Peterbilt's 386 Hybrid

Ignore the "viper green" paint and the "hybrid" labels, and this Model 386 looks like a lot of road tractors out there.

November 2007, - Test Drives

by Tom Berg, Senior Contributing Editor - Also by this author

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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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