A Cummins-Peterbilt SuperTruck demonstrator notched 10.7 mpg with a fully loaded tractor-trailer, representing a 75% fuel-economy increase and a 43% greenhouse gas (GHG) emissions reduction compared to a 2009 baseline truck.
“The project team successfully completed both the drive cycle and 24-hour objectives of the SuperTruck program,” Jennifer Rumsey, Cummins Inc.’s Vice President of Engineering, Engine Business, noted in an interview with SAE Magazines.
Powered by a Cummins engine, the Peterbilt Class 8 demonstrator truck averaged just under 9.9 mpg in 2012 when driven on a 312-mi (502-km) route along U.S. Highway 287 from Fort Worth to Vernon, TX, going roundtrip to minimize possible tailwind benefits. That same route was driven in December 2013 when the latest version of the SuperTruck tallied 10.7 mpg.
“The terrain is generally rolling hills with elevation changes up to 600 feet. Per the Department of Energy (DOE) program guidelines, the gross vehicle weight was 65,000 lb and the cruise speed was set to 64 mph,” Landon Sproull, Chief Engineer at Peterbilt Motors Co., explained to SAE Magazines.
In 2010, the U.S. DOE initiated the SuperTruck program to improve long-haul Class 8 vehicle freight efficiency, a metric using payload weight and fuel efficiency defined as ton-miles per gallon. Three projects are under the SuperTruck umbrella: the Cummins-Peterbilt project as well as a separate program involving Daimler Trucks North America and another program involving Navistar Inc.
The Cummins-Peterbilt SuperTruck demonstration showcased an 86% improvement in freight efficiency, exceeding the DOE’s program goal of 68% freight efficiency versus a 2009 baseline tractor-trailer with the same gross vehicle weight traveling the same highway route.
Peterbilt’s Model 579 underwent assorted changes to create a fully integrated aerodynamic tractor-trailer unit. According to Sproull, the alterations included opening the bumper for improved cooling and aero-performance; adding a NASCAR-style chin splitter with an air dam beneath the bumper; creating sculpted returns from the trailer bogie to the bumper; mounting a trailer tail; and installing full skirts on the trailer that retract between the truck and trailer tandems for low-speed clearance events, such as curbs, railroad tracks, and inclined loading docks.
One of the key focus areas for engineers was airflow optimization around the gap between the tractor and trailer. “We did this by lengthening the rear sleeper extenders and adding a nose fairing to the trailer to decrease the gap,” noted Sproull.
Other fuel-efficiency improvements included weight-reducing material changes to the tractor-trailer.
Chassis changes included a mono-leaf front axle (replacing a taper-leaf front axle), a 6x2 tandem configuration (vs. a 6x4), magnesium crossmembers (replacing aluminum crossmembers), variable gauge steel frame rails (replacing constant cross-section steel rails), ceramic aluminum brake drums (vs. cast iron drums), an aluminum fifth wheel (replacing steel fifth wheel), and super-single tires with aluminum wheels (replacing dual tires with steel wheels), according to Sproull.
Lightweight materials also were used on the trailer, including an aluminum frame and subfloor, a lightweight steel tandems assembly, ceramic brake drums, and super-single tires with aluminum wheels. In addition, the front of the trailer skirts and the trailer front’s “horse collar” are made from durable, lightweight carbon fiber.
Modifications to the production Cummins ISX15 diesel six-cylinder engine elicited 50% thermal efficiency, a 20% improvement from baseline 2009 engine efficiency.
Said Rumsey, “The work encompassed single cylinder engine developments, combustion analysis and iteration, cycle analysis, friction and parasitic power reduction developments, and multi-cylinder development combined with parameter optimization techniques to attain high efficiency.”
Engine revisions included calibration optimization, new piston bowl geometry, new specifications for fuel injectors and the turbocharger, as well as an engine-integrated Waste Heat Recovery (WHR) system.
The system’s primary components are exhaust heat-collecting heat exchangers, a condenser, a pump, and a turbine expander that is connected through a gearbox to the crankshaft via a drive belt. “The WHR system has been in development at Cummins for the past 10 years,” said Rumsey. “SuperTruck enabled the system’s successful integration with an entire vehicle system.”
A commercially viable WHR system is edging closer to production reality. “The next cost-effective design will integrate function and reduce the package size and weight,” said Rumsey.
Several companies collaborated to design, analyze, and integrate the engine package, vehicle exhaust system, and high-efficiency cooling module. Modine developed the heat exchangers. Exa Corp. provided vehicle cooling module analysis. Cummins developed the system architecture and control methodologies while its Turbo Technologies Division provided the turbine expander, and Peterbilt handled vehicle system integration, according to Rumsey.
Gerard DeVito, Director of Product Management, Automated Transmission and Hybrid at Eaton, noted that the Cummins-Peterbilt SuperTruck’s test-drive in Texas was performed with an experimental version of Eaton’s production UltraShift PLUS automated manual transmission.
“Eaton developed a transmission that enabled fuel savings through engine downspeeding,” meaning lowered engine rpm at cruise power, DeVito noted. The SuperTruck’s transmission features small-step ratios less than 30% in the top gears while increasing the overall ratio spread to approximately 20:1 as well as precision dry-sump lube technology.
During the SuperTruck’s prove-out test-drive, Eaton’s in-development lubrication system used less oil than a traditional automated manual transmission. Compared to Eaton production transmissions using a similar lubrication system, oil consumption is reduced by one-third vs. traditional automated manual transmissions.
SuperTruck also served as the test bed for a fuel-cell auxiliary power unit to reduce engine idling. The solid-oxide fuel cell (SOFC) used the truck’s diesel fuel and ambient air for catalytically creating electrical power.
According to Rumsey, “The SuperTruck team successfully integrated and demonstrated SOFC no-idle power generation on the first demonstration vehicle, but several shortfalls in key performance metrics as well as the supply of a vital fuel de-sulfuring component have halted progress toward production launch.”
Additional SuperTruck activities are on the agenda. Said Rumsey, “There are still some ‘extra credit’ tests that we as a team are curious to answer, including the fuel-economy effect at increased or decreased payload.”
Cummins’ engineers are looking for further efficiency improvements in the final SuperTruck deliverable. “Our developments are focused now on extending the engine efficiency outlook to a 55% efficient engine system,” Rumsey said.