Clarifies March diesel engines story
A
little clarification regarding the March article “Power Supply.” The SCR (selective catalytic reduction) is actually a complete package of components that doesn’t “remove” NOx (nitrogen oxide) from the exhaust but rather initiates a chemical reaction that converts NOx into harmless nitrogen
and water by way of the additional fluid injected into the exhaust. Conversion rates of 90%-plus are achieved in some current applications, and an engine doesn’t “lose power” as this fluid reducing agent, DEF (diesel exhaust fluid), is diminished. Rather an engine can be programmed — most frequently because of an emissions law requirement in effect — to operate at a reduced power level, necessitating
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that an operator expediently get the onboard DEF level back up to a usable level. The point being of course that an operator cannot continue to operate an engine with no DEF being used, thereby generating excessive emissions. This is a common product characteristic of on-road trucks where additional DEF is readily available at the next fill-up, but the EPA has taken a more lenient approach for applications where the dangers of not being able to operate a particular engine outweigh the hopefully short time when it might be generating a little more emissions than usual. And for products that would “power down” because the DEF level is too low, there are typically several warnings that would let the operator know that this action is coming their way if they don’t replenish things soon. John Fischer Palatine, Ill.
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Editor’s note: Correspondent Michael Crowley responds: Thanks for your letter regarding the section on MAN diesels in the “Power Supply” story in the March issue of WorkBoat. Critiquing the story, you state that SCR “doesn’t remove NOx” from exhaust gases but converts NOx into harmless nitrogen. That’s an accurate description of what happens with SCR. However, then it’s no longer NOx but something else, which is what the article said. Secondly, you say that an engine with an SCR aftertreatment system “doesn’t lose power” as the reducing agent is diminished but is programmed to operate at reduced power levels. That’s true, to the point that eventually there might only be enough power for what some call the “limp home mode,” which would be about 20% power. That seems to be a great description of an engine that’s losing power. Just so we are clear, the reference in the story to an engine with an SCR aftertreatment system losing power is for on-road and off-road vehicles. A diminished reduction agent has no effect on a marine engine’s output. It will not cut the engine’s power. www.workboat.com • JUNE 2021 • WorkBoat
