Methodology for Automotive Crankshaft Design using Analytical and Flexible Models 2013-36-0590
The crankshaft is one of the most important moving components of an internal combustion engine. It is responsible for transforming the oscillating piston movement into rotating movement by the connecting rods. During engine running, the crankshaft is submitted to axial, bending and torsional loads, which results in high stressed regions on the component. Due to the phased cylinder combustions, the crankshaft has high levels of torsion loads and the excessive torsional vibration is one of the main causes of failures in crankshafts and engine accessories, as pulleys, belts and gears.
This paper presents an analytical method for previewing the crankshaft stresses by considering the component as simple a cylindrical shaft, applying the radial and the torsional loads on the crankpins and supporting at the main bearing positions. The loads are applied on the beam model and the stresses are calculated by integrating the static equilibrium equations for the hyper static models for bending and torsion load cases. For an existing crankshaft model calculations, the input data can be extracted from CAD models (mass properties) and from simple CAE or analytical analyses (stiffness parameters).
The input combustion and inertial loads, radial forces and torque, are calculated by Newton-Euler method and the torsional vibration torques are determined in the frequency domain by using the Fourier transformation, convolution integral and the state equations. Moreover, a simple method for best torsional damper parameters estimation was implemented, in order to reduce the vibration amplitudes.
Torsional dampers are used in order to reduce the crankshaft torsional vibration amplitudes. Two types of dampers were implemented in the methodology proposed: tuned rubber damper and unturned viscous damper. Basically, the torsional damper is an additional inertia introduced in the system at the front end of the crankshaft. The main difference between both types is the fact that in the viscous damper, there is no stiffness between the damper ring and the damper hub, but only damping provided by the viscous fluid, commonly a silicone fluid with high viscosity. In this case, no further natural frequency is added in the system. In the other hand, the tuned damper have the stiffness and the damping parameters provided by the rubber ring, which is placed between the damper hub and the damper inertia ring. An additional natural frequency appears in the system (lower than the first mode frequency of the original system) with lower vibration amplitude.
In the proposed methodology, the calculations are performed for the entire engine cycle (two crankshaft revolutions) and for the critical engine operation conditions: maximum torque, maximum power and maximum speed. The critical crankangle is identified for each operation condition and the fatigue safety factor is estimated based on Goodman criteria.