Hydraulic Hybrid Research

EPA is a research leader in the application of hydraulics in vehicles. Hydraulic hybrid technology uses a hydraulic energy storage and propulsion system in the vehicle. This hydraulic system captures and stores a large fraction of the energy normally wasted in vehicle braking and uses this energy to help propel the vehicle during the next vehicle acceleration. The hydraulic system also enables the engine to operate more efficiently when it is needed.

Hydraulic hybrids draw from two sources of power to operate the vehicle - the diesel or gasoline engine and the hydraulic components. In other words, a typical diesel-powered or gasoline powered vehicle can be fitted with hydraulic components as a secondary energy storage system. The primary hydraulic components are two hydraulic accumulator vessels (a high-pressure accumulator capable of storing hydraulic fluid compressing inert nitrogen gas and a low-pressure accumulator) and one or more hydraulic pump/motor units.

Benefits of Hydraulic Technology: Hydraulic drivetrains are particularly attractive for vehicle applications that entail a significant amount of stop-and-go driving, such as urban delivery trucks or school buses. A major benefit of a hydraulic hybrid vehicle is the ability to capture and use a large percentage of the energy normally lost in vehicle braking. Hydraulic hybrids can quickly and efficiently store and release great amounts of energy due to a higher power density. This is a critical factor in maximizing braking energy recovered and increasing the fuel economy benefit. While the primary benefit of hydraulics is higher fuel economy, hydraulics also increase vehicle acceleration performance. Hydraulic hybrid technology cost-effectively allows the engine speed or torque to be independent of vehicle speed resulting in cleaner and more efficient engine operation.

Future of Hydraulics: Hydraulic hybrid systems create a unique opportunity to optimize engine operations. EPA has produced research concept vehicles that demonstrate the hydraulic technology. One concept vehicle is an urban delivery truck that uses hydraulic "launch assist." This delivery truck retains its conventional engine and transmission, but adds on a hydraulics package optimized for fuel economy. The next generation of hydraulic vehicles involves fully integrating hydraulic technology. In this configuration, the "full" hydraulic hybrid replaces the conventional drivetrain with a hydraulic drivetrain and eliminates the need for a transmission and transfer case. Using the full hydraulic drive in conjunction with EPA's clean diesel combustion technology is projected to improve fuel economy even more.

EPA also has achieved major breakthroughs in designing hydraulic accumulators and pump/motors to be more efficient, smaller, and lighter for motor vehicle applications, which will help improve fuel efficiency. EPA currently has cooperative research and development agreements with several private sector partners to further the development of hydraulics.