The purpose of an SCR dosing system is to inject Adblue into the exhaust system, where it breaks down into ammonia. Critical factors are that precisely the correct quantity is injected, and also that the system is reliable.
One of the biggest challenges in designing an SCR dosing system is to prevent blockages. Urea, which is the only thing in Adblue apart from water, has two undesirable attributes:
- It crystallises
- It turns into solids at high temperature
Unfortunately this means that, in badly designed dosing systems, blockages are common, especially if they are only run infrequently, such as in the case of standby generators.
One of the main distinctions in dosing systems is between those which use compressed air to spray the Adblue, and those that are ‘airless’. Compressed air has two advantages in that, firstly, it cools the Adblue down, and secondly, it can be used to purge the injection nozzle when the engine is shut down, so that residues of Adblue are removed before they can cause a blockage. The only drawback of ‘air assisted’ systems is that they require a compressor, but for applications with intermittent use, they are ideal.
Since truck manufacturers began researching SCR, the technology involved in dosing systems has improved enormously, and at the same time various mass-produced components have become available at affordable prices. Blackthorn’s dosing system is air-assisted and makes use of pumps, nozzles and metering units from the truck industry, connected to a programmable logic controller which provides a user interface. The result is a modular, reliable, affordable system which can be used on any diesel or gas engine in the power generation or marine sectors.