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The Importance of Controlling Low-Temperature NOx

The Importance of Controlling Low-Temperature NOx

Apr 10, 2023
Posted by Keith Howard, Strategic Technology Manager

Euro 7 will contain tight limits for all key pollutants, combined with a widening of test conditions and extended emissions durability requirements. These changes have significant implications for the design of engines and aftertreatment systems, which, in turn, place additional demands on the engine oil. The use of high performance, aftertreatment compatible engine oils will be essential for compliance with Euro 7 requirements. Significant reductions in the levels of nitrogen oxides (NOX) will be a key part of the legislation for heavy duty (HD) vehicles, and light duty (LD) diesel vehicles will need to comply with the stricter gasoline limit due to the technology neutral approach in Euro 7. This article explains what NOX is, how it is controlled, and its impacts on engine oil requirements.

NOx Defined

NOx covers a range of nitrogen oxides: the key two for vehicle emissions being nitric oxide (NO) and nitrogen dioxide (NO2). More recently, concerns have been raised over nitrous oxide (N20) as a potent greenhouse gas. NOX is a result of the reaction of nitrogen and oxygen during combustion, especially during the lean-burn operation of diesel engines, which are a significant contributor to urban NOX levels. NOX exposure has been associated with respiratory illnesses including asthma. It also reacts with volatile organic compounds to form smog. Successive rounds of legislation have tightened limits on NOX emissions over the years in order to combat these effects. 

NOx Reduction and Problems at Low Temperatures

There are two key technologies for diesel NOX reduction: Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR). With EGR, a controlled amount of the exhaust gas is cooled and fed back into the intake. This dilutes the air and reduces the peak combustion temperature, thereby limiting the chemical reaction responsible for creating NOX. This approach has been used for several decades and is a relatively low-cost method to achieve moderate NOX reduction.  

More recent legislation has required much higher levels of NOX reduction and has led to the widespread use of SCR technology within aftertreatment systems, especially on heavy-duty diesel vehicles. SCR involves the dosing into the exhaust stream of a precisely measured amount of reductant in the form of a urea solution, commonly known as Adblue or diesel emission fluid (DEF). This solution breaks down and releases ammonia, which then reacts over a catalyst to transform NOX into harmless nitrogen and water. At higher operating temperatures, these systems are capable of conversion efficiencies exceeding 95 percent. SCR systems usually incorporate an ammonia slip catalyst at the downstream side to mop up any small amounts of unreacted ammonia.

Euro 6 HD Aftertreatment System Including Downstream SCR System

Low temperature operation, however, is a problem as the catalyst operates with limited efficiency below its lower operating or "light-off" temperature. Euro 7 will have a much wider range of on-road test conditions including low temperature urban operation and idling, combined with much lower HD NOX limits. This will create a major NOX compliance challenge. Original equipment manufacturers (OEMs) are responding by "close coupling," which involves placing an additional SCR catalyst very close to the engine to increase the temperature as much as possible (previous system layouts place the SCR catalyst further downstream, behind the particulate filter). It may also be necessary to build in an electrical catalyst heater to reduce NOX at start-up, although this will need a large amount of electrical energy which will increase fuel consumption and CO2 emissions.

Potential Euro 7 HD heavy Heavy-Duty Aftertreatment System with Close-coupled SCR


Ultra-low NOx and Engine Oils

Close coupling of the SCR system creates an emissions durability issue as the catalyst is often subjected to very hot exhaust gases containing small amounts of contaminants from the engine oil. At these high temperatures, contaminants can react and poison the catalyst surfaces, permanently reducing the NOX conversion efficiency and increase the light-off temperature. This could lead to NOX levels exceeding limits. Euro 7 will place much more emphasis on long term compliance, with heavy duty aftertreatment systems expected to work effectively at least 875,000km. Much stricter emissions tests as part of regular roadworthiness checks will be introduced along with onboard monitoring (OBM) via sensors in the exhaust system.

The use of high performance, aftertreatment compatible engine oils as part of a thorough and regular maintenance regime will be essential, not just for the warranty period, but for the full useful life of the vehicle. These oils are designed to minimize contamination of aftertreatment and are key to enabling low tailpipe emissions. They also ensure that the engine is kept clean with minimal wear, also essential for low emission levels. Their use throughout the vehicle's life will be a vital for long-term compliance with strict limits. 
OEMs specify the oils for their latest low-emission engines based on extensive research and test data. These oils combine high quality base stocks with additive technologies designed to provide maximum protection and efficiency with minimum effect on aftertreatment.  

Our View

By using high performance, OEM-specified engine oils, owners and operators will ensure that their vehicles run efficiently, reliably and with ultra-low emission levels for many years of service. 

For more information on Euro 7, please contact your Lubrizol representative.


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