Simulation Cuts Time

Computer simulation reduced the time needed to develop a revolutionary engine lube centrifuge from approximately three years that would have been needed in the past to just one year. Centrifuges have been used as bypass filtration devices to remove contaminants from engine lube oil for several decades.

Fleetguard, which today is known as Cummins Filtration, used computational fluid dynamics (CFD) several years ago to identify inefficiencies in traditional designs and develop a new design that would provide cleaner oil and reduced engine wear. More recently, the company again used CFD to modify this design to improve manufacturability while maintaining the performance improvements of the earlier design.

Using the traditional approach of building machined or stereolithography prototypes and evaluating them on a test bench would have taken several years to go through the 40 or so configurations that were simulated prior to reaching the final design. CFD made it possible to evaluate the same number of designs in less than three months with minimal prototype fabrication. CFD also provides far more information and design insight than physical testing, which makes it possible to move much more quickly from the initial concept to the optimized design.

Increasingly stringent environmental requirements, coupled with increasing engine power density and customer expectations for long service intervals, are causing diesel engine lube oil to come under ever-increasing chemical, thermal, and particle mass loads. With improvements in oil formulations and full-flow filtration technology reaching maturity, engine manufacturers are increasingly looking towards bypass centrifuges to reduce their oil particulate loads and improve engine life.

A bypass centrifuge diverts a small percentage of oil pump output and cleans that oil to a higher level than the full-flow filter, before returning it directly back to the sump. These types of bypass systems are sometimes referred to as "kidney loops". Traditional centrifuges use a rotor that is typically driven in rotation at speeds from 4,000 to 10,000 rpm. Prior to studying new designs, Cummins Filtration engineers were interested in evaluating the performance of these early designs, which had been developed long before the advent of computer simulation.

Building virtual prototypes

CFD was selected as its primary analysis tool because of the extensive information and insight that can be obtained by studying the resulting complex flow phenomena, and because of the unique ability to quickly evaluate the effect of changing geometry or flow parameters on performance.

CFD can also facilitate detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop a prototype for a new design, thereby reducing design cycle times and costs. Modeling centrifuge operation with CFD is challenging because the centrifugal acceleration at the perimeter of the rotor typically ranges from 2,000 to 6,000 g's, which causes radial pressure gradients that are many orders of magnitude higher than the axial gradients.

Fleetguard engineers selected Fluent CFD software from Fluent Inc., Lebanon, NH (acquired by ANSYS Inc. earlier this year), because it demonstrated the ability to consistently reach solutions on these challenging models. Furthermore, its discrete phase particle model enables direct assessment of centrifuge particle removal efficiency vs. particle size.

Cummins Filtration first analyzed the traditional engine lube centrifuge design, which consists of a cylindrical vessel with a central hub, rotating on a fixed shaft which also supplies oil to the vessel through a drilled port. Flow is introduced into the centrifuge through the shaft, from where it flows upward to the centrifuge inlet hole at the top of the hub.