INSTANT EXPERT
AdBlue MORE THAN WEE & WIND
Modern Euro 6 diesel cars represent the most cost-effective motoring for many people, leading Rob Marshall to explain what the AdBlue consuming SCR system is all about.
UNDERSTANDING ADBLUE & SCR SYSTEMS
Once upon a time, diesels were promoted by governments that latched onto carbon dioxide (CO2) as the most harmful gas emitted by motorcar exhausts. Businesses and motorists alike lapped up the tax incentives and revelled in their fuel efficiency. As heavy-oil fuelled cars became more dominant, it was alleged that diesel exhaust fumes were causing notable air quality problems.
Pulling out the pollution
The majority of anti-pollutant measures on diesel engines are focussed on posttreatment technologies. These convert gases within the exhaust system, before they exit the tailpipe. Like a petrol engine, catalytic converters appeared on many models from the mid-1990s. These units facilitate chemical reactions but do not change themselves.
Diesels employ two-way catalytic converters, which convert poisonous carbon monoxide and hydrocarbons to carbon dioxide and water. Unlike the three-way catalytic converters used by petrols, diesel cannot treat Nitrous Oxide (NOx) gases, due to the exhaust fumes’ higher oxygen content. As NOx gases pass through the exhaust system untreated, another way has to be found to address them.
A partial solution is to use an Exhaust Gas Recirculation Valve (EGR), which allows some exhaust gases to enter the engine’s cylinders. While this seems to be a crazy idea, it reduces not only the temperatures at which NOx emissions form but also the quantity of fresh nitrogen and oxygen that enter the engine through the air filter. EGR, therefore, reduces the rate at which NOx forms at the source. To be even more effective, the exhaust gases have their temperatures reduced by the cooling system, before being drawn through the intake. These EGR coolers have become virtually universal.
With ever-tightening regulations, Euro 5 emissions focussed on tacking soot emissions. This resulted in diesel particulate filters (DPFs) becoming widespread to trap unburnt carbon particulates (soot) and, under certain conditions, burn them off into mainly harmless carbon dioxide and ash. The oxidising catalytic converter aids this process, by helping to increase the temperatures within the DPF.
Euro 6 arrives
The sixth stage of European antipollution legislation focussed mainly on waging a war on NOx emissions.
Many (but not all) Euro 6 cars also tend to possess not one but two EGR circuits. The conventional high-pressure system w
