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Perkins showcased three different hybrid power solutions at bauma: the hydraulic hybrid is in the foreground, the mechanical hybrid in the middle, and the electric hybrid furthest away. (Gehm)

Unique hybrids required

Power systems expert at Perkins stresses that on-highway electric solutions cannot be directly applied to the more-rugged and varied applications in off-highway.

Hybrid powertrains still account for a small portion of the off-highway vehicle market—less than 1%, according to Oliver Lythgoe, product marketing executive at Perkins—but interest is most certainly intensifying. Engine makers cannot delay R&D activities in this area, lest they be left behind.

“Outside of forklift trucks, electrical machines are a very small part of the market as well,” Lythgoe told Truck & Off-Highway Engineering at the bauma 2019 trade show in Munich. But that will not always be the case, he added. “We have to be able to help OEMs plot out those strategies and make sure that when the market changes, either through economics or regulation, that they’re ready.”

Perkins has conducted research projects in this space for about 10 years now, Lythgoe said, focusing on fundamental technologies such as battery ruggedization as well as on system integration. In the past two years, these activities have “ramped up considerably,” he said, running a number of technologies in test cells in its labs, and creating an engineering team dedicated to working with OEMs on hybrid and electric solutions.

“OEMs are very pressurized, it’s not a new thing. But the waves of regulation coming from different governments around the world are a constant pressure on OEM R&D departments,” Lythgoe explained. “It’s not just emissions, it’s other safety issues, lighting, noise, RoHS, all sorts of compliance challenges…And now with stakeholders asking, ‘By the way, can you develop some hybrid and electric versions, please?”—that comprises a completely different skillset on an incredibly burdened team and already-stretched resources.”

The size of Perkins’ dedicated team ultimately will be determined by customer interest, starting at the bauma show in April. “We’ve had a lot of decent conversations here,” he said. “If we end up running 15 projects with customers this year, then it’s going to look very different, where we’re running three or four [teams].”

Hybrid architectures aplenty
The duty cycles, operating conditions and packaging constraints for off-highway machines drive the need for specific configurations that are highly customized to the individual application. These factors differ to such an extent between off-highway machines and other sectors such as automotive, truck and marine that there are limited opportunities for technology transfer.

“Particularly different is the amount of g shock that machines experience—physically the solutions could not directly carry over, but also in terms of performance,” Lythgoe explained. “The urban driving cycle relies on congestion and braking at traffic lights—that’s where the inefficiencies are that hybrids solve. None of those things exist off-highway. So, the opportunities for efficiency improvement are in some other place in an excavator or wheel loader.”

The operating cycles from one piece of equipment to another also vary enough that one solution does not exist for all construction machines. “The right solution for a telehandler might not be the same as for a road roller, which might not be the same for an excavator or a wheel loader. There’s quite a range of optimal solutions.”

Perkins highlighted three different hybrid power solutions at bauma, all based on the company’s Syncro 2.8-L Stage V engine and all generating 75 kW (100 hp) of peak power to provide a direct comparison.

The first is a hybrid-electric setup using a motor generator with the engine, lithium-ion batteries to store up to 6 kWh, and 48-V electrics. Long-duration energy storage makes this a suitable option for machines with less-predictable operating cycles, “but it likes to release energy quite slowly,” Lythgoe explained. “So if you want to release a lot of energy, you need a big battery, which becomes hard to package in a machine and gets expensive.”

A mechanical hybrid stores energy in a high-speed flywheel that can be delivered back to the machine. This setup is particularly useful in hybridizing machines that run a cyclic operation and need very intense bursts of additional power. Installation size is another benefit compared to hybrid-electric: “In a very small space you can package this into an existing machine,” he said. But the system has an energy-storage capacity of only 0.05 kWh.

The third solution is a hydraulic hybrid that stores energy (0.04 kWh) as high-pressure hydraulic fluid in accumulators, which could be the most practical and cost-effective hybrid solution for some machines, Lythgoe said.

“Many machines already have lots of hydraulic architecture in place—pumps, valves, motors, electronic controls—so the effect on the machine will be quite minimal and quick to implement. Another thing is it’s technology that everybody understands,” he said. “However, if it’s not a cyclical operation, it’s not going to help you.”

The market will decide
The solutions don’t end there. According to Lythgoe, there are eight different hybrid architectures that could be viable for one type of off-highway machine or another, including “several different flavors of electrical hybrids,” he noted.

Micro hybrids provide a small amount of electrical boost with not much recovery but in an inexpensive package. Series hybrids only deliver power from an electric motor run by a battery or a gasoline-powered generator—or versions that integrate renewables. “For fixed installations in telecoms, for example, you could integrate solar panels or wind,” Lythgoe said.

Parallel hybrids have a much more-sophisticated transmission to be able to switch between power sources: the electric motor and internal combustion engine, which can provide power simultaneously. Pure electric machines are another option, as is a “limp home” range extender that incorporates “an undersized engine to get out of trouble or to just work that extra hour in the day,” he said.

“Even in the electrical options, there’s quite a lot of differences in voltages—48 volt or 400 volt, 600 volt or 700 volt. Some are AC and some DC, and these are fundamental questions to how you build a machine, how much it costs, how efficient it is, and also what kind of technician you need to maintain it.”

Recharging in remote locales poses a challenge. “Some cases it will be solvable,” he said. For example, in small wheel loaders commonly used in agricultural applications, charging points can be placed at fixed points on a farm where machines always return. “But there’ll be other [applications] that are almost impossible to resolve. If you are operating the excavator that is building the electrical substation, it’s hard to see how you’d get around that.”

Lythgoe noted that “real technology” is currently running in its test cells, but he could not share which customers and which machines are involved. And plans for technology rollout will be guided by market demands, he said.

“We’re going to listen to our customers and see where they want to go, where they’re being driven by their stakeholders. If it’s, ‘Hey, let’s get fuel economy now,’ then it’s going to be one set of technologies. If it’s, ‘Let’s take a slightly longer view but get to zero tailpipe emissions,’ then it’s going to be a different set of technologies. We have to be guided by what the market wants rather than what we want sell.”

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