Achieving
Tier-4/Stage
IIIB emissions regulation compliance represents one of the biggest and costliest challenges faced by
OEMs and users of mobile diesel-powered equipment. Ditto for the supplier base
that supports them. Over the years, the world's engine builders have worked
wonders, but there is no getting around the fact that the power density of the
required Tier-4 engines is going to be substantially less than that of the
engines they will replace.
The new engine systems are going to take up more machine
volume for the same output largely due to the requirement for after-treatment
equipment that wasn't previously required. So, it is quite understandable that
the industry has been intensely focused on the vehicle packaging changes
required to accommodate both the physically larger engines and the new
after-treatment devices they need.
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This singular focus may be understandable, but it's also shortsighted because it tends to neglect the potential contribution of other systems toward the goal of preserving and enhancing overall vehicle efficiency. Hydraulics, for example, are the single largest consumer of energy on most diesel-powered construction and off-highway equipment, yet most of the new Tier-4-compliant machines are still using legacy components and systems.
An optimized Tier-4 hydraulic system could provide substantially increased power density along with enhanced reliability and expanded control options. But to do that, it would have to operate at significantly higher pressure than today's equipment, as most of the losses in a hydraulic system are directly related to flow volume. In other words, the use of higher pressures means that less fluid is required to do the same amount of work. Since less flow means less energy is wasted as heat, reducing the flow makes the system more efficient.
Higher pressures also allow physically smaller system components to deliver the same performance as larger components operating at lower pressures. Theoretically, one could upgrade the performance of a legacy design without adding weight by increasing operating pressure. This is possible if the system components could handle it.
But if the goal is to optimize the efficiency of the whole machine, and not just the hydraulic system, then all of the Tier-4 components will have to be physically smaller than their current generation counterparts. Ideally, an optimized Tier-4 hydraulic system would consume less energy, occupy less volume, and perform more useful work than a legacy system - all at a cost that delivers increased value for the OEM and ultimately, the end user.
Compliant Design
Designing such Tier-4 components is a challenge,
particularly when it needs to be done in a tight economy. One example of how
suppliers have met this challenge that illustrates both the process involved
and the results that can be achieved is Eaton Corp.'s new 620 Series
of open-circuit piston-type pumps.
The development process for this medium-duty pump began
with the goal of delivering a smaller pump. During design, the question focused
on how to make it smaller. Should the pump be axially shorter or radially
thinner than existing products?
Eaton
engineers began by looking at the installed base of piston-type pumps on
construction and off-road equipment. Unlike pumps used on truck applications,
which tend to be sensitive to diameter, most of the pumps used in construction
and off-road equipment are powered front to back, making length the more
critical dimension.
Extensive
customer interviews and other quantifiable research reinforced the need for a
shorter pump that was also lighter. Customers wanted a pump that could be
installed in existing engine compartments even when a physically larger
Tier-4/Stage IIIB engine was used.
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Two additional design requirements dictated customers were looking for a 280 bar (4,060 psi) continuous pressure capability (350 bar peak) and the ability to operate at 2,200 rpm rather than the common legacy standard 2,000. This corresponded well with preliminary design goals for the pump which included increased power density, a physically smaller package, higher operating pressure capabilities, extended bearing life, improved hydraulic efficiency, lighter weight and quieter operation.
Trade-Offs
There are three basic ways to
make a piston-type pump physically shorter. The case walls can be made thinner,
ports can be relocated, or the swash plate angle reduced. The first two options
deliver minimal length reduction, and thinner case walls often adversely impact
the pump's operating pressure capabilities and noise signature. Changing the
swash plate angle delivers significant length reduction but at the cost of
reduced mechanical efficiency.
Faced
with these facts, Eaton engineers decided to take a balanced approach to
designing the new 620 pump, one that would maximize the potential benefits of
the available length reduction strategies. They began by optimizing the case
design for operation at 280 bar while avoiding the common tendency to
over-design the components.
Using
simulation and finite element analysis tools, they designed the case and end
cover to achieve the required levels of strength and stiffness for 280 bar
operation. This included adding internal stiffening ribs to help control
vibration and reduce the pump's noise signature. This option was balanced
against the engineers' contribution to the goal of making the pump quieter.
By
far the most difficult challenge in designing the 620 pump was finding a way to
overcome the reduction in mechanical efficiency produced by changing the swash
plate angle from the de facto 18A degrees industry standard to the 15A degrees required to meet
the length reduction goal. That change, together with the case optimization,
allowed the engineers to make the 620 27 mm shorter (289 versus 317 mm) and 8
lb lighter than the most widely used 18A degrees pump in its category.
But
that achievement came at a price. A smaller swash plate angle produces a
shorter piston stroke that reduces mechanical efficiency. So, the next task was
to optimize the pump's volumetric behavior to compensate for as much of the
lost mechanical efficiency as possible.
Once
again, extensive simulation and computer analysis was applied to the design of
the valve plate and particularly to the metering notches to minimize
cross-porting while optimizing flow. Many notch geometries, spacings and
locations were evaluated during the development process using computational
fluid dynamics tools.
Real-World Functionality
To back the computer-generated design
recommendations, the most promising designs were prototyped and tested under
real-world operating conditions before finalizing the production version.
Candidate designs also had to meet real-world
requirements for noise and vibration signatures, bearing life and reliability.
For example, the higher internal forces generated by 280 bar operation required
development of a bimetallic valve plate to reduce friction on the bearing
surface while increasing wear resistance on the other. Bearing design was also
optimized to achieve a 13,600 hour B-10 life while handling the higher internal
forces created by the new design as well as the upgraded 2,200 rpm maximum
input speed.
As
a result of the various design iterations and engineering and customer reviews
employed during the 620 pump's development, the pump has 28 percent fewer parts
than Eaton's existing family of open-circuit pumps while delivering higher
performance.
A
good example of this is evidenced in the 620's use of a single-acting control
piston located in a bore machined directly into the pump's single split-line
housing. The piston is an advanced design using a proprietary anti-friction
coating to minimize particulate build-up and reduce response time.
Tony
Welter is construction and mining segment director at Eaton Corp.