By: Jim Park
CatalIST makes creative use of existing systems, relying less on advanced technology for its impressive gains.
The unveiling of Navistar’s SuperTruck in late September completes the first round of the Department of Energy’s fuel efficiency improvement demonstration project. Like the three others before it, Navistar’s truck, dubbed “CatalIST,” exceeded the DOE’s goals by some margin. CatalIST achieved a freight efficiency improvement of 104% compared to the baseline vehicle, a Brake Thermal Efficiency rating of 50.5% and it achieved 13 mpg in real-world over-the-road and urban duty-cycle test runs.
Aerodynamics played a huge role in the efficiency gains, while advanced technology such as kinetic energy recovery, off-engine accessory drives, variable speed compressors and lightweighting shared the rest of the glory. Navistar also brought some clever solutions to traditional problems that, as far as we can tell, are unique among the four SuperTruck projects.
“We engineered the aerodynamic design back to front,” says Dean Opperman, chief engineer for advanced vehicles and the supertruck group at Navistar. “We are pretty well tapped today on what we can do with the tractor alone until we clean up the trailer. Once we did the trailer, speaking in relative numbers, a 2-3% improvement on the tractor became an 8-10% improvement because of the overall reduction in drag on the rear of the trailer.”
Engineers got closer to the highly desirable teardrop shape by lowering the ride height of the air-over-spring steer axle suspension, developed by Hendrickson, and the trailer suspension giving the truck an airfoil-like top profile.
“It’s really tough to get trailer manufacturers to cross that bridge,” says Opperman. “At speeds above 50 mph, we drop the front and rear suspension by 1.5 to 2 inches. The boat tail at the rear helps to reduce the low pressure area at the base of the trailer, too. That relatively small shape change is very good for air flow.”
Additionally, the windshield is highly raked and curved for a very smooth frontal profile. The side fairings on the tractor are highly sculpted and the trailer features nose treatment and the Ventix Drag Reduction System from Wabash, which is essentially a vented side skirt. There is also a bogey fairing in front of the trailer axles.
Overall, Opperman says the aero treatments yielded a 50% reduction in aero drag compared to the baseline unit.
Navistar made wide use of kinetic energy recovery and off-engine accessory drives to reduce or eliminate load on the engine. The air compressor, for example, is clutched and runs only when the truck is not under power, such as when braking or coasting. Rather than a traditional compressor governor, Navistar’s system engages the compressor clutch any time the pressure is below 120 psi so the air reservoirs are constantly being topped up. Opperman says the GPS-based predictive cruise control system’s three-mile look ahead capability constantly searches for opportunities to engage the compressor and to harvest energy through an engine-mounted 15-kilowatt generator that powers a bank of 48-volt batteries.
The 48-volt battery is linked with a unique three-voltage bus for specific purposes. There’s a 48-volt bus for charging and energy storage, a 12-volt bus that powers hardware such as lighting, radio, etc., and finally there’s a 24-volt system dedicated to a 24-volt super-capacitor that provides the power for the 24-volt starter. The truck’s engine shuts off when the vehicle is stopped at a red light, but restarts the moment the throttle is applied. The big super-capacitor releases a lot of energy for each start cycle. These are very light and recharge very quickly.
The truck has a lot of extra electrical energy available, so it’s put to work driving things like the 48-volt HVAC compressor and two 48-volt fans mounted ahead of the AC condenser in the cooling package. That eliminates some engine load, weight and system complexity. There are two evaporator units, one for the cab, the other for the sleeper. With the lighter cooling load from the sleeper alone, the system can run efficiently all night on the energy recouped during the previous day’s operation.
The driveline on the truck is a 6×2 tag configuration, with unique two-way load-biasing capability. It will automatically load the driven axle to a higher weight by transferring suspension pressure to that axle at launch or whenever a traction event is detected. At highway speed more weight is shifted to the tag axle because those tires are typically rib tires which have lower rolling resistance than a traction tire.
The drive axle ratio is 1.91:1, which Opperman calls the “one of the tallest axle ratios ever produced for a commercial vehicle.” It’s from Dana, by the way. That’s coupled to a direct drive transmission, giving the truck an engine speed of 1,050 rpm at 65 mph.
Another unique feature of Navistar’s CatalIST is a pressurized cooling system; not the 15-25 psi we see in normal trucks, but significantly higher.
This permits higher coolant temperatures without localized boiling. That’s good from a durability perspective, but higher operating temperatures (210-220 degrees F) also improve engine efficiency and more importantly almost eliminate fan-on time.
“Predictive cruise pitches in here to optimize cooling needs when climbing a hill by “preconditioning” the engine by speeding up the water pump to cool the engine to a lower temp before the big power demand begins,” Opperman says.
As far as weight reduction goes, engineers managed to carve 2,700 pounds off of the tractor and 1900 pounds from the trailer. Some weight went back on for the aero kit, but the net weight reduction was in the order 3,500 pounds for a calculated 10% improvement in freight efficiency (increased payload potential).
As we have noted in previous SuperTruck reports, the improved efficiency provided by low-friction lube and bearings, 6×2 drive axles, etc. and the vast reduction in aerodynamic drag has lowered the horsepower required to maintain highway cruise speed. Opperman says the CatalIST requires a mere 75 hp to move 65,000 pounds down a flat road at 65 mph. That’s less than a third of what was needed 15 years ago with a classic-styled truck.
“Because the torque and horsepower requirement is so small, thanks to the aero improvement and low-friction drivetrain, the backup torque, driveabilty and gradeability is not as much of an issue here,” he says. “That said, we can’t down-rate too far because you still need torque and horsepower to climb hills and pull the weight at speed.”
This truck used a 475 hp, 1,850 lb-ft, N13 engine equipped with a prototype exhaust-based waste heat recovery system using ethanol as the working fluid. Opperman says at 75 horsepower at highway cruise speed there’s not a lot of engine heat generated so there’s not a lot of waste heat to go after.
“The technologies are fighting each other,” he says. “It’s very difficult to harvest energy in an efficient way. In the test cell we have seen between 1% and 2% fuel economy improvement out of a WHR system, but in a real-life road application there’s not a lot of heat to recover.”
Engineers managed 50.3% thermal efficiency with WHR while their baseline truck was 42.5%. That improvement came with WHR, but the thermal efficiency improvement without WHR was still 49.5%. That’s a lot of complex technology for an improvement of just 0.8%.
Navistar’s SuperTruck, like the three that were unveiled previously, demonstrates the gains that are possible with aerodynamics and some advanced technology. But Navistar’s effort showed some real creativity in the use of the variable ride height, the 48-volt charging system, the high-pressure cooling system and the two-way load biasing 6×2 drive axles, rather than relying on advanced technology like WHR.
What we see on this truck has some near immediate potential for some dramatic energy savings.
“We’re just scraping the surface of what we have available to us far as operating knowledge,” says Opperman. “When we start putting smart systems in place, such as this cooling system, it gives us levers to optimize the vehicle in ways that look very realistic.”