OTEKON 14 7. Otomotiv Teknolojileri Kongresi 26 27 Mayıs 2014, BURSA COMPARISON OF TWO MODELING TECHNIQUES FOR PISTON- LINER INTERACTION IN TERMS OF PISTON SECONDARY MOTION USING AVL EXCITE Çağatay Kocaoğlu*, M.Selçuk Tabak** *AVL Research&Engineering Turkey, **Ford-Otosan ABSTRACT With increasing demand for better NVH characteristics of vehicles, demand for more advanced and accurate NVH simulation methodologies are increasing. Noise radiating from powertrains constitutes one of the three pillars of Powertrain NVH. With piston dynamics having the potential of affecting the radiating noise especially at low engine speeds and being hard to assess, this paper is dedicated to the comparison of the capabilities of two different piston-liner interaction techniques utilized in a specialized Multi-Body Dynamics simulation tool, AVL Excite. In one of the techniques, piston-liner interaction is modeled with a simple joint - between liner and conrod small end where piston is attached as point mass- that is basically a linear spring-damper system. In the other one, a 3D piston FE (Finite Element) model is included and piston-liner interaction is achieved via an elastohydrodynamic (EHD) joint that is capable of modeling the oil film between the liner and the piston. Analyses have been performed for idle speed and rated power speed at hot conditions. Surface velocity results are taken from points on the cylinder block surface around TDC height where it is more likely to observe the undesired piston slap phenomenon and both modeling techniques are investigated. Keywords: Piston Secondary Motion, Multi-Body Dynamics, Vibration, NVH, Elastohyrodynamics (EHD) 1.INTRODUCTION Customer satisfaction is a must in automotive industry where competition is fierce and satisfaction from the vehicle is affected from the engine performance (fuel economy, durability, NVH) characteristics. With increasing demand for improved fuel economy, reduced engine noise and less durability issues, it has become crucial to have a better understanding of the piston secondary motion in internal combustion engines, since it affects engine friction, contributes to engine radiated noise via piston slap and may cause piston durability and liner scuffing problems. In this paper, the focus is given on the NVH aspect of piston secondary motion. Piston secondary motion is the transverse and rotational motion of the piston during the reciprocating motion. Transverse motion occurs in the thrust antithrust plane and rotational motion takes place around the piston pin axis. The reason for this secondary motion is the transient forces and moments on the piston and the piston-bore clearance. The previously described transverse and rotational motion of the piston causes the piston to come in contact with the liner and vibration due to this contact may be experienced as noise known as piston slap. Piston slap has a variety of definitions and the definition that takes into account only the first and largest slap occurring shortly after TDC seems to be the most common one. In this paper, piston slap term is used to describe all the piston-to-liner impacts, which all differ in intensity but cause unwanted cylinder block vibration. In the cases where the block surface presents low mobility in the face of piston slap, friction and scuffing may still be affected from piston slap and at that point the importance of the oil film between the piston skirt and the cylinder wall becomes apparent. Existence of oil film provides lubrication, prevents liner scuffing and has a damping effect on piston slap. As the study conducted by Shah, Mourelatos and Patel [1] showed, piston secondary motion is greatly affected by the presence of oil film. Therefore, it is crucial to include the oil film in the prediction models. All the aforementioned points present us the need for accurate modeling and prediction of piston secondary motion, which can be of tremendous benefit if incorporated into the early engine design phase. The study presented here, tries to address that need and makes an effort to contribute to our understanding of the piston secondary motion by comparing two modeling techniques for piston-liner interaction in terms of secondary motion prediction capabilities. The models used here include the following: distortion of the cylinder liner under assembly and thermal loads, thermal expansion of the piston, piston topography
(barrel profile, ovality), dynamic stiffness of the entire engine assembly (with accessories omitted), oil film and roughness of the surfaces (piston-liner). Two speed cases are investigated in hot conditions: - Idle speed (low inertia forces, low combustion pressures) - Speed at rated power (high inertia forces, high combustion pressures) Results are investigated in time-domain (in crank angles) and in frequency domain for a full engine operating cycle (720 degrees). 2.METHODOLOGY A Multi-Body Dynamics (MBD) program specialized for powertrain design and analysis, AVL EXCITE has been used for the analyses conducted on an I6 Diesel engine. For piston-liner interaction, two different joints - GUID (Piston/Liner Guidance) joint & EPIL (Elastic Piston- Liner Contact) joint - with different complexities and capabilities have been used. GUID joints are used to connect the conrod small end (where the piston mass is attached as a point mass) and the liner via a simple spring-damper system that has linear stiffness and damping properties acting in the plane normal to the guidance line [2] (Fig.1). With EPIL joints, the joint force is modeled with a highly nonlinear function of the relative displacements and the relative velocity of the connected nodes along with the oil film history. Reynolds equation (a simplified form of the Navier-Stokes) is used by the program to simulate the elastic piston-liner contact [2]. For the surface contact model, the Greenwood/Tripp boundary contact model has been utilized within the EPIL joint by using experienced-based parameters for the contact model. The complete analysis workflow for the EPIL joint approach can be seen in Fig.2. For GUID joints, piston and liner profiles are not needed; performing only dynamic reduction is enough. Fig.2 Analysis workflow for EPIL joint models Fig.1 GUID joint [3] For the GUID joint approach, point mass modeling of the piston has been utilized, whereas 3D FE models of piston and piston pin have been introduced to use EPIL joints for piston-liner interaction. Since it incorporates EHD for the oil film characteristics, the contact behavior between piston and liner is best represented by EPIL joints in AVL EXCITE. Due to computational requirements, a simplified engine FE model (consisting of flywheel housing, mounts, cam cover, oilpan, cylinder head and cylinder block) has been used in dynamic reduction. Other than the powerunit, cranktrain and conrod dynamic reductions have been performed and the reduced models have been introduced to AVL EXCITE for the model with GUID joints. For the EPIL joint approach, dynamically reduced models of piston and piston pins have been included in addition to the previous ones. To achieve a higher accuracy of the simulation, profiles of the piston and the liner are necessary to be inputted into the EPIL joint. Hot condition of the running engines has been investigated, therefore hot profiles of the liner and the piston have been obtained in several steps: - Heat transfer analysis of the piston to obtain temperature distribution on the piston - Thermal expansion analysis of the piston - Hot profile calculation based on the cold profile and the thermal expansion of the piston - Heat transfer analysis of the cylinder block-head assembly to obtain the temperature distribution - Bore distortion analysis based on the calculated thermal loads combined with the assembly (tightening) loads - Liner profile extraction from the bore distortion analysis Calculated hot profiles of the piston skirt and the liner can be seen in Fig.3a and Fig.3b, respectively.
and anti-thrust sides). Furthermore, various other results regarding piston secondary motion have been obtained and reported in this paper. Here, to avoid a results section of unnecessary length, results of every cylinder are not presented. Since the results of all the cylinders show the same characteristics, results from a single cylinder will be sufficient for our purposes. Fig.4 shows the surface velocity results from thrust side (TS) and anti-thrust side (ATS) in both domains for the rated power speed. Fig.3a Piston Skirt Hot Profile Fig.4a Surface velocities in time-domain Fig.3b Liner Hot Profile 3.ESULTS & DISCUSSIONS Results have been obtained for surface velocities in both time and frequency domains from points on the cylinder block surface at around the TDC locations (at both thrust Fig.4b Surface velocities in frequency-domain
As the results presented in Fig.4 show, EPIL and GUID joints predict almost the same surface velocities at rated power speed where inertia forces and combustion pressures are high. The lateral forces exerted on the liner at rated power speed are given in Fig.5. Although they are not exactly the same, it is obvious that they are very well aligned. Fig.5 Lateral forces on the liner at rated power speed The results presented above lead to the conclusion that using EPIL- or GUID-joints at high engine speeds -where inertia and combustion loads are high and the piston secondary motion is expected to decrease with the hydrodynamic action in the oil film becoming more pronounced as Liu et al. [4] showed is of small difference. As the following results presented in Fig.6 suggest, idle speed case characteristics are much different than the rated power speed case. Fig.6b Surface velocities in frequency-domain Fig.6 shows the surface velocity results from thrust side (TS) and anti-thrust side (ATS) in both domains for the idle speed. Fig.7 Lateral forces on the liner at idle speed Fig.6a Surface velocities in time-domain When the idle speed results for EPIL and GUID joints are compared, surface velocities and lateral force exerted on the liner (see Fig.7) are quite different for both approaches. EPIL joint modeling of the piston-liner interaction results in significant oscillations of the block surface velocities. Also, lateral force exerted on the liner shows severe peaks at two points during the engine operating cycle. The only possible reason for this difference is clearly the piston secondary motion which can be modeled with EPIL joints in contrast to the GUID joint where piston is modeled as point mass and piston-liner interaction is based on simple linear springdamper system - using 3D flexible piston FE model and oil film between the piston and the liner. The previously presented rated power speed results weren t able to show the difference in the modelling capabilities of both joint types in terms of piston secondary motion. This can be explained by the fact that there is a significant difference between idle and rated
power speed cases regarding inertia and combustion loads acting on the piston. That, combined with the decreased piston secondary motion due to the increased hydrodynamic action of the oil film at high speeds, makes it clear that it is normal and as expected to see almost exact surface velocity results for both joint types at rated power speed. The following figure, Fig.8, where EPIL joint results of piston tilting angle and piston lateral position are plotted for both speed cases, shows the decrease in piston secondary motion at high speeds due to hydrodynamic effects as previously mentioned. Fig.8 Piston Transverse and Rotational Motions As it can be seen, lateral displacements within the cylinder and rotational motion around the piston pin are much higher at idle speed, which makes piston secondary motion a problem for NVH at low speeds. Although not investigated here, this becomes a more severe problem especially at cold start conditions where piston to liner clearances are much higher. 4.CONCLUSIONS In this study, our aim has been to assess the piston secondary motion prediction capabilities of two different joints used in AVL EXCITE for piston-liner interaction: - EPIL joint with the ability to incorporate elastohydrodynamics into the piston-liner interaction, - GUID joint, which is a simple spring-damper system with linear stiffness and damping properties. Results have shown that piston secondary motion (transverse and rotational motion of the piston) is decreased at high engine speeds. Decreased secondary motion combined with high inertia and combustion loads on the piston resulted in both types of joints displaying almost identical cylinder block surface velocity results at rated power speed. At idle speed, piston secondary motion levels are much higher and EPIL joint results differ from GUID joint results in that they exhibit large oscillations of surface velocities along with a different characteristic of the lateral force exerted on the liner. Represented results confirm that EPIL joint modeling of the piston-liner interaction is more capable of predicting piston secondary motion. Confirmation of the results with measurement data remains to be done. If the case turns out to be that reasonable correlation can be achieved, then powertrain radiating noise analyses are likely to become more accurate as they are strongly linked to the surface velocity levels of the powertrain. REFERENCES 1. Paras Shah, Zissimos P. Mourelatos and Prashant Patel, Piston secondary dynamics considering elastohydrodynamic lubrication, SAE paper 2007-01-1251 2. AVL EXCITE PowerUnit User s Guide, v2009.3, p.404 3. AVL EXCITE PowerUnit Basic Engine Dynamics Training Material 4. K Liu, Y B Xiu and C L Gui, A comphehensive study of the friction and dynamics of the piston assembly, Proceedings of Institute of Mechanical Engineers Vol.212, 1998, pp.221-226