Two in one
The lithium-ion battery’s high “energy density,” meaning minimal size and weight, and its ability to accept a charge quickly, make it the battery of choice. It can handle high discharge rates for high peak motor power, too. “It’s the size and weight of the batteries that limit how much you can use the hybrid system to help out acceleration and braking,” Gosbee says. “They also limit how far you can run on pure electric power.”
Mack’s hybrid design is used mostly on Class 8 refuse trucks. However, for over-the road or regional Class 8, two basic designs are in development. One is likely to be a significantly upgraded version of an Eaton system used on medium-duty trucks built by Daimler Trucks, Kenworth and Peterbilt. The other will come from ArvinMeritor in 2012 or 2013.
The two systems will be based on significantly different architecture. The Eaton system is a parallel hybrid, says Dmitri Kazarinoff, vice president of hybrid systems development at Eaton Vehicle Group. It will make use of an ordinary mechanical transmission, for example a 10-speed Ultrashift. The electric motor will be installed between the clutch and transmission, with the motor driving the transmission input shaft. This means the motor-generator will always drive and be driven (during braking) through the transmission. This design allows the transmission gearing to multiply the motor’s torque during startup. Disengaging the clutch allows the motor to launch the vehicle from rest with the engine disconnected from the rest of the drivetrain.
The system will include a controller-inverter that will allow battery power to be converted to AC to operate hotel loads running at 120 volts, as well as the hybrid electric motor. Eaton is partnering with Daimler and Peterbilt to develop the system.
ArvinMeritor will introduce a series-parallel hybrid system. It will use a simple mechanical transmission, says Chad Mitts, director of product line management, hybrids and new technology. Combined with an electric motor, the system will drive the wheels through a driveshaft and conventional drive axle. The gearbox will have only two gears, which will be used only above 48 mph.
However, at less than 48 mph, the system will operate in what Mitts described as “silent mode,” or full electric drive, using the electric motor only. At low speeds, the diesel will run only to charge the batteries. The diesel can stay off for about 20 minutes at a time, especially useful in cities or ports where emissions and noise must be limited.
What makes all this practical is the system’s 180-kilowatt motor-generator, which will produce 240 hp. Part of the reason the truck will be able to run in silent mode is that, especially with such a large motor-generator, much of the braking energy will be channeled into the batteries rather than being lost.
Mitts adds that the powerful motor-generator also means that “no friction clutch is needed for startability.” The motor will be located at the rear of the transmission, he adds, between it and a conventional driveshaft. When running in silent mode, the transmission will simply be out of gear.
ArvinMeritor is partnering with Walmart, Navistar and Cummins to develop the system.
Mack’s system is a “blended hybrid,” says Dave McKenna, director of powertrain sales and marketing. Its motor-generator, installed near the clutch, has components that enable power generated during braking to be stored in lithium-ion batteries. “Prior to this, friction linings merely produced wasted heat,” he says.
Mack has combined the motor-generator and batteries with an I-shift, fully automated transmission. The hybrid system launches the vehicle using electric power. “An electric motor has plenty of torque at 1 rpm to get the vehicle moving without input from the diesel engine,” McKenna says. The motor is integrated with the I-shift, allowing the standard drivetrain to begin to take over at 5 to 7 mph. Also, the hybrid system allows the I-shift to skip gears appropriately, improving both performance and smoothness of acceleration. The system not only saves fuel, it also helps brake linings and drums last longer, lengthening brake job intervals by 2.5 to 3.5 times. “Payback is almost immediate,” McKenna says.
More savings accrue, he says, from the system’s ability to use the “immediate peak torque” of the electric motor to eliminate using the clutch to start the vehicle from rest. Instead, it’s used only to disconnect the engine from the rest of the drivetrain, allowing the diesel to idle with the truck stopped or in the early stages of acceleration. Cutting normal startup slip time virtually eliminates clutch wear.