Stored braking energy saves fuel

Tom Shelley reports on a revived idea to reduce fuel costs in heavy vehicles and construction equipment

Storing braking energy in a hydraulic accumulator, then releasing it to assist acceleration, could improve heavy vehicle fuel economy by up to 35%. The basic idea is not new: Ford and Eaton announced a system called Hydraulic Launch Assist in 2002, but the roots of the idea go back to the invention of the hydraulic accumulator tower in the mid-nineteenth century, which stored energy to operate lock gates and cranes. Bladder accumulators have been in widespread use since the 1940s. The breakthrough in the present development – by Australian company Permo-Drive – has been to improve control so that it can be used in a bus, heavy truck or construction equipment. The system is called the Regenerative Drive System (RDS). In its basic form, the driveshaft is equipped with a pump/motor. When a vehicle is braking, braking energy is used to pump oil into a hydraulic accumulator, pressurised with nitrogen. When the vehicle subsequently accelerates, the compressed gas in the accumulator pushes oil through the pump/motor to assist the engine by adding energy to the drive train. The control system simultaneously reduces engine output while the stored energy is returned in order to maximise fuel saving. Hydraulic systems intrinsically have higher energy densities than electric systems, especially when these involve batteries. A hybrid hydraulic system with an accumulator is about half the weight of a hybrid electric system – even one using the most advanced batteries – and about four-fifths that of a system based on a super capacitor for energy storage. Because of the relative maturity of hydraulic components, hybrid hydraulic systems with accumulators are less expensive, and have long times between major component replacement, whereas rechargeable batteries have strictly limited working lives. System efficiency is also said to be significantly higher. The environmental footprint of a hydraulic system is less than that of an electric system involving batteries, since these require careful recycling. For safety reasons, the RDS automatically discharges to zero pressure when the vehicle is switched off. Other claimed benefits are: up to 35% improvement in fuel economy depending on drive cycle; reduced carbon dioxide and NOx emissions; a doubling of brake life, reduced braking noise; and improved acceleration. One of the most interested groups of potential users is the military. World War II battles were lost by armies running short of fuel, both in North Africa and in Russia. And designers of military vehicles are always encouraged by technologies that enable a faster get away when under attack. Mario van Eck, chief operating officer of Permo Drive says: “In normal use, to save fuel, the RDS provides power to replace some of the engine power. But to provide the ‘dash capability’, as the US military calls it, the RDS can be changed to ‘power’ at the flick of a switch – and will then add its power to that from the engine.” Permo-Drive has only 20 employees. While it can supply kits of parts to equip demonstrator and test vehicles, it is more interested in licensing its technology. Three FMTV (Family of Medium Tactical Vehicles) trucks,, one 6x6 and two 4x4s fitted with the RDS have been prepared and delivered to the US Army to complete a 21,000 mile test programme at their Aberdeen Test Center. There are programmes to evaluate the technology on buses, refuse trucks, delivery vehicles and construction equipment, especially items that are fitted with hydraulic transmission systems. The technology is easily extended for use on cranes and lifting devices, where energy can be gathered during lowering, and released during lifting. Permo-Drive Pointers * Braking energy pumps oil into an accumulator, which can be released later to aid acceleration. * Intrinsically higher energy density means that system weight is half that of battery-based hybrid electric systems. * Capital and over life maintenance costs are lower, by avoiding battery costs.