Exhaust Gas Recirculation vs Selective Catalytic Reduction

October 22, 2008

To Our Customers and Friends,

For most of us,change is as scary as the zombie in a Halloween movie. But come 2010,most of us will have to make changes to our engines to comply with the new standards set forth by the Environmental Protection Agency (EPA). What will you do? Which engine will you choose? The following article is a comparison of 2 types of available engines that comply with the EPA emissions standards.


Making sense of it all

Heavy-duty trucks comprise about 10 percent of nitrogen oxide emissions in the United States

In 2010, the U.S. Environmental Protection Agency will be changing its emissions standard to two-tenths of a gram of nitrogen oxides(NOx) per horsepower hour.

This significant reduction will force the diesel engine industry to react strongly to meet the requirements.

It's not too soon to start thinking about which EPA-compliant engine you'll be buying: a smaller, lighter one that requires urea as an additive, or a larger, heavier one that doesn't.

So -- let's start with the basics:

What is SCR? Selective Catalytic Reduction (SCR) - a form of after treatment to reduce NOx outside the engine. SCR feeds a small amount of ammonia-containing urea solution into the exhaust stream of a diesel engine. The ammonia combines with NOx to form harmless byproducts that are then discharged through the tailpipe. This reaction will reduce NOx emissions up to 90%. Urea (also called Ad-Blue) is a liquid reductant produced from synthetic ammonia and carbon dioxide. Currently, 90% of the urea produced worldwide is used as a fertilizer. In order to utilize the SCR system, trucks would need to have an on board tank.

What is Cooled EGR? Exhaust Gas Recirculation (EGR)- is a NOx emissions reduction technique that re-circulates a portion of an engine's exhaust gas back to the engine cylinders. Engines employing EGR recycle part of the engine exhaust back to the engine air intake. The oxygen depleted exhaust gas blends into the fresh air entering the combustion chamber. Reducing the oxygen produces a lower temperature burn, reducing NOx emissions by as much as 50%. The recycled exhaust gas can then be cooled. This "cooled EGR", can create an even greater reduction in emissions by further lowering the combustion temperatures. When used with a DPF (diesel particle filter), emissions can be reduced up to 90%.

Now that you understand the basics of the two systems, let's see what happens when we compare them side by side, weighing the pros and cons:

Selective Catalytic Reduction (SCR)

Enhances thermal efficiency & fuel economy

It reduces heat rejection and cooling system stresses, allowing for a smaller radiator and cooling fan and extending oil-drain intervals

The smaller, lighter engine may equate to increased payload and less expensive injection system

Can reduce emissions up to 90%

Ideal where fuel economy and weight are primary considerations and trucks operate on main travel lanes

Availability of urea - searching for suppliers may add out-of-route miles

Consumption of urea is unpredictable, since its mixing ratio varies with driving conditions

Vehicles will be fitted with an NOx sensor to ensure the urea level is not neglected. Failure to maintain the urea tank will result in a minimum 40% reduction in torque output if the additive runs out

The urea system cost doesn't really scale with engine size. It's a fixed cost and as you move down in engine size, it starts to account for a larger percentage of the engine cost

Least effective in stop start situations such as city operations where the constant acceleration creates the most NOx

Unknown price stability of urea

Exhaust Gas Recirculation (EGR)

Ideal for users running less traveled routes and those eager to avoid replenishing urea

No additive, no extra tanks, and the loss of payload and fuel capacity associated with SCR

No risk of experiencing a power down event due to NOx monitoring the addition of EGR coolers increases overall engine size and the additional heat loads could mean an increase of 10 to 30% cooling capacity might be required

Technical risk related to the SCR catalyst and doser is eliminated

The simpler fueling requirements are easier for hired drivers, thus good for small-fleet driver retention

When combined with a DPF can reduce emissions up to 90%Engines are larger and possibly heavier, depending on power rating

Larger radiator and fan are needed to handle small increase in heat rejection

The fuel cost is higher than the cost of fuel plus urea in an SCR system

2010 PLANS (announced to date):

Caterpillar - out of heavy-duty truck engine market

Cummins - EGR

Detroit Diesel - SCR using urea solution & catalyst

International - EGR

Mack - SCR using urea solution & catalyst

Paccar - Unannounced

Volvo - SCR using urea solution & catalyst

There are notable advantages & disadvantages to both systems. It seems to come down to dollars and cents. For operators, the major considerations are around the price stability over the operating life of the vehicle and the hassle factor associated with the purchase, storage and filling of the urea tank, along with your own belief on the future cost of Urea.

Whatever you decide, we are here. We want to help you make the best decision and find the best solution. So give us a call or stop by today!

Marc Hess

Best Trucks KC

Reprint courtesy of MHC

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