March 2007, TruckingInfo.com - Test Drives
Think of the U.S. Air Force, and what usually comes to mind is high-performance jet airplanes. But this branch of the military also uses thousands of trucks to support its aviation missions at bases around the world. And soon a few trucks will go on duty with advanced hybrid systems, thanks to efforts by Volvo Powertrain and its sister companies.
Last month, Volvo Group – the company's Swedish parent – hosted a briefing on its hybrid vehicles projects. Mack Trucks, which is building six heavy trucks for the Air Force, showed off a pair of them – both Granite dump trucks. An RD-based heavy tanker used for refueling aircraft is now undergoing evaluation, while another Granite dump truck and two LE trash collection trucks are planned.
The briefing and demonstration took place in Washington, D.C., to underscore the importance of government participation and funding at this point in development. Contracts worth $6.8 million are paying for this project; Volvo plans to offer the systems in commercial trucks by 2009, and believes their advantages – such as fuel savings of up to 35 percent – will offset the hybrid components' extra costs and make them worth buying.
The ability of hybrid systems to reduce fuel consumption is key to western countries achieving energy independence from oil-rich, but politically troubled regions, said Leif Johansson, Volvo Group's president and chief executive officer. This, and fuel efficiency in general, will offset developing countries' growing demand for petroleum and bide time while hydrogen power – perhaps the energy source of the future – comes on line.
Sten-Ake Aronsson, who heads Volvo Powertrain in Hagerstown, Md., said this system is a parallel hybrid with an 11-liter MP7 diesel, an automatic clutch, a liquid-cooled electric motor that doubles as a generator, and a 12-speed automated mechanical transmission.
It will launch using electric energy only, diesel only, or both; but the diesel is needed for much of the work, which is why the system is called "mild." The driveline and tandem rear axles are standard Mack components. In operation, the motor helps launch the truck from a standstill, then, during braking, converts the truck's inertia to alternating current. This is inverted to direct current and stored temporarily in a bank of ultra-capacitors; when the truck moves out from a stop, the DC electricity is inverted back to AC to spin the motor-generator.
The truck launches smartly, and the driver hardly knows it's getting an electric boost. That was my impression from behind the wheel the next day on part of the parking lot surrounding RFK Stadium. If it were not for the chromed bumper and two big chromed boxes hung from the frame behind the cab and the smooth, self-shifting transmission, the GI driver might not know how special a vehicle this is.
The transmission is an adaptation of Volvo's just-introduced I-Shift. Mack people – like Guy Rini, director of advanced propulsion – simply call it an automated mechanical transmission, partly because Volvo Trucks is a rival even if it is a sister company. Also, this tranny has some mechanical differences: Its clutch is split away to remotely mount on the engine and ahead of the motor-generator, and its top gear is a 1 to 1 direct-drive for vocational use whereas I-Shift has an overdrive top gear for highway use.
There is no clutch pedal because the clutch engages and disengages automatically. All the driver does is start the engine, release the parking brake, move the shift selector from Neutral to Drive, and step on the accelerator. The truck moves out and the engine begins working almost immediately, maybe fooling the driver into thinking the diesel's doing all the work. It would be, if the ultra-capacitors were discharged, but the voltmeter on the instrument panel varied between 580 and 610, indicating there was a lot of juice available.
The average is 600 volts, said Tim Jacobson, a Volvo Powertrain engineer who's been working on this project for several years. The engine began roaring so soon because the transmission was programmed to start out in 1st or 2nd, Jacobson explained. As road speed matches the engine's 700-rpm idling speed in the start-out gear, it revs up and begins assisting. This was at about 3 mph in 2nd.
He showed me how to get the electric motor to do more of the launching: Put the selector in Manual and thumb a rocker switch on the selector until the readout on the dash says "6," the new start-out gear. Then step on the accelerator, and the truck moves out on motor alone until it reaches about 6 mph – the road speed it would be moving at 700-rpm idle – and at that point the engine began revving and doing more of the acceleration work. I soon discovered I preferred this because it's quieter and, of course, we were using more of the stored electric energy. The 160-horsepower motor is strong – one of the things that makes this system so driveable, Volvo Group's Johansson had said.
The start-out gear can be programmed to suit the operation, Jacobson said. Now it's set up for a trash collection truck. "We've gotten it to launch, go to 12 mph, stop for the next pickup, then launch again. We've gotten it to match an Allison" automatic in performance, he said. This is a dump truck, though, and the programming might be changed to suit a different sort of operation.
What will that be? Hauling dirt, gravel and other materials in support of "civil engineering" activities on Air Force bases, and picking up exploded ordnance on bombing ranges, said Capt. Jim Muldoon, one of the service's project managers. A dump truck is one application, he said. "There are others, but we'll learn a lot from this." Aside from saving fuel, the Air Force wants mechanics and operators to get experience with this technology, he said. Maintenance should be simpler because the motor-generator is also the starter and alternator; the third dump truck will also have electric-driven power steering, air conditioning and other accessories.
Other types of trucks will have greater use for the power generation aspect of the hybrid system, because the Air Force has weapons transporters that need 24 steady volts and up to 400 amperes to power their equipment. Right now, separate engine-driven generators supply this power, "but why use a generator when you've got a perfectly good one under the hood?"
Electricity in the hybrid dump trucks is used strictly for propulsion. The diesel powers the hydraulic pump and hoist for the Galion-Godwin dump body through a rear-engine power take-off, Mack's Rini explained. And while a hybrid trash truck might get away with a smaller diesel because it stops often to generate electric juice for launching, the dump truck would need a standard-size diesel because it doesn't stop as often.
But the engine can be derated somewhat because of the electric launch assist. The 11-liter MP7's normal ratings begin at 325 horsepower, but in these hybrids it's set for 315. Add in the motor's 160 horses and there's 475 on tap. That won't make the truck fly, but it does take off smartly, and that gives this mild hybrid a real role in preserving the "wild blue yonder."