Norrhydro, based in Tampere, Finland, and Volvo Construction Equipment are moving ahead with field trials of a new excavator concept that engineers say could revolutionize machine hydraulic performance. If proven successful and widely adopted, the system could improve overall productivity of various types of off-road equipment and help meet the mobile industry’s sustainability targets and electrification goals.
Key components of the design, which is based on a Volvo EC300E 30-ton excavator, include Norrhydro’s patented NorrDigi multi-chamber digital hydraulic actuators, a common-pressure rail (CPR) circuit comprising high and medium-pressure lines, and hydraulic accumulators for energy storage — which enable energy-efficient recovery of kinetic energy and the ability to supply peak power on demand.
Unlike traditional cylinders with two chambers — one pushing and the other pulling — the NorrDigi digital hydraulic actuator uses four chambers that can be connected in up to 16 different combinations, depending on the force required by the desired operation. In essence, this makes the device a variable-displacement actuator, said Teemu Sarvijärvi, technology development manager at Norrhydro.
The NorrDigi has the capability to vary the displacement in a discrete manner by adjusting the pressure connection of each chamber. Inside the four-chamber cylinder two positive forces are opposed by two negative forces. When all four cylinder areas differ in size and the CPR has two pressure levels, 16 different steady state force combinations are possible.
Directly mounted onto the cylinder is a valve manifold containing the necessary connections between its chambers and the CPR. Proportional valves control flow to and from each cylinder chamber. In contrast to conventional cylinders that supply the maximum force every stroke, the NorrDigi, only generates the speed and force that a task demands. “This makes it a very efficient system,” said Sarvijärvi, as it uses only a fraction of the energy for the same machine action compared to a traditional system.
Improving excavator efficiency
With growing interest in electrification and carbon-neutral operations, engineers are sharpening their focus on ways to improve the efficiency of industrial and mobile equipment. Excavators offer plenty of room for improvement in terms of energy efficiency in hydraulic systems, said Kim Heybroek of Volvo CE, in a paper published in the International Journal of Fluid Power. The single largest power loss normally comes from the inability to recover energy from overrunning loads, for instance when lowering a boom or decelerating the swing motion.
Another issue is pressure compensation losses resulting from operating multiple functions with different pressure levels in parallel. In many excavator systems, this is addressed using two pumps working at isolated pressure levels whenever possible, and together only when needed for faster operation of certain functions. Typically, installed pump power capacity exceeds the engine’s power capacity. Because the maximum torque of the engine then limits pump displacement, the pumps frequently run at non-optimal efficiency, said Heybroek.
Also, because high function velocities require high pump flows and small components are preferred over large components for cost reasons, the engine is typically forced to operate at high rpm where it has relatively poor efficiency and is noisier. Finally, in many excavator duty cycles, the work power profile is transient and high power is seldom used for more than a few seconds at a time.
The new Volvo/Norrhydro architecture addresses all of these issues. Using multi-chamber cylinders for the work hydraulics eliminates the problem of losses in parallel operation because the effective cylinder area is adapted to the load condition of each function individually. Moreover, the cylinders allow for energy recuperation from overrunning loads.
Recovered energy may be used directly by the other functions or stored in hydraulic accumulators connected to the CPR. The test EC300E excavator, via the CPR, also uses variable hydraulic motors to recover energy from the swing function. Energy storage capacity means the accumulators can provide a power boost when needed, and the engine and pumps do not need to be sized to provide peak power. Such a system is commonly considered a series hybrid.
The multi-chamber cylinders and digital hydraulics system offer numerous benefits. Because it uses less flow than a conventional system, hydraulic pumps can be smaller. It removes the need for a main control valve. The CPR is a shared circuit, making plumbing is simpler, as it is no longer necessary to route individual hose lines from the pump to every actuator. And the system generates less heat, which means that the hydraulic cooler may be smaller or possibly eliminated.
While multi-chamber digital actuators are more complicated than typical hydraulic cylinders, reliability is not an issue, said Sarvijärvi, as the units have been subjected to long-term cyclic and fatigue tests and field trials. While there are more valves on the cylinders, the overall design eliminates the main control valve in the system and the pumps only control charging the common rails. “So it’s not that complex a system,” he said.
Controllability matches or exceeds that of conventional cylinders. The hydraulic controls are mounted right at the cylinder, creating a very stiff system. When a valve opens fluid flows directly to the cylinder, providing instant power. “So it’s a very reactive system as well,” he said.
In addition, pressure sensors throughout the system provide ample data, letting OEMs and operators offer data management and predictive maintenance capabilities. And position sensors housed in each cylinder provide the exact boom location all the time. That opens the possibility of automated movements or deep control to meet customer requirements, said Sarvijärvi.
Because the system uses less energy, the power source can be downsized. On traditional ICE machines, that means a smaller engine. And the engine can be run at a near-constant, optimum speed, which has a significant impact on both noise and emissions. For electric vehicles, it can mean smaller electric motors and battery packs. Or it can significantly extend service time between charges.
The hybrid system is said to be particularly efficient in applications with high inertia loads, both linear and rotary, such as lowering or braking. And with higher available power, cycle times can be shortened.
The results to date are significant. Initial field trials show fuel efficiency improvements of 45 to 60% and productivity increases of up to 12% versus conventional machines performing the same duty cycles. Further field trials and engineering refinements are ongoing at Volvo’s excavator R&D center in Korea. The technology may offer even greater benefits in equipment like wheel loaders, said Sarvijärvi. And the multi-chamber cylinder design is scalable, making it suitable for a wide range of equipment.
If widely adopted, digital hydraulic actuators and related technology could boost productivity while radically cutting fuel bills and CO2 emissions in construction applications. The innovation is expected to accelerate the introduction of e-mobility across Volvo CE’s larger excavators.
In addition to partnering with Volvo, Norrhydro plans to offer the technology to other OEMs, said Sarvijärvi. “Volvo wants these kind of technologies to be used industry wide.” It will benefit all customers and users, and increased demand would lead to higher production volumes and lower costs, he said.
The technology is also applicable to stationary, industrial applications — anywhere there is potential energy available. But Norrhydro’s main focus is on mobile machines for construction, forestry, mining, and material handling applications, he said. The hybrid system may be commercially available in 2024.
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