Limit cycles in diesel/controllable pitch propeller propulsion systems using load control 1

Abstract

Marine diesel/controllable pitch propeller propulsion systems using load control can experience limit cycling or hunting. Simplified fourth-order and eight-order models are developed to study the properties of these control systems. The backlash inherent in mechanical-hydraulic type engine governors is taken as the principal nonlinearity in the system. Describing function analysis and digital simulation are used to demonstrate the existence and characteristics of the limit cycles. The numerical example is based upon the characteristics of a 1000 foot Great Lakes bulk carrier with 8000 horsepower per shaft. It is shown that an active load control feature, which couples the pitch control to the engine governor, is a major cause of the limit cycles with 15 to 20 second period observed on these vessels. The describing function analysis with the fourth-order model is effective in predicting the limit cycle existence and period. The sensivity of the limit cycle characteristics to the system parameters is presented. Results are presented for both describing function analysis and simulation for each model for comparison.