SCR Systems for Diesel Engines

W. Addy Majewski

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Abstract: Urea-SCR technology has been adopted as a NOx reduction strategy from most types of mobile diesel engines. Urea-SCR systems typically include an SCR catalyst, an auxiliary oxidation catalyst, and urea injection system which supplies an aqueous urea solution upstream of the SCR catalyst. High NOx reductions depend on the catalyst temperature window and on complex, electronically controlled urea injection control strategy.

• Introduction
• System Configuration
• Urea Consumption and Replenishment
• Emission Performance
• Low Temperature Operation
• DPF Integration
• Commercial SCR Systems

Introduction

NOx reduction systems based on the selective catalytic reduction (SCR) technology have been developed and commercialized for a number of mobile diesel engine applications in jurisdictions with stringent diesel emission standards, beginning with the EU (Euro IV/V, /), Japan (JP ), and the United States (US EPA ). The use of ammonia has been practically ruled out, due to safety concerns, and urea (in water solution) has been commonly used as the preferred reductant.

SCR-only NOx Control. In applications with emission limits of moderate stringency, the most attractive SCR emission strategy involves a calibration of the engine for low PM (injection timing, high injection pressures), and using the SCR catalyst to reduce the increased NOx, Figure 1. This approach can also meet some applicable PM emission limits—as was the case with many SCR applications on Euro IV/V engines or on EU Stage IV and US Tier 4 nonroad engines. Due to the advanced injection timing a fuel economy improvement could be realized, making urea-SCR more attractive than the competing EGR technology that brings a fuel economy penalty. Urea-SCR may also have a fuel economy advantage over NOx adsorber catalysts—another competing NOx reduction technology—due to the fuel economy penalty resulting from adsorber regeneration. However, any fuel savings in SCR engines are offset to some degree by the cost of urea.

The SCR-only strategy was widely used in Europe to simultaneously meet the Euro IV/V limits for both NOx (3.5/2 g/kWh, respectively) and for PM (0.02 g/kWh) [623]. The engines were calibrated for low PM emission levels, below 0.02 g/kWh, while engine-out NOx was elevated to about 9-11 g/kWh. SCR aftertreatment was then used to bring down NOx emissions to below 2 g/kWh. The required NOx conversion efficiency of the SCR system was about 80-85% for Euro V and only about 65% at the Euro IV stage. The need for a diesel particulate filter was eliminated, resulting in smaller size, complexity, and cost of the emission aftertreatment system. The Euro V calibration could provide fuel savings of some 3-5%.

SCR systems had also been considered a potential solution for meeting the US - heavy-duty NOx fleet average standards of about 1.1 g/bhp-hr without the use of EGR [980]. In this case, the emission control system combined an SCR catalyst and a diesel particulate filter (DPF) to meet the US PM limit of 0.01 g/bhp-hr. According to cost analyses, SCR aftertreatment presented the most cost-effective technology for meeting the US emission standards. This is illustrated in Figure 2, which presents the results of an analysis by DaimlerChrysler [977]. In terms of fuel economy, -compliant SCR+DPF package could provide a 6% advantage over the MY baseline, comparing very favorably to the EGR+DPF alternative. Also, the total life cycle cost change for SCR compared favorably with the competing EGR and NOx adsorber paths over the entire analyzed range of cost for urea solutions. Notwithstanding the apparent cost benefit, EGR technology was used for NOx control in US - engines, while SCR was adopted three years later, in .

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In light-duty application, a comparison between urea-SCR and the NOx adsorber paths to meet the US Tier 2 Bin 2 emission standards conducted by Ford concluded that urea-SCR could provide a cost advantage, both in terms of system cost and operating costs [].

SCR+EGR NOx Control. Applications with more stringent emission standards require simultaneous use of SCR and EGR for NOx control. The SCR+EGR configuration has been used in US engines to meet the NOx standard of 0.2 g/bhp-hr. Relative to the standard, the limit required over 90% NOx reduction over the transient FTP cycle. In applications with such high NOx reduction requirements the SCR technology has no extra capacity to handle any increased engine-out NOx levels resulting from fuel efficient engine calibration [622] and it is necessary to combine SCR with EGR to meet these more demanding NOx limits.

Further benefits of using EGR in SCR engines include better NOx control at low temperatures, such as in applications tested over low temperature test cycles [], and a more robust configuration for meeting OBD requirements.

SCR Issues. The application of SCR technology to mobile engines requires solving a number of technical, regulatory, and urea distribution infrastructure problems. The following are the most important issues:

• Low temperature performance: SCR catalysts in mobile applications operate under a much wider temperature window than in stationary engines. Low temperature catalyst activity continues to be a problem, both in terms of NOx conversion efficiency and catalyst durability (deactivation by ammonium nitrate and/or sulfate). Furthermore, the choice of catalyst formulation is limited, as concerns have been raised in the USA and Japan about possible health impacts of vanadium emissions from vanadia-based systems.
• Control strategy: The transient operation in mobile diesel engines presents challenges in the development of urea injection strategies and makes it more difficult to control ammonia slip and other secondary emissions (such as N2O or NH4NO3). High efficiency, low NOx SCR systems require advanced closed-loop SCR control strategies and improved NOx sensor technology.
• Urea distribution infrastructure: The SCR technology requires a urea distribution infrastructure that makes it possible to replenish the on-board urea tank at reasonable intervals.
• Regulatory compliance: The diesel engine can run without SCR reductant []. To ensure emission compliance, SCR systems must be designed to make it very difficult to operate without urea. Since replenishing the urea solution is an out-of-pocket expense for the vehicle operator, SCR systems are an obvious tampering target.

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I have searched the internet and this forum and can't totally understand how the scr actually works. My first question is is def being injected into the scr constantly when the engine is running? Second, what is in the scr? Is it built like a muffler or a dpf or something else? Third, Does the scr get hot when the dpf is not regenerating? Last, can the scr be cleaned like a dpf or it that not necessary? I went to the Cummins store and asked all these questions. No one seems to comprehend how this stuff really works. I walked out a little more confused than I started. Any help understanding this would be greatly appreciated. Obviously this stuff is here to stay and I am going to have to live with it. I hope to understand it enough to learn how to avoid down time. Thanks for reading.

DEF is not being Dosed or injected all the time.

SCR stands for Selective Catalyst Reduction - Do some Google searches and you can find plenty of reading to do. Basically DEF is injected and atomized ahead of the catalyst to covert NOX- (oxides of Nitrogen) to less harmless- Carbon Dioxide-(CO2) and Water. The exhaust temperatures or SCR temp must be above 500 degrees F for sufficient NOX conversion to occur.

A catalyst does not require normal cleaning or routine service as it is not a filter but a flow thru catalyst- Similar to an automotive catalytic converter. The only reason cleaning wound be needed is if it were contaminated with heavy amounts of oil or coolant discharge from the engine that caused the unit to become restricted or plugged.

Maintenance to the system would include- keeping the fill cap in place and fill the system with good clean DEF- Change and DEF filters as per Manufacturer recommendation. Think of the SCR-DEF a completely separate system from the DPF. They serve two completely different purposes to the treatment of the Diesel Engine exhaust.

Thanks for the reply. I have tried to read all I can about how the scr works and all the information is vague at best. I now understand the concept and it seems to make a lot of sense. It is unfortunate that egr has not been eliminated since the dpf and scr would appear to achieve the same results without running soot back through the engine.

Mostly correct though the catalyst or sootcan rather does require an occasional clean based on age. It does eventually fill with unburned soot and other contamination over time even when operateing normally. Typically idleing a lot causes this much faster as the engine cant get to the proper burn off temp and not all the soot is correctly converted. In a normal system it needs to be cleaned and/or replaced every 500k or so to keep everything operateing at peak.

So what triggers the def to be injected and how often and for how long is it injected? I understand the dpf a little better. It clogs up a bit and triggers unburnt fuel to be injected to create the reaction that turn soot to ash.

The SCR typically should never see any soot unless the DPF were cracked or damaged and soot was able to pass thru. Of course I speak generally of Cummins product emissions. There is no recommended maintenance cleaning of an SCR. I see many original with no issues closing in on and past the million miles.

Now DOC will have soot build that is cleaned or washed during a DPF maint. Oil and coolant buildup- otherwise known as face plugging is common with Oil and coolant consuming engines.

There has been some reduction of EGR% over the years and DEF has been increased to compensate. I have heard of some trials of strictly DEF and No EGR oversea's but have seen nothing close in North America. EPA restrictions pretty much made it impossible to reach requirements without EGR.

The Engine and aftertreatment ECU monitor NOX levels out of the engine and out of the tailpipe via NOX sensors measured in PPM. The goal is to have as close to 0 PPM as possible coming out of the tailpipe. DEF is dosed once the exhaust temps reach a certain threshold and adjusts the dosing rate as needed to achieve adequate levels. The NOX output varies greatly based on engine operating conditions. As long as conditions are met DEF will continue to Dose until the engine is shutdown or temps fall back out of operating range.

To a point yes. It may be diffrent with the newer ones but i know all three pete dealers ive been in have told me that as a rule the soot can should be cleaned at about 500k as they do tend to fill up. Im sure if the system is maintained correctly it likely can go a million plus but every one of the epa 13 engines i have used over the years typically start to show problems at 500ishK. Then again a lot of companys seem to just let the EGR and def maintaince slide until the truck shows problems then kick it down the road for a new one.

Mine for example only had 600k and the can was full to near bursting and had to be cleaned.