CLEANING UP THE ACT: REDUCING NOX EMISSIONS VIA SELECTIVE CATALYTIC REDUCTION

In this article, AutoMate’s Clint Flower discusses SCR technology: why it is important, how it works, and what you need to know when you see it in your shop

When the Volkswagen Dieselgate scandal first broke in September of 2015, few people understood the profound effect this event was to have on the direction of vehicle technology in the global car industry. Three years on, the shockwaves continue to reverberate internationally, as automakers scramble to clean-up their diesel-engines and turn to various solutions to achieve this.
As a result, technicians will increasingly find themselves dealing with systems designed specifically to reduce diesel engine emissions, particularly as the more complex diesel vehicles from the last decade begin filtering into the independent aftermarket.
One such system is SCR, or Selective Catalyst Reduction: an exhaust gas after-treatment system used by manufacturers of diesel powered vehicles to reduce the oxides of nitrogen (NOx) produced by their engines during combustion.
SCR is now employed widely across the industry, particularly in the aftermath of Dieselgate, and is often paired with Diesel Particulate Filters to achieve maximum NOx reduction without reducing vehicle performance.
It’s interesting to note that a great number of the vehicles implicated in the Dieselgate saga were not fitted with SCR, with Volkswagen attempting to eliminate the expense of installing such technology into their vehicles by using the “defeat” software instead. Indeed, installation of SCR requires substantial engineering of the vehicle floor-pan to accommodate the various delivery lines and reservoirs that form part of the system, meaning it cannot be easily retrofitted and must be ‘designed-in’ from the drawing-board stage of development.
It’s also worth noting that different manufacturers have different names for their SCR systems, for example, Mercedes refers to their system as BlueTec, while the PSA Group (Peugeot and Citroen) call their system BlueHDi.
So, what is SCR?
Fundamentally, SCR utilises a chemical reaction known as selective reduction. This means that, out of all exhaust gas pollutants produced by the engine, only the NOx is specifically targeted and reduced.
To achieve this goal, a reducing agent is injected into the exhaust gas flow upstream of the NOx catalyst that reacts (reduces) NOx into pure nitrogen and water. The primary reducing agent used in this reaction is ammonia, which is extremely toxic in nature and difficult to store. (See Graphic 1)
Thus, manufacturers use a diluted form known as “Aqueous Urea’. A common brand of aqueous urea reductant is “AD BLUE”. This is a synthetically manufactured reductant using a 32.5 percent solution of urea in water. As a side benefit, using Aqueous Urea provides additional Oxygen in the exhaust gas stream that aids in the conversion of Carbon Monoxide into safer Carbon Dioxide.
Urea based reductants must be kept within a very narrow temperature range, as most will freeze if their temperatures drop to around minus 10 degrees Celsius. These same reductants will also break down at temperatures of over 70 degrees Celsius and are very susceptible to bacteria growth (Urea is after all, an organic liquid). Vehicles using SCR must have a storage system that can accommodate the characteristics of urea based reductants.
The reducing agent or reductant tank contains a delivery module, a level evaluation unit and the heater control module. Due to the low freezing point of the reductant, heating elements are built into the tank, delivery module and delivery lines to prevent solidification of the Urea solution. (See Graphic 2)

Reductant is drawn from the heated pot via a low-pressure pump. When the ignition is switched on and the operating conditions for the SCR system are met, the pump delivers reductant to the injector at pressures of around five bar. Aqueous urea needs to be vaporised in order to react with NOx, and the injection pressure and exhaust gas temp achieves the right conditions.
When the driver switches the engine off, an integrated reversing valve is activated by the ECM, drawing the reductant out of the injector and delivery line then back into the storage tank. If the reducing agent was left in the lines and allowed to freeze, not only would the NOx output of the engine exceed the emission standards but the expansion of the urea solution may cause permanent damage to the SCR system.
Reductant pressure is monitored by the ECM to determine the optimal current supply to the supply pump, thereby maintaining a pre-defined delivery rate to the injector.
The level sender unit used in the reductant tank is designed as a multi sensor array – and is mounted directly into the heater pot. A designated control module evaluates the level signals from each individual sensor and transmits actual level data to the engine control module via PWM signal.
Each sensor is of the capacitive design, and calibrated to measure different properties of the reducing agent as well as the fluid level. Because the UREA solution is of a dielectric nature; contamination, biological growth, reductant breakdown and quality can be determined by the capacitance of the fluid. (See Graphic 3)
The role of a delivery injector in the SCR system is to deliver a metered quantity of reductant directly into the exhaust gasses downstream of the close-coupled DPF.
The ECM is responsible for the operation and monitoring of the SCR injector and its control circuit. If a failure occurs and reductant is unable to be introduced into the exhaust gas, the ECM may prevent engine start if the failure warnings are repeatedly ignored by the vehicle driver. (See Graphics 5 & 6)
Shortly after injection and mixing has occurred, the aqueous urea solution undergoes chemical reactions with the exhaust gas – known as “Thermolysis” and “Hydrolysis.” The aqueous urea separates into ammonia and oxygen.

NOx reduction takes place in the reduction catalytic converter, which is designed to accelerate the selective reduction reaction. It is the reaction between the NOx and ammonia that forms nitrogen and water. The design of the reduction catalytic converter is very similar to that of an oxidation catalytic converter using a honeycomb-shaped ceramic body. The converter’s substrate can be coated with a variety of different ceramic materials including copper zeolite – an accelerant for NOx reduction.
SCR converters can operate from as low as 200 degrees Celsius, although normal operating temperatures typically exceed 350 degrees Celsius. If the exhaust temperature drops below this, the chemical reaction is no longer effective – so reduction of the NOx can’t occur. Exhaust gas temperature sensors located upstream and downstream of the reduction catalyst monitor and report changes in temperature to the engine control module.
A dedicated NOx sensor is mounted in the exhaust flow directly downstream of the reduction converter. This sensor works in a similar way to a wideband or lambda probe. NOx sensor signal values are very small, so a control module is needed to amplify and convert current values to network messages which are received by the ECM. (See Graphic 4)
The moral of the story is that diesel emission control is only going to get more complex as worldwide emission standards are tightened and enforced. Staying ahead of your customers’ needs by learning about this rapidly evolving technology is therefore vital.
AutoMate’s online, on-demand training program contains over 370 videos on topics including SCR and other important systems, making it an ideal tool for all technicians with an eye for their continuous professional development in a changing world.

Written by Clint Flower – Automotive Technical Trainer and Presenter – with Harrison Boudakin for AutoMate Training an industry leading provider of online, on-demand digital training.

Visit www.automatetraining.com for a free 14-day trial.