Selective Catalytic Reduction Purge System Vacuum Control
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Solution Overview
Problem
Current purge procedures for reductant delivery units in selective catalytic reduction systems are inefficient, leading to residual fluid in injectors that can freeze and cause damage, and the creeping mechanism of urea crystals breaches seals, potentially causing leaks and decomposition of AdBlue.
Innovation Solution
A purge procedure utilizing a pumping mechanism with multiple modes of operation and a valve system that generates pressurized fluid and vacuum to efficiently remove fluid from the injector, including multiple vacuum sequences and a control strategy to maximize fluid evacuation and reduce sealing load, ensuring effective removal of trapped fluid from off-axis volumes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a simple purge procedure is used to remove fluid from the injector, then the device complexity is reduced, but the fluid removal efficiency is insufficient and residual fluid remains in the injector
Solution Approach 1:
The purge procedure employs periodic vacuum cycles where the pump alternates between vacuum and atmospheric pressure states. The valve switches between open and closed positions to create periodic vacuum suction that draws residual fluid from the injector, achieving complete fluid removal through repeated cyclic action rather than continuous flow
Solution Approach 2:
The system uses pneumatic principles by generating vacuum pressure through the pump to create a pressure differential that draws fluid from the injector. The valve controls the connection between the injector and atmosphere, allowing vacuum suction to remove residual fluid that cannot be removed by simple gravity drainage
2Reliability
If the injector is left with residual fluid to prevent freezing damage, then the reliability against freezing is improved, but the risk of decomposition from hot soak exposure increases
Solution Approach 1:
The purge procedure is executed before the engine shuts down and before hot soak conditions can develop. By removing residual fluid in advance during the shutdown sequence, the system prevents both freezing damage (by ensuring complete removal) and decomposition risk (by eliminating fluid that could be exposed to high temperatures during hot soaks)
3Reliability
If O-rings and injector cups are used to seal the injector, then the sealing function is improved, but the creeping mechanism of urea crystals can still breach the seal
Solution Approach 1:
The purge procedure removes the root cause of seal breaching by extracting residual fluid from the injector before it can accumulate and freeze. By completely evacuating the injector volume through periodic vacuum cycles, the system eliminates the fluid that would otherwise form urea crystals and creep along seal surfaces, thereby preventing seal failure
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 enhanced purge procedure significantly improves the efficiency of fluid removal from the reductant delivery unit, reducing the risk of fluid damage from freezing and decomposition, and minimizing the creeping mechanism, thereby protecting injector components and maintaining system integrity.
Implementation Method 1
the pumping mechanism generates a vacuum when the valve portion is in the closed position, and the pumping mechanism directs fluid away from the injector
Implementation Method 2
the pumping mechanism directs pressurized fluid to the injector
Implementation Method 3
the urea is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO)
Implementation Method 4
The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2)
Implementation Method 5
a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O)
Implementation Method 6
At the fluid boundary layer, if there has been a minimal bypass of the sealing joint, fluid evaporates and leaves behind urea in its solid form
Data Source
AI summary
A purge procedure which is part of an injector, that may be used as part of a reductant delivery unit (RDU), where the RDU is part of a selective catalytic reduction system for injecting diesel exhaust fluid into an exhaust system, to control exhaust emissions. The RDU delivers a reductant carrier to the engine exhaust system. The purge process includes a control strategy to improve the quality of the purge cycle (i.e., increase the amount of fluid evacuated). The sequence of the purge event is adjusted in order to generate a strong vacuum in the fluid supply line and the injector—this enhances the efficiency of the purge by increasing the initial flow rates through the injector. However, upon opening the injector, the pressure inside the fluid path increases to a level just below the ambient pressure outside the injector, therefore the gas flow rate is substantially reduced.