LP-EGR Control in Variable Displacement Engines
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Solution Overview
Problem
In variable displacement engines, the long transport delay of low-pressure EGR systems leads to difficulties in providing a desired amount of EGR, especially during transient conditions, resulting in engine misfires and combustion instability due to varying EGR tolerance levels when transitioning between operating modes.
Innovation Solution
Implementing a dual fixed EGR schedule system, where a higher EGR percentage is applied during variable displacement mode and a lower EGR percentage during non-variable displacement mode, with delayed transition out of VDE mode until EGR levels in the air induction system decrease to prevent exposure to higher EGR levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed EGR percentage is used during VDE mode operation, then EGR delivery is improved for the current mode, but engine misfires occur when transitioning to non-VDE mode due to excess EGR dilution
Solution Approach 1:
The EGR percentage is changed from a fixed value to a dynamic value that adapts based on engine operating mode. The system implements separate EGR schedules (first EGR schedule for VDE mode, second EGR schedule for non-VDE mode) that are dynamically selected based on whether cylinders are deactivated, allowing the EGR delivery to match the engine's current EGR tolerance and prevent misfires during mode transitions
Solution Approach 2:
The EGR percentage parameter is changed based on the engine operating mode. When transitioning from VDE to non-VDE mode, the system changes the EGR percentage from the first fixed percentage to the second fixed percentage, accounting for the reduced EGR tolerance when all cylinders are active. This parameter change ensures combustion stability is maintained across different operating conditions
2Object-affected harmful factors
If the engine transitions out of VDE mode during tip-out, then NVH issues are reduced, but combustion instability occurs due to lingering high EGR levels in the air induction system
Solution Approach 1:
The system performs a preliminary action by delaying the transition out of VDE mode until the EGR level in the air induction system has decreased to an acceptable threshold. This preliminary delay ensures that high EGR levels are cleared from the system before transitioning to non-VDE mode, preventing combustion instability and misfires while still ultimately achieving the NVH benefit
Data Source
AI summary
Methods and systems are provided for controlling LP-EGR flow in a variable displacement engine. In one example, a method may include providing a higher fixed LP-EGR percentage relative to a total intake air flow during a VDE mode, and providing a lower fixed EGR percentage relative to the total intake air flow during a non-VDE mode. Further, during a tip-out when operating in the VDE mode at a load below a threshold, transitioning out of the VDE mode after transitioning the LP-EGR from the higher fixed percentage to the lower fixed percentage.