Two in one

By John Baxter | November 01, 2009

Electric and diesel muscle combine in hybrid systems that store energy, cut fuel usage and reduce operating costs. If you want to see the face of tomorrow’s powertrain, look at what’s on the road today.


In the near future, hybrid Class 8 trucks combining powerful motor-generators in the drivetrain and a new type of potent battery will make over-the-road trucks significantly more fuel-efficient than today’s models. The improved efficiency will emerge largely from the ability to store energy recovered during braking.

Navistar shows off this Walmart International tractor, incorporating Arvin-Meritor's hybrid system, during a Hybrid on the Hill event last June in Washington D.C.


Fuel savings will come from generating power for in-cab hotel loads while the truck is running, or with the engine in an efficient, loaded, fast-idle operating mode, and storing it. This energy will provide the power needed to cool the cab and run onboard devices like refrigerators, televisions and microwaves via the batteries. The process will reduce the need for idling and its harsh effect on engine oil. More savings will accrue from lower maintenance costs because hybrid systems help in easing the workload of high-maintenance clutches and foundation brakes.


Hybridization is likely to be a big fuel saver in all applications except cross-country routes, where minimal braking is needed.


Hybrids so far retain the standard service brake system. The traditional air brakes are necessary both because the hybrid braking effect works only through the drive wheels and because, at least at this stage of development, electric motors can’t create the abrupt, powerful stopping force needed for hard braking.


A lot of hybrids are already out there. Kenworth makes T270 Class 6 and T370 Class 7 diesel-electric hybrid trucks for use in pickup and delivery, utility and service applications. The Kenworth hybrids feature a Paccar PX-6 engine, integral transmission-mounted motor-generator and frame-mounted 340-volt battery pack. Peterbilt and Freightliner make similar medium-duty hybrids. International is selling a medium-duty truck and school bus parallel hybrid system, says Darren Gosbee, director of hybrid powertrain strategy and execution.


Even smaller engines can be combined with hybrid technology for additional fuel savings, practical because of hybrid systems’ ability to add torque and horsepower for acceleration and hill climbing. Kenworth says its goal is to enhance fuel economy by up to 30 percent in pickup and delivery applications and up to 50 percent in utility and service operations.


Two components that make hybrids practical today are direct-drive motor-generators and lithium-ion batteries. The motor generators are high-voltage alternating current devices with liquid cooling. AC motors are more efficient and powerful than the direct current motors used in starters. Liquid cooling allows a smaller, more compact and, therefore, more efficient device with shorter current paths.


Gosbee says medium-duty hybrids run at 300 to 350 volts, but may be introduced at up to 600 volts if the safety issues can be handled. That’s the same operating voltage estimated by Sharon Gunn, chief product manager of hybrid technology for Mack and Volvo, at the June Hybrid on the Hill event in Washington, D.C., organized to clue Congress into the technology’s value. Higher voltage means greater efficiency and increased power.


Bill Conn, Peterbilt’s manager of advanced concepts, says the truck maker will use the Eaton Corp. system that offers 310 lb.-ft. of torque and 60 hp from the electric motor. While that may not sound like much torque, Conn says, “With an electric motor, you get all this torque at zero rpm. Because of that fact, you can easily move an 80,000-pound vehicle with the motor.” This is true when the motor is driving through a 10-speed transmission used in Peterbilt hybrid prototypes.


Lithium-ion batteries are another key component. Gosbee says, “Hybrid energy is about recovering as much energy as possible and re-using it. Recovery is a significant way to save energy. But batteries will need to get smaller and lighter to store enough of this energy.” strives to maintain an open forum for reader opinions. Click here to read our comment policy.