DoD’s fleet of diesel powered vehicles and equipment generates greenhouse gases, criteria pollutants, soot, and noise. These emissions are hazardous to the environment and the equipment operators. Rising fuel prices and logistics of delivering the fuel to forward deployed bases increase cost and risk of heavy equipment operations. This project evaluated the benefits and readiness of existing early-commercial hybrid platforms, and also helped establish a military link to the hybrid vehicle manufacturing industry. The project also provided data on which platforms would benefit most from the hybrid technology. Two heavy duty hybrid vehicles were assessed on six parameters: fuel economy, noise levels, break wear, maintainability, drivability, and ease of use.
Heavy duty hybrid vehicles feature a supplemental power system for the diesel internal combustion engine. The supplemental system is typically either electric or hydraulic. With an electric hybrid, braking (kinetic) energy is captured by using the propulsion system to apply a load to the drive axle during braking, and converted into electrical energy via a generator. The vehicle stores that energy in on-board batteries for driving the wheels at another time. Hydraulic hybrids are similar in approach, except that they store braking energy with high pressure accumulators that assist with vehicle propulsion when needed. These systems avoid the requirement for large battery systems and complex electrical controls.
The project team purchased and deployed four test trucks for the demonstration, including one pair of refuse trucks and one pair of utility trucks. Each test pair included a conventional truck baseline, and a hybrid truck of equivalent make, model year, and production run. The hybrid equipped utility truck is an electric configuration with battery storage, regenerative braking, launch assist, and engine-off power.
Refuse Truck Test Summary
The refuse truck equipped with the hybrid launch assist system failed to meet the critical performance objective for fuel economy. This result is not related to the hybrid system, but is due to the mild driving conditions characteristic of most non-tactical truck applications on DoD installations. The project team feels the same truck, if placed in a severe duty cycle, would yield entirely different results. The hydraulic hybrid also fell short of the noise reduction objective for outdoor noise, showing a 20 percent increase in noise rather than a reduction. Recommendation is for fleet management to further investigate candidate cycles and next generation hydraulic hybrid systems prior to finalizing procurement plans.
Utility Truck Test Summary
The hybrid utility truck successfully achieved four of six performance objectives, including fuel economy, noise, maintainability, and ease of use. The truck fell short of the drivability and brake wear objectives. Operators expressed drivability characteristics as a considerable annoyance. The project team concludes drivability will improve with further refinement and engineering for next generation trucks. As with drivability, brake wear can be reduced with further optimization to meet the objective. Hybrid electric utility truck is considered acceptable by the project team, with the recommendations that purchasing agents specify the driving performance requirements for the intended application.
The hybrid utility truck fell short of meeting two objectives, drivability and brake wear. Poor drivability (specifically accelerations and hill-climbing ability) is critical to ensuring broad scale acceptance by the users. This symptom is related to the early commercial nature of the technology, and will be resolved through further manufacturer refinement and engineering.
Fleet managers are the responsible entity that must integrate heavy hybrid technology into the vehicle fleets. The decision to purchase hybrid technologies means the agency receives fewer trucks at a higher cost. With incremental costs prohibitively high, the industry must see discounts or other incentives.
Training is the other item that is critical to successful technology implementation and integration. Fleets will realize greater support, and improved chance of user acceptance, if operators understand operating concepts and best operational practices. Also, in the interest of safety, all service and maintenance training should accompany delivery of all new hybrid trucks. The high energy systems are potentially hazardous and could result in injury or death unless fleet management takes proper precautions and ensures mechanics are trained for servicing the energy storage systems. Training must also be provided to emergency responders and vehicle operators on the potential hazards related to the use of battery packs and high pressure accumulator systems, which present potential for electrocution, toxic gas inhalation, or overpressures if compromised by fire or physical damage. Crews must be trained to watch for the hybrid labels and be instructed in the procedures to de-energize electric or gas lines that are compromised and present a hazard.