DETROIT — What appears to be a practical, low-tech method to recover braking energy in 8,000-lb. to 11,000-lb. (3,600-kg to 5,000-kg) GVW vehicles in urban stop/start application was introduced by Ford Motor Co. at the recent Detroit International Auto Show here.

The system, jointly developed with Eaton Corp., captures braking energy through a hydraulic pump-motor unit, where it is stored in dual accumulators that contain nitrogen, isolated from the hydraulic fluid, at pressures up to 5,000 psi (345 bar). The retained energy then can be released back through the hydraulic motor to the driveshaft, supplementing engine power, particularly to launch the vehicle from rest.

A Ford engineer explains that the energy thereby stored during normal deceleration from 32 mph (51 km/h) to stop (without use of the brakes) can then accelerate the vehicle back up to 25 mph (40 km/h) — resulting in overall energy efficiency of close to 80%.

The pump-motor, control systems, two cylindrical accumulators (pressure vessels) and 5 gals. (23L) of fluid weigh about 450 lbs. (204 kg). The system is mounted under the body on one side of the frame, with the pump-motor in line with the driveshaft. The system is expected to add about $2,000 to vehicle prices and save 25% in fuel bills.

Ford engineers say systems of almost any size are possible, with garbage trucks as one plausible application — they often go less than 100 ft. (30 m) between stops.

One downside mentioned is hydraulic pump-motor noise — although this is being addressed and must be considered in context with the commercial use such vehicles will see.