Natural Gas Engine Fuel Control for Catalyst Stability
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Solution Overview
Problem
Spark ignition internal combustion engines, particularly those fueled by natural gas, face challenges in controlling the air/fuel ratio to maintain optimal pollutant conversion efficiency in the three-way catalyst, as existing strategies are not effective in preventing deactivation of methane residue conversion and are sensitive to deviations in the air/fuel ratio.
Innovation Solution
A method using an electronic control unit to implement a feedback loop control strategy with oscillating air/fuel ratio patterns, employing lambda sensors with different sensitivity characteristics and a model-based estimation block to manage oxygen storage in the catalyst, ensuring continuous oscillation between rich and lean combustion phases to maintain optimal catalyst operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a stoichiometric air/fuel ratio is supplied to maintain optimal catalyst conversion, then pollutant conversion efficiency is improved, but the system becomes highly sensitive to small deviations that cause rapid degradation in conversion efficiency
Solution Approach 1:
The patent applies periodic action by implementing a wobbling control strategy that deliberately oscillates the air/fuel ratio around the stoichiometric point. This periodic variation prevents the catalyst from deactivating while maintaining high average conversion efficiency. The control system continuously adjusts the air/fuel ratio between slightly rich and slightly lean conditions, creating a periodic pattern that keeps the catalyst active and responsive.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the air/fuel ratio parameter within a controlled range around stoichiometry. Rather than maintaining a fixed stoichiometric ratio, the system varies this parameter periodically to prevent catalyst deactivation. This parameter modulation allows the system to adapt to different operating conditions while maintaining effective pollutant conversion.
2Reliability
If an oscillating air/fuel ratio control strategy (wobbling) is used to prevent catalyst deactivation, then methane residue conversion is improved, but the control becomes challenging due to the need for precise calibration to balance all emission species
Solution Approach 1:
The patent implements feedback control by using a lambda sensor to continuously monitor the actual air/fuel ratio in the exhaust gases. This feedback signal is fed back to the control unit, which adjusts the injection quantity accordingly. The feedback mechanism automatically adapts the oscillating control pattern to actual operating conditions, eliminating the need for complex manual calibration and ensuring optimal performance across different scenarios.
Solution Approach 2:
The control system performs self-service by automatically adjusting its own operation through the feedback loop. The lambda sensor measurements directly inform the control unit's decisions about injection quantity, allowing the system to self-regulate the air/fuel ratio oscillations without external intervention or complex calibration procedures. This self-adjusting capability simplifies the overall control strategy.
3Ease of manufacture
If a lambda sensor at the engine outlet is used to detect oxygen levels for wobbling control, then the control strategy is simple to implement, but difficult calibration is required to achieve desired behavior after disturbances such as fuel cut-offs
Solution Approach 1:
The patent applies preliminary action by implementing a model-based estimation that predicts the catalyst's oxygen storage state and anticipated emission behavior before actual disturbances occur. The control unit uses this predictive model to pre-adjust the air/fuel ratio oscillation pattern, preparing the system for upcoming disturbances such as fuel cut-offs. This proactive approach eliminates the need for complex post-disturbance calibration.
Solution Approach 2:
The patent introduces an intermediary element in the form of a model-based estimation block that mediates between the simple lambda sensor input and the complex control decisions required for disturbance management. This intermediary model translates simple sensor signals into sophisticated control actions, bridging the gap between implementation simplicity and adaptive performance without requiring complex calibration.
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 approach effectively stabilizes the catalyst around the stoichiometric condition, preventing deactivation and improving emission trade-offs, especially during transient operations and after disturbances, without requiring hardware modifications or additional devices, thus enhancing the efficiency and adaptability of natural gas engines.
Implementation Method 1
The latter is able either to oxidize carbon oxides (CO) and hydrocarbon residues (HC) or to reduce nitrogen oxides (NOx)
Implementation Method 2
based on a control strategy which uses, as a basis, a value indicative of an actual air/fuel ratio determined on the basis of a signal emitted by a lambda sensor arranged between the engine and the catalyst
Data Source
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AI summary
A method for controlling fuel supply is provided in an assembly (1), which has a spark ignition internal combustion engine (2), an exhaust after-treatment system (10) with a three-way catalyst (14), and an injection device (2) controlled to supply fuel to the engine (2) according to a set-point (λset), indicative of a desired air/fuel ratio during combustion of a mixture of fuel and air in the engine (2); the set-point (λset) has a wobbling pattern so as to operate the engine (2) with alternating lean combustion phases and rich combustion phases; a model simulates the catalyst (14) by a plurality of cells (Cell1, Cell2, ..., Celln), which correspond to respective areas arranged in series in the catalyst (14) and are associated to respective oxygen levels (Ocell1, Ocell2, ... Ocelln), estimated by the model; a parameter (Ocell1), indicative of at least one of the estimated oxygen levels, is compared with an upper and a lower threshold (S1,S2); the wobbling pattern is set so as to bring the engine (2) to the lean combustion phase when the parameter (Ocell1) becomes lower than the lower threshold (S2) and to the rich combustion phase when the parameter (Ocell1) becomes higher than the upper threshold (S1).