by Steve Meyer, Contributing Editor
Sometimes mechatronic challenges come in unexpected places. Heavy hauling trucks are the latest transportation segment where an electric makeover provides major benefits.
For the world’s largest retailer, WalMart—with more than 11,000 store locations in the U.S.—getting products to customers is a major logistics challenge. The company executes its logistics with more than 6,500 trucks, 55,000 trailers and 7,000 drivers. The fleet logs hundreds of millions of miles per year. Finding ways to reduce operating costs in the fleet of trucks is a major objective that benefits both the environment and the bottom line.
The “WalMart Advanced Vehicle Experience,” or WAVE, is a demonstration program funded by Walmart to extend the performance of truck technology. Improving truck performance is a complex mechatronics problem that starts with wind drag and mechanical losses. A truck is, after all, a mechatronic system that uses energy to perform work through a variety of systems. The surprising solution came from exploring advanced hybrid electric drivetrain technology to improve the overall efficiency, which directly enabled the aerodynamic improvement by eliminating the traditional radiator at the front of the cab.
The new truck sports a unique driver cab that is highly aerodynamic. The driver’s seat is mounted in the center of the cab giving the driver a full panoramic view and reducing the cab’s front to a much smaller profile. Fenders and wheel covers are smooth, flat surfaces that complement the overall aerodynamic style of the truck.
All of this design work translates to a 20% reduction in coefficient of friction and an estimated 10% improvement in fuel efficiency—a huge gain in a class of vehicle that is not typically known for finesse. The aerodynamic leap is even more significant when considering the fact that the narrow cab feature is only possible because the hybrid drivetrain eliminates the need for a radiator heat exchanger at the front of the truck. The potential for performance improvement also needs to be considered across the entire fleet of more than 2 million heavy haulers operating in the U.S. every day.
Among many “firsts” in this project is the first 53-ft-long standard trailer made using a round front end that coordinates with the cab to improve aerodynamics and increases cargo space at the same time. These are single, 53-ft-long piece structures, a first in carbon fiber fabrication and a first in the trucking industry. Lightweight carbon fiber provides the necessary strength while reducing the trailer weight by 4,000 lb. This creates additional freight capacity in a trailer that meets the current standards of the Department of Transportation for this category of carrier.
Heavy hauling trucks that move goods from large distribution centers to local retail operations make up a significant portion of the transportation market in the U.S. According to DoT statistics, there are 2.45 million trucks with loaded curb weights in excess of 33,000 lb logging roughly 165 billion miles per year and getting 5.89 mpg fuel efficiency.
That’s a lot of mileage, a lot of goods being moved, and in the past, a lot of atmospheric pollution. A major improvement in this category of transportation has enormous impact on the environment and the cost of shipping goods in all sectors of the economy. Huge investments in technology have been made over the years to improve the efficiency and reduce emissions and incremental gains have been made. Hybrid electric technology, as demonstrated in WalMart’s WAVE, has the potential to transform the way we bring products to the point of sale.
EV Drivetrain Terminology
There has been a lot of confusion about the terminology of electric vehicle drivetrains. The creation of the first production dual drivetrain hybrid by Toyota was an amazing accomplishment that was very much unexpected. In the past, most assessments of hybrid electric vehicle technology suggested that the dual drivetrain, having both an internal combustion engine and an electric drive motor attached to the rear axle, would be expensive and complex to produce.
That accomplishment led to some ambiguity in the description of electric vehicle drivetrains. In the past, any electric vehicle was either a plug-in electric or a hybrid, since no dual drivetrains existed. In order to differentiate terms in the current environment, we have the drive-by-wire vehicles, which are either plug-in electrics that run exclusively from battery packs, or series hybrids, which substitute some of the battery with an on board generator run by a small combustion engine.
Plug-in electrics are like giant radio controlled cars, where batteries power an electric motor that powers the wheels. Plug-in electrics are attractive due to their inherent simplicity and lack of maintenance. The tradeoff is that they are generally limited to a range of 40 to 120 miles per charge, depending on the vehicle. Obviously, Tesla is trying to address the range limit by reducing the cost of the battery, but today, this is still an expensive solution.
In the series hybrid concept, a combustion engine runs a generator to recharge the batteries and the engine is not connected to the wheels mechanically. The generator arrangement can also be referred to as a range extender. The benefit is that the range extender weighs less than the equivalent battery pack. The negative for EV purists is that the range extender is a source of pollution, although most hybrids meet the Very Low Emission Vehicle standard.
Electric vehicles produce 3 to 4 times more torque at the wheel over the internal combustion engines, which lose most of their energy as heat. In addition, the electric vehicles can recover the force of deceleration, converting the energy to a battery charge, which is also not possible in combustion drivetrains.
Given the difficulty in coming up with an electric vehicle passenger car that is competitive with internal combustion, it’s hard to imagine that high horsepower vehicles, like freight trucks or buses, would make sense. The irony is that larger vehicles do not have the space and weight limitations that make it so challenging to produce a good EV solution. The on-board generator technology plays a crucial role in making the size, weight and cost of the vehicle competitive over a battery-only solution.
Enter the turbine generator
Today’s heavy hauler takes a 450-hp engine and a multi-speed transmission to generate the torque necessary to move the freight. By pairing a 300-kW GVM motor from Parker Hannifin’s Electromechanical Div. with a standard 10-speed truck transmission, equivalent power to the wheels is available. The water-cooled motor and inverter are compact and extremely efficient providing all the benefits of an EV for the truck.
Capstone Turbine has been perfecting the micro turbine generator for more than 25 years with impressive results. Capstone’s micro turbine generator provides a compact package that generates electricity from natural gas or diesel with only one-fourth of the emissions of the California Air Resources Board standards. Because the emissions are so low, no catalytic converter of other exhaust treatment is required.
The turbine operates in the same way as a jet engine: high speed, high efficiency, with one important distinction—there are no mechanical bearings and no oil lubrication required. The rotor is a single shaft rotating on frictionless air bearings. These features result in the elimination of the radiator that is required for cooling a conventional engine—and eliminate significant maintenance costs compared to other types of combustion engine.
The combination of the Capstone micro turbine generator and Parker traction motor offers a drivetrain solution that is hard to beat: multi-fuel, high efficiency, compact and practically maintenance free. The on-board generation of power is the key enabling aspect of the hybrid drivetrain. The generator with 50 gal of diesel, a fraction of what is normally carried on a large truck, can be converted to 325 kWh of energy, enough to run the fully loaded truck an estimated 217 miles. 325 kWh of lithium battery storage would weigh 6625 lb and would cost roughly $150,000, eliminating batteries as an option.
The WAVE Improvements
While the prototype vehicle has not yet undergone full road testing, a 35% reduction in fuel consumption translates to incredible cost savings in operation and a dramatic reduction in pollutants. The micro turbine burns fuel cleanly with nitrous oxide and diesel particulates 75% below California Air Resource Board mandated levels. With all the performance and driver improvements, this truck looks to be the WAVE of the future.
Parker Hannifin Corp.