FEM ANALYSIS OF CONNECTING ROD FOR STATIONARY ENGINE P. Brabec, P. Kefurt, C. Scholz, R. Voženílek Technical University of Liberec, Hálkova, Liberec, Czech Republic BEZ MOTORY, a.s., Plotiště nad Labem, Hradec Králové, Czech Republic This submission addresses the computation of the strength and distortion characteristics of a stationary engine connecting rod. Respecting distortions, we primarily monitored the clearance between the connecting rod and the crank pin. The finite element method was used in resolving the strength and distortion issues. Introduction This submission shows the implementation of the FEM software for the assessment of the strength and distortion characteristics of a connecting rod. Connecting rods transfer energy from pistons to crankshafts and convert the linear, reciprocating motion of a piston into the rotary motion of a crankshaft. From the viewpoint of functionality, connecting rods must have the highest possible rigidity at the lowest weight. Generation of the Model Prior to any calculations for the connecting rod itself, preparatory calculations were carried out. The first calculation served to determine the contact pressure between the crank pin bearing brass and the connecting rod big end. The pattern of the radial and tangential stress is nearly constant both in the direction of the curvature and the Fig. : View of a stationary engine made by BEZ MOTORY, a.s. bearing width. The other supporting calculation determined the moment of inertia of the connecting rod assembly which was used to substitute the connecting rod mass with a sliding and rotary part. This substitution facilitates the calculation of connecting rod inertia forces since it converts its mass into a simple alternative system with a division into two points. The sliding part is concentrated into the gudgeon pin centre line together with which it performs the advance motion, and the rotary part is concentrated into the crank pin centre line with which it performs the rotary motion. However, the substitution of the connecting rod with the two points still does not fulfil all of the conditions of the dynamic equivalence of the original and alternative system. Only the condition of maintaining the total weight and the position of the centre of gravity is met but it does not fulfil the requirement that the original and alternative systems have the same moment of inertia with respect to the axis passing through the centre of gravity. For the determination of
the original connecting rod s moment of inertia the D CAD software was used which further determined the mass and the position of the centre of gravity. T Y X Connecting rod-centre of gravity inertia ellipsoid. T The following figure shows the results arising from the connecting rod being substituted with two points. m. kg ; b. mm ; a.. mm m A 9.9 kg (. m) ;. kg (. m) m B The subtitution of the connecting rod with only two points creates an error which is corrected by adding a compensation moment expressed as M J. The following chart shows a pattern of the compensation moment. The maximum value of the compensation moment was ±.99 Nm at 9 and degrees. The size of the moment in view of the connecting rod dimensions is not significant. With a new design of the connecting rod this deviation J can be eliminated completely provided that the radius of inertia is a b. i x A M B m A m B a b M (Nm) - - - - - Fig. : The pattern of the compensation moment with the connecting rod being substituted with two points dependent on crankshaft angular displacement
When the model was created, the sliding fit was simulated among all components by means of contact elements. For symmetry, only one half of the connecting rod assembly was modelled and the conditions of symmetry utilized on the relevant areas of the section. In the operation of a four-stroke internal combustion engine under normal conditions the two most significant states of stress acting on the connecting rod are caused primarily by forces from a) max. combustion pressure b) sliding masses at max. speed x' [m/s] - x'' [m/s ] - - - 9 - Fig. : The pattern of the piston speed and acceleration dependent on the crankshaft angular displacement Fig. : The view of the connecting rod assembly with marked points for functional clearance readouts to make calculations between the connecting rod bushing and the crankshaft pin.
a) Results max. combustion pressure was set at. MPa and it acts closely behind the top dead centre. Simultaneously, inertia forces resulting from the sliding masses of the piston assembly act here. Note: X-axis corresponds to the connecting line from the big end and small end centres. Fig. : The computational results for stress from max. pressure and inertia forces,,, vůle (mm),,, 9 Fig. : The pattern of clearance between the connecting rod bearing and the crank pin dependent on alpha angle for the stress from max. combustion pressure and inertia forces (for the version with max. possible fit clearance of. mm) The results indicate that the highest stress occurs in the connecting rod stem (compression stress). The contact point between the connecting rod bearing and the crank pin within about degrees of the perimeter is evident in Fig.. a ( )
b) Results max. inertia forces from sliding masses predominate in the cylinder content exchange period when pressures in the cylinder in the piston top dead centre are insignificant: Note: X-axis corresponds to the connecting line from the big end and small end centres. Fig. : Computational results for stresses from inertia forces only.,,,, vůle (mm),,,, 9 a ( ) Fig. : The pattern of clearance between the connecting rod bearing and the crank pin dependent on alpha angle for stresses from inertia masses only (for the version of max. possible fit clearance of. mm). This type of stress results in the tensile stress acting on the connecting rod stem. Fig. shows a wider contact point between the connecting rod bearing and
the crank pin within the range of degrees of the perimeter. The pattern of the graph (See Fig., ) indicates that the clearance of. mm between the specified components will not result in a functional clamping of the crankshaft pin in the connecting rod bushing. The most dangerous stresses for the clamping (taking up the clearance in points and See Fig., 9) of the crankshaft pin is caused by the inertia force in top dead centre. Conclusion The results clearly indicate that the connecting rod is not significantly strength stressed and a worse variant is the stress at max. combustion pressure of. MPa. Respecting distortions it is important to monitor the area around the lower end of the connecting rod. Here, on the other hand, the second variant proves to be worse due to the forces arising at max. speed under the influence of sliding masses. As far as distortions are concerned, it is vital to avoid such distortions of the connecting rod big end which would take up the bearing clearance. As a result the most critical variant was calculated which is possible for the min. fit clearance (. mm) between the connecting rod and the crankshaft. The computational results are shown in the following graph (Fig.9). As the graph pattern indicates, it is obvious that the functional clearance of the fit has not been taken up in points and even in this case.,,, Clearance size:. mm. mm, vůle (mm),,,, 9 Fig. 9: The pattern of clearance between the connecting rod and the crank pin dependent on alpha angle for inertia mass stress only (for variant of min. and max. clearance possible). This submission shows the importance of the solution of the connecting rod big end distortions in view of the changes in the bearing clearance at the most important variants of stress. This variant is frequently unfairly overlooked and primary importance is, instead, attached to the strength stress which, however, does not always have to be the limiting one. a ( )
References - http://www.bezmotory.cz/en.pdf - BEROUN, S. BRABEC, P. MRNUŠTÍK, L. SCHOLZ, C. - VOŽENÍLEK, R.: VÝZKUMNÉ A VÝVOJOVÉ PRÁCE v rámci řešení PROJEKTU MPO Č. FD-K/ (Etapa roku ). ZPRÁVA SM /. TU v Liberci, Liberec. - BRABEC, P. SCHOLZ, C. - VOŽENÍLEK, R.: FEM analysis of connecting rod. In: XIII. International Scientific Meeting Motor Vehicles & Engines. Str. + CD. Department for motor vehicles and engines FME Kragujevac, Kragujevac. Serbia ISSN 9X. Published results were acquired using the subsidization of the Ministry of Education of the Czech Republic, project M Josef Božek Research Centre for Engine and Vehicle Technologies II.