Hydraulics as an Emissions Compliance Solution

DN Staff

February 19, 2011

7 Min Read
Hydraulics as an Emissions Compliance Solution

AchievingTier-4/StageIIIB emissions regulation compliance represents one of the biggest and costliest challenges faced byOEMs and users of mobile diesel-powered equipment. Ditto for the supplier basethat supports them. Over the years, the world's engine builders have workedwonders, but there is no getting around the fact that the power density of therequired Tier-4 engines is going to be substantially less than that of theengines they will replace.

The new engine systems are going to take up more machinevolume for the same output largely due to the requirement for after-treatmentequipment that wasn't previously required. So, it is quite understandable thatthe industry has been intensely focused on the vehicle packaging changesrequired to accommodate both the physically larger engines and the newafter-treatment devices they need.


Thissingular focus may be understandable, but it's also shortsighted because ittends to neglect the potential contribution of other systems toward the goal ofpreserving and enhancing overall vehicle efficiency. Hydraulics, for example,are the single largest consumer of energy on most diesel-powered constructionand off-highway equipment, yet most of the new Tier-4-compliant machines arestill using legacy components and systems.

Anoptimized Tier-4 hydraulic system could provide substantially increased powerdensity along with enhanced reliability and expanded control options. But to dothat, it would have to operate at significantly higher pressure than today'sequipment, as most of the losses in a hydraulic system are directly related toflow volume. In other words, the use of higher pressures means that less fluidis required to do the same amount of work. Since less flow means less energy iswasted as heat, reducing the flow makes the system more efficient.

Higher pressures also allow physically smaller systemcomponents to deliver the same performance as larger components operating atlower pressures. Theoretically, one could upgrade the performance of a legacydesign without adding weight by increasing operating pressure. This is possibleif the system components could handle it.

Butif the goal is to optimize the efficiency of the whole machine, and not justthe hydraulic system, then all of the Tier-4 components will have to bephysically smaller than their current generation counterparts. Ideally, anoptimized Tier-4 hydraulic system would consume less energy, occupy lessvolume, and perform more useful work than a legacy system - all at a cost thatdelivers increased value for the OEM and ultimately, the end user.

CompliantDesign

Designing such Tier-4 components is a challenge,particularly when it needs to be done in a tight economy. One example of howsuppliers have met this challenge that illustrates both the process involvedand the results that can be achieved is Eaton Corp.'s new 620 Seriesof open-circuit piston-type pumps.

The development process for this medium-duty pump beganwith the goal of delivering a smaller pump. During design, the question focusedon how to make it smaller. Should the pump be axially shorter or radiallythinner than existing products?

Eatonengineers began by looking at the installed base of piston-type pumps onconstruction and off-road equipment. Unlike pumps used on truck applications,which tend to be sensitive to diameter, most of the pumps used in constructionand off-road equipment are powered front to back, making length the morecritical dimension.

Extensivecustomer interviews and other quantifiable research reinforced the need for ashorter pump that was also lighter. Customers wanted a pump that could beinstalled in existing engine compartments even when a physically largerTier-4/Stage IIIB engine was used.


Twoadditional 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 commonlegacy standard 2,000. This corresponded well with preliminary design goals for the pump which included increased power density, a physically smallerpackage, higher operating pressure capabilities,extended bearing life, improved hydraulic efficiency, lighter weightand quieter operation.

Trade-Offs

There are three basic ways tomake 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 optionsdeliver minimal length reduction, and thinner case walls often adversely impactthe pump's operating pressure capabilities and noise signature. Changing theswash plate angle delivers significant length reduction but at the cost ofreduced mechanical efficiency.

Facedwith these facts, Eaton engineers decided to take a balanced approach todesigning the new 620 pump, one that would maximize the potential benefits ofthe available length reduction strategies. They began by optimizing the casedesign for operation at 280 bar while avoiding the common tendency toover-design the components.

Usingsimulation and finite element analysis tools, they designed the case and endcover to achieve the required levels of strength and stiffness for 280 baroperation. This included adding internal stiffening ribs to help controlvibration and reduce the pump's noise signature. This option was balancedagainst the engineers' contribution to the goal of making the pump quieter.

Byfar the most difficult challenge in designing the 620 pump was finding a way toovercome the reduction in mechanical efficiency produced by changing the swashplate angle from the de facto 18A degrees industry standard to the 15A degrees required to meetthe 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 8lb lighter than the most widely used 18A degrees pump in its category.

Butthat achievement came at a price. A smaller swash plate angle produces ashorter piston stroke that reduces mechanical efficiency. So, the next task wasto optimize the pump's volumetric behavior to compensate for as much of thelost mechanical efficiency as possible.

Onceagain, extensive simulation and computer analysis was applied to the design ofthe valve plate and particularly to the metering notches to minimizecross-porting while optimizing flow. Many notch geometries, spacings andlocations were evaluated during the development process using computationalfluid dynamics tools.

Real-WorldFunctionality

To back the computer-generated designrecommendations, the most promising designs were prototyped and tested underreal-world operating conditions before finalizing the production version.

Candidate designs also had to meet real-worldrequirements for noise and vibration signatures, bearing life and reliability.For example, the higher internal forces generated by 280 bar operation requireddevelopment of a bimetallic valve plate to reduce friction on the bearingsurface while increasing wear resistance on the other. Bearing design was alsooptimized to achieve a 13,600 hour B-10 life while handling the higher internalforces created by the new design as well as the upgraded 2,200 rpm maximuminput speed.

Asa result of the various design iterations and engineering and customer reviewsemployed during the 620 pump's development, the pump has 28 percent fewer partsthan Eaton's existing family of open-circuit pumps while delivering higherperformance.

Agood example of this is evidenced in the 620's use of a single-acting controlpiston located in a bore machined directly into the pump's single split-linehousing. The piston is an advanced design using a proprietary anti-frictioncoating to minimize particulate build-up and reduce response time.

TonyWelter is construction and mining segment director at Eaton Corp.





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