Researchers from Bosch Rexroth described a hydrostatic regenerative brake system in their paper “Hydraulic Hybrid Systems for Commercial Vehicles,” which was presented during the SAE 2007 Commercial Vehicle Engineering Congress. Their system captures the large amounts of energy expended in braking operations and temporarily stores it using hydraulic accumulators.
According to the Bosch Rexroth researchers, hydraulic accumulators offer a number of advantages for capture and storage of brake energy, including the ability to use hydraulic compounds that are already in serial production, and the need to make only relatively minor changes to retrofit a vehicle. They also noted that accumulators are superior to both fuel cells and batteries, which have a low power density due to their high internal resistance. In addition, the cyclical charging and discharging that would be necessary for heavy-duty applications with repeated cycling can severely limit fuel cell and battery service life.
With the hydrostatic regenerative braking system described in the Bosch Rexroth paper, braking energy is captured and stored by pumping oil into a hydraulic bladder accumulator that contains a set amount of gas. As the oil enters the accumulator, the volume available for the gas shrinks, causing the gas to become more compressed and raising both its pressure and temperature. When the vehicle accelerates, the system discharges oil back out of the accumulator to help drive the motor. In the accumulator, the discharge of oil expands the volume available for the gas, which causes both the pressure and temperature of the gas to decrease.
Researchers noted the importance of an electronic control unit to receive operator input from the brake pedal, accelerator, etc., as well as input from the vehicle CAN and sensors, and actuate the regenerative braking system to capture or release energy as needed.
Driving cycles and basic performance requirements are decisive factors for the estimation of potential energy savings available from a regenerative braking system. According to the Bosch Rexroth researchers, the system is only a benefit if sufficient brake energy is present and if the means to recover that energy can be implemented economically. Factors to consider include total vehicle mass, including load; rolling and air resistance; velocity at the beginning and end of the braking operation; deceleration; and the braking interval or frequency. They identified a number of vehicle types that could potentially benefit from the application of a hydrostatic brake system, including forklift trucks, refuse trucks, inter-urban and local delivery trucks, and city buses.
The Bosch-Rexroth researchers used a diesel engine model with consumption characteristic maps in the AMESim simulation tool to simulate the use of a hydraulic regenerative braking system integrated into a 10-t (11-ton) lift truck with hydrostatic drive. A loading cycle was defined, including unloaded reversing, driving, loading up to a 5-t (5.5-ton) load, reversing with load, and driving, with the truck accelerating up to its maximum speed of 23 km/h (14 mph). Results, as illustrated, reflect a fuel savings of up to 25% with the integration of a hydrostatic regenerative braking system into the hydraulic circuit.