Exhaust Catalyst Air-Fuel Switching to Suppress Downstream NOx
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Solution Overview
Problem
Existing exhaust purification systems for internal combustion engines face challenges in preventing the emission of NOx from downstream catalysts due to HC poisoning, which reduces the catalyst's reactivity and leads to incomplete removal of oxygen and NOx, even when the oxygen storage amount is below a certain threshold.
Innovation Solution
An exhaust purification system that controls the air-fuel ratio of the exhaust gas flowing into the upstream catalyst to maintain the oxygen storage capacity of both upstream and downstream catalysts by alternating between lean and rich air-fuel ratios, preventing NOx emission and maintaining catalyst reactivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the air-fuel ratio is continuously set to lean air-fuel ratio to increase oxygen storage amount of downstream catalyst, then oxygen storage amount increases, but NOx flows out from upstream catalyst and downstream catalyst cannot sufficiently remove NOx
Solution Approach 1:
The control device periodically alternates between lean air-fuel ratio and rich air-fuel ratio instead of maintaining a continuous lean condition. This periodic switching allows the downstream catalyst to accumulate oxygen during lean phases while preventing NOx buildup by flushing with rich phases, thereby resolving the contradiction between oxygen storage and NOx removal capability
Solution Approach 2:
The system dynamically changes the air-fuel ratio parameter between lean and rich states based on the oxygen storage amount of the downstream catalyst. When oxygen storage reaches a threshold, the system switches to rich mode to prevent NOx emission, and switches back to lean mode when oxygen storage decreases, optimizing both oxygen accumulation and NOx control
2Quantity of substance
If fuel cut control is periodically executed to maintain oxygen storage amount of downstream catalyst, then oxygen storage is maintained, but unburned gas periodically flows into downstream catalyst causing HC poisoning
Solution Approach 1:
The system implements periodic fuel cut control to maintain oxygen storage in the downstream catalyst, but combines it with periodic rich air-fuel ratio phases that flush unburned gas before it can cause HC poisoning. This modified periodic approach maintains oxygen while protecting catalyst reactivity
Solution Approach 2:
The system converts the potentially harmful rich air-fuel ratio phases (which could cause unburned gas accumulation) into a beneficial flushing mechanism that prevents HC poisoning. By strategically timing rich phases after fuel cut periods, the unburned gas is burned off in the upstream catalyst before reaching the downstream catalyst, transforming a potential harm into a protective effect
3Object-generated harmful factors
If the air-fuel ratio is switched to rich air-fuel ratio to prevent NOx flowout from upstream catalyst, then NOx purification is improved, but oxygen storage amount of downstream catalyst decreases
Solution Approach 1:
The control device uses periodic switching between lean and rich air-fuel ratios, where lean phases allow the downstream catalyst to accumulate oxygen and rich phases prevent NOx from the upstream catalyst. This periodic alternation resolves the contradiction by distributing the functions of oxygen storage and NOx prevention across different time periods
Solution Approach 2:
The system maintains continuous NOx purification capability by ensuring that the upstream catalyst operates in rich mode frequently enough to prevent NOx accumulation, while the downstream catalyst accumulates oxygen during lean modes to maintain its purification function. The continuous alternation ensures both catalysts remain effective
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
The system effectively suppresses NOx emissions from the downstream catalyst by fluctuating the air-fuel ratio to manage oxygen storage, ensuring continuous purification efficiency and preventing catalyst deactivation.
Implementation Method 1
when the oxygen storage amount of the downstream side exhaust purification catalyst becomes a switching reference storage amount or less
Implementation Method 2
unburned HC is physically adsorbed on the surface of the precious metal carried on the downstream side exhaust purification catalyst (HC poisoning)
Implementation Method 3
the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst is switched to an air-fuel ratio richer than the stoichiometric air-fuel ratio
Data Source
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AI summary
An exhaust purification system of an internal combustion engine comprises an upstream side catalyst (20), a downstream side catalyst (24), a downstream side air-fuel ratio sensor (41) provided between the upstream side catalyst (20) and the downstream side catalyst (24), and a control device (31) 31 able to control an air-fuel ratio of exhaust gas flowing into the upstream side catalyst (20) as air-fuel ratio control. In the air-fuel ratio control, the control device (31) switches the air-fuel ratio of the exhaust gas to the lean air-fuel ratio when the output air-fuel ratio of the downstream side air-fuel ratio sensor (41) becomes the rich judged air-fuel ratio or less and switches the air-fuel ratio of the exhaust gas to the rich air-fuel ratio when the oxygen storage amount of the upstream side catalyst (20) becomes the switching reference storage amount or more. During the air-fuel ratio control, the control device (31) increases the concentration of NOx in the exhaust gas flowing into the upstream side catalyst (20) when the oxygen storage amount of the downstream side catalyst (24) becomes the limit storage amount or less as control for increasing NOx. As a result, NOx is kept from flowing out from the downstream side catalyst (24).