Hybrid technology is ideal for pickup and delivery trucks like this Kenworth T270 because they usually run in urban areas and see a lot of braking.
Imagine being able to start up a steep grade without the usual shudder and rocking of the cab. That’s what I experienced when Eaton Corp. allowed truck industry editors to drive Class 8 and smaller rigs with hybrid drivetrains.
After bringing a fully loaded test vehicle to a dead stop on a grade that was much steeper than drivers normally encounter, I gently released the brake and I steadily depressed the throttle. A powerful electric motor quietly nudged the vehicle into the smoothest steep-grade start I have ever experienced. This was possible because the clutch wasn’t needed – electric motors produce maximum torque at zero rpm, unlike even the best diesel.
This illustrates just one potential advantage of a hybrid drivetrain. They can eliminate clutch wear while making the driver’s job easier and more pleasant. But what is the future of the technology? Long-haul truckers may be running hybrids sooner than you think.
Medium-duty hybrids are a reality. Peterbilt and Kenworth both introduced versions last summer. Peterbilt has Class 6 and 7 hybrids for a number of applications, as well as a Class 8 refuse truck. Kenworth has Class 6 and 7 hybrids combining the Paccar PX-6 engine and an Eaton hybrid drivetrain. The two drivetrain giants, Eaton Corp. and ArvinMeritor, along with vertically integrated truck manufacturer Mack and its parent company, the Volvo Group, have likewise developed workable and efficient hybrid drivetrains. One reason all have gone this route is the tax credits that make investment in new technologies more affordable.
Gary Moore, assistant general manager for marketing and sales at Kenworth, says the trucks qualify for “$6,000 and $12,000” buyer tax credits, respectively. The Peterbilt hybrids qualify for the tax credit, too, says Peterbilt Chief Engineer Landon Sproull. But the larger impetus for saving has come from hybrids’ ability to capture formerly wasted energy.
You’ve probably noticed that when you run through areas that require a lot of stops and starts your fuel economy will drop by 40 percent or more. It takes a lot of energy to accelerate a truck from resting to highway speeds. In interstate driving, accelerating one time often allows you to travel hundreds of miles. But when driving on secondary streets with cross traffic, you may stop 5 to 10 times a mile or more, and this makes a tremendous difference in fuel economy. When using the service brakes or engine brake to slow and stop, almost all the energy you’ve used to accelerate the truck will be thrown into the environment as waste heat.
The hybrid system’s most critical function is to slow the truck (or control its speed downhill) without throwing the energy away in the service brakes and engine brake. A hybrid drivetrain can recover that energy instead of tossing it out. The hybrid’s batteries are like a bank account the truck can put energy into, then withdraw when it’s most necessary.
What makes hybrid drives possible is the fact that an electric motor can instantly be reversed and turned into a generator. Add to a motor-generator a lithium-ion battery pack that can store a lot of energy, and you have the makings of a hybrid system. The only additional requirement is for digital controls to appropriately apply electric power and reverse the motor to cause it to generate power.
The motor-generator usually is connected to the input shaft of the transmission, right behind the clutch. When the driver applies the brakes in a normal manner (as opposed to a hard-braking event) the hybrid system activates the generator function of the motor alone. It produces braking torque by using the motor to oppose the rotation of the transmission input shaft, thus generating power, with the energy being supplied by the moving vehicle. The power generated is used to charge the battery pack.
When the vehicle resumes acceleration, the controls reverse the motor-generator, turning it into an electric motor that draws energy back out of the battery pack to be applied to the drivetrain. This assists the diesel engine in accelerating the truck. Not only will less fuel be needed to get the truck back up to its cruise speed, acceleration rates also are improved.
Tammy Packard, product strategy manager for commercial vehicle systems at ArvinMeritor, says the system is even tuned to allow the diesel to operate more efficiently: “This dual-mode system allows the diesel engine to operate at a constant speed and power output, functioning either as a generator for the batteries or as a power source for the drive axles, depending on what’s necessary. In either case, the engine can run constantly at its ‘sweet spot,’ delivering the most efficient mix of fuel economy and torque output.”
Research by Eaton Corporation reveals that the best Class 8 vocations for hybrids are those like central distribution operations in metropolitan areas where the truck sees lots of city traffic. However, the system also has proven capable of saving 4 to 5 percent of the fuel bill in places like Nebraska and New Mexico, where there is undulating terrain that causes the driver to accelerate to climb hills, then use the brakes on the way back down. “That 4 to 5 percent of fuel is a big number when the truck runs 120,000 miles a year,” says Kevin Beatty, business unit manager for hybrid power systems at Eaton.
Mack President and CEO Paul Vikner says “while the per-truck impact would be more modest on long-haul vehicles that don’t do as much braking, we believe hybrid systems will ultimately be able to reduce these vehicles’ fuel consumption by up to 10 percent.” That’s a significant savings for the nation as a whole when applied to the aggregate of nearly 2 million highway tractors on U.S. roads today.
Mack and its parent company, Volvo, have been working jointly on hybrid technology. Leif Johansson, president and CEO of Volvo Group, calls the latest Volvo hybrid system “fourth-generation hybrid technology” and believes the technology will catch on fast, partly because Volvo has developed standardized hybrid components that will cost less because they will be mass produced.
The Volvo system is known as ISAM (Integrated Starter Alternator Motor). The diesel engine and electric motor are connected mechanically in parallel so either one may drive the truck while the other is disconnected. The separate starter also is eliminated as the hybrid motor can easily crank the diesel. Volvo has designed the system so it can operate the engine accessories electrically and only as needed, saving more fuel.
Beatty says Eaton is still searching for the “sweet spot” where there is enough benefit for the cost of hybrid drivetrains in Class 8s: “As soon as we prove this out, we’ll put it into the market.”
In addition to providing enough energy storage to help run the truck, high-capacity hybrid battery packs also might be used in the future to eliminate idling without an APU. “The same batteries used to propel the vehicle during the day can power the HVAC and accessories during a sleeper shift,” Packard says.
Beatty agrees, adding that “with enough energy storage, you could operate the hotel loads with no idling.” This would give you the ability to “comply with idling and noise ordinances,” Packard says.
Still another advantage in certain areas would be operation as a zero-emissions vehicle. Packard detailed ArvinMeritor’s work with Cummins and International Truck and Engine Corp. to “design and integrate” a hybrid drivetrain for Wal-Mart’s linehaul trucks. This is part of Wal-Mart’s corporate goal to reduce fuel consumption on new vehicles by 50 percent by 2015. Packard says the system will be designed to allow “using battery power alone to propel the vehicle.” This won’t benefit the trucker directly, but she says “the ability to operate in zero-emissions mode may be increasingly important in urban areas or jurisdictions that want to restrict access for vehicles that emit greenhouse gases.”
And it may give a boost to the move toward hybrids across the spectrum, including in long-haul applications. “Since nearly two-thirds of all new Class 8 trucks are bought by only 100 large companies,” Packard says, “Wal-Mart believes it can influence truck design by specifying vehicles that are more energy efficient and reduce pollution. Hybrid vehicle development is an important part of this strategy.”
Vikner mentions a recent executive order mandating federal hybrid and alternative fuel vehicle purchases and believes that if the federal government offered private-sector truck buyers a new set of temporary incentives to buy hybrids modeled on the federal tax credits now provided for hybrid passenger cars, hybrid trucks would become a “significant contributor” to national prosperity.
Peterbilt will be building some hybrid 386 trucks this fall. “The performance of all hybrids will improve in the future as battery production and technology advance,” Sproull says. “As production goes up, cost will come down, and more applications can be pursued. Also, as energy density increases and we can move more energy back and forth without limiting the life of the cells, overall performance can be improved.”
Spec’ing the Drivetrain for Fuel Economy
Of particular importance is the transmission and rear-axle ratio. Make sure to work with your salesperson to ensure he or she knows exactly how you will be operating. The tire size (rpm/mile), transmission top gear ratio and rear-axle ratio must all be coordinated so the engine will be in its sweet spot at the speed where you normally cruise. As Peterbilt Chief Engineer Landon Sproull says, “Many drivers know that a lower cruise speed can improve fuel economy from an aerodynamic standpoint. But the wrong gearing may actually reduce fuel economy since the engine won’t be running efficiently.”
All the engine and truck makers offer a system that monitors driver