Hyped up over hybrids

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Seems like everything is hybrid these days. My dad had a bachelor's degree in agriculture from Michigan State University, and I remember him telling me about how crops were bred to produce hybrid species that were highly resistant to certain diseases, drought, and other conditions — and that was more than 40 years ago.

Ever since the introduction of hybrid cars, we see the word hybrid popping up everywhere. A news release arrived awhile back describing a new hybrid excavator from Doosan Infracore, Atlanta, scheduled for launch in North America in 2012.

Why is this excavator a hybrid? The release explained that the excavator “will be equipped with a diesel engine, electric swing motor, electric converter, and an ultra-capacitor that will store excess energy during turning and light-duty operations. The reserve electricity will then be used to assist the power of the engine during heavier work; allowing the engine to maintain low revolutions and high efficiency combustion during acceleration.”

Now I’m not surprised that Doosan is replacing the conventional hydraulic swing drive with an electric drive. Hydraulic gear motors are normally used for this function. And although gear motors are powerful, reliable, inexpensive, and extremely compact, they’re not especially efficient. Plus, the rotary output of a hydraulic motor can easily be accomplished with an electric motor — even if the electric package must be larger and heavier. So it stands to reason that if efficiency is your primary goal, you’d consider using an electric motor for the swing drive.

However, Doosan could probably save all that trouble by using a moreefficient bent-axis piston motor instead of the gear motor and use an accumulator instead of the “ultra capacitor” for energy storage. Plus, the hydraulic solution would eliminate the likelihood of overheating the motor and avoid the potential dangers of electricity. After all, excavators are often used near water and out in the rain.

But what I found more puzzling was that Doosan made no mention of using a hydraulic fan drive to cool the excavator’s diesel engine. Hydraulic fan drives can substantially improve fuel efficiency and reduce emissions of diesel engines. How substantial? According to Gary Gotting, of High Country Tek, a manufacturer of electronic controls (including those for hydraulic fan drives), testing has shown that simply replacing a vehicle’s conventional belt-driven fan with a hydraulic fan drive can cut fuel consumption by 14.5%.

For the record, I contacted Doosan to find out what type of drive this machine will use for its cooling system. I was told it uses a conventional belt driven fan drive. Therefore, if Doosan’s goal is to produce the “greenest” excavator on the market, I think they’re missing out if they don’t investigate the virtues of hydraulic fan drives. These fuel savers have been popping up in all kinds of off-highway equipment. They’re just not getting all the hype of hybrid electric drives.

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Alan Hitchcox

Alan joined Hydraulics & Pneumatics in 1987 with experience as a technical magazine editor and in industrial sales. He graduated with a BS in engineering technology from Franklin University and...
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