24-11-2012, 12:04 PM
Dynamic Load Analysis and Optimization of Connecting Rod
DynamicLoadAnalysis.pdf (Size: 4.55 MB / Downloads: 100)
INTRODUCTION
BACKGROUND
The automobile engine connecting rod is a high volume production, critical
component. It connects reciprocating piston to rotating crankshaft, transmitting the thrust
of the piston to the crankshaft. Every vehicle that uses an internal combustion engine
requires at least one connecting rod depending upon the number of cylinders in the
engine.
Connecting rods for automotive applications are typically manufactured by
forging from either wrought steel or powdered metal. They could also be cast. However,
castings could have blow-holes which are detrimental from durability and fatigue points
of view. The fact that forgings produce blow-hole-free and better rods gives them an
advantage over cast rods (Gupta, 1993). Between the forging processes, powder forged or
drop forged, each process has its own pros and cons. Powder metal manufactured blanks
have the advantage of being near net shape, reducing material waste. However, the cost
of the blank is high due to the high material cost and sophisticated manufacturing
techniques (Repgen, 1998). With steel forging, the material is inexpensive and the rough
part manufacturing process is cost effective. Bringing the part to final dimensions under
tight tolerance results in high expenditure for machining, as the blank usually contains
more excess material (Repgen, 1998).
OBJECTIVES AND OUTLINE
The objective of this work was to optimize the forged steel connecting rod for its
weight and cost. The optimized forged steel connecting rod is intended to be a more attractive option for auto manufacturers to consider, as compared with its powder-forged
counterpart.
Optimization begins with identifying the correct load conditions and magnitudes.
Overestimating the loads will simply raise the safety factors. The idea behind optimizing
is to retain just as much strength as is needed. Commercial softwares such as I-DEAS and
ADAMS-View can be used to obtain the variation of quantities such as angular velocity,
angular acceleration, and load. However, usually the worst case load is considered in the
design process. Literature review suggests that investigators use maximum inertia load,
inertia load, or inertia load of the piston assembly mass as one extreme load
corresponding to the tensile load, and firing load or compressive gas load corresponding
to maximum torque as the other extreme design load corresponding to the compressive
load. Inertia load is a time varying quantity and can refer to the inertia load of the piston,
or of the connecting rod. In most cases, in the literature the investigators have not
clarified the definition of inertia load - whether it means only the inertia of the piston, or
whether it includes the inertia of the connecting rod as well. Questions are naturally
raised in light of such complex structural behavior, such as: Does the peak load at the
ends of the connecting rod represent the worst case loading? Under the effect of bending
and axial loads, can one expect higher stresses than that experienced under axial load
alone? Moreover, very little information is available in the literature on the bending
stiffness requirements, or on the magnitude of bending stress.
DYNAMIC LOAD ANALYSIS OF THE CONNECTING ROD
The connecting rod undergoes a complex motion, which is characterized by
inertia loads that induce bending stresses. In view of the objective of this study, which is
optimization of the connecting rod, it is essential to determine the magnitude of the loads
acting on the connecting rod. In addition, significance of bending stresses caused by
inertia loads needs to be determined, so that we know whether it should be taken into
account or neglected during the optimization. Nevertheless, a proper picture of the stress
variation during a loading cycle is essential from fatigue point of view and this will
require FEA over the entire engine cycle.
The objective of this chapter is to determine these loads that act on the connecting
rod in an engine so that they may be used in FEA. The details of the analytical vector
approach to determine the inertia loads and the reactions are presented in Appendix I.
This approach is explained by Wilson and Sadler (1993). The equations are further
simplified so that they can be used in a spreadsheet format. The results of the analytical
vector approach have been enumerated in this chapter.
This work serves two purposes. It can used be for determining the inertia loads
and reactions for any combination of engine speed, crank radius, pressure-crank angle
diagram, piston diameter, piston assembly mass, connecting rod length, connecting rod
mass, connecting rod moment of inertia, and direction of engine rotation.
VERIFICATION OF ANALYTICAL APPROACH
The analytical approach used in this study was verified with the results obtained
from ADAMS/View -11. A simple slider crank mechanism as shown in Figure 2.4 was
used in ADAMS. This mechanism will be referred to as ‘slider-crank mechanism-1’ and
its details have been tabulated in Table 2.1. The crank OA rotates about point O and the
end B of the (connecting rod) link AB slides along the line OB. The material density used
is 7801.0 kg/m3 (7.801E-006 kg/mm3). Crank OA rotational speed is 3000 rev/min
clockwise.
All these details were input to the DAP. Results were generated for the clockwise
crank rotation of the ‘slider crank mechanism-1’. It is to be noted that the gas load is not
included here since the purpose is just to verify the DAP. However, it is just a matter of
superimposing the gas load with the load at the piston pin end in DAP, when it is used for
the actual connecting rod analysis.
DYNAMIC ANALYSIS FOR THE ACTUAL CONNECTING ROD
Now that the DAP has been verified, it can be used to generate the required
quantities for the actual connecting rod which is being analyzed. The engine
configuration considered has been tabulated in Table 2.2. The pressure crank angle
diagram used is shown in Figure 2.10 obtained from a different OEM engine (5.4 liter,
V8 with compression ratio 9, at speed of 4500 rev/min). These data are input to the DAP,
and results consisting of the angular velocity and angular acceleration of the connecting
rod, linear acceleration of the connecting rod crank end center and of the center of
gravity, and forces at the ends are generated for a few engine speeds.
Results for this connecting rod at the maximum engine speed of 5700 rev/min
have been plotted in Figures 2.11 through 2.14. Figure 2.11 shows the variation of the
angular velocity over one complete engine cycle at crankshaft speed of 5700 rev/min.
Figure 2.12 shows the variation of angular acceleration at the same crankshaft speed.
Note that the variation of angular velocity and angular acceleration from 0o to 360o is
identical to its variation from 360o to 720o. Figure 2.13 shows the variation of the force
acting at the crank end. Two components of the force are plotted, one along the direction
of the slider motion, Fx, and the other normal to it, Fy. These two components can be
used to obtain crank end force in any direction. Figure 2.14 shows similar components of
load at the piston pin end. It would be particularly beneficial if components of these
forces were obtained along the length of the connecting rod and normal to it. These
components are shown in Figure 2.15 for the crank end and Figure 2.16 for the piston pin
end.
FEA WITH DYNAMIC LOADS
Once the components of forces at the connecting rod ends in the X and Y
directions are obtained, they can be resolved into components along the connecting rod
length and normal to it. The components of the inertia load acting at the center of gravity
can also be resolved into similar components. It is neither efficient nor necessary to
perform FEA of the connecting rod over the entire cycle and for each and every crank
angle. Therefore, a few positions of the crank were selected depending upon the
magnitudes of the forces acting on the connecting rod, at which FEA was performed. The
justification used in selecting these crank positions is as follows:
The stress at a point on the connecting rod as it undergoes a cycle consists of two
components, the bending stress component and the axial stress component. The bending
stress depends on the bending moment, which is a function of the load at the C.G. normal
to the connecting rod axis, as well as angular acceleration and linear acceleration
component normal to the connecting rod axis. The variation of each of these three
quantities over 0o–360o is identical to the variation over 360o-720o. This can be seen from
Figure 2.20 for the normal load at the connecting rod ends and at the center of gravity. In
addition, Figure 2.12 shows identical variation of angular acceleration over 0o–360o and
360o-720o. Therefore, for any given point on the connecting rod the bending moment
varies in an identical fashion from 0o–360o crank angle as it varies from 360o–720o crank
angle .
Quasi-Dynamic FEA
The same mesh that was used for static FEA, as presented in the section above,
was also used for quasi-dynamic FEA. Convergence was checked at locations where high
bending stresses are expected. In this case they were checked at locations 12 and 13,
about 87.6 mm from crank end center, as shown in Figure 3.5. As discussed in Chapter 4,
locations 12 and 13 (with reference to Figure 3.5) experience considerably high bending
stresses. Figure 3.6 indicates that convergence of stress σxx was achieved with a mesh
that uses 1.5 mm uniform global element length and 1 mm local element length.
Test Assembly FEA
The mesh used for static FEA used 1.5 mm global element length and 1 mm local
element length at chamfers. The mesh used for assembly FEA was even finer. The mesh
was generated with an element length of 1 mm between the ends of the connecting rod
and 1.5 mm at the cap.