Simulation helps improve valve trains
|In the typical finger-follower valve train, the cam pushes against a rocker arm that pivots on one side by a hydraulic valve lifter fixed to the cylinder head. The other side of the rocker pushes against the valve stem. A valve spring pushes on the valve stem and maintains contact between valve-train components. Thus, the valve opens and closes the port based on motion generated by the cam.|
The old homegrown Fortran modeled valve motions in just one dimension. It also had trouble accurately representing dynamic lateral forces. Another drawback was that complicated design changes usually necessitated writing new code. Use of general-purpose multibody analysis codes called LMS DADS overcomes such difficulties. The software, from LMS International, Leuven, Belgium (www.lmsintl.com), boasts special features such as cam contact and combustion-force elements, helical-spring models, and flexible bodies. The software models 3D systems and imports them from most major CAD packages.
Engineers define joints, constraints, and forces in DADS, and the software solves the nonlinear motion equations and determines loads, positions, velocities, and accelerations at each time step of the simulation. The software displays results as graphs, and photorealistic 3D animations reveal flexible deformations of moving engine components.
DaimlerChrysler engineers model valve trains in DADS by defining a cam body connected to a cylinder head by a bushing element. The cam drives the rocker by a cam-contact element. The rocker pivots on a ball representing the hydraulic-valve lifter on one side and a contact element connected to the valve stem on the other side. This lets the rocker and valve stem separate and impact intermittently. The valve is divided into two bodies connected with a translational spring-damper actuator (TSDA) element. This element gives engineers a way to account for valve-stem compression and friction. Another contact element represents the valve seat. In the early stages, a TSDA element models the valve spring. Later work uses a more sophisticated helical-spring element. When components must be treated as deformable, the structural mass and stiffness results are imported from an FEA.
The helical spring is created in the coil preprocessor, which acts as a mesh preprocessor and FE solver, eliminating the need to define the spring for commercial FE codes. Engineers enter basic spring data such as material properties, dimensions, and coil centerline. Beam elements arranged along the centerline then model the spring. The solution often requires considerable computational resources, especially for a complete valve train. To sidestep this requirement, the preprocessor creates a coarse model, a one-dimensional flexible body with mass points connected by springs and dampers. It is simpler but still demonstrates the correct dynamic behavior.
The coil preprocessor calculates the mass distribution and stiffness. And DADS combustion elements model the gas forces acting on the valve disk. DaimlerChrysler engineers have developed algorithms that describe the stiffness of the oil and air cushion inside the hydraulic valve lifter. DADS calculates the needed masses and moments of inertia. Later in the design process, finite-element modeling determines the stiffness of various components, particularly the lever.
The result is a model of a single valve assembly that can simulate the performance of most valve trains. For in-depth investigations, engineers model entire cylinder heads with flexible camshafts. After testing, they compare simulation data with valve-acceleration measurements taken from prototypes. Initial measurements usually correlate well and engineers can use them to fine-tune the model for an even closer match. Once validated, the model helps evaluate the valve-train design, generates improvements, and perfects the valve lifts.
For example, engineers can use the model to investigate contact forces on the hydraulic valve-lifter ball, the spherical area that takes the brunt of forces from the lever. If stresses are too high, or there is slippage (possible if side forces are too high), there can be problems such as abrasion of the ball. Simulation exposes these problems, which are nearly impossible to measure, and evaluates possible solutions. Models can also assist in calculating the critical speed of the engine, i.e., the rpm level at which valve-train components lose contact with each other. Exceeding this speed risks engine damage.
It's not easy being a valve-train engineer
Valve-train engineers develop the linkages and mechanisms, calculate the loads, and determine how much, how fast, and with what force intake and exhaust valves will open and close. High engine speeds complicate the task, as do the dynamic effects they create. For example, valve acceleration during opening and closing is not a simple curve that follows the geometry of the cam. Dynamic excitation and oscillations, among other things, enter into the calculations for loads on valve-train components.
Designing a valve train for a new engine is much like building a bridge without knowing what it will carry or the length of its span. In the early phases, valve-train specialists work with engineers responsible for overall engine development to determine performance goals and basic geometrical constraints. Then they define the valve-train's qualities, both kinematic and dynamic, and send them to the engineering team calculating performance and gas-exchange figures. They, in turn, work out the valve timing and gas forces that create loads on the valves. These go back to the valve-train engineers so they can do dynamic simulation. Later, when assembling prototypes, valve-train engineers tell the test department what to measure. They also use test results to validate and fine-tune simulation models.