Touareg TDI Clean Diesel: AdBlue in detail

February 16, 2009 -- Filed under Technology by Louis-Alain Richard

I’ve already written about the pollution control systems on the 2-litre 4-cylinder TDI engine which uses, in turn, an oxidation catalytic converter, a particulate filter, the recirculation of exhaust gases and an NOx trap. All this makes the TDI engine the cleanest diesel of all time.

A different approach is used in the Touareg TDI’s 3-litre V6 engine: a new pollution control system with urea injection completes the treatment process of exhaust gas. This the solution that has been dubbed AdBlue.

I have already broached the subject here, but those who would like a little more information can continue reading.

Modified engine

Since its launch in 1989, a lot of effort has gone into making TDI technology what it has become today. In fact, TDI now meets standards that haven’t even been adopted yet in any jurisdiction! Research and development have produced an injection system that operates at nothing less than 2000 bar (30,000 lb/in²), and the recent decision to fit the combustion chamber with pressure sensors means injection and combustion can be monitored in real time. These modifications are instrumental in drastically reducing pollution even before it has reached the exhaust system.

Furthermore, gases that are still untreated are taken care of in the exhaust’s pollution control system, which has been cleverly designed to fit snugly underneath the vehicle. Indeed, fitting everything in was a complex problem, since it was essential to preserve the Touareg’s ability to conquer rough terrain (see video), even with a TDI engine. It’s too bad the whole apparatus isn’t readily visible, because it’s very much an impressive sight to behold!

The principles behind the treatment of exhaust

Once the exhaust gas has been forced out of the combustion chamber, and although it’s relatively clean, it still has to be treated in order to come out odourless with no smoke and a minimum of NOx. The first step is for an oxidation catalytic converter to transform partly burned hydrocarbons and carbon monoxide (CO) into carbon dioxide (CO₂) and water (H₂O), the same process employed in the gas engine.

The gas is then pushed through a particulate filter that removes any soot and stores it in small porous metallic tubes. This soot is usually burned off when the vehicle is running. Both the converter and the filter are standard equipment with all modern Volkswagen TDI engines.

To reduce NOx to a minimum, a third step is necessary. As the exhaust comes out of the particulate filter, a 32.5% solution of liquid urea is injected into the gas stream. The heat from the exhaust breaks down the solution into ammonia, which in turn helps to break down the NOx into harmless nitrogen (N₂) and oxygen (O₂) in the last catalytic converter, the NOx trap. Two NOx sensors monitor the reaction and adjust the quantity of urea solution injected. This is the process that is specific to AdBlue.

New components

Precision and consistency are two critical factors in the AdBlue injection process. A small reactor has been specially designed to inject the exhaust gas with urea solution in pulsated spurts. For better results, the exhaust and urea solution must be well mixed before reaching the NOx trap.

The 3-litre V6 TDI’s numerous sensors play a crucial role in the treatment of exhaust gas. The new NOx sensors measure the concentration of NOx before the exhaust reaches the particulate filter and after the NOx trap: the exact quantity of AdBlue solution required can thus be calculated. Furthermore, pressure and temperature sensors calculate the amount of soot in the particulate filter and can initiate a regeneration process if needed.

Although designed with an integrated AdBlue solution reservoir, the main fuel tank has retained its capacity at 100 litres. The 22.5 litres of solution are stored in two reservoirs: the 7-litre active reservoir located right under the fuel tank filler neck, and the 15.5-litre passive reservoir located under the floor. A 5-bar pump propels the AdBlue solution to the active reservoir towards the reactor, while the passive reservoir only serves to ensure sufficient quantities of solution are available. Another pump ensures the exchange of solution between the two reservoirs.

Filling up with AdBlue solution is done through a small opening located beside the fuel filler neck under the same gas tank flap. As AdBlue solution only needs to be added when you come in for regular maintenance, this is done during routine visits. The solution will also be sold at certain service stations. Because AdBlue solution freezes below minus 11°C, a heating system ensures the proper temperature is always maintained in the active reservoir, the dosage module and the pump.

Hidden beauty

As I’ve already mentioned, it’s too bad that this incredible pollution control apparatus is so well hidden underneath the chassis. Most people will never get to admire the intrinsic beauty of this masterpiece of research and technology. In fact, this technology has made the clean diesel engine every bit as impressive as the know-how that makes modern engines so powerful -the 600 hp of luxury sedans come to mind.

However impressive, I believe even this kind of power is outshone by the pollution control system, although it’s still easier to sell power than pollution control. My hope is that the launch of a vehicle like the Touareg TDI and other TDI vehicles will eventually change our scale of values.

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