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With an electrically driven wheel head, ZF engineers developed a drive solution for applications in agricultural machinery.

Hybrid variety in off-highway

Perhaps the most interesting point about hybrids in the off-highway market is the sheer variety of architectures and energy storage choices.

“Other industries, especially passenger cars, feature similar architectures of hybrid electric vehicles,” stated Mark Kuhn, Vice President for Ricardo Strategic Consulting, in interview with SAE Magazines. “Not so in off-road, where you have hydraulics, mechanical energy recovery, and ultracapacitors as well as batteries.”

He pointed out that some wheeled loaders from John Deere, such as the 644 K or 944 K, do not store any energy at all, but re-deploys any regenerative braking energy instantaneously.

“When they designed this, energy storage (e.g., batteries) was the most expensive component and eliminating it reduced the cost while still providing substantial benefit,” he said. Coming up with novel ways to reduce cost and provide benefit is the driver behind variety.

“John Deere chose electric hybrid technology for several reasons,” said John Chesterman, Product Marketing Manager, for John Deere Construction & Forestry. Beyond reducing fuel consumption are a number of other reasons including: decoupling the engine speed from vehicle speed, traction control, rim pull control, durability, and ease of operation.

“The 944K with individual wheel motors can control the wheels from spinning out like they do on the other competitor’s loaders in this size class in heavy digging or pushing applications. This reduces tire slicing and tire costs for our customers,” said Chesterman. He also noted that they use oil cooled, brushless ac electric motors and generators that are sealed from dirt and moisture, and using motors means no reverse shifting.

“We simply reverse the direction of the electric motor,” he said, quite a convenience with a typical v-pattern truck load with four reversals. “[Our customers] have found that the John Deere hybrid wheel loaders are much easier to operate and reduce the amount of fatigue over the course of a day of operating.” He added that users of the 944K Hybrid are happy with the low fuel consumption, overall performance, and ease of operation.

“I think the ability to apply hybrid electrics for off-road construction equipment is an easier transfer of that technology from on- to off-road. As a result, if you look at all the hybrid architectures that are in equipment today, most of them are hybrid electrics,” said Ricardo's Kuhn. There is lower technical risk because it is more of a known quantity.

ZF is a good example of this. “We can exploit all kind of trends within our development landscape. [Many types of] energy storages are analyzed and monitored by the corporate advanced engineering of ZF,” explained Udo Kneitz, Executive Vice President ZF Off-Highway Systems.

He sees both hybridization and electrification as growing in off-highway. “This trend is to be seen especially in the agricultural segments where PTO-powered implements are very common,” he said.

This approach has its pay-off in the agricultural segment with its ZF-TERRA+ system. This is an interesting idea for electrical hybridization, incorporating a generator module integrated into a transmission housing to power auxiliary electrical loads. As an enhancement to the TERRA+, ZF offers a control unit for intelligent energy and hybrid management. The control unit is intended to avoid vehicle conditions with poor efficiency, thus achieving best operational performance.

For ZF electrification and hybridization are elements of a comprehensive plan for improving efficiency. According Kneitz, other baseline technologies were needed as well.

“With the development of fully mechanical-hydrostatical power split CVT transmissions we did a first step along our technology roadmap,” he said, referring to the TERRAMATIC CVT for agricultural equipment. The latest version is rated at 450 hp.

“To upgrade driveline technology in our eyes is the basis for any meaningful hybridization in terms of saving energy or fuel and increasing efficiency,” he said. He also noted that ZF TERRA+ is only one example for the electrification of implements. Adding a 60-kW generator system integrated into this TERRAMATIC transmission is yet another.

Kneitz also acknowledges the potential in other industries beyond agriculture. “We will see also hybrid systems for construction machinery driveline systems and drives on future construction sites. This technology evolution needs to be driven by market demand with reasonable total costs of ownership on the one hand and governmental emissions regulations on the other,” he said. “The ZF hybrid module works as a parallel hybrid with an efficient electric machine, providing up to 85 or 120 kW performance, depending on the size.” As far as he is concerned, as soon as the market signals its readiness, ZF will be too.

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