Compensation of sub-harmonic vibrations during engine idle by variable fuel injection control

Abstract

Today, in many passenger cars and light trucks, the conventional driveline is extended by a dual mass flywheel (DMF). The DMF reduces driveline oscillations by mechanically decoupling the transmission from the periodic combustion events that excite the engine crankshaft. Existing engine control systems are designed for conventional single mass flywheel (SMF) systems. In the future, to facilitate the optimal control of engines equipped with advanced DMF systems, such conventional control systems may require adaptation, modification or even replacement. The basic task of idle speed control systems is to maintain a defined setpoint of rotational engine speed independent from engine operating conditions (e.g. load disturbances). Due to the torque reactions of the DMF, control systems designed for engines with SMF can be disturbed, leading to unstable engine idle (e.g. sub-harmonic vibrations, oscillations following load rejection, etc.) In this approach, an optimised solution for idle speed control regarding conventional combustion engines equipped with DMF is introduced. The enhanced control system is based on conventional PID-control strategies with improved fuel injection scheduling. Using incremental (i.e. tooth-to-tooth) engine speed, critical dead times, which can lead to limit cycles in non-linear closed-loop control circuits, are minimised. Limit cycles, which are distinguishable as sub-harmonic vibrations at the same frequency, are effectively reduced by improving load rejection at idle. The implementation of these solutions in current engine management systems requires no additional sensors or other hardware.