Engine-off Vacuum Decay Leak Detection for Fuel Tank Alpha Error Reduction
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Solution Overview
Problem
Conventional natural vacuum leak detection systems have overly sensitive pass/fail thresholds, leading to high alpha error rates, where good fuel systems are incorrectly identified as leaking, particularly in compact vehicles with small fuel tanks.
Innovation Solution
The engine-off vacuum decay method involves maintaining a pressure below the threshold before engine shutdown, monitoring the vacuum switch post-shutdown, and determining a pass if a predetermined time elapses before the pressure exceeds the threshold, using a low-level purge flow and damping coil current to prevent poppet resonance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional natural vacuum leak detection is used, then leak detection capability is provided, but the pass/fail threshold is too low causing high alpha error rates
Solution Approach 1:
The system performs preliminary actions by maintaining sub-atmospheric pressure in the fuel tank before engine shutdown and establishing a baseline vacuum level. This preliminary conditioning allows the system to distinguish between normal pressure fluctuations and actual leaks, thereby reducing false positives while maintaining detection sensitivity.
Solution Approach 2:
The invention changes the detection parameter from absolute vacuum level to vacuum decay rate. By monitoring how quickly vacuum is lost over time rather than just the vacuum level itself, the system can accommodate normal variations in fuel tank pressure while detecting abnormal pressure changes indicative of leaks, thus reducing alpha errors.
2Measurement precision
If the pass/fail threshold is lowered to detect smaller leaks, then detection sensitivity increases, but good systems are incorrectly identified as leaking
Solution Approach 1:
The system employs feedback mechanisms by continuously monitoring vacuum levels and comparing them against dynamically adjusted thresholds based on historical data and system conditions. This feedback loop allows the system to learn normal pressure variations and adjust detection criteria accordingly, maintaining high sensitivity while reducing false positives through adaptive thresholding.
Solution Approach 2:
The invention introduces dynamic elements by making the detection threshold adaptive rather than fixed. The system adjusts detection criteria based on real-time conditions such as temperature, fuel level, and engine operating history, allowing the threshold to move with system conditions rather than remaining static, thereby reducing false positives while maintaining detection capability.
3Productivity
If vacuum threshold is set to detect medium leakage, then more leaks are detected, but diagnostic accuracy decreases due to alpha errors
Solution Approach 1:
The system segments the detection process into multiple stages: initial vacuum establishment, stabilization period, decay monitoring phase, and result determination. Each stage has specific criteria and thresholds, allowing the system to evaluate different aspects of system behavior separately. This segmentation enables comprehensive leak detection while maintaining diagnostic accuracy by requiring multiple criteria to be met simultaneously.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method reduces alpha error by providing additional opportunities for systems to pass the leak test, even with medium leakage, thereby improving diagnostic accuracy and reducing unnecessary repairs.
Implementation Method 1
Over time, vacuum develops in a fuel tank due to gas law effects, especially due to cooling of the tank.
Implementation Method 2
vacuum develops in a fuel tank due to gas law effects, especially due to cooling of the tank
Data Source
AI summary
A technique is provided for detecting leaks in a fuel system such as an automotive fuel system. The technique complements an on-board diagnostics evaporative leak monitor that uses natural vacuum leak detection (NVLD). The technique utilizes the same switch and valve utilized by NVLD.Before engine shut-down, the system maintains a vacuum in the fuel tank and also provides a low-level purge flow. Upon engine shut-down, a timer is started and the NVLD switch is monitored to determine how long the vacuum is maintained in the tank. If the vacuum is maintained longer than a predetermined time period, then the system determines that the leak test is passed. If the vacuum decays faster than the predetermined time period, then the NVLD test is performed and system determines that the system passes the leak test if the NVLD test is passed.