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NOx-Conversion Comparison of a SCR-Catalyst Using a Novel Biomimetic Effervescent Injector on a Heavy-Duty Engine
Abstract:
NOx pollution from diesel engines has been stated as causing over 10 000 pre-mature deaths annually and predictions are showing that this level will increase [1]. In order to decrease this growing global problem, exhaust after-treatment systems for diesel engines have to be improved, this is especially so for vehicles carrying freight as their use of diesel engines is expected to carry on into the future [2].
The most common way to reduce diesel engine NOx out emissions is to use SCR. SCR operates by injecting aqueous Urea solution, 32.5% by volume (AUS-32), that evaporates prior the catalytic surface of the SCR-catalyst. Due to a catalytic reaction within the catalyst, NOx is converted nominally into Nitrogen and Water.
Currently, the evaporative process is enhanced by aggressive mixer plates and long flow paths. The mixer plates create extra exhaust back pressure and cool the exhaust gases, which decreases engine and catalyst efficiency, resulting in overall poor NOx conversion (<40%) and higher CO2 production under real life drive cycle conditions.
To achieve future emission legislation targets, SCR efficiency has to be improved, especially under low catalyst temperature conditions. It should also be noted that Ammonia slip has to be avoided as it is now legislated against [3].
In this study a novel biomimetic effervescent aqueous urea injector, its design inspired by the natural spray phenomenon of the Bombardier Beetle, is used to compare against a market-leading aqueous urea doser in comparable exhaust conditions with and without aggressive mixer plates being installed.
The novel biomimetic effervescent injector operates by increasing the temperature of the Urea solution inside a constant volume chamber to its saturated vapour pressure. At the required time of injection an electromagnetically controlled outlet valve opens exposing the now superheated fluid to lower pressure conditions, this results in a hot, effervescent spray being ejected which rapidly breaks up into very fine droplets (<20um) projected at high velocity (60m/s) into the exhaust stream targeted at the face of the SCR catalyst.
The novel biomimetic effervescent injector shows great potential with a significant higher NOx conversion rate and lower ammonia slip compared to the market-leading doser on a heavy-duty engine.