Hybrids keep the pressure up

Tom Shelley reports on the latest approaches to applying hybrid technologies to construction equipment and refuse vehicles.

By their nature, commercial vehicles tend to consume large quantities of fuel, meaning that they are extremely expensive to run. This has meant that the need to develop systems to mitigate the problem is being addressed urgently. The solution generally arrived at has been the hydraulic hybrid system, which takes advantage of the stop-start nature of commercial vehicles by storing the kinetic energy created by braking in a hydraulic accumulator and immediately using it when accelerating again. A number of systems have been developed for this purpose. One is Parker Hannifin's Stored Energy Management System (SEMS), which was developed especially for use with vehicles such as refuse trucks and short-haul delivery lorries that frequently stop and start in the course of their normal working life. SEMS incorporates a conventional Parker hydraulic accumulator, which is automatically charged every time the vehicle brakes; alternatively, the accumulator can be powered via an electronically controlled pump attached to a power take-off (PTO) point. The accumulator is normally linked to a hydraulic converter that reduces the stored high pressure energy to the exact level required by the different items of ancillary equipment on each vehicle. Overall system management is then achieved by Parker's IQAN software based hydraulic control technology, which combines in-cab HMI units and joysticks with specialised sensors, I/O and interfaces, to create a powerful yet flexible method of monitoring and controlling hydraulic systems. Another, similar system is Bosch Rexroth's Hydrostatic Regenerative Braking (HRB) system. Here, the system goes into action mode when the vehicle's driver presses the brake pedal, causing a hydraulic unit to push hydraulic fluid into a high-pressure reservoir. Each time the driver brakes, energy which would usually be lost, is stored. When accelerating, the electronically-controlled hydraulic pressure reservoir releases the pressure, thus relieving the load on the diesel engine. As a result, the engine consumes less fuel, generates fewer exhaust gases and functions more quietly. HRB functions through the use of a hydraulic axial piston unit that is integrated in the mechanical drive train by means of a gearbox. Upon braking this unit acts as a pump and uses the released brake energy to charge a hydraulic accumulator (bladder accumulator) with hydraulic oil. Electronics control this procedure via a hydraulic valve control block and give the signal to reverse the process when the vehicle begins to move.The pressurised oil is then released from the accumulator in a controlled manner and flows back through the axial piston unit. This unit is driven by the oil flow and thus functions as a motor, delivering its power to the mechanical drive train while a pressure relief valve ensures maximum safety. Rexroth's HRB can be retrofitted in the chassis as an add-on system without major modifications – even in vehicles without hydraulics. The latest development of this type, however, comes from Haldex Hydraulics and is able to take the energy from a descending loader bucket, and use it to help propel the vehicle. Combined with energy storage in an accumulator, full scale laboratory tests suggest that in service, in a vehicle, it should be able to achieve fuel savings in excess of 50%. While other hydraulic hybrid systems capture braking energy, store it and redeploy the stored energy for propulsion, however, the Haldex Energy Management System is the only hydraulic hybrid that provides energy management the Haldex Hydraulics Energy Management System aims to integrate all functions on a piece of construction equipment, and do everything by hydraulics. Jeff Maney, director of advanced solutions engineering at Haldex Hydraulics in Rockford, Illinois explained: "Our system is strictly series" and that unlike a conventional hydrostatic swashplate transmission system, "In our system, the high pressure port is always the high pressure port. The flow changes direction." The system is open loop, also unlike a conventional hydrostatic transmission, which is closed loop with pressure in the loop dictated by load rather than system control. The EMS system aims to maintain a constant supply pressure and some load on the engine at all times. Even when the machine is stationary, Maney said that the swashplate in the unit attached to the engine is kept at a shallow angle, while the swashplates in the motors attached to the wheels are at zero angle and the units are at full pressure. The surplus high pressure oil is used to slowly add charge to the accumulator. Movement is initiated by controlling the swash plates at the wheels which leads to the pump swashplate angle being increased to deliver more high pressure oil. He said that this means that they "draw torque rather than torque being pushed towards them". This should mean that the system is very quick to respond, and thus easier to control. Hydraulic systems which have to be pressurised before anything happens, sometimes have a tendency to oscillate if demand changes are made very quickly. Traction control and anti-lock braking is, built in, as Maney explains: "If a wheel starts to slip, the torque is backed off until it stops, whether it is forwards or backwards torque." The overall goal of the control system is to avoid energy waste. Hence, energy from a descending bucket on a loader can be stored or delivered to help accelerate the vehicle. Surplus energy is delivered to the hydraulic accumulator during braking events and released from it when extra energy is required for forward acceleration or to perform lifting actions. This principle is used in other hybrid systems, of course, but others we are aware of do not try to integrate everything at once. This accumulator is sized at six US gallons on the 180HP powered system on the development test bed, which is considered to be large enough to store energy for the demand side of an operating cycle, but not so large as to introduce excessive weight. Maney said that the test system can absorb up to 550 HP, which is more than three times the engine input. Compared to parallel hybrid systems, component sizes will be larger but complexity and the number of component is reduced, which also eases control. The next stage, Maney said, "Is to work with a client", developing the system on either a loader, excavator, forklift or refuse truck, although the system could in theory be applied to any hydraulically-actuated machine.