Modern truck engines deliver promised performance

July 2011 Column

In response to new EPA emission standards, diesel engine manufacturers selected one of two choices to achieve a reduction in emissions of oxides of nitrogen (NOx). They chose either an in-cylinder approach through enhanced exhaust gas recirculation (EGR) over 2007 levels, or EGR combined with an engine aftertreatment approach using selective catalytic reduction (SCR) technology.

Engines used by truck manufacturers produced after Jan. 1, 2010, will have met the emission standards set by the U.S. Environmental Protection Agency. Prior to this round of emissions regulations, the EPA lowered emissions in 2004 and 2007. More recently, Washington announced plans to draft regulations for minimum fuel economy and limited carbon dioxide emissions that will go into effect in 2014. Those fuel economy and CO2 standards are now being finalized by the EPA and the National Highway Traffic Safety Administration. They're expecting manufacturers to rely on the use of existing products and technology to achieve those goals.

What's the difference?
Both the SCR and the enhanced EGR approaches use exhaust gas recirculation to circulate a portion of an engine's exhaust back into the engine cylinders to achieve a reduction in emissions. Both methods also use a diesel particulate filter to remove particulate matter from the engine exhaust.

The SCR technology approach introduces a chemical called a reducing agent into the exhaust stream, and a catalytic converter to achieve the NOx reduction required by the EPA. The enhanced EGR approach recirculates a larger amount of the engine's exhaust back into the engine where the NOx emission reduction occurs. The enhanced EGR approach is often referred to as an in-cylinder approach.

SCR technology reduces NOx by mixing the oxides of nitrogen in the exhaust gases with a solution of urea and de-ionized water commonly called diesel exhaust fluid (DEF). The exhaust passes through a decomposition tube, where the DEF is introduced through a dosing valve. The exhaust then passes through a chamber known as the SCR catalyst. Here the DEF reacts with the NOx, breaks it down, producing oxidizing ammonia. The oxidizing ammonia chemically reacts with the NOx, converting the pollutants into nitrogen, water and tiny amounts of carbon dioxide. The SCR catalyst contains a honeycomb-like substrate, which also reacts with the pollutant, converting it into nitrogen and water.

Enhanced EGR reduces NOx by increasing the amount of engine exhaust gases recirculated into the engine cylinder, slowing and cooling the combustion process. Since the exhaust gases have already combusted, they don't burn again. These gases displace some of the normal intake, slowing and cooling the combustion process, which reduces NOx formation. This process doesn't require introducing DEF into the exhaust stream to reduce NOx, but it does require more fuel to be injected into the diesel particulate filter for active regenerations. The increased heat created with the enhanced EGR approach can also require a greater cooling capacity for the engine.

Read more at http://www.fleetequipmentmag.com/Item/89156/modern_truck_engines_deliver_promised_performance.aspx

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Modern truck engines deliver promised performance

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