To improve the efficiency of its Class 8 heavy-duty trucks, Walmart U.S. Logistics, in partnership with truck and component manufacturers, built the following prototype tractors, according to its 2011 Global Responsibility Report.

Like many emerging technologies, Walmart says, these trucks might never enter operation or remain in operation long as they go through multiple iterations before being cleared for commercialization.

Hybrid assist

Walmart's hybrid assist tractors use a traditional diesel drivetrain in conjunction with an electric motor to recover energy normally lost during braking to provide additional torque in high-load situations such as going up hills. The onboard batteries are also used to run components such as heating, air conditioning and the electrical system when the engine is off.

This model was the company's first hybrid tractor and was built in partnership with Peterbilt and Eaton Corp. During the course of a year, Walmart gathered data and highlighted key areas of opportunity (e.g., batteries) needed to achieve an acceptable return on investment. Hurdles for the batteries include cost, size and weight, and capacity.

Wheel-end hybrid assist

The concept of this hybrid, built in partnership with Freightliner, is similar to the hybrid assist, except for the placement of the electric motor. This tractor has two wheel-end motors on the second axle, putting the energy right where it is needed and avoiding energy loss from the front of the tractor.

Already under development, the next generation of this tractor will attempt to resolve the inability to add future enhancements, such as stationary recharge of the hybrid battery due to current motor placement.

Full propulsion hybrid

This configuration is a dual-mode hybrid with the ability to run completely on its electric motor when traveling less than 48 mph. When it passes that mark, it switches over to a diesel engine. This hybrid, built by Meritor, was the first of its kind in this class when unveiled.

Similar to the hybrid assist tractors, the next generation of this hybrid drive system is under development, being paced by the design of a battery system with enhanced performance and improved robustness.

Natural gas

Walmart has been testing five Westport liquefied natural gas trucks in California for the past three years and has just taken possession of a Westport/Cummins Alpha 12-liter natural gas engine. This new truck will run on compressed natural gas and will also be tested in California. Although available fueling infrastructure is a challenge, the technology performance and potential sustainability benefits encourage further review.

Technology truck

Walmart and Freightliner worked together to build an advanced, aerodynamic tractor with the latest in efficiency and predictive technologies. Testing will allow for the evaluation of individual components as well as the interplay of multiple components. This tractor was built with further development in mind and will be retrofitted with new advancements as they become available.

Walmart reported a 69% improvement in fleet efficiency in 2011 over its 2005 baseline. The company delivered 65 million more cases and drove 28 million fewer miles by increasing pallets per trailer and better managing routes, it says in its 2011 Global Responsibility Report. It added more than13,000 skirted trailers, and its efficiency improvement equates to avoiding nearly 41,000 metric tons of CO2 emissions, the equivalent of taking about 7,900 cars off the road, the company says.

To read the company's full Global Responsibility Report, click here.