But climb inside this Pete and look around, and you'll get some hints of how this truck's different.

There's a keypad transmission selector that's linked to a self-shifting gearbox, which is nothing new, but still unheard of in most fleets. There's also a special readout on the dash and a rocker switch over to the right that has something to do with the air-conditioning system, but you wouldn't notice them without some study.

Crank up the engine, release the parking brakes, punch Drive and hit the accelerator, and you can tell it's operating a little differently. The engine stays at idle for a second or two as the truck starts rolling, then begins revving as though to catch up. Touch the brake pedal and little numbers in that readout on the dash begin advancing, from 48 to 50 to 52, and you'll wonder what it means, unless you have an engineer like Bill Kahn along to explain it.

Kahn works in Peterbilt's Advanced Concepts department and helped design this tractor as part of the builder's hybrid program. Wal-Mart Transportation, which last year embarked on an ambitious quest for greater fuel efficiency, saw the vehicle as a way to do it. The Model 386, which combines an aerodynamic nose with the 379/389 cab, is like several hundred already in the fleet, except of course for its Eaton electric hybrid apparatus and an electric A/C system designed by Peterbilt and Delphi engineers.

Kahn rode shotgun on a warm May day when I took the 386 Hybrid on a drive out of Peterbilt's Denton, Texas, headquarters. He guided me along a series of interstate and state roads where traffic caused us to slow down and speed up, and a lot of traffic signals where the tractor's electric propulsion could be used. We pulled a loaded test van that gave me a good idea of the tractor's capabilities.

The numbers in the dashboard readout show the percentage of charge in the hybrid system's batteries, he said. They rise during braking because a generator between the engine and transmission is making the electricity as it drags on the driveline to slow the vehicle; air-disc service brakes add stopping power when needed. Electricity flows from the batteries and back through the generator, which now acts as a motor, to provide propulsion. This is mostly when starting out from a dead stop, but if there's enough juice in the batteries they'll also help climb hills while on the move.

The engine, a 430-horsepower Caterpillar C13, sends power through the driveline to move the truck under most circumstances, with the electric motor assisting. Because it doesn't have to work quite as hard to overcome inertia at "launch," and at other times, the engine consumes less fuel. Kahn and his colleagues figure the fuel savings at 5 to 7 percent, which many fleet managers would kill for.

Wal-Mart Transportation, based next to the huge retail store chain's headquarters in Bentonville, Ark., is looking for even higher gains, according to Mike Disney, logistics environmental sustainability manager. The 386 Hybrid is part of the company's announced intention of finding ways to cut fuel usage by 25 percent by the end of this year. By 2015 it intends to reduce fuel usage by 50 percent.

Advanced aerodynamics on tractors and trailers and reduced engine idling are other methods it's already identified. The company is experimenting with trailer skirts to reduce wind drag, and has equipped most of its 7,000 sleeper-cab tractors with Thermo King TriPac auxiliary power units.

"The APUs have cut idling a huge amount," Disney said, "by millions of hours per quarter. The APUs provide an 8 percent efficiency gain," which amounts to about a half a mile per gallon, so the fleet now averages 6.8 to 6.9 mpg. The hybrid fits into that scheme because its engine shuts down while its electric A/C system is at work.

Like the hybrid propulsion system, the A/C system runs at 340 volts DC. The dash switch lets the driver set its output at low or high, and the batteries supply power to run an electrically driven, variable speed, scroll-type refrigerant compressor (there is no engine-driven compressor). A condenser and fan are in a frame-mounted box outside. The system operates quietly and emits no fumes as it sends chilled air through vents in the dash and the sleeper box. A diesel-fired heater warms the sleeper in cold weather.

While the truck's parked, that dash readout shows the number of A/C minutes remaining, based on battery charge. The engine remains shut down until the system's controls determine that the batteries need charging, whereupon it cranks the engine and lets it spin the big motor-generator.

When it's done, the engine goes through a "soft" shutdown - something that Eaton and Cat engineers devised to avoid disturbing a sleeping driver. There's noticeably less shaking as the engine quits turning than with a normal shutdown.

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The engine can generate enough power in 4.5 minutes to run the A/C for 50 minutes when the outside temperature is 95 degrees F and humidity is at 50 percent, Kahn said. The stop-start engine operation allows the truck to stay legal even in places where laws limit idling; often that limit is five minutes, so the 4.5-minute running time complies under those circumstances. More engine use would be needed under hotter and/or more humid conditions.

This 386 Hybrid is unique in the Wal-Mart fleet. Disney said the tractor will not go into regular service, but instead is used for presentations (such as to the company's board of directors the day after I drove it) and to benchmark it against standard tractors. Meanwhile, another diesel-electric hybrid is in the works. It's being prepared by International, Cummins and ArvinMeritor, and will be based on International's new ProStar tractor. ArvinMeritor says it's due out in January 2009.

The green 386 also serves to familiarize drivers with the hybrid concept. "Guys that have driven it really like it," Disney said. "It's a paradigm change for them - there's no noise. They're used to noise when a truck takes off, and there isn't any" with the hybrid.

There's no noise at first because the motor silently begins moving the truck from a standstill, and the engine begins revving as its idle speed matches road speed and more power is needed to accelerate. Then revs rise and fall as the transmission changes gears and road speed builds. To my ears, engine noise after launch is pretty much normal, and was made worse because the 10-speed UltraShift tended to hang in each gear and didn't upshift until 1,900 or so rpm. This should be alterable by reprogramming the transmission's electronic controls, but in the meantime, I began prompting progressive upshifts by punching the M (for manual) button and then the Up arrow at lower revs.

The electric motor-generator makes as much as 60 horsepower and 340 pounds-feet. With a really light foot you can make the motor work longer on launch, but acceleration then would be really slow. Once I was able to use electric-only power while backing up, and the motor emitted a slight whine while the engine stayed at idle. Otherwise I never heard the motor, and while I could feel it work on launch, it was imperceptible while under way.

A really useful feature is the motor-generator's "hill-holder" ability. Kahn pointed it out when he noticed I was using the trailer brake to keep us in place on a slight upgrade as we waited to venture onto a busy street - from a Wal-Mart Super Center, appropriately enough. "You can use the hill holder here," he said. "The motor produces torque at zero rpm, so it can keep the truck from rolling backward. Just put your foot on the accelerator and release the brake." I did, and it worked. During this activity the clutch is disengaged from the still-idling engine so its facings aren't burned, and there's no harm to the motor-generator, either.

The Eaton UltraShift, with its automatically operated clutch, takes away much of the work of driving a heavy truck. It lets its driver concentrate on traffic and maneuvering through it, and seeing to his/her deliveries. Tests and fleet experience show it saves fuel because it makes everybody an economical driver.

Not so at Wal-Mart. "Our drivers are seasoned with many miles behind them, so the automatic doesn't do as well with us," Disney said. "We know it's part of the hybrid system and we have to accept it, but the manual transmission does better with our drivers for fuel economy," which of course is the point of the hybrid.

"At this point it's not cost-effective, and there's no return on investment," he said of the hybrid. "It's very costly" - how much neither he nor Kahn would say - "but it's doing what we want it to at this point. We're trying to understand what it can and cannot do, and seeing who (among suppliers) can provide best efficiency with the best return on investment. We want a production truck; we don't want a one-off vehicle."

The current high cost of the hybrid's "lithium-ion battery modules," as Kahn calls them, is primarily what makes the hybrid truck unaffordable for now, and its efficiency gains don't qualify it for any federal subsidies at this time. But he believes cost will come down as more of the batteries are used in other diesel- and gasoline-electric hybrids and volume production of the batteries gets under way. That might be two or three years away, which would jibe with Peterbilt's and Eaton's prediction of 2010 production for hybrid road tractors.

Much closer are other Peterbilt hybrids: a medium-duty public-utility bucket truck, with eight Model 335-based units now built and being outfitted with bodies for test; and a midrange delivery truck, with four 330 chassis with van bodies entering limited service. These use Cummins-made Paccar PX diesels with Eaton electric hybrid systems.

There's also a heavy trash-truck hybrid using a Model 320 low-cabover chassis; it has a Cummins ISL and Eaton's Hydraulic Launch Assist. Twelve of these are now being tested in fleets, and production could start next year. So watch down the street to see if a big green truck shows up to fetch your garbage.

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