Most dump trucks operate in urban settings, with a lot of stop-and-go. This is also where hybrid-drives offer the greatest fuel savings achieved by capturing braking energy and using it to relaunch the vehicles. Peterbilt has assembled a Class 7 Model 335 dump-truck chassis employing Eaton's electric system.
The term “pickup & delivery” is associated with parcels, freight and beverages; not dirt, gravel, stone and all the other commodities hauled by a dump truck. But most dumpers operate in urban settings, where stop-and-go is a large part of their days. This is also where hybrid-drives offer the greatest fuel savings achieved by capturing braking energy and using it to relaunch the vehicles.
People at Peterbilt Motors realize this and they've assembled a Class 7 Model 335 dump-truck chassis employing Eaton's electric system.
The truck was among two dozen Eaton-equipped vehicles at the Hybrid Truck Users Forum's annual ride-and-drive event, held last October at the Bosch proving grounds near South Bend, Ind. Track time was preceded by two days of informational sessions downtown where attendance of more than 550 set a new record. This, organizers said, showed ever-increasing interest in hybrid-drive trucks and buses by builders and operators.
In development and field use, electric hybrids are a couple of years ahead of hydraulic-drive systems. That's why most of the 35 hybrids available for demonstration driving were electrics, and 23 of those used Eaton's system.
Eaton's 24th was a Class 8 Peterbilt 320 trash truck using Hydraulic Launch Assist, a simpler and potentially less costly system. Bosch Rexroth, the meeting's official sponsor, showed its own hydraulic system in a pair of heavy trash chassis (one displayed and one for a ride-only demo) plus a Class 3 International military utility truck with a prototype hydraulic setup (for display only).
Eaton's electric system uses a 60-horsepower motor mounted in the driveline ahead of a 6-speed UltraShift automated mechanical transmission with an automatically operated clutch. During drifting and braking, the motor becomes a generator that drags on the driveline to produce electricity, which the system stores in a bank of lithium-ion batteries. During launch, electricity courses back through the motor and helps push the truck into motion. This takes some load off the vehicle's diesel engine, thus saving 30 percent or more in fuel.
Prices for diesel fuel have dropped $1.50 to $2 per gallon since last summer — the upside of the current worldwide recession — so a hybrid's monetary savings now aren't as great. But it's almost certain that as the economy comes back, fuel prices will go up again. So there's still a decent business case to be made for a hybrid, assuming the truck runs at least 20,000 to 30,000 miles a year and a buyer claims available federal, state or local incentives to offset the hybrid system's premium cost.
Driving an Eaton-equipped hybrid truck is pretty much like driving a regular one with an automated transmission: You start the engine, punch D-for-Drive on the selector pad, release the brakes, step on the accelerator and go. The clutch engages smoothly, and if you're easy on the pedal, electric power gets you underway until juice from the batteries is depleted. Then the engine, which has been idling, quickly revs up and takes over.
If you've impatiently mashed the “gas,” clutch engagement is still smooth, but the engine will cut in almost immediately and you'll take off quicker.
Of course, you haven't saved as much fuel as you could've, but that's true even without the hybrid system. If you start out on an upgrade, the engine will also begin working rather soon. The electric motor makes up to 60 horsepower, and its 310 pounds-feet of torque are available at zero rpm, so it can kick off smartly.
This is a “parallel” system, so both the motor and the engine run through the tranny, and they'll work alone or together. If you're conscientiously light on the pedal, the motor will go through several gears, whirring as it revs up and down. It'll go to 3rd gear and, if you nurse it, to 4th and almost 25 mph. It might go faster on a downgrade, but other-wise 24 or 25 is about the limit in electric-only mode, and the engine then cuts in.Speed requires horsepower and the diesel has much more of it — up to 300 in this case — so it does all of the propulsion work on afreeway or highway.
That's how any Eaton medium-duty electric hybrid operates, and so did this truck. The system's operating voltage is 340, and that's carried by thickly insulated orange cables. This is the industry standard color; most emergency responders have been told about it, and so should anyone who drives or maintains a truck like this. The system has built-in safeguards to keep people from an accidental shock, and there's a big orange push-button switch at the control box that shuts off the high voltage.
The short dump box was empty, so demands on the power train weren't serious; with a load, the diesel would've cut in sooner in any circumstance. The hybrid system is only for propulsion, but it could conceivably power an electric motor to run the hydraulic pump that could power the hoist, a snow plow, and other hydraulic mechanisms on a truck. If there's not a lot of regeneration from braking, the diesel spins the motor-generator to produce electricity so the batteries should never run low. For now, this truck's hydraulic dump hoist is run by a standard diesel-powered PTO and pump.
The engine in this truck always runs and does not shut down during pauses at arterial stops and other light-load driving situations. But it could, and does in another 335 truck shown at the HTUF event. Peterbilt and Eaton engineers programmed electronic controls to turn off the engine if it finds itself idling for more than a few seconds. The fuel-saving engine-off mode could be extended if the truck's power steering and other accessories were electric powered, and that's something that's in the near future for electric hybrid trucks.
Eaton's electric-drive system is strong enough for Class 8 trucks, and it was used with a 10-speed UltraShift in a Peterbilt 386 road tractor being evaluated by Wal-Mart Transportation. Thus it could conceivably be employed in heavier dump and mixer trucks and help them save fuel, too. So could a hydraulic hybrid system, possibly at lower cost than electric hybrids with their expensive lithium-ion batteries. We'll see what continued development brings us.
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