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Reliability Evaluations of an Innovative Oil Pump under Crankshaft Torsional Vibrations


Fig. 1 - A crack in the VOP rotor after the engine test

Pierburg Pump Technology (PPT) is a company specializing in the development and the production of mechanical and electrical oil pumps, mechanical and electrical water pumps and vacuum pumps. PPT is collaborating with most of the automakers worldwide in order to develop new products that fulfil the requirements of the Euro5 and Euro6 standards. Remarkably, these environmental restrictions have caused a radical change in the design of oil pumps which now must be able to optimize the engine lubrication while at the same time reducing the fuel consumption. In order to achieve this, PPT is developing the new generation of oil pumps based on the new vane concept with variable displacement (VOP), instead of the traditional gear design. In fact, through this design revolution, PPT, as well as some other competitors leader in the pumps development for the automotive market, could provide fuel consumption reductions of up to 2.5%, with corresponding CO2 reductions. Nevertheless, these vane pumps are significantly more complex and less robust of the traditional geared ones and for this reason, they require a long engineering phase in which the support of the most advanced simulations technologies is even more important every day.

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Fig. 2 - Magnification of the worn and cracked rotor engagement face

An example of the PPT engineering approach
As an example of the PPT engineering approach, we describe here the activities of the Calculation and Simulation group during the development of an oil pump for a new Euro5 engine. These activities involve both calculation and testing. Based on the requirements of the customer, one of the world’s leading automakers – the R&D group of PPT has decided to develop, a vane oil pump with variable displacement in order to achieve the required fuel consumption reduction while maintaining the same performance level as is provided by a traditional pump. This VOP pump was designed for a new gasoline engine; this new engine was based on an existing engine, but with a modified injection system, the power has been doubled.

During this design phase, the PPT team made all preliminary verifications of the design following the internal standard calculation procedure, using on both commercial and in-house software. This procedure is based on both commercial and in-house software and allows the design team to verify that the pump is well-designed from the standpoints of kinematics, dynamics, hydraulics and structural. Early testing of prototypes confirmed a successful design, however there were some failures observed in a following validation phase when installed on the actual engine.

In particular, the breakages have affected the pump rotor that cracked generally at 1/3 of the total tests duration or, in some cases, even before. As a first step, a SEM investigation of the cracked surfaces of the parts have been performed in order to investigate deeper the failure mode and to guess the main causes of breakages. Through this investigation it has been clarified that the failures have been the result of a classical fatigue phenomenon which starting point is likely located on the rotorcrankshaft (CS) engagement face, where the surfaces have also a ductile aspect due to the several impact loads between the two parts.


Fig. 3 - Comparison of the CS torsional vibrations for the Euro4 and Euro5 engines

The simulation work
Based on these results, a complete calculation loop involving hydraulic, kinematic, dynamic and structural evaluations was conceived by the R&D simulation group in order to verify the suspicion that the crankshaft’s irregular motion could be the main cause of the failures. Although, even if some preliminary calculations were performed in the design phase, it was absolutely necessary to investigate whether the crankshaft torsional vibrations, which was not included in the previous verification simulation, could significantly increase the level of load in the VOP. In order to do so, the CS acyclisms in various working conditions were measured by the customer, readily verifying easily that the irregularity of the motion in the “new” Euro5 engine was much higher than that in the “old” Euro4 [2].


Fig. 4 - CS-rotor contact forces and rotor speeds vs CS rotation angle from MBs

Moreover, some CFD analyses were carried out, to estimate the oil pressure level and to verify that no unwanted fluidodynamic effects were taking place while the pump was working [3]. Based also on these results, some MB simulations were performed modelling the complete oil pump and applying as external loads the oil delivery pressures previously estimated through the CFD analyses. Through this modeling and the imposition of the torsional vibrations on the CS, the dynamic forces between the VOP parts in presence of acyclisms have been estimated.

In this way also the contact forces between the rotor and the crankshaft, which should be the main cause of the rotor failures, have been evaluated in several working conditions [1]. Moreover, the detailed analysis of these loads has clarified that the “Evo” engine causes a dramatic increase of in the CS-rotor impacts due to its high motion irregularity. As confirmation of this, the maximum values of the CS-rotor contact forces have been estimated to be 5 times higher for the Euro5 engine application than the ones of the Euro4.

The forces evaluated in this way were then used to perform an FEM simulation in order to evaluate their effect on the stress level in the rotor in general and in the rotorcrankshaft interface in particular. In order to do so, the dynamic problem schematized in the MB has been reduced to an equivalent static one applying the “d’Alembert Principle” [4-5], so that the inertial torques are distinguished from the reaction torque due to the constraints and the loads. For this reason, the forces, the angular accelerations and the friction and inertia torques in specific high-stressing instants have been recollected from the MB and implemented in ANSYS. The FEM simulations so completed have confirmed that the crack initiation starts on the rotor engagement surface in contact with the CS. Remarkably, the FEAs have also demonstrated that the failures are essentially caused by the abnormal loads coming from the engine and not by a lack of robustness in the rotor. As further confirmation of this, the level of stress in the current Euro4 application has been estimated through the repetition of the MB and FEA calculations, confirming that the dramatic increase of the engine motion irregularity could cause rotor breakages for the pump already in production also.


Fig. 5 - CFD evaluation of the VOP oil pressure


Fig. 6 - Equivalent stress in the rotor from FEAs


Fig. 7 - Stress reduction in the rotor and consequent SF increase as result of redesign activity

The rotor redesign to avoid the failures
In addition to this, the calculations loop here described has enabled the evaluation of the possible benefits of some design modifications through the repetition of the MBs and the FEAs. In particular, the variation of the CSrotor clearance at the engagement surfaces, the increase of the critical strength section of the rotor and increase of the number of the engagement surfaces have been evaluated as possible modifications for the reduction of the stresses in the rotor. Through the evaluation of these, it has been verified that a proper combination of all these modifications will increase the part SF from 1 up to almost 2. In addition to this, a decrease of the motion irregularity has been finally agreed to with the customer in order to increase even more the safety of the part and to avoid further failures. As final confirmation of the whole reliability analysis activity, the significant improvement of the rotor safety was confirmed by the following tests on the engine. These test were successfully passed even for the worst working conditions. Summarizing, the multidisciplinary analysis so performed has allowed PPT to find and to examine the causes of failures of a rotor for an innovative oil pump, allowing PPT to describe them very carefully to the customer. In this way, the engineering choices already taken during in the first design phase have been confirmed, allowing PPT to propose and evaluate only the design modifications that could significantly increase the reliability of the part. Finally, through this approach, PPT has managed to propose and validate through calculations the final proposed pump design, thus avoiding a long tests phase that could significantly increase the time and costs of the product development.


Articolo pubblicato sulla Newsletter EnginSoft Anno 7 n°4

Alessandro Testa, Raffaele Squarcini, Matteo Gasperini, Riccardo Maccherini
Pierburg Pump Technology Italy

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