An
increasing corporate focus on energy efficiency is a significant design trend
affecting mobile hydraulics based on the belief that fuel costs will continue
to rise and there will continue to be upward pressure on fuel prices as global
demand for energy increases. End users' interest in sustainability and the
impact of greenhouse gas regulations adds this focus on energy efficiency.
For a company such as Caterpillar - a manufacturer of
construction and mining equipment, diesel and natural gas engines, and
industrial gas turbines - looking specifically at the fuel burned in the
aggregate of all its machines, around 60 to 70 percent of that fuel used is
consumed by the hydraulic system. So as Caterpillar looks to control owning and
operating costs of its machinery, a major area to hone in on is hydraulic
system efficiency.
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Tana Utley, vice president and chief technology officer at Caterpillar Inc., says that, "We view hydraulics as an important part of our future because it's hard to find a technology with greater power density that is continuously variable. Hydraulics will be part of our future platforms for a long time because the technology allows a very flexible routing of power."
Key Areas of Focus
To understand Caterpillar's energy-conscious approach to
hydraulics, let's first look at the three "hows" driving Caterpillar plans.
According to Utley, the first "how" is thinking about
hydraulic system efficiency and it starts with attention to details such as all
of the bends, leak sources, radii and transitions in flow that can be targeted
to improve efficiency at the component level.
The
second "how" entails looking at the system architecture, and how the components
are put together. Beyond that, Caterpillar looks at how to control this
architecture, which includes the integration of electronics into hydraulic
systems.
The
final "how" is to enable capabilities for even greater owning and operating
cost benefits. Hydraulics, and especially electro-hydraulics, is the platform
for semi-autonomous and fully autonomous functions on Caterpillar machines.
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There are six key areas where Caterpillar is focusing to increase hydraulic system energy efficiency:
- minimizing metering losses;
- improving component and system efficiency;
- recycling lost energy;
- reducing parasitic losses;
- optimizing engine operating points; and
- improving operator and worksite efficiency
To minimize metering losses, one area of focus is the integrated electro-hydraulic valve control that enables intelligent control of valve position to minimize restrictions. "We are doing that today in our hydraulic systems," says Utley.
Another area of focus is hydraulic excavators that use a cross-sensing system with two separate hydraulic implement pumps. This system enables Caterpillar to separate the circuits and the pressures during certain operations to control metering losses.
Though fixed and variable displacement piston pumps have been around for a long time, another issue Caterpillar is working on is the next generation of variable piston pumps, with a focus on volumetric and mechanical efficiency. Utley says the objective here is to be able to deliver higher pressure and quicker response times. "We are also developing this family of pumps in concert with our engines of the future so that our engines will run at their optimum operating points," she adds.
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"With what we are facing with EPA's Tier 4 regulations, hydraulics and how we tune our hydraulics are beginning to have an important influence, not just on overall machine performance, but also in how we address those regulatory challenges," Utley says. "We use computational fluid dynamics (CFD), a mathematical model of how fluid flows through a system, to know - before we cut any iron - the complete design of the system and we can analyze how the fluid flows through the circuits. The designer is able to see expansion and losses as the hydraulic fluid expands, quantify the losses, and change the orifice size and the resulting fluid flow."
To enable the recycling of lost energy, Caterpillar has begun using regeneration circuits in its hydraulic excavators including both the boom and the stick that save energy during the boom-down and stick-in operations. According to Utley, this increases efficiency, and reduces cycle times and pressure losses that result in lower fuel costs.
"Our wheeled excavator uses a dedicated variable displacement piston pump and a fixed displacement hydraulic piston motor to power the swing mechanism," Utley says. "This provides a recuperative technology with a closed hydraulic circuit that allows us to maximize swing performance without reducing power to the other hydraulic components. Swing braking energy is transferred directly back to the engine where it can be used in other circuits."
Other Energy Improvements
Fans
consume a lot of energy, so Caterpillar has begun using hydraulic demand fans
which are electronically controlled.
The units can vary fan speed to cool the engine as needed, thus saving fuel.
"We also carefully design lines and tubes to make sure
that the fluid flow is very smooth and is as linear as we can possibly make it,
especially at connection points and transitions," says Utley. "Computation
fluid dynamics also helps us to understand where losses occur, and to improve
our design to reduce losses."
One
of Caterpillar's goals is to reduce the top engine rpm, especially in
applications such as hydraulic excavators.
"Historically
we have had a high engine operating speed to accommodate the heavy hydraulic
demands that we get in those applications and to ensure we have sufficient pump
response," says Utley. "Instead of tuning the engine to run all of the time at
a high rate of speed, we are now tuning the engine at a lower rpm, which
enables us to optimize the engine performance. It also produces the very best
fuel consumption at a lower engine rpm where it more naturally runs at a sweet
spot."
Another
energy question continually facing Caterpillar is how to supply the right kind
of power for a heavy transient demand in a hydraulic system. Currently,
Caterpillar believes the answer is to speed the engine up temporarily which
allows it to run at the sweet spot most of the time and provide the transient
power only when needed.
Hydraulic Efficiency
Looking
beyond direct hydraulic efficiency, Caterpillar also considers the efficiencies
of what the hydraulic system itself enables. This includes technologies such as
Caterpillar's Accugrade system, which uses electro-hydraulic systems as a
fundamental building block. Accugrade and auto-dig features rely on the ability
of the machine to sense the position of the blade and control it based on the
difference between what the computer thinks is the blade's ideal position and
the actual position.
Utley
says it is not uncommon using Accugrade technology to see a 20 to 30 percent
improvement in the efficiency of an earth-moving job. "In hydraulic efficiency,
a five or 10 point improvement is tremendous," she says. "We have achieved 20
percent with a basic electro-hydraulic system; adding the positioning
technology has enabled us to implement innovations such as Accugrade to drive
additional improvement gains."
Hydraulics
are not just used by Caterpillar to power machine work, but also for operator
comfort.
"Think
of the geometry of these machines when the operator is carrying a load and must
drive over bumpy surfaces," explains Utley. "On a tough worksite, the machine
begins to bounce, which can cause the operator to slow down and even start
dumping out the contents of the machine's bucket."
One
of the technologies Caterpillar uses on its 938H wheel loader is a ride control
system that offers an option of using four nitrogen-charged accumulators set to
different pressures that enables the machine's hydraulics to provide optimum
cushioning over uneven terrain at all loads and speeds. "Since hydraulics is a
means of distributing power," says Utley, "it takes hydraulic energy to do
that. But in order to optimize energy consumption, we activate the system only
when the operating speed is greater than 3 mph and the operator is in carry
mode."
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