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A few clarifications on the different ways of depolluting a clean diesel engine
Following Andrew’s post, I dug around a bit to delve deeper into this question. You know, I consider myself capable of grasping complex concepts, provided one takes the time to explain things clearly to me. That being said, why can’t I make heads or tails of all these new diesel engine terms? BlueTEC, AdBlue, EGR, SCR… what do they actually mean? I’m guessing that I’m not the only one struggling with these concepts. So here’s an attempt to clear things up.
First, you should know that all engines, whether they are in a car or a truck, must meet certain pollution emission standards. These standards differ for gas and diesel engines, nor are they the same for passenger vehicles and highway transport vehicles.
Second, while depolluting gas engines is relatively simple – usually all it takes is a catalytic converter – diesel engines need a bit more help. There are two methods of doing this: EGR and SRC.
EGR: technology used in the Jetta TDI Clean Diesel
Exhaust Gas Recirculation (EGR) involves recirculating a portion of the engine’s exhaust gas, a process that has been around since the 1970s, although at the time it was used only on gas engines. In theory, the EGR system routes a portion of the exhaust gas (5% to 30%) back into the intake manifold via a special tube and EGR valve. The goal of this process is to reduce the production of nitrogen oxides (NOx) by lowering the fuel’s peak combustion temperature. To lower this temperature, the EGR system adds to the air-fuel mix a certain amount exhaust gas that remains inert during combustion and which absorbs some of the heat produced.
In the case of the VW 2.0 TDI 140 hp, there are actually two EGR circuits to maximize the amount of recirculated gas. Keep in mind that a diesel can suck up as much air as it wants. So admitting a large quantity of recycled gas is a great way to reduce polluting emissions.
However, to meet the very strict BIN5/LEV2 standards, diesel engines employing EGR technology require an additional system for reducing these oxides – a catalyser that can store any residual nitrous oxide emissions. This storage catalyser self-regenerates – just like the particulate filter – during normal driving.
SCR: the other depolluting technology
The other depollution system for diesel engines is called Selective Catalytic Reduction (SCR). This process involves treating exhaust gas with a urea-based fluid additive sold under the name of AdBlue. For specialists, AdBlue is called AUS32 and contains 32.5% urea diluted in water. Once injected into the exhaust system, this additive can transform up to 80% of nitrogen oxides (NOx) into water vapour and pure nitrogen (N2). An additional catalytic reactor, positioned downstream of the urea injection, completes the reaction.
According to some sources, the consumption of AdBlue varies from 3% to 5% compared to that of diesel. What this means is that the AdBlue tank has to be refilled at regular intervals, which typically coincide with regular vehicle maintenance.
In both cases, a particulate filter is necessary for removing most of the soot upstream of the other systems.
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