Variable Cam Timing for Pre-ignition Control in Boosted Engines
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Solution Overview
Problem
Boosted engines operating in blow-through mode are prone to pre-ignition events, which can lead to abnormal combustion, engine degradation, and turbo lag, as existing mitigation strategies like cylinder enrichment or reducing boosting device output either exacerbate issues or compromise engine performance.
Innovation Solution
Implementing a system that adjusts valve overlap in response to pre-ignition indications, reducing positive valve overlap to limit engine load during blow-through operations and incrementing counters for pre-ignition events, allowing for targeted mitigating actions such as enrichment and throttle adjustments based on mode-specific thresholds to manage pre-ignition and maintain performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If valve overlap is increased to reduce turbo lag, then engine response speed is improved, but pre-ignition risk increases
Solution Approach 1:
The valve overlap duration is made dynamically adjustable rather than fixed. The control system monitors pre-ignition indicators and dynamically modifies valve overlap timing to prevent pre-ignition while maintaining turbo responsiveness. This allows the system to adapt valve timing based on real-time combustion conditions.
Solution Approach 2:
The invention changes the parameter of valve overlap timing based on detected pre-ignition conditions. When pre-ignition is detected, the system modifies valve timing parameters to reduce or eliminate valve overlap, thereby preventing the harmful effect while allowing normal operation during non-problematic conditions.
2Reliability
If cylinder enrichment is used to mitigate pre-ignition, then pre-ignition frequency is reduced, but catalyst temperature increases
Solution Approach 1:
The invention extracts the pre-ignition mitigation action from the cylinder level and moves it to the valve timing level. Instead of enriching the cylinder mixture, the system modifies valve overlap timing to prevent pre-ignition at its source, thereby avoiding the harmful thermal effects on the catalyst while still achieving reliability improvement.
Solution Approach 2:
Valve timing acts as an intermediary control mechanism between the turbocharging system and combustion process. By adjusting valve overlap, the system indirectly controls pre-ignition tendency without directly altering fuel composition or catalyst environment, thus preventing both pre-ignition and catalyst overheating.
3Object-affected harmful factors
If boosting device output is reduced to prevent pre-ignition, then pre-ignition severity is decreased, but engine power decreases
Solution Approach 1:
The system takes preliminary action by detecting pre-ignition indicators before full pre-ignition events occur. By advancing the control action based on predictive indicators, the system can adjust valve timing proactively to prevent severe pre-ignition while maintaining normal boosting operation and engine power output.
Data Source
AI summary
Methods and systems are provided for addressing pre-ignition occurring while operating with blow-though air delivery. A variable cam timing device used to provide positive intake to exhaust valve overlap is adjusted in response to an indication of pre-ignition to transiently reduce valve overlap. Pre-ignition mitigating load limiting and enrichment applied during a blow-through mode is adjusted differently from those applied when blow-through air is not being delivered.