements from the battery bank, inserting a resistor in one of the circuits to give two or more different speeds, or using a simple rheostat or potentiometer in the circuit to allow varying speed over a given range. Another method is to use an electronically controlled switching device such as an SCR (silicon-controlled rectifier) or a PWM (pulse width modulation) control.
A different approach uses a brushless permanent magnet ac (PMAC) motor in conjunction with an inverter to convert the dc battery power to ac and provide the necessary motor control. One such system is Parker Hannifin's Global Vehicle Motor (GVM), a PMAC brushless motor. Parker’s project manager, Ciprian Ciuraru explains, “These motors are managed by high voltage motor controllers (inverters) suitable for hybrid and electric vehicles. The inverters are suitable for four-quadrant, synchronous or asynchronous ac motor control, providing speed, torque, and dc-voltage control modes.”
Managing the System
Supporting the trend toward decoupling hydraulic systems on mobile equipment from the vehicle’s motive power are solutions such as Parker’s Electro-Hydraulic Pump (EHP) system. It includes the company’s brushless PMAC motor directly connected to a hydraulic pump controlled by a high-performance electric drive designed for mobile equipment.
Parker Hannifin’s Electro-Hydraulic Pump (EHP) system, designed for mobile-equipment applications, consists of a hydraulic pump directly coupled to an electric motor and controlled by a variable-speed electric drive.
When used with a vehicle having an internal-combustion engine, the engine drives a generator that charges the battery. The vehicle’s work functions then become independent of the engine, with the EHP system’s electric motor driving the hydraulic pump. During load-lowering functions, the EHP regenerated electrical energy back into the battery.
Prototype and Real-World Examples
Here are some examples of electro-hydraulic drives in mobile equipment:
Bucket lift hydraulics. An electric version of the bucket lift, built by Versalift USA, Waco, Tex., drives its hydraulic pump with a battery-powered electric motor, saving fuel while also reducing emissions and noise. It employs a variable-speed dc motor that receives power from separate onboard batteries to drive its hydraulic pump. At the jobsite, the operator turns off the chassis engine and turns on the machine’s aerial master control switch, causing a series of on-demand switches located on various hydraulic valves to activate the motor-pump. It can operate for up to an hour without running the vehicle’s engine, reducing fuel consumption and emissions.
This Versalift bucket lift retains its IC engine but drives its hydraulic pump with a battery-powered electric motor, saving fuel while also reducing emissions and noise.
The Versalift electric drive is available in nine models, with three different voltages to meet a range of hydraulic lift requirements. A 12-V system uses two 12-V absorbed glass mat (AGM) batteries wired in parallel and a 12-V dc motor to provide flow to 2 gpm at up to2,500 psi.
AGM uses a battery construction with boron silicate mats between plates instead of a gelled or liquid electrolyte. AGM batteries are more expensive than the liquid variety but offer advantages. Because they contain no liquid to freeze or expand, they’re practically immune from freezing damage. AGMs also have a very low self-discharge rate—typically from 1% to 3% per month. As a result, they can remain idle for much longer periods than standard batteries without having to be recharged.
Versalift also offers higher-voltage systems when more power is needed. A 36-V system uses three AGM batteries wired in series and a 36-V dc motor to provide 3 gpm at up to 2,250 psi for the lift or 5 gpm at 2,000 psi for hydraulic tools. A 48-V system uses four of the batteries in series and a 48-V dc motor to provide up to 6 gpm at 3,000 psi. All systems use a high-efficiency hydraulic gear pump, and all batteries are sealed and maintenance-free.
All-electric excavators. Excavators and other types of construction equipment present an opportunity for electro-hydraulic operation, with battery-powered motors sometimes providing motive power as well as driving the hydraulic pumps. In one application, Danfoss Editron, a segment of Danfoss Power Solutions, has delivered electrification technology to support Pon Equipment AS in developing a fully battery-operated, 25-ton electric excavator. Pon, a major Caterpillar dealer in Europe, worked with Danfoss to convert a standard Cat 323F diesel machine to the Danfoss Editron drivetrain, a software-led system. The excavator, dubbed the Cat 323 FZ, can operate for up to seven hours on a single battery charge under nominal load, according to Danfoss.
This modified Caterpillar 323F excavator uses battery packs to supply a Danfoss Editron drives for propulsion and for driving the machine’s hydraulic system.
Unlike its diesel-powered counterpart—which emits up to 52 t/yr of CO2—the Cat 323 FZ produces zero emissions and is significantly quieter than the diesel version, which is important for meeting noise restrictions in urban areas. The Danfoss Editron powertrain relies on smart software controls suitable for hybrid and full-electric machines with power ratings of 30 kW to 2,000 kW.
Another example is a new mini-excavator being introduced by Mecalac, Annecy-le-Vieux, France, which combines all-electric power with hydraulics for work functions. Mecalac specializes in construction equipment for urban jobsites, where compact size and quiet operation are paramount. The Mecalac e-12 is a modified versin of the company's 12MTX model in which designers replaced the internal combustion engine with a lithium iron phosphate (LiFePO4) battery pack. The e-12 also addresses the European construction market’s trend toward zero-emission standards for small- to mid-sized vehicles while delivering greater efficiency and reliable performance.
Mecalac’s e12 all-electric mini-excavator is a modified version of the company’s 12MTX model, but with a lithium iron phosphate battery pack for power instead of an-internal combustion engine.
The battery pack has an eight-hour working range and a service life three times longer than conventional batteries. It delivers 146 kW of power and can be charged in about seven hours. The e-12 reportedly uses two variable-speed electric motors: one to drive the wheels and the other to drive the hydraulic system’s main pump. This matches power delivered to the hydraulics to that required by the load and conserves energy.
Electric Refuse Truck. A demonstration model of an all-electric refuse truck, the Mack LR Battery Electric Vehicle (BEV), will be put into operation by the New York City Department of Sanitation in 2020. The vehicle is powered by Mack’s integrated electric powertrain, which consists of two 130-kW variable-speed motors that produce a combined 496 peak horsepower and 4,051 lb-ft of torque, available from zero rotational speed. Power travels to the wheels through a two-speed Mack Powershift transmission and Mack’s 52,000-lb. proprietary S522R rear axles.
Mack LR Battery Electric Vehicle (BEV), uses a pair of lithium nickel manganese cobalt oxide battery packs to power all functions instead of a diesel engine. The machine will be put into operation in New York City next year.
Hydraulic systems for the LR BEV’s Heil DuraPack 5000 body and all other accessories are driven electrically through 12-, 24-, and 600-V circuits. Electrical power comes from four NMC lithium-ion batteries (lithium nickel manganese cobalt oxide), which are charged by a 150 kW, SAE J1772-compliant charging system.
Mack officials say refuse and recycling collection is an ideal application for BEVs because vehicles operate on predetermined routes and return home at the end of a shift, minimizing concerns about range and recharging. Their frequent stop-and-start operation also provides a significant opportunity to recapture energy through regenerative braking. Because such fully electric trucks produce zero emissions and considerably less noise, they are conducive to nighttime operation, particularly in urban environments.