Heavy-duty Hybrids Are Here

With the concentration of media attention on the passenger car hybrid market, it is easy to overlook the significance of hybrids in the commercial vehicle segment.

Most commonly, this type of motive power has shown up in inner-city applications such as transit buses and delivery vehicles, where the start-and-stop nature of the daily routine lends itself very well to the use of high-torque electric motors, with internal combustion engine assistance, of course. But the technology offers many other advantages over the traditional power approach.

Fuel savings and their attendant pollution-reduction effects are clearly at, or near, the top of the list of benefits, but the preservation of components prone to wear is another, perhaps lesser-known advantage.

A look at the offering from major hybrid module supplier ZF, best known in automotive circles for its transmissions, provides some insight into how these systems work.

For applications in commercial vehicles, ZF offers hybrid modules with the product name DynaStart. With up to 120 kW, this main component of a parallel hybrid drive is very powerful and provides an additional torque of up to 1,000 newton meters, which converts to nearly 740 ft.-lbs.

Interestingly, ZF also offers DynaStart as a ready-to-install module, something that the passenger car market will never see, including a separating clutch, which makes purely electric driving possible.

A separating clutch decouples the electric motor and the transmission from the combustion engine and, consequently, provides for purely electric driving. For generator operation, the electric motor is also used as a power source.

The braking effect generated during generator operation is integrated into the brake management system so that the service brakes are used less, and thus last longer.

In the case of this automatic transmission, which is aimed at use in delivery trucks, an electric motor with 60 kW supports the combustion engine. The overall system consists of transmission, electric motor, inverter, high-voltage battery, and power routing. It can cover all the functions of a full hybrid system.

Already during the development of the new automatic transmission, the integration of a parallel hybrid version has been considered for use in bus applications. Instead of the torque converter, an electric motor with a power output of up to 120 kW is accommodated in the clutch bell housing.

Recovering some of the energy expended in braking is a key part of the savings accorded through the use of hybrid technology.

The Bosch Rexroth Hydrostatic Regenerative Braking system (HRB) for heavy commercial vehicles reduces fuel consumption by up to 25% percent, says the company.

A gearbox links a hydraulic variable axial piston unit to the mechanical drive train (drive shaft) to convert kinetic into hydraulic energy when braking. The axial variable piston unit acts like a pump and converts the released braking energy into hydraulic energy by loading a hydraulic bladder accumulator with hydraulic fluid. This process is controlled by an electronic controller together with a hydraulic valve manifold.

During acceleration the entire process is reversed: the pressurized fluid is discharged in a controlled manner from the accumulator and flows back through the variable axial piston unit. The latter is driven by the fluid flow and, acting like a motor, gives up its energy to the mechanical drive train. A pressure relief valve in the system ensures the highest level of safety for both processes.

A bonus for the vehicle operator is that the braking force imparted by the process significantly reduces brake wear. And, also as a result, the production of fine dust from brake wear and the CO2 emissions from the combustion engine are reduced.

The parallel HRB can be integrated into essentially any commonly available commercial vehicle frame. This also enables retrofitting of existing vehicle fleets.

Of course, that is the component approach to making a hybrid truck.

On the branded “off the shelf” vehicle side, it is notable that Kenworth has embarked on a fuel efficiency strategy that has hybrid technology as a key component.

“Paccar has targeted an ambitious goal of 30% improvement in vehicle fuel efficiency for selected medium duty applications over the next seven years, utilizing hybrid technology as a key contributor to achieving this objective,” says Mark C. Pigott, Paccar chairman and chief executive officer in 2006. “Paccar’s comprehensive global hybrid program is aimed at commercializing energy-management systems that provide benefits to both our customers and the environment.”

Paccar began production in 2008 of medium- duty hybrids from its Peterbilt and Kenworth brands, while Peterbilt also moved into production of its hydraulic hybrid refuse truck this year. Peterbilt also has said it is planning production of its Class 8 hybrid, based on its Model 386 truck, in 2009.

In September of this year, Kenworth Truck Company introduced a new T370 Class 7 diesel-electric hybrid tractor for local haul applications, including beverage, general freight, and grocery distribution, that the company says reduces fuel consumption by 25%.

Produced at the Paccar plant in Ste. Therese, Que., the Kenworth T370 tractor is powered by a Paccar PX-6 engine and features an integral transmission-mounted motor/generator, a frame-mounted 340-volt battery pack, and a dedicated power management system. Advanced powertrain controls monitor driving conditions and automatically select the ideal power mode, switching among electric-only, combined diesel and electric, and diesel-only power modes. Electricity generated through regenerative braking is stored and used for acceleration, assisting the diesel engine.

Virtually every report says that, other than the change in driving feel that is a consequence of the regenerative braking systems, there is little or no outward change in operation for the drivers.

Still, the technology remains almost exclusively in the domain of the early adopters.

Currently, there are only about 1,000 hybrid trucks either on order or on the road in the U. S. They are aided by a U. S. federal congestion mitigation air quality fund that pays up to 80% of the premium for them, which still puts them on the knife-edge of purely monetary (as opposed to ecological) payback. Estimates put the add-on technology in hydraulic and electric hybrids at 50% to 80% higher than the original cost, though the goal is to get that to 20%.

Still, the technology has its proponents. Chief among these are some large corporations, municipalities, and a few service fleets that are adding hybrid vehicles partially to test their viability in true conditions, but also to make a socio-political statement.

Further to this point, the largest assembly of the world’s greenest trucks, buses and technologies assembled in South Bend, Ind. in October at the Hybrid Truck Users Forum. More than 35 vehicles were on hand for test drives, including dump trucks, utility trucks, and even a 60-foot articulated bus.

The event attracted a record 582 registered attendees — a 35% increase from last year — a near doubling to 36 Ride and Drive vehicles, and three days of events focused on the cleanest trucks on the road today.

“Industry commitment has clearly reached a new level based on the production-ready hybrid medium-and heavy-duty trucks showcased during a convoy and ride and drive in South Bend,” says Bill Van Amburg, senior vice-president at Calstart, which organized the event. “Every major truck maker and the world’s leading hybrid system makers took part and made a strong statement to users and policy makers that hybrids are here and real.”