NOx emissions and SCR

[dlmetzger]

I'm puzzled about the use of urea (Selective Catalyst Reduction, aka SCR) to reduce NOX emissions. I've read several articles about its use (most recently Diesel Progress May 2008). It seems that it has been well developed in Europe's heavy duty trucks.



Lowering NOX and particulate emissions in diesel engines is difficult. The following is probably over simplified. However, to lower particulate (and increase efficiency), the compression ratio and injection pressures must be increased but at the expense of higher NOX emission. To lower NOX emission, the compression ratio must be lowered but at to expense of greater particulate and lower efficiency.



If SCR is used and it is very effective at reducing NOX, why not go that route. Urea is not a cost driver (availability may be a problem initially). It allows the engine manufacturers to concentrate on better engine efficiency and better mileage.



I understand the EPAs concern about end-users not refilling the tank, but it seems like that could be addressed. If I had to spend $10-20 (I don't know its exact cost) to fill the urea tank every oil change and got 20 mpg on my unloaded truck, I'd do it.

 

[betterthanstock]

It's been discussed before, and the decision to not use SCR was made while diesel was about $1. **/gallon.

At that price, Cummins thought no one would want to spend money on urea.

The EPA was also opposed to it, thinking that it was too easy to fool the system.



Now, at $5 and climbing, maybe the competition will force them to re-certify it for 2010 with urea and better mileage than the present. I doubt it though. Honda is working on a proprietary catalyst without urea, that they say also gets decent mileage.

2009 Honda Accord diesel to hit 52 mpg! - AutoblogGreen

 

[sag2]

The biggest drawback to Urea is the manufacturer has to certify the truck will pass emissions for the time/mileage specified by the state or federal requirements. Unless the vehicle comes with a very large Urea tank, there is no way to meet the standard. You and I may say we will fill it, but there is no way to make sure we do.

 

[betterthanstock]

The biggest drawback to Urea is the manufacturer has to certify the truck will pass emissions for the time/mileage specified by the state or federal requirements. Unless the vehicle comes with a very large Urea tank, there is no way to meet the standard. You and I may say we will fill it, but there is no way to make sure we do.

20 November 2006



The US Environmental Protection Agency (EPA) published a draft guidance document outlining the intended approach for the certification of light-duty diesel vehicles and heavy-duty diesel engines using selective catalytic reduction (SCR) systems for NOx control. As manufacturers intend to use urea-SCR technology for meeting the light-duty Tier 2 and the heavy-duty 2010 diesel emission standards, the EPA has been developing criteria for accepting the SCR technology and ensuring compliance with NOx emission standards.



In the SCR technology, a NOx reductant such as water-based urea solution is stored in an onboard tank and injected upstream of the SCR catalyst. As urea is consumed in the SCR reaction, it must be replenished at an interval of about 6,000 - 12,000 miles. If the vehicle is operated without urea, the SCR catalyst becomes inactive, resulting in significantly increased NOx emissions. The EPA guidelines address the issues of vehicle compliance (to ensure that vehicles are not operated without urea) and urea availability (to ensure that drivers will find urea when they need it) in vehicles equipped with SCR systems.



The vehicle compliance criteria are divided into five categories: (1) driver warning system, (2) driver inducement, (3) identification of incorrect reducing agent, (4) tamper resistant design, and (5) durable design.



The driver warning system must include a dashboard indicator such as a warning light, as well as an audible alarm, which are activated 1500 miles prior to the urea tank becoming empty, and gradually escalate the warning intensity as the urea tank approaches empty level. In case the urea tank becomes empty, the driver inducement system will ensure that users replenish urea. The EPA provides several examples of alternative “driver inducement” strategies. The “No engine restart after restart countdown” strategy limits the number of engine restarts once urea reaches a certain minimum level. The “No start after refueling” strategy prevents engine restart after refueling if the urea level is too low. The “Fuel lockout” strategy makes it impossible to refuel the vehicle with empty urea tank. Some form of vehicle performance degradation can be also used as a driver inducement strategy. However, the driver inducement strategies should not create safety concerns, noted the EPA.



The identification of incorrect reducing agent must ensure that the urea tank is filled with the correct urea solution, as opposed to, for example, water or diluted urea. This task can be achieved through the use of urea quality sensors or NOx emission sensors. The tamper resistant design must cover the driver warning system, driver inducement system, as well as the urea dosing system (for instance, to prevent any attempts to block the flow of urea to the SCR catalyst).



To address urea availability, the EPA requires that urea be available through dealerships and truck stops. A back-up plan must also exist, including toll-free phone numbers to help locate urea, and urea availability at other commercial outlets, e. g. , department stores.





The guideline also requires that manufacturers implement educational and outreach programs for potential customers and the service industry. Additional issues mentioned in the guideline include the need to establish a urea quality specification, and freezing protection.



The EPA will accept comments on the draft guidance by December 15, 2006.

 

[betterthanstock]

So you see, by the time the EPA even warmed over to the idea, the 07. 5 trucks were already in the showroom.

 

[dlmetzger]

Thanks for the info. Hind sight is always 20/20. Cummins made the call based on the info at the time. It would be interesting if there was some OEM change to make urea an option, but I'm not holding my breath and probably wound not be cost affective.

 

[ackermanN]

Urea, in solid form at least, is an explosive when combined with diesel fuel,eg Oklahoma City Federal bldg

destruction. That may have something to do with it also.