Shutting off the cylinders

In a drive to improve fuel economy and reduce emissions, General Motors has designed a V8 application that shuts off cylinders when they’re not needed by deactivating their valves.

However, engine design needs to be carefully evaluated and modified in order to accommodate cylinder deactivation which has a major impact on the bearing loads, crank journal vibration and torsional resonance.

GM engineers decided to streamline the development process using multibody computer simulation. Historically, the problem with this method when modeling an engine is that multibody analysis has regarded physical structures as rigid bodies without any flexibility. However, this issue has been addressed recently by software that allows flexible bodies to be incorporated into multibody simulation.

After evaluating several simulation programs, GM selected DADS/Engine, a dynamic simulation tool from LMS that allows a 3D system to be modeled or imported from CAD packages and enables engineers to define joints, constraints, and forces on the system.

DADS/Engine automatically solves the non-linear equations of motion and reports loads, positions, velocities, and accelerations at each time step of the simulation. Modules are included for simplifying the simulation of Helical Springs, CAM Contact, Flexible Crankshafts, Combustion Forces, and Hydrodynamic Bearings.

GM developed a multibody dynamic model of the crankshaft and engine block for a V6 engine, and, using MSC/Nastran, created a finite element model of the cylinder block and crankshaft. The results of the analysis provided valuable design information. For example, positions, velocities and acceleration at any mesh grid can be extracted from the solution set. The vertical crank pin loads can be plotted in parallel to the cylinder axis and lateral crank pin loads are plotted perpendicular to the cylinder axis. The display of the vertical bearing loads showed the ignition order as well as the influence of the flexplate and torque converter. The analysis also shows the minimum film thickness and maximum film pressure as a function of crank angle. Fluid structure interaction is performed in the finite element bearing approach to generate detailed bearing design parameters. The journal equilibrium velocity is computed along with corresponding nodal tractions and cavitation conditions. Using these tractions, new structural deflection values are computed and deflection rates are estimated. These deflection values and rates are used to compute new tractions and cavitation conditions as before. This process is repeated until convergence is achieved.

GM engineers have used this model to evaluate six engine designs, including several cylinder deactivation models. The model was also used to design an engine for a parallel hybrid electric vehicle that uses an internal combustion engine and an electric motor.

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