How Efficient Hydraulic Systems Reduce Fuel Consumption on Refuse and Recycling Trucks
In 2009, households and institutions in the United States generated 243 million tons of municipal solid waste(MSW). Every day, the average American tosses away 4.34 pounds of trash. Canadians actually tossed more than Americans – in 2008, approximately 1,031 kilograms (kg) of waste was disposed per person – or approximately 2.82 kg (6.22 pounds) per day. Although recycling rates are on the rise in both countries, and more facilities now convert solid waste into energy, the vast majority is still collected and transported to a landfill via refuse and recycling trucks.
For many years, the refuse and recycling vehicles of choice in North America have been a truck-mounted body with a collection chamber at the rear. One or two riders step off the truck at the collection location, stop and lift bags, bins or any number of receptacles and dump it into the chamber. When the collection area is sufficiently filled, the truck’s hydraulic system is engaged and hydraulic cylinders move a compression plate to pack and move the waste into the truck body. When the body is full, the truck is driven to a transfer station or landfill to unload and repeat the process.
Over time, other truck body styles have become more prominent, including front end loaders and automated side loaders. Regardless of body style, all trucks perform the same basic function of loading, compressing and transporting refuse materials to the appropriate facility.
Hydraulic systems background
The typical refuse and recycling truck hydraulic system has traditionally consisted of a gear pump coupled to the engine crankshaft in the front of the truck. Gear pumps are among the simplest and most cost effective type of pump, both of which are primary factors in their selection. The gear pump is a fixed displacement pump, and the output flow is in constant relation to the engine rpm. The higher the engine rpm, the more flow is being pumped throughout the system at all times.
Furthermore, the typical refuse or recycling truck requires substantial lengths of fluid conveyance lines due to the location of the pump (front of the truck) relative to the valves that control the working cylinders (middle to rear of the truck). The long length of fluid conveyance lines coupled with several adapters and the open-centre valves all mean the system is vulnerable to pressure drop. In a circuit, pressure drop creates heat and requires added energy to run the system. Reducing (or eliminating) pressure drop is key to ensuring an energy efficient hydraulic system is at the heart of any refuse and recycle truck.
Prior to 2007, the historic and relative low cost of diesel fuel offset nearly all incentives for body manufacturers to switch to higher efficiency hydraulic systems. As in any open market industry, business has become more competitive, and increases in the cost of truck bodies are difficult to pass on in a value-selling process. Furthermore, the benefits of highly efficient hydraulic systems (lower fuel consumption) provided benefits for the end user, not the body manufacturer. So, low cost won out. But recent trends in diesel fuel cost have shifted the market.
In North America, the cost of diesel fuel remained low until the mid-2000s. Then a dramatic rise in diesel fuel costs in 2007 pushed end users to be much more aware of overall efficiency as well as durability.
The pump solution
As stated earlier, reducing pressure drop is key toward improving system efficiency. A simple way to reduce pressure drop is to use a variable displacement style pump and to control the pump – having it in a zero-flow state
when hydraulic functions are in the nonworking mode. A sound choice for the pump is an axial piston style with load sensing controls. For rear loader and commercial style front loader trucks, the hydraulic work circuit is engaged less than 20 percent of the time during a daily route.
By using a load-sense style piston pump, pressure drop is essentially eliminated during the non-working portion of the day and fuel savings are significant. Multiple end users that have employed a piston pump-equipped truck have reported from 7 to 15 percent fuel savings for both front-loader and rear-loader style trucks when compared to a truck equipped with a gear pump. Piston pumps are easily adaptable to most any style refuse/recycle truck hydraulic circuit, including the traditional open centre systems employed on the majority of trucks in service today.
The cost of a piston pump versus a gear pump is usually slightly higher, but at 7 to 15 percent fuel savings (2 to 5 gallons per day), the payback period is months, not years.
A further advantage of a piston pump is that it can be outfitted with a positive shut-down valve in case of hose rupture. Because the pump can go to a zero-flow state, a positive blocking valve will allow full system shutdown without damage to the pump. As a result, the vehicle can not only be driven to a repair centre, but the chance of avoiding a costly oil spill – and potential fines or clean-up costs – on a public roadway is also reduced.
Finally, piston pumps have been used for many years in the refuse and recycling industry, not only in Europe but on several thousand rear-loader trucks in North America. End users report that the piston pumps are more durable and last far longer than gear pumps. One large municipality has utilized Eaton piston pumps on hundreds of collection trucks since 1997 and has reported that the piston pumps typically last for the 8 to 10 year lifespan of each truck, while an average of 2.3 gear pumps per truck were needed over same time span.
Fuel savings benefits can be equally applied to CNG engines. A further benefit of piston pumps on trucks equipped with a CNG engine is that torque control can be added, either hydro-mechanically or electronically, to match peak torque and prevent engine stall at low RPM. The inherent low torque and torque response for CNG engines are a good fit for using a piston pump with torque-limiting control.
As fuel costs continue to rise, end users are expected to continue seeking value added solutions to reduce operating costs. Using a variable displacement piston pump is a proven solution that meets the rigours of the refuse and recycling truck industry and has proven itself many times over to reduce fuel consumption.
A piston pump is readily applied to any refuse or recycling truck hydraulic circuit, either with current open-centre valves or completely integrated with a full load sensing system for further optimization and smooth control over the entire hydraulic circuit.
The end user will realize measurable fuel savings to lower not only their cost of operation, but corresponding emissions, both of which also contribute to significant environmental improvements.