Hydraulic hybrids

Kinetic Energy Recovery Systems can cut fuel consumption in some heavy-duty vehicles.

The world price of oil has fluctuated in recent months, reaching highs of almost $150 a barrel before plunging back, as this issue went to press, to less than $70. Although there's no obvious trend in such a wild market, the noises from oil companies mean it is a safe prediction that future prices will have an upward tendency as stocks dwindle and demand increases.

This leaves the owners and operators of all oil-thirsty vehicles in an unenviable position. In particular, those companies with fleets of commercial vehicles, buses and off-highway equipment are going to see their operating margins steadily erode as the cost of fuel rises.

At present, although there are various technologies in development, there is no commercially viable option that allows a complete switch away from the use of oil-derived fuels. There are, however, a number of emerging systems that allow the operators of vehicles to reduce both fuel consumption and unwanted emissions, while improving the functionality, reliability and performance of on-board equipment. These systems are especially relevant to vehicles that are subject to constant stop-start operations, such as short-haul buses, refuse collection vehicles and large-wheeled construction and materials handling machinery, including dump trucks and wheel loaders.

Capturing kinetic energy

Constant stop-start operation produces considerable kinetic energy. This would normally be wasted as heat, especially under deceleration and braking. Capturing this energy using conventional hydraulics technology enables it to be stored and then returned to the vehicle systems, perhaps to aid subsequent acceleration thereby reducing the energy required from the engine, or to power ancillary equipment. A refuse collection vehicle (RCV) can, for example, use stored energy to drive the hydraulic refuse compacting and packing mechanisms. This enables a significant reduction of engine speeds and operating noise.

In many applications - especially those where high power densities are required - these hydraulic hybrid systems offer a more efficient alternative to those driven by electric motors. Hydraulics technology such as Parker's Energy Recovery System can be used to capture and transfer high levels of energy extremely quickly compared with similarly sized electric systems, which generally require long periods over which batteries have to be charged. Hydraulic systems are also likely to have a longer operating life than battery-powered devices.

One area, however, where electric hybrids currently offer better economy is in vehicles where consistent levels of power are required for extended periods at near constant speeds, such as long-distance cruising.

Hybrid hydraulic powertrain technology normally incorporates a hydraulic system alongside the petrol or diesel engine to enable the task of powering the vehicle to be shared. Although a number of powertrain arrangements are possible, the simplest is where the conventional vehicle transmission and driveline components are replaced by a system that works in much the same way as a hydrostatic transmission.

Typically, an output shaft from the vehicle's engine is used to drive a hydraulic pump that in turn supplies pressure to hydrostatic motors; these are then connected via a gearing mechanism to the vehicle powertrain to drive the wheels. These motors then, under braking, act as pumps to charge accumulators, where energy is stored before being released back to the powertrain, transmitting torque to the driveshaft and propelling the vehicle.

The energy storage devices, or accumulators, each comprise a vessel divided into two compartments, which are separated by a flexible bladder or membrane. In one compartment, an inert gas, normally nitrogen, is stored under high pressure. Fluid from the hydraulic circuit, under system pressure, enters the second compartment and bears against the bladder or membrane, compressing the gas and allowing energy to be stored. Energy is then released when required through a conventional valve arrangement. The ability of gas to store energy increases exponentially as pressure rises; this factor, combined with the inert property of nitrogen, makes it an extremely efficient and safe medium.

In the Parker Energy Recovery System, vehicle braking activates a specially designed hybrid controller, which harnesses the energy from the wheels by pumping fluid from a reservoir into the accumulator. This is an extremely efficient process, enabling over 70 per cent of the energy normally wasted during braking to be used, minimising the load on the engine, helping to reduce fuel consumption. It's also worth noting that the hydrostatic motors, when acting as pumps during vehicle braking, also help to slow the vehicle down by inducing drag on the rotating drive-train; a feature that helps to reduce brake wear.

If the energy stored in the accumulator falls below a predetermined level, the vehicle engine can be used to provide supplementary power. In practice, however, on vehicles with frequent stop-start cycles, this is rarely required as even gentle braking is sufficient to maintain the stored energy at high levels.

The benefits of this technology include better fuel consumption, which in applications such as dump trucks can be over 40 per cent, plus lower brake wear. It is also possible to reduce the size of the vehicle engine as this can be sized for peak speeds, rather than for low-end torque; in something like a short haul delivery truck, for example, it may be possible to reduce engine capacity by up to 25 per cent.

Hydraulic hybrid technology

Although the key benefits of hydraulic hybrid technology are fuel and cost savings it can also be used to overcome other challenges. For example, RCVs used in city centres and built-up areas need to be fuel-efficient with low exhaust emissions; the best way to achieve this is to operate vehicles at night or the early hours of the morning when traffic conditions are light so that start-stop operations can be minimised. RCVs are, however, inherently noisy as their diesel engines need to run at speeds of around 1,400rpm to drive the high-pressure hydraulic refuse compacting and packing systems - delivering over 40t of force and running at pressures of up to 210 bar - and to give optimum cycle times without the risk of stalling the engine. Unfortunately, high operating noise means that these vehicles have to be used at busy times of day, increasing operating costs and reducing productivity.

Energy recovery systems, such as Parker's Stored Energy Management System (SEMS), offer a potential solution, as they do not rely on noisy engine power for their operation. Indeed, the first vehicles with hydraulic hybrid technology are currently being introduced in Europe, in the form of rear-loading RCVs that incorporate a virtually silent waste packing system. As this functions while the vehicle engine is idling, noise levels are cut by up to 50 per cent; fuel efficiency is also improved by around 5 per cent, with a corresponding reduction in emissions.

The SEMS energy recovery and storage technology is key to the function of the refuse compaction and packing mechanism and integrates computerised control, in-cab HMI and conventional hydraulics into a system that is easy to install and use.

The compacting and packing mechanism is powered in a similar way to that found in engine energy recovery systems, with vehicle braking being used to charge a hydraulic accumulator; this is connected to a hydraulic converter that reduces stored high-pressure energy to the precise output levels required by the items of ancillary equipment on each RCV.

Benefits of KERS

The latest hydraulic hybrid energy recovery technology can provide benefits for all those involved in the construction and operation of vehicles with regular stop-start cycles.

For original equipment manufacturers, hydraulic hybrid systems can be incorporated into existing vehicles without major modifications, minimising the cost of new technology while adding value to the product. Similarly, for end users, the technology can deliver real savings in fuel consumption and brake wear while reducing both emissions and noise pollution.

Although hydraulic hybrid systems are better suited to certain types of commercial on- and off-highway vehicles, they look set to provide an extremely cost effective and reliable option for many companies, at least until such time as a commercially viable alternative to the internal combustion engine can be found.

Further information

www.parker.com [new window]

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