When the U.S. Army needs to move ammunition and other suppliescross-country, its soldiers often turn to a Heavy Expanded Mobility Tactical Truck, or HEMTT. These four-axle, diesel rigs can carry payloads of up to 13 tons off-road, on steep grades that look better suited for a tank. HEMTTs have little in common with their civilian trucks, except for one thing — they can both benefit from better gas mileage that doesn't come at the expense of power.
The HEMTT's maker, Oshkosh Truck Inc., went back to the drawing board three years ago to create a more fuel-efficient version of the truck, which it dubbed the HEMTT A3. Rather than trying to squeeze incremental improvements out of a conventional diesel engine, Oshkosh instead created a brand-new, hybrid-electric diesel powertrain.
Called ProPulse, this hybrid system propels the vehicle with four 140-hp ac induction motors, one mounted on each of the vehicle's four axles. A 305 kW synchronous generator, 400-hp diesel engine and a bank of ultracapacitors together serve as the vehicle's power pack.
This hybrid system isn't like anything you'd find on a Toyota Prius. "Unlike consumer hybrids, we don't have a mechanical drivetrain at all," notes Dan Binder, Oshkosh Truck's technical director. The HEMTT A3 has no transmission, no drive shaft, no transfer case and no torque converter. Instead, each motor directly drives a set of wheels, via the differential on each axle.
Currently undergoing field testing in the Nevada desert, the A3 has already shown impressive fuel economy compared to the current diesel versions of the HEMTT, which run on a 450-hp diesel engine. According to Binder, the A3 offers at least 20 percent better fuel economy. "And under some driving conditions, the savings is more like 40 percent," he says.
At the same time, the A3 brings other improvements to the table. Among them is the improved ability to transport the relatively compact A3 on a C-130 aircraft. The vehicle's electric power plant also lets the A3 act as a mobile 300 kW generator, one capable of powering a field hospital. "That has been a huge performance upgrade," says Binder.
All this should make the Army happy when the truck is ready to go into service in three years. But the A3 already has something to offer engineers, particularly those who need to shoehorn a powerful electric drive and energy storage systems into a small, hot space.
At first glance, the sizeable A3 doesn't seem like it would pose many problems in the packaging department. It looks like there's plenty of room for motors and drives. But looks can be deceiving. "Once we packaged the payload, cab, tires and the structural components, there was really very little room left for the motors and drives," Binder says.
In early A3 prototypes, Oshkosh engineers tried to get by with off-the-shelf liquid-cooled motors and air-cooled drives. But these components proved too big. "We found out that there simply wasn't anything on the the commercial market that would fit in our packaging space," Binder says.
Making matters worse, the A3's motors and drives needed to withstand a harsh environment since they are mounted underneath the vehicle, right next to the axles. Ambient temperatures climb as high as 55C, with internal coolant temperatures of 85C. The motors and drives also have to be hermetically sealed against dust and moisture. "The A3 has to ford 60 inches of water, which submerses the drive system," Binder notes.
For the motors, Oshkosh worked with Rockwell Automation,which has an engineering group dedicated to hybrid vehicles, to develop a compact, liquid-cooled model that fit within the available space. Binder says the 140-hp induction motors occupy about the same space as a conventional 50-hp air-cooled motor. "It's all about managing the heat," he says.
Oshkosh also initially tried to use air-cooled power electronics, but found they just didn't provide the heat dissipation or environmental protection. "Ultimately," says Binder, "we had to go with custom, liquid-cooled solution." Oshkosh again turned to Rockwell to develop these custom drives for the A3.
Rockwell wound up creating a liquid-cooled, line-regenerative drive system that takes up about 1 ft3 of space and weighs less than 75 lb. "This is a phenomenal
accomplishment," says Jerry Pollack, new business development manager for the company's hybrid group. He estimates that a comparable air-cooled set-up in a typical industrial setting would occupy a 2 × 3 × 7-ft cabinet.
One key to shrinking the drive came down to direct bonding all the power electronics to a chill plate. Rockwell has been direct bonding electronic components — like rectifiers, transistors and diodes — to cold-plate materials for more than a dozen years now. This capability came in handy on the A3. According to Pollack, the direct bonding of the power electronics to the chill plate eliminated two thermal interfaces per electronic component, not only saving space but also improving heat transfer. "Direct bonding tightly couples the electronics with the cold plate for maximum heat transfer," Pollack says, noting that the cooling loop removes more than 90 percent of the device's watts.
More thermal and packaging advantages came from changes to the input reactor used on the drives. Had these been air-cooled they would have been 10 × 10 × 12 inches, nearly as big as the entire drive package, Pollack reports. But Rockwell managed to shrink this component too, by focusing on the cooling. "We found a unique, proprietary way to restructure the wound reactor core in such a way that we could add liquid cooling," he says, but refuses to say much more about the method.
Pollack estimates that direct bonding the power electronics and shrinking the input reactor together accounted for 75 percent of the size reduction in the drive package.
Another important space-saving technology related to the cooling, Oshkosh kept the plumbing to a minimum by sharing cooling loops when possible. So the motors and drives share a water-ethylene-glycol cooling loop. The motor is additionally cooled by a loop containing the differential oil.
Controlling the ProPulse system is an Allen-Bradley PowerFlex 700 motor controller, which handles the torque and speed regulation for the motors. Like the custom drive components, it needed only minor hardening to withstand the 30g shock and vibration loads that Oshkosh engineers specified.
Another way in which the A3 departs from consumer hybrids is in its energy storage system. Rather than batteries, the A3 has a bank of ultracapacitors to store excess energy created by the generator and the vehicle's regenerative braking system. The energy is put to use during acceleration — the four motors need 560 hp to run full-out, while the A3 generator only produces about 400 hp. "The ultracapacitors offset any mismatch between supply and demand during acceleration events," Binder says.
During regenerative braking, the ultracapacitors also capture the energy overflow from the generator. Binder says Oshkosh has a unique take on regenerative braking:
When the ultracapacitors become full during steep declines, the system quickly bypasses them. It instead sends the excess braking energy into the generator, which then functions in an engine braking mode with the help of a Jake Brake. According to Binder, these energy storage and engine braking functions are usually separate. "I believe we're the first to integrate them," says Binder. "The transition between energy capture and engine braking is seamless to the driver."
And that seamless transition is one of the reasons Oshkosh picked ultracapacitors over batteries in the first place. "Ultracapacitors don't store as much energy as batteries but what they do store is more useful from the standpoint of a quick turnaround," Binder says, pointing out that the ultracapacitors can fully charge and drain in seconds.
Batteries have another drawback in terms of size and weight. To get equivalent energy storage, batteries would have weighed four or five times more than the ultracapacitors, Binder estimates.
Ultracapacitors finally play a small role in keeping the troops safe. Binder explains that they helped Oshkosh build a "zero-voltage maintenance mode." Flipping a switch on the dashboard quickly drains the ultracapacitors. Otherwise mechanics would have to contend with up to 600V when working on the drive system or generator.
Other Uses Possible
While the A3 is a military vehicle through and through, the technology it uses could have civilian applications, too — especially as diesel emissions standards tighten. Binder says heavy trucks, as well as construction, mining and agriculture equipment, could reap the benefits of a hybrid-electric drive. The ones that benefit the most would have duty cycles something like a military vehicle — garbage trucks and concrete mixers, for example. Or the hybrid technology could work in emergency vehicles, which can benefit from the acceleration provided by electric drives.
Binder points out the A3 has been optimized for a military duty cycle in which all the off-road driving results in lots of throttle activity, as well as numerous starts and stops. Because the ProPulse decouples engine speed from the propulsion, the A3's diesel hums along at nearly a constant speed. "The sweet spot is about 1,800 rpm," he says.
Binder attributes efficiency gains to the fact that the engine doesn't need to ramp up and down much in response to the vehicles' difficult duty cycle. "You lose most of your efficiency due to frequent changes in engine speed," Binder says. It's hard to say just how much more efficient the ProPulse-driven A3 is compared to a conventional diesel, since dynomometer testing presupposes a mechanical drivetrain. "Dyno testing isn't a good fit for a hybrid," Binder argues. "But in field testing, we're finding that a 400-hp A3 has the same or better performance than a conventional truck with a 455-hp engine." And then the fuel economy improvement of 20-40 percent speaks volumes about improved efficiency, too.
Rockwell's Pollack says there's also been lots of interest lately in adding small, liquid-cooled drives and motors to agriculture, mining, forestry and construction
equipment — but not as a way to propel the entire vehicle. "The OEMs are investigating electric drives as a way to power accessories and take some of the load off their conventional diesel engines," he says.
And not all of the civilian uses have to be mobile ones. Pollack says much of what Rockwell learned on the Oshkosh project translates into stationary applications, too. He's seen a recent up-swing in liquid-cooled drives for HVAC applications, for instance. "Liquid-cooled drives generally start to make sense at 200 hp and up," says Pollack. Above 700 hp, they can actually demonstrate a price advantage over air-cooled. Between 200 and 700 hp, their advantage usually comes down to one of the total installed cost, Pollack explains.
In some circumstances, though, liquid-cooled drives can make sense at even lower horsepowers. Some users may want to redirect heat into "less-valuable space outside a plant" rather than having to manage it inside — often in air-conditioned rooms — Pollack says. Others may have dirty manufacturing environments and require sealed enclosures that could interfere with air cooling. Still others may be space-limited. "If you're running the drives in a clean, cool environment and need less than 200 hp, I'd use air-cooled," he says. But if all those things aren't true, liquid cooling may be for you.
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