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Ricardo Brings Flywheel Energy Storage to Excavators, But Don't Call It a Hybrid

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Photo by Jeff Rubenstone for ENR
Ricardo's flywheel can store power from an excavator's hydraulic system and then sent it back when needed using the hydraulic pump motor seen here on this acrylic model.
Photo by Jeff Rubenstone for ENR
The flywheel can be seen in the center of the housing cylinder, where it will be able to spin freely in a magnetic field.
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Engine manufacturer Ricardo normally doesn't go seeking attention from the press. While it supplies engines to everyone from the biggest heavy equipment makers to McLaren sports cars, the U.K.-based company is normally happy to keep its name off the engines of its OEMs. But Ricardo was proudly touting its new flywheel energy-storage technology at CONEXPO 2014, claiming the system will bring real-world fuel economy improvements of 10% to excavators.

"10% real-world improved fuel economy. That's for real digging in real life and real work in real holes across a range of different duty cycles," says David Rollafson, Ricardo vice president for global innovation and intellectual property.

Ricardo's design uses a hydraulic pump motor which takes excess energy from lowering an excavator's boom arm during a dig cycle to spin up a 4-in.-long, 6-in.-dia. flywheel. The flywheel spins up to 45,000 rpm, and that energy is then released back through the hydraulic pump motor when required, reducing the need to rev up the engine. "Instead of running the engine at operating speed the whole time, we control the engine, so it will be on idle most the of the time," explains Rollafson. "When we start digging, we dump the energy from the flywheel through the pump motor to drive the hydraulics. and when the flywheel's kinetic energy is exhausted, we rev up the engine and it works like a normal excavator."

The flywheel in Ricardo's engine isn't mounted on a shaft. It's a free-floating cylinder suspended within a ring of powerful magnets. There is a one-millimeter vacuum gap separating the flywheel from the casing. Energy is transferred to and from the flywheel by the magnetic field. "It's a bit spooky," says Rollafson. "But if the flywheel on the inside spins clockwise, the outer ring goes counterclockwise." With the flywheel in a vacuum-sealed container with no shaft seals, there's no need for a compressor to maintain the vacuum. The sealed flywheel unit can be replaced in the field with another unit without the need for a vacuum pump to repressurize it onsite.

Using energy-storage systems during excavator duty cycles is nothing new. In fact there are quite a few hybrid excavators on the floor of CONEXPO this year that use a variety of methods. But Rollafson says the large, industry leading OEMs Ricardo is dealing with are a bit cagey about marketing engines with this flywheel as hybrids. "It's interesting talking to OEMs, who say 'You won't sell a hybrid excavator,'" says Rollafson. "Why not? 'You see them at exhibitions, but you won't sell any, because people will not buy it with a battery. If the excavator has a battery hybrid, the secondary markets, Eastern Europe, Africa, India, won't touch it.'" According to Rollafson, at least one of the OEMs they are dealing with will not be calling their flywheel-equipped excavator a hybrid. "Their thinking is that if they don't call it a hybrid, they can put flywheels into every machine and sell on fuel economy alone."

There are ways other than flywheels of storing energy on excavators mechanically, without a battery. Caterpillar's 336F H hybrid excavator, also on display at CONEXPO, uses nitrogen-charged gas accumulator tanks to store energy that would otherwise be lost when the crane slows down. But Rollafson doesn't see Ricardo's flywheel approach as competing with that technology. "A flywheel can deliver an immense amount of power in a short time, but it won't keep spinning forever. Gas-powered accumulators can capture energy and store it for longer, but you're not going to get the same burst of power as a flywheel. It's a tradeoff for different applications."

In addition to excavators, Ricardo is also bringing their flywheel-equipped engines to wheel loaders and mining trucks. According to Rollafson, the flywheel adds about $7,000 to $7,500 to the cost of the engine, but he expects the fuel economy savings will pay that back. While he declined to name the major OEMs Ricardo is working with, he says hybrid excavators with Ricardo's flywheel technology should be available on the market in about 18 months.

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