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Safety Checks - After Treatment Systems


Recent changes in emissions legislation have driven manufacturers to develop cleaner and more efficient engine systems. This could take the form of an EGR (Exhaust Gas Recirculation) or an SCR (Selective Catalytic Reduction).

The SCR system essentially consists of a catalyst (CAT) in the exhaust system and a means dosing metered amounts of urea (Adblu) into the CAT. A reaction takes place that ultimately reduces the amount of NOx emitted from the exhaust.

A typical system requires an awareness of the basic function and some special considerations in owning and operating a bus or coach with such a system.

The cleanliness of the Adblu itself is critical to the operation of the system; the metering device (Dosing Unit) is sensitive to pollutants or particulate matter. Diesel in particular can ultimately damage the CAT too, therefore care when filling or choosing a decanting method is required.

A dedicated container for Adblu should never be used for screen wash, fuel cleaning products or any other liquid. Care should always be taken when storing Adblu liquid.

The dosing system is generally air controlled and includes in-line air filters that require maintenance. Adblu, if exposed to the outside atmosphere in the form of a leak, is prone to natural crystallisation, this can create blockages in the dosing system if not maintained properly. Always look out for signs of this crystallising effect as it usually indicates a leak in the system.

The Adblu line also has a filter gauze which should be clear of debris or crystallised Adblu. A block in this filter would cause the system to default and cease delivery of Adblu, resulting in zero Nox reduction and a fault in the system. It would also de-rate the engine to 60% torque.

As with many modern electronically controlled systems a dashboard warning light is usually the first indication that the system has a fault, specialist diagnostic tools will direct you to the source of the problem and get the vehicle back on the road as soon as possible.