Test Drive Helps Kenworth Refine Its New Truck, Trailer Designs
By Bruce Harmon, Managing Editor
This story appears in the Feb. 16 print edition of Transport Topics. Click here to subscribe today.
RENTON, Wash. — In the struggle to squeeze a few more miles out of each tank of fuel, truck makers are designing tractors with the aid of computer models and wind tunnels more generally used to tweak race cars or airplanes.
The result is a generation of highly aerodynamic trucks that don’t seem to leave designers much more room to make further improvements, at least on the tractors. But manufacturers continue investigating ways to make trailers less of a drag.
“We need to do something with trailers,” said Brian Bowe, a test driver and engineer at Kenworth Truck Co.’s research and development center here.
Bowe has tested trailers with fairings and boat-tails that reduce wind turbulence, which can cut fuel use as much as 15%.
“You can see a two-gallon difference in just one trip,” he said.
Irregularly shaped flatbed loads and ocean-shipping containers are among the biggest problems, Bowe said during a test run, one of many that Kenworth does as it refines designs and factors affecting economy that range from trailer setbacks to management of an engine’s soot-filter burn-offs.
All truck manufacturers test their truck designs, sometimes running at constant speeds on oval tracks, sometimes putting them through their paces on the highway to simulate a fleet’s actual use.
Kenworth, which introduced one of the first aerodynamically Class 8 trucks — the T600 — has developed real-world, on-highway testing almost to an art form.
Engineers at the research center have been testing their trucks and competitors’ trucks over a set of standardized runs since 1985, when diesel fuel averaged 92 cents a gallon. That early step was “a key part in our history,” said Dan Kieffer, manager of the center.
The T600, introduced that year, was the first product of the company’s aerodynamic-conscious design and testing program. Subsequent improvements and new models are the result of research that includes on-highway testing designed to come as close as possible to the way the tractor is going to be used in the real world.
A run with engineer Bowe down Interstate 5 from Seattle to Portland, Ore., provides a good look at the sort of standardized testing that truck manufacturers perform. It’s one part of a rigorous and standardized cycle, known as the TMC Type IV protocol, that the company uses to figure a truck model’s average fuel economy.
The complete test cycle has three parts:
• A 494-mile run down I-5 from Renton to Portland, then east on Interstate 84 through the windy Columbia River Dalles to Baker, Ore., and back. It’s 10 hours on the engine, with three driver breaks. This segment gets a 50% weight in figuring the overall average.
• Contributing 30% to the average is an hour-and-a-half run at a steady 65 mph on Paccar’s 1.6-mile banked-oval test track.
• A high-load segment, 212 miles up to Snoqualmie Pass on Interstate 90 through the Cascade Mountains, a 3,000-foot altitude gain. Going up, the truck averages 27 to 34 mph, and on the return, “You’re on the jake all the way down,” Kieffer said. They do it three times, putting five hours on the engine. This whole segment contributes 20% to the weighted average.
Averaged, the three tests “give a good indication of the lifetime fuel economy of a linehaul truck,” Kieffer said.
Testing procedures have been kept consistent for thousands of runs, providing a “very consistent and repeatable” set of data for making comparisons of various factors, Kieffer said.
To isolate the single factor to be tested, other factors are standardized. Test trucks are fueled at the same time of day before and after the test — in the morning, before the trucks go out, and the following day, allowing the fuel to cool to the same temperature. The fuel can heat up to 120 degrees when the truck has run all day. When it gets hot it expands, so filling the tanks immediately afterward would throw off results by 6 or 7 gallons.
Tire pressure is kept at 110 PSI on the steer wheels, 100 PSI on drive and trailer wheels, and tire circumferences are measured, to allow for wear that would throw off distance measurements and the cruise control.
The trailer is loaded with 46,000 pounds of concrete blocks, the same weight every time, bringing the total vehicle weight to 65,000 pounds.
The truck used in this test is a SmartWay-compliant T660 with a 425 horsepower Cummins ISX engine, an 18-speed Eaton transmission, pulling a 53-foot Wabash DuraPlate trailer.
On the highway, Bowe aims for a consistent speed, setting the cruise control at 1 mph over the limit and using it about 95% of the time. In heavy traffic, he stays in the left lane to avoid on-off traffic.
Besides the truck’s standard instrumentation, the test truck is equipped with two panels of driver-information displays, the Cummins Road Relay system and Kenworth’s Driver Information Center.
It displays a bar graph comparing miles per gallon at any moment with overall trip miles per gallon and provides a visual cue when the engine is running at its optimal speed. It also shows total engine hours, and the percentage of time idling.
Other manufacturers have similar systems. Volvo’s Performance Bonus Guide includes a display of “$” signs when engine speed is in the most fuel-efficient “sweet spot.” Mack Trucks has a vehicle management and control system it calls V-MAC IC, and Detroit Diesel Corp. offers its DDEC VI, which monitors all engine functions including exhaust aftertreatment systems.
Kenworth’s Paccar stablemate has a Driver Information Display. Navistar Inc. plans to upgrade the driver information display on International trucks this spring.
Bowe watches the gauges, to keep engine speed below 1,500 revolutions per minute, and makes sure the turbo boost doesn’t exceed 25 pounds per square inch. He keeps an eye on the exhaust temperature so he can avoid “active” particulate filter regeneration, which requires a dose of fuel, using about a gallon.
Sometimes a driver can use “passive” regeneration, switching off the extra fuel dose and burning off built-up soot in the filter as the engine heats up when it’s climbing a hill. “But you have to know your route,” Bowe said.
Kenworth pushes the driver information systems for its customers, because, as Bowe said, “without instruments, you’re just driving.”
And as the truck turns off I-5 and hits heavier traffic in Renton, the gauges tell the driver what this congestion is costing. On a clear day with mostly light traffic, the truck averaged 7.13 mpg, but as it crawls along in the bumper-to-bumper jam, the trip average drops steadily, finishing at 7.05 mpg.
