Cam synchronization algorithm for engine with variable cam timing

#1. Field of the Invention

#The invention relates to a system and method for controlling multiple camshafts in a variable cam timing engine.

#2. Background of the Invention

#FIG. 1 is block diagram of an automotive vehicle having two VCT mechanisms and a control system for controlling the mechanisms. Note that additional VCT mechanisms can be included as described below.

#FIG. 2 is a cross-section view of one of the VCT mechanisms shown in FIG. 1.

#FIGS. 3A-3B are flowcharts of a method of controlling camshafts of VCT mechanisms in an engine in accordance with the present invention.

#An engine may be configured in various ways according to the present invention. For example, the engine may have multiple VCT phaser devices that are expected to move in unison or with certain prescribed relationships. This can occur in Intake Only, Exhaust Only, Dual Equal (intake and exhaust shifted together), and Dual Independent (intake and exhaust shifted independently, possibly through different ranges and directions) VCT systems. Further, the engine may have multiple banks, each with its own VCT systems.

#As discussed above, transient VCT shifting is relevant because the VCT position directly affects airflow through the cylinder port. Accurate estimates of air mass and residual gas inducted into the cylinder are used for air/fuel control and to correct spark advance, both of which affect emissions and fuel economy. Characterization of the engine can accommodate transient non-ideal VCT phase position as the actuators attempt to place the cam shafts at the desired operating point, but this problem becomes more difficult to handle if actuators in different banks have positions that differ significantly during transients, thereby resulting in bank imbalances. Additionally, for dual independent (DI) VCT systems, there may be benefits to maintaining the intake-to-exhaust valve “overlap” (timing between intake valve opening and exhaust valve closing) during transients, and this overlap will depend on phaser shifting velocities.

#The present invention describes an algorithm for synchronizing the operation of all phasers of a DI-VCT system (two phasers for an “in-line” engine, or four phasers for a “V” engine, as just two specific examples) when engine conditions cause the phasers to move in different ways, or with differing responses. Note that this synchronization is precluded under certain engine operating conditions, where such control action may be more detrimental to driver satisfaction or could cause increases in emissions, for example, than benefits gained in emissions or fuel economy.